Author name code: schuessler
ADS astronomy entries on 2022-09-14
author:"Schuessler, Manfred" 
------------------------------------------------------------------------  

Title: Loss of toroidal magnetic flux by emergence of bipolar
    magnetic regions
Authors: Cameron, R. H.; Schüssler, M.
Bibcode: 2020A&A...636A...7C
Altcode: 2020arXiv200205436C
  The polarity of the toroidal magnetic field in the solar convection
  zone periodically reverses in the course of the 11/22-year solar
  cycle. Among the various processes that contribute to the removal of
  "old-polarity" toroidal magnetic flux is the emergence of flux loops
  forming bipolar regions at the solar surface. We quantify the loss of
  subsurface net toroidal flux by this process. To this end, we determine
  the contribution of an individual emerging bipolar loop and show that
  it is unaffected by surface flux transport after emergence. Together
  with the linearity of the diffusion process this means that the
  total flux loss can be obtained by adding the contributions of all
  emerging bipolar magnetic regions. The resulting total loss rate of
  net toroidal flux amounts to 1.3 × 10<SUP>15</SUP> Mx s<SUP>-1</SUP>
  during activity maxima and 6.1 × 10<SUP>14</SUP> Mx s<SUP>-1</SUP>
  during activity minima, to which ephemeral regions contribute about 90
  and 97%, respectively. This rate is consistent with the observationally
  inferred loss rate of toroidal flux into interplanetary space and
  corresponds to a decay time of the subsurface toroidal flux of about
  12 years, also consistent with a simple estimate based on turbulent
  diffusivity. Consequently, toroidal flux loss by flux emergence is a
  relevant contribution to the budget of net toroidal flux in the solar
  convection zone. The consistency between the toroidal flux loss rate
  due to flux emergence and what is expected from turbulent diffusion,
  and the similarity between the corresponding decay time and the length
  of the solar cycle are important constraints for understanding the
  solar cycle and the Sun's internal dynamics.

Title: Solar activity: periodicities beyond 11 years are consistent
    with random forcing
Authors: Cameron, R. H.; Schüssler, M.
Bibcode: 2019A&A...625A..28C
Altcode:
  Power spectra of solar activity based on historical records of sunspot
  numbers and on cosmogenic isotopes show peaks with enhanced power
  apart from the dominant 11-year solar cycle, such as the 90-year
  Gleissberg cycle or the 210-year de Vries cycle. In a previous paper
  we have shown that the overall shape of the power spectrum is well
  represented by the results of the generic normal form model for a
  noisy and weakly nonlinear limit cycle, with parameters all determined
  by observations. Using this model as a null case, we show here that
  all local peaks with enhanced power, apart from the 11-year band,
  are consistent with realization noise. Even a 3σ peak is expected
  to occur with a probability of about 0.25 at least once among the 216
  period bins resolved by the cosmogenic isotope data. This casts doubt
  upon interpretations of such peaks in terms of intrinsic periodicities
  of the solar dynamo process.

Title: Solar activity: intrinsic periodicities beyond 11 years
Authors: Cameron, Robert; Schuessler, Manfred
Bibcode: 2019arXiv190305398C
Altcode:
  Power spectra of solar activity based on historical records of sunspot
  numbers and on cosmogenic isotopes show peaks with enhanced power
  apart from the dominant 11-year solar cycle, such as the 90-year
  Gleissberg cycle or the 210-year de Vries cycle. In a previous paper
  we have shown that the overall shape of the power spectrum is well
  represented by the results of the generic normal form model for a
  noisy and weakly nonlinear limit cycle, with parameters all determined
  by observations. Using this model as a null case, we show here that
  all local peaks with enhanced power, apart from the 11-year band, are
  consistent with realisation noise. Even a $3\sigma$ peak is expected
  to occur with a probability of about 0.25 at least once among the 216
  period bins resolved by the cosmogenic isotope data. This casts doubt
  upon interpretations of such peaks in terms of intrinsic periodicities
  of the solar dynamo process.

Title: Origin of the hemispheric asymmetry of solar activity
Authors: Schüssler, M.; Cameron, R. H.
Bibcode: 2018A&A...618A..89S
Altcode: 2018arXiv180710061S
  The frequency spectrum of the hemispheric asymmetry of solar activity
  shows enhanced power for the period ranges around 8.5 years and
  between 30 and 50 years. This can be understood as the sum and beat
  periods of the superposition of two dynamo modes: a dipolar mode with
  a (magnetic) period of about 22 years and a quadrupolar mode with a
  period between 13 and 15 years. An updated Babcock-Leighton-type dynamo
  model with weak driving as indicated by stellar observations shows
  an excited dipole mode and a damped quadrupole mode in the correct
  range of periods. Random excitation of the quadrupole by stochastic
  fluctuations of the source term for the poloidal field leads to a
  time evolution of activity and asymmetry that is consistent with the
  observational results.

Title: Solar Magnetoconvection and Small-Scale Dynamo
Authors: Borrero, J. M.; Jafarzadeh, S.; Schüssler, M.; Solanki, S. K.
Bibcode: 2018smf..book..275B
Altcode:
  No abstract at ADS

Title: Observing and modeling the poloidal and toroidal fields of
    the solar dynamo
Authors: Cameron, R. H.; Duvall, T. L.; Schüssler, M.; Schunker, H.
Bibcode: 2018A&A...609A..56C
Altcode: 2017arXiv171007126C
  Context. The solar dynamo consists of a process that converts poloidal
  magnetic field to toroidal magnetic field followed by a process that
  creates new poloidal field from the toroidal field. <BR /> Aims:
  Our aim is to observe the poloidal and toroidal fields relevant to
  the global solar dynamo and to see if their evolution is captured by
  a Babcock-Leighton dynamo. <BR /> Methods: We used synoptic maps of
  the surface radial field from the KPNSO/VT and SOLIS observatories,
  to construct the poloidal field as a function of time and latitude; we
  also used full disk images from Wilcox Solar Observatory and SOHO/MDI
  to infer the longitudinally averaged surface azimuthal field. We show
  that the latter is consistent with an estimate of the longitudinally
  averaged surface azimuthal field due to flux emergence and therefore
  is closely related to the subsurface toroidal field. <BR /> Results: We
  present maps of the poloidal and toroidal magnetic fields of the global
  solar dynamo. The longitude-averaged azimuthal field observed at the
  surface results from flux emergence. At high latitudes this component
  follows the radial component of the polar fields with a short time
  lag of between 1-3 years. The lag increases at lower latitudes. The
  observed evolution of the poloidal and toroidal magnetic fields is
  described by the (updated) Babcock-Leighton dynamo model.

Title: Solar Magnetoconvection and Small-Scale Dynamo. Recent
    Developments in Observation and Simulation
Authors: Borrero, J. M.; Jafarzadeh, S.; Schüssler, M.; Solanki, S. K.
Bibcode: 2017SSRv..210..275B
Altcode: 2015SSRv..tmp..113B; 2015arXiv151104214B
  A number of observational and theoretical aspects of solar
  magnetoconvection are considered in this review. We discuss recent
  developments in our understanding of the small-scale structure of
  the magnetic field on the solar surface and its interaction with
  convective flows, which is at the centre of current research. Topics
  range from plage areas in active regions over the magnetic network
  shaped by supergranulation to the ubiquituous `turbulent' internetwork
  fields. On the theoretical side, we focus upon magnetic field generation
  by small-scale dynamo action.

Title: Observing and modelling the poloidal and toroidal magnetic
    fields of the global dynamo
Authors: Cameron, Robert; Duvall, Thomas; Schüssler, Manfred;
   Schunker, Hannah
Bibcode: 2017SPD....4830601C
Altcode:
  The large scale solar dynamo is a cycle where poloidal flux is
  generated from toroidal flux, and toroidal flux is generated from
  poloidal flux. The toroidal and poloidal fields can be inferred from
  observations, and the Babcock-Leighton model shows how differential
  rotation and flux emergence explain the observed evolution of the
  fields.

Title: The solar magnetic field: from complexity to simplicity
    (and back)
Authors: Schüssler, Manfred
Bibcode: 2017SPD....4820201S
Altcode:
  Observations reveal a stunning complexity of the magnetic field due
  to its interaction with turbulent convection. Numerical simulations
  and observations strongly suggest that most of the small-scale field
  is generated by small-scale dynamo action. The fundamental nature
  of this process makes it potentially relevant in a broad variety
  of astrophysical settings.On the other hand, the global nature of
  the 11-year cycle reveals a surprising simplicity. This suggests a
  description of the global dynamo process in terms of relatively simple
  concepts. During the last decades, studies of magnetic flux transport
  at the solar surface provided crucial information about the workings
  of the dynamo process. They confirm the visionary approach proposed
  Babcock and Leighton. A recent update of their model permits a full
  study of the space spanned by the few remaining parameters in order to
  identify the regions with solar-like solutions.Observations of other
  cool stars suggest that the relatively slow rotation of the Sun puts
  it near to the threshold for which global dynamo action ceases. This
  suggests a further simplification of the dynamo model in terms of
  a generic normal form for a weakly nonlinear system. Including the
  inherent randomness brought about by the flux emergence process leads
  to a stochastic model whose parameters are fixed by observations. The
  model results explain the variability of the solar cycle amplitudes
  from decadal to millennial time scales.However, the true complexity
  of the processes cannot be ignored. Simulations indicate that the
  connection between the toroidal field in the convection zone and
  the magnetic flux emerging at the surface is highly complex and
  non-trivial. This is an important "loose end" of Babcock-Leighton-type
  dynamo models. Furthermore, internal differential rotation, convective
  flows, meridional flows, and tilt angles are largely unknown in stars
  other the Sun and presently cannot be reliably inferred from theoretical
  models or simulations. Consequently, models for the solar dynamo cannot
  be easily extended to stars with different rotation rate, structure,
  or evolutionary state.

Title: Understanding Solar Cycle Variability
Authors: Cameron, R. H.; Schüssler, M.
Bibcode: 2017ApJ...843..111C
Altcode: 2017arXiv170510746C
  The level of solar magnetic activity, as exemplified by the number
  of sunspots and by energetic events in the corona, varies on a wide
  range of timescales. Most prominent is the 11-year solar cycle,
  which is significantly modulated on longer timescales. Drawing from
  dynamo theory, together with the empirical results of past solar
  activity and similar phenomena for solar-like stars, we show that
  the variability of the solar cycle can be essentially understood in
  terms of a weakly nonlinear limit cycle affected by random noise. In
  contrast to ad hoc “toy models” for the solar cycle, this leads
  to a generic normal-form model, whose parameters are all constrained
  by observations. The model reproduces the characteristics of the
  variable solar activity on timescales between decades and millennia,
  including the occurrence and statistics of extended periods of very
  low activity (grand minima). Comparison with results obtained with
  a Babcock-Leighton-type dynamo model confirm the validity of the
  normal-mode approach.

Title: The solar magnetic field: from complexity to simplicity
    (and back)
Authors: Schüssler, Manfred
Bibcode: 2017AAS...23030001S
Altcode:
  The Sun is the only astrophysical object that permits a detailed study
  of the basic processes governing its magnetic field. Observations
  reveal stunning complexity due to the interaction with turbulent
  convection. Numerical simulations and observations strongly suggest
  that most of the small-scale field is generated by a process called
  small-scale dynamo action. The fundamental nature of this process
  makes it a candidate for magnetic field generation in a broad variety
  of astrophysical settings.On the other hand, the global nature of the
  11-year cycle (as exhibited, for instance, by the polarity laws of
  sunspot groups and the regularly reversing axial dipole field) reveals
  a surprising simplicity. This suggests a description of the global
  dynamo process underlying the solar cycle in terms of relatively simple
  concepts. Insufficient knowledge about the structure of magnetic field
  and flows in the convection zone requires the introduction of a variety
  of free parameters (or even free functions), which severely impairs the
  explanatory power of most such models. However, during the last decades,
  surface observations of plasma flows and magnetic flux emergence,
  together with studies of magnetic flux transport, provided crucial
  information aboutthe workings of the dynamo process. They confirm
  the visionary approach proposed already in the 1960s by Babcock and
  Leighton. A recent update of their model permits a full study of the
  space spanned by the few remaining parameters in order to identify
  the regions with solar-like solutions.Observations of other cool
  stars show that the magnetic activity level decreases strongly with
  stellar rotation rate. The relatively slow rotation of the Sun puts
  it near to the threshold at which global dynamo action ceases. This
  suggests a further simplification of the dynamo model in terms of
  a generic normal form for a weakly nonlinear system. Including the
  inherent randomness brought about by the flux emergence process leads
  to a stochastic model whose parameters are fixed by observations. The
  model results explain the variability of the solar cycle amplitudes
  from decadal to millennial time scales.

Title: An update of Leighton's solar dynamo model
Authors: Cameron, R. H.; Schüssler, M.
Bibcode: 2017A&A...599A..52C
Altcode: 2016arXiv161109111C
  In 1969, Leighton developed a quasi-1D mathematical model of the solar
  dynamo, building upon the phenomenological scenario of Babcock published
  in 1961. Here we present a modification and extension of Leighton's
  model. Using the axisymmetric component (longitudinal average) of
  the magnetic field, we consider the radial field component at the
  solar surface and the radially integrated toroidal magnetic flux in
  the convection zone, both as functions of latitude. No assumptions
  are made with regard to the radial location of the toroidal flux. The
  model includes the effects of (I) turbulent diffusion at the surface
  and in the convection zone; (II) poleward meridional flow at the
  surface and an equatorward return flow affecting the toroidal flux;
  (III) latitudinal differential rotation and the near-surface layer of
  radial rotational shear; (iv) downward convective pumping of magnetic
  flux in the shear layer; and (v) flux emergence in the form of tilted
  bipolar magnetic regions treated as a source term for the radial surface
  field. While the parameters relevant for the transport of the surface
  field are taken from observations, the model condenses the unknown
  properties of magnetic field and flow in the convection zone into a
  few free parameters (turbulent diffusivity, effective return flow,
  amplitude of the source term, and a parameter describing the effective
  radial shear). Comparison with the results of 2D flux transport
  dynamo codes shows that the model captures the essential features of
  these simulations. We make use of the computational efficiency of the
  model to carry out an extended parameter study. We cover an extended
  domain of the 4D parameter space and identify the parameter ranges
  that provide solar-like solutions. Dipole parity is always preferred
  and solutions with periods around 22 yr and a correct phase difference
  between flux emergence in low latitudes and the strength of the polar
  fields are found for a return flow speed around 2 m s<SUP>-1</SUP>,
  turbulent diffusivity below about 80 km<SUP>2</SUP>s<SUP>-1</SUP>,
  and dynamo excitation not too far above the threshold (linear growth
  rate less than 0.1 yr<SUP>-1</SUP>).

Title: Solar Dynamics, Rotation, Convection and Overshoot
Authors: Hanasoge, S.; Miesch, M. S.; Roth, M.; Schou, J.; Schüssler,
   M.; Thompson, M. J.
Bibcode: 2017hdsi.book...85H
Altcode:
  No abstract at ADS

Title: Solar Cycle 25: Another Moderate Cycle?
Authors: Cameron, R. H.; Jiang, J.; Schüssler, M.
Bibcode: 2016ApJ...823L..22C
Altcode: 2016arXiv160405405C
  Surface flux transport simulations for the descending phase of
  Cycle 24 using random sources (emerging bipolar magnetic regions)
  with empirically determined scatter of their properties provide a
  prediction of the axial dipole moment during the upcoming activity
  minimum together with a realistic uncertainty range. The expectation
  value for the dipole moment around 2020 (2.5 ± 1.1 G) is comparable
  to that observed at the end of Cycle 23 (about 2 G). The empirical
  correlation between the dipole moment during solar minimum and the
  strength of the subsequent cycle thus suggests that Cycle 25 will
  be of moderate amplitude, not much higher than that of the current
  cycle. However, the intrinsic uncertainty of such predictions resulting
  from the random scatter of the source properties is considerable and
  fundamentally limits the reliability with which such predictions can
  be made before activity minimum is reached.

Title: The turbulent diffusion of toroidal magnetic flux as inferred
    from properties of the sunspot butterfly diagram
Authors: Cameron, R. H.; Schüssler, M.
Bibcode: 2016A&A...591A..46C
Altcode: 2016arXiv160407340C
  Context. In order to match observed properties of the solar cycle,
  flux-transport dynamo models require the toroidal magnetic flux to be
  stored in a region of low magnetic diffusivity, typically located at
  or below the bottom of the convection zone. <BR /> Aims: We infer the
  turbulent magnetic diffusivity affecting the toroidal field on the basis
  of empirical data. <BR /> Methods: We considered the time evolution of
  mean latitude and width of the activity belts of solar cycles 12-23 and
  their dependence on cycle strength. We interpreted the decline phase
  of the cycles as a diffusion process. <BR /> Results: The activity
  level of a given cycle begins to decline when the centers of its
  equatorward propagating activity belts come within their (full) width
  (at half maximum) from the equator. This happens earlier for stronger
  cycles because their activity belts are wider. From that moment on, the
  activity and the belt width decrease in the same manner for all cycles,
  independent of their maximum activity level. In terms of diffusive
  cancellation of opposite-polarity toroidal flux across the equator,
  we infer the turbulent diffusivity experienced by the toroidal field,
  wherever it is located, to be in the range 150-450 km<SUP>2</SUP>
  s<SUP>-1</SUP>. Strong diffusive latitudinal spreading of the toroidal
  flux underneath the activity belts can be inhibited by an inflow toward
  the toroidal field bands in the convection zone with a magnitude of
  several meters per second. <BR /> Conclusions: The inferred value of
  the turbulent magnetic diffusivity affecting the toroidal field agrees,
  to order of magnitude, with estimates based on mixing-length models for
  the solar convection zone. This is at variance with the requirement of
  flux-transport dynamo models. The inflows required to keep the toroidal
  field bands together before they approach the equator are similar to the
  inflows toward the activity belts observed with local helioseismology.

Title: Solar Dynamics, Rotation, Convection and Overshoot
Authors: Hanasoge, S.; Miesch, M. S.; Roth, M.; Schou, J.; Schüssler,
   M.; Thompson, M. J.
Bibcode: 2015SSRv..196...79H
Altcode: 2015SSRv..tmp...24H; 2015arXiv150308539H
  We discuss recent observational, theoretical and modeling progress
  made in understanding the Sun's internal dynamics, including its
  rotation, meridional flow, convection and overshoot. Over the past
  few decades, substantial theoretical and observational effort has
  gone into appreciating these aspects of solar dynamics. A review of
  these observations, related helioseismic methodology and inference and
  computational results in relation to these problems is undertaken here.

Title: Three-dimensional simulations of near-surface convection
    in main-sequence stars. III. The structure of small-scale magnetic
    flux concentrations
Authors: Beeck, B.; Schüssler, M.; Cameron, R. H.; Reiners, A.
Bibcode: 2015A&A...581A..42B
Altcode: 2015arXiv150504739B
  Context. The convective envelopes of cool main-sequence stars harbour
  magnetic fields with a complex global and local structure. These fields
  affect the near-surface convection and the outer stellar atmospheres
  in many ways and are responsible for the observable magnetic activity
  of stars. <BR /> Aims: Our aim is to understand the local structure in
  unipolar regions with moderate average magnetic flux density. These
  correspond to plage regions covering a substantial fraction of the
  surface of the Sun (and likely also the surface of other Sun-like stars)
  during periods of high magnetic activity. <BR /> Methods: We analyse
  the results of 18 local-box magnetohydrodynamics simulations covering
  the upper layers of the convection zones and the photospheres of cool
  main-sequence stars of spectral types F to early M. The average vertical
  field in these simulations ranges from 20 to 500 G. <BR /> Results:
  We find a substantial variation of the properties of the surface
  magnetoconvection between main-sequence stars of different spectral
  types. As a consequence of a reduced efficiency of the convective
  collapse of flux tubes, M dwarfs lack bright magnetic structures in
  unipolar regions of moderate field strength. The spatial correlation
  between velocity and the magnetic field as well as the lifetime
  of magnetic structures and their sizes relative to the granules
  vary significantly along the model sequence of stellar types. <P
  />Movies associated to Fig. A.1 are available in electronic form at <A
  href="http://www.aanda.org/10.1051/0004-6361/201525788/olm">http://www.aanda.org</A>

Title: Three-dimensional simulations of near-surface convection
    in main-sequence stars. IV. Effect of small-scale magnetic flux
    concentrations on centre-to-limb variation and spectral lines
Authors: Beeck, B.; Schüssler, M.; Cameron, R. H.; Reiners, A.
Bibcode: 2015A&A...581A..43B
Altcode: 2015arXiv150504744B
  Context. Magnetic fields affect the local structure of the photosphere
  of stars. They can considerably influence the radiative properties near
  the optical surface, flow velocities, and the temperature and pressure
  profiles. This has an impact on observables such as limb darkening
  and spectral line profiles. <BR /> Aims: We aim at understanding
  qualitatively the influence of small magnetic flux concentrations
  in unipolar plage regions on the centre-to-limb variation of
  the intensity and its contrast and on the shape of spectral line
  profiles in cool main-sequence stars. <BR /> Methods: We analyse
  the bolometric and continuum intensity and its angular dependence
  of 24 radiative magnetohydrodynamic simulations of the near-surface
  layers of main-sequence stars with six different sets of stellar
  parameters (spectral types F to early M) and four different average
  magnetic field strengths (including the non-magnetic case). We also
  calculated disc-integrated profiles of three spectral lines. <BR />
  Results: The small magnetic flux concentrations formed in the magnetic
  runs of simulations have a considerable impact on the intensity and
  its centre-to-limb variation. In some cases, the difference in limb
  darkening between magnetic and non-magnetic runs is larger than the
  difference between the spectral types. Spectral lines are not only
  broadened owing to the Zeeman effect, but are also strongly affected by
  the modified thermodynamical structure and flow patterns. This indirect
  magnetic impact on the line profiles is often bigger than that of the
  Zeeman effect. <BR /> Conclusions: The effects of the magnetic field on
  the radiation leaving the star can be considerable and is not restricted
  to spectral line broadening and polarisation by the Zeeman effect. The
  inhomogeneous structure of the magnetic field on small length scales and
  its impact on (and spatial correlation with) the local thermodynamical
  structure and the flow field near the surface influence the measurement
  of the global field properties and stellar parameters. These
  effects need to be taken into account in the interpretation of
  observations. <P />Appendix A is available in electronic form at <A
  href="http://www.aanda.org/10.1051/0004-6361/201525874/olm">http://www.aanda.org</A>

Title: The Cause of the Weak Solar Cycle 24
Authors: Jiang, J.; Cameron, R. H.; Schüssler, M.
Bibcode: 2015ApJ...808L..28J
Altcode: 2015arXiv150701764J
  The ongoing 11 year cycle of solar activity is considerably less
  vigorous than the three cycles before. It was preceded by a very deep
  activity minimum with a low polar magnetic flux, the source of the
  toroidal field responsible for solar magnetic activity in the subsequent
  cycle. Simulation of the evolution of the solar surface field shows
  that the weak polar fields and thus the weakness of the present cycle
  24 are mainly caused by a number of bigger bipolar regions emerging at
  low latitudes with a “wrong” (i.e., opposite to the majority for
  this cycle) orientation of their magnetic polarities in the north-south
  direction, which impaired the growth of the polar field. These regions
  had a particularly strong effect since they emerged within +/- 10^\circ
  latitude from the solar equator.

Title: A Method for Measuring Active Region Filling Factors on
    Solar-Type Stars
Authors: Giampapa, Mark Steven; Andretta, Vincenzo; Beeck, Benjamin;
   Reiners, Ansgar; Schussler, Manfred
Bibcode: 2015TESS....120101G
Altcode:
  Radiative diagnostics of “activity” in the Sun and solar-type stars
  are spatially associated with sites of emergent magnetic flux. The
  magnetic fields themselves are widely regarded as the surface
  manifestations of a dynamo mechanism. The further development of
  both dynamo theory and models of the non-radiative heating of outer
  stellar atmospheres requires a knowledge of stellar magnetic field
  properties. In this context, it becomes important to determine the
  surface distribution, or at least the fractional coverage of, magnetic
  active regions as one critical constraint for dynamo models. But,
  while information on the spatial distribution of activity on stellar
  surfaces can be gathered in some special cases (mostly rapid rotators),
  such measurements have always been elusive in more solar-like stars. We
  discuss the challenges and results obtained from a method that relies
  on the non-linear response of the two principal He I triplet lines
  (at 1083 nm and 587.6 nm) to infer useful constraints on the fractional
  area coverage of magnetic active regions on solar-type stars.

Title: The crucial role of surface magnetic fields for the solar
    dynamo
Authors: Cameron, Robert; Schüssler, Manfred
Bibcode: 2015Sci...347.1333C
Altcode: 2015arXiv150308469C
  Sunspots and the plethora of other phenomena occurring in the course of
  the 11-year cycle of solar activity are a consequence of the emergence
  of magnetic flux at the solar surface. The observed orientations
  of bipolar sunspot groups imply that they originate from toroidal
  (azimuthally orientated) magnetic flux in the convective envelope
  of the Sun. We show that the net toroidal magnetic flux generated by
  differential rotation within a hemisphere of the convection zone is
  determined by the emerged magnetic flux at the solar surface and thus
  can be calculated from the observed magnetic field distribution. The
  main source of the toroidal flux is the roughly dipolar surface
  magnetic field at the polar caps, which peaks around the minima of
  the activity cycle.

Title: MHD Simulations of Near-Surface Convection in Cool
    Main-Sequence Stars
Authors: Beeck, Benjamin; Schussler, Manfred; Reiners, Ansgar
Bibcode: 2015csss...18..467B
Altcode: 2014arXiv1408.1802B
  The solar photospheric magnetic field is highly structured owing
  to its interaction with the convective flows. Its local structure
  has a strong influence on the profiles of spectral lines not only by
  virtue of the Zeeman effect, but also through the modification of the
  thermodynamical structure (e.g. line weakening in hot small-scale
  magnetic structures). Many stars harbor surface magnetic fields
  comparable to or larger than the Sun at solar maximum. Therefore, a
  strong influence of the field on the surface convection and on spectral
  line profiles can be expected. <P />We carried out 3D local-box MHD
  simulations of unipolar magnetized regions (average fields of 20, 100,
  and 500G) with parameters corresponding to six main-sequence stars
  (spectral types F3V to M2V). The influence of the magnetic field on
  the convection and the local thermodynamical structure were analyzed
  in detail. For three spectral lines, we determined the impact of the
  magnetic field on the disc-integrated Stokes-I profiles. Line weakening
  has in many cases a stronger impact on the spectral line profiles than
  the Zeeman effect. Moreover, for some stars, the correlation between
  the magnetic field and the vertical velocity strongly influences the
  line shapes. These effects can impair determinations of stellar magnetic
  fields since currently used methods neglect the local structure of the
  magnetic field and its interaction with the convective flows. The MHD
  simulations presented can be used to quantify these effects and thus
  help to improve magnetic field measurements of cool main-sequence stars.

Title: Generalized Investigation of the Rotation-Activity Relation:
    Favoring Rotation Period instead of Rossby Number
Authors: Reiners, A.; Schüssler, M.; Passegger, V. M.
Bibcode: 2014ApJ...794..144R
Altcode: 2014arXiv1408.6175R
  Magnetic activity in Sun-like and low-mass stars causes X-ray coronal
  emission which is stronger for more rapidly rotating stars. This
  relation is often interpreted in terms of the Rossby number, i.e., the
  ratio of rotation period to convective overturn time. We reconsider
  this interpretation on the basis of the observed X-ray emission and
  rotation periods of 821 stars with masses below 1.4 M <SUB>⊙</SUB>. A
  generalized analysis of the relation between X-ray luminosity
  normalized by bolometric luminosity, L <SUB>X</SUB>/L <SUB>bol</SUB>,
  and combinations of rotational period, P, and stellar radius, R,
  shows that the Rossby formulation does not provide the solution with
  minimal scatter. Instead, we find that the relation L <SUB>X</SUB>/L
  <SUB>bol</SUB>vpropP <SUP>-2</SUP> R <SUP>-4</SUP> optimally describes
  the non-saturated fraction of the stars. This relation is equivalent
  to L <SUB>X</SUB>vpropP <SUP>-2</SUP>, indicating that the rotation
  period alone determines the total X-ray emission. Since L <SUB>X</SUB>
  is directly related to the magnetic flux at the stellar surface, this
  means that the surface flux is determined solely by the star's rotation
  and is independent of other stellar parameters. While a formulation
  in terms of a Rossby number would be consistent with these results if
  the convective overturn time scales exactly as L_bol<SUP>-1/2</SUP>,
  our generalized approach emphasizes the need to test a broader range
  of mechanisms for dynamo action in cool stars.

Title: Effects of the Scatter in Sunspot Group Tilt Angles on the
    Large-scale Magnetic Field at the Solar Surface
Authors: Jiang, J.; Cameron, R. H.; Schüssler, M.
Bibcode: 2014ApJ...791....5J
Altcode: 2014arXiv1406.5564J
  The tilt angles of sunspot groups represent the poloidal field source
  in Babcock-Leighton-type models of the solar dynamo and are crucial for
  the build-up and reversals of the polar fields in surface flux transport
  (SFT) simulations. The evolution of the polar field is a consequence
  of Hale's polarity rules, together with the tilt angle distribution
  which has a systematic component (Joy's law) and a random component
  (tilt-angle scatter). We determine the scatter using the observed tilt
  angle data and study the effects of this scatter on the evolution of
  the solar surface field using SFT simulations with flux input based
  upon the recorded sunspot groups. The tilt angle scatter is described
  in our simulations by a random component according to the observed
  distributions for different ranges of sunspot group size (total
  umbral area). By performing simulations with a number of different
  realizations of the scatter we study the effect of the tilt angle
  scatter on the global magnetic field, especially on the evolution of
  the axial dipole moment. The average axial dipole moment at the end
  of cycle 17 (a medium-amplitude cycle) from our simulations was 2.73
  G. The tilt angle scatter leads to an uncertainty of 0.78 G (standard
  deviation). We also considered cycle 14 (a weak cycle) and cycle 19
  (a strong cycle) and show that the standard deviation of the axial
  dipole moment is similar for all three cycles. The uncertainty mainly
  results from the big sunspot groups which emerge near the equator. In
  the framework of Babcock-Leighton dynamo models, the tilt angle scatter
  therefore constitutes a significant random factor in the cycle-to-cycle
  amplitude variability, which strongly limits the predictability of
  solar activity.

Title: Comparison of solar photospheric bright points between Sunrise
    observations and MHD simulations
Authors: Riethmüller, T. L.; Solanki, S. K.; Berdyugina, S. V.;
   Schüssler, M.; Martínez Pillet, V.; Feller, A.; Gandorfer, A.;
   Hirzberger, J.
Bibcode: 2014A&A...568A..13R
Altcode: 2014arXiv1406.1387R
  Bright points (BPs) in the solar photosphere are thought to be the
  radiative signatures (small-scale brightness enhancements) of magnetic
  elements described by slender flux tubes or sheets located in the darker
  intergranular lanes in the solar photosphere. They contribute to the
  ultraviolet (UV) flux variations over the solar cycle and hence may
  play a role in influencing the Earth's climate. Here we aim to obtain
  a better insight into their properties by combining high-resolution
  UV and spectro-polarimetric observations of BPs by the Sunrise
  Observatory with 3D compressible radiation magnetohydrodynamical
  (MHD) simulations. To this end, full spectral line syntheses are
  performed with the MHD data and a careful degradation is applied
  to take into account all relevant instrumental effects of the
  observations. In a first step it is demonstrated that the selected
  MHD simulations reproduce the measured distributions of intensity at
  multiple wavelengths, line-of-sight velocity, spectral line width,
  and polarization degree rather well. The simulated line width also
  displays the correct mean, but a scatter that is too small. In
  the second step, the properties of observed BPs are compared with
  synthetic ones. Again, these are found to match relatively well,
  except that the observations display a tail of large BPs with strong
  polarization signals (most likely network elements) not found in the
  simulations, possibly due to the small size of the simulation box. The
  higher spatial resolution of the simulations has a significant effect,
  leading to smaller and more numerous BPs. The observation that most BPs
  are weakly polarized is explained mainly by the spatial degradation,
  the stray light contamination, and the temperature sensitivity of the Fe
  i line at 5250.2 Å. Finally, given that the MHD simulations are highly
  consistent with the observations, we used the simulations to explore
  the properties of BPs further. The Stokes V asymmetries increase with
  the distance to the center of the mean BP in both observations and
  simulations, consistent with the classical picture of a production
  of the asymmetry in the canopy. This is the first time that this has
  been found also in the internetwork. More or less vertical kilogauss
  magnetic fields are found for 98% of the synthetic BPs underlining
  that basically every BP is associated with kilogauss fields. At the
  continuum formation height, the simulated BPs are on average 190 K
  hotter than the mean quiet Sun, the mean BP field strength is found to
  be 1750 G, and the mean inclination is 17°, supporting the physical
  flux-tube paradigm to describe BPs. On average, the synthetic BPs
  harbor downflows increasing with depth. The origin of these downflows
  is not yet understood very well and needs further investigation.

Title: Physical causes of solar cycle amplitude variability
Authors: Cameron, R. H.; Jiang, J.; Schüssler, M.; Gizon, L.
Bibcode: 2014JGRA..119..680C
Altcode:
  The level of solar activity varies from cycle to cycle. This
  variability is probably caused by a combination of nonlinear and
  random effects. Based on surface flux transport simulations, we
  show that the observed inflows into active regions and toward the
  activity belts provide an important nonlinearity in the framework of
  Babcock-Leighton model for the solar dynamo. Inclusion of these inflows
  also leads to a reproduction of the observed proportionality between
  the open heliospheric flux during activity minima and the maximum
  sunspot number of the following cycle. A substantial component of
  the random variability of the cycle strength is associated with the
  cross-equatorial flux plumes that occur when large, highly tilted
  sunspot groups emerge close to the equator. We show that the flux
  transported by these events is important for the amplitude of the polar
  fields and open flux during activity minima. The combined action of
  inflows and cross-equatorial flux plumes provides an explanation for
  the weakness of the polar fields at the end of solar cycle 23 (and
  hence for the relative weakness of solar cycle 24).

Title: Can Surface Flux Transport Account for the Weak Polar Field
    in Cycle 23?
Authors: Jiang, Jie; Cameron, Robert H.; Schmitt, Dieter; Schüssler,
   Manfred
Bibcode: 2014crh..book..289J
Altcode:
  No abstract at ADS

Title: Three-dimensional simulations of near-surface convection in
    main-sequence stars. II. Properties of granulation and spectral lines
Authors: Beeck, B.; Cameron, R. H.; Reiners, A.; Schüssler, M.
Bibcode: 2013A&A...558A..49B
Altcode: 2013arXiv1308.4873B
  Context. The atmospheres of cool main-sequence stars are structured
  by convective flows from the convective envelope that penetrate the
  optically thin layers and lead to structuring of the stellar atmospheres
  analogous to solar granulation. The flows have considerable influence on
  the 3D structure of temperature and pressure and affect the profiles
  of spectral lines formed in the photosphere. <BR /> Aims: For the
  set of six 3D radiative (M)HD simulations of cool main-sequence
  stars described in the first paper of this series, we analyse the
  near-surface layers. We aim at describing the properties of granulation
  of different stars and at quantifying the effects on spectral lines of
  the thermodynamic structure and flows of 3D convective atmospheres. <BR
  /> Methods: We detected and tracked granules in brightness images
  from the simulations to analyse their statistical properties, as well
  as their evolution and lifetime. We calculated spatially resolved
  spectral line profiles using the line synthesis code SPINOR. To enable
  a comparison to stellar observations, we implemented a numerical
  disc-integration, which includes (differential) rotation. <BR />
  Results: Although the stellar parameters change considerably along the
  model sequence, the properties of the granules are very similar. The
  impact of the 3D structure of the atmospheres on line profiles is
  measurable in disc-integrated spectra. Line asymmetries caused by
  convection are modulated by stellar rotation. <BR /> Conclusions:
  The 3D structure of cool stellar atmospheres as shaped by convective
  flows has to be taken into account when using photospheric lines to
  determine stellar parameters.

Title: Three-dimensional simulations of near-surface convection in
    main-sequence stars. I. Overall structure
Authors: Beeck, B.; Cameron, R. H.; Reiners, A.; Schüssler, M.
Bibcode: 2013A&A...558A..48B
Altcode: 2013arXiv1308.4874B
  Context. The near-surface layers of cool main-sequence stars are
  structured by convective flows, which are overshooting into the
  atmosphere. The flows and the associated spatio-temporal variations of
  density and temperature affect spectral line profiles and thus have an
  impact on estimates of stellar properties such as effective temperature,
  gravitational acceleration, and abundances. <BR /> Aims: We aim at
  identifying distinctive properties of the thermodynamic structure of
  the atmospheres of different stars and understand their causes. <BR />
  Methods: We ran comprehensive 3D radiation hydrodynamics simulations
  of the near-surface layers of six simulated stars of spectral type
  F3V to M2V with the MURaM code. We carry out a systematic parameter
  study of the mean stratifications, flow structures, and the energy
  flux in these stars. <BR /> Results: We find monotonic trends along
  the lower main sequence in granule size, flow velocity, and intensity
  contrast. The convection in the M-star models differs substantially
  from that of the hotter stars, mainly owing to the more gradual
  transition from convective to radiative energy transport. <BR />
  Conclusions: While the basic mechanisms driving surface convection
  in cool stars are the same, the properties of the convection vary
  along the main sequence. Apart from monotonic trends in rms velocity,
  intensity contrast, granule size, etc., there is a transition between
  "naked" and "hidden" granulation around spectral type K5V caused by
  the (highly non-linear) temperature dependence of the opacity. These
  variations have to be taken into account when stellar parameters are
  derived from spectra. <P />Appendix A is available in electronic form
  at <A href="http://www.aanda.org">http://www.aanda.org</A>

Title: Limits to solar cycle predictability: Cross-equatorial
    flux plumes
Authors: Cameron, R. H.; Dasi-Espuig, M.; Jiang, J.; Işık, E.;
   Schmitt, D.; Schüssler, M.
Bibcode: 2013A&A...557A.141C
Altcode: 2013arXiv1308.2827C
  Context. Within the Babcock-Leighton framework for the solar dynamo, the
  strength of a cycle is expected to depend on the strength of the dipole
  moment or net hemispheric flux during the preceding minimum, which
  depends on how much flux was present in each hemisphere at the start of
  the previous cycle and how much net magnetic flux was transported across
  the equator during the cycle. Some of this transport is associated
  with the random walk of magnetic flux tubes subject to granular and
  supergranular buffeting, some of it is due to the advection caused by
  systematic cross-equatorial flows such as those associated with the
  inflows into active regions, and some crosses the equator during the
  emergence process. <BR /> Aims: We aim to determine how much of the
  cross-equatorial transport is due to small-scale disorganized motions
  (treated as diffusion) compared with other processes such as emergence
  flux across the equator. <BR /> Methods: We measure the cross-equatorial
  flux transport using Kitt Peak synoptic magnetograms, estimating both
  the total and diffusive fluxes. <BR /> Results: Occasionally a large
  sunspot group, with a large tilt angle emerges crossing the equator,
  with flux from the two polarities in opposite hemispheres. The largest
  of these events carry a substantial amount of flux across the equator
  (compared to the magnetic flux near the poles). We call such events
  cross-equatorial flux plumes. There are very few such large events
  during a cycle, which introduces an uncertainty into the determination
  of the amount of magnetic flux transported across the equator in any
  particular cycle. As the amount of flux which crosses the equator
  determines the amount of net flux in each hemisphere, it follows that
  the cross-equatorial plumes introduce an uncertainty in the prediction
  of the net flux in each hemisphere. This leads to an uncertainty in
  predictions of the strength of the following cycle.

Title: No evidence for planetary influence on solar activity
Authors: Cameron, R. H.; Schüssler, M.
Bibcode: 2013A&A...557A..83C
Altcode: 2013arXiv1307.5988C
  Context. Recently, Abreu et al. (2012, A&amp;A. 548, A88) proposed
  a long-term modulation of solar activity through tidal effects
  exerted by the planets. This claim is based upon a comparison of
  (pseudo-)periodicities derived from records of cosmogenic isotopes
  with those arising from planetary torques on an ellipsoidally deformed
  Sun. <BR /> Aims: We examined the statistical significance of the
  reported similarity of the periods. <BR /> Methods: The tests carried
  out by Abreu et al. were repeated with artificial records of solar
  activity in the form of white or red noise. The tests were corrected
  for errors in the noise definition as well as in the apodisation and
  filtering of the random series. <BR /> Results: The corrected tests
  provide probabilities for chance coincidence that are higher than
  those claimed by Abreu et al. by about 3 and 8 orders of magnitude
  for white and red noise, respectively. For an unbiased choice of the
  width of the frequency bins used for the test (a constant multiple of
  the frequency resolution) the probabilities increase by another two
  orders of magnitude to 7.5% for red noise and 22% for white noise. <BR
  /> Conclusions: The apparent agreement between the periodicities
  in records of cosmogenic isotopes as proxies for solar activity and
  planetary torques is statistically insignificant. There is no evidence
  for a planetary influence on solar activity.

Title: Modelling stellar activity cycles using deep-seated dynamos
    and surface flux transport
Authors: Işık, Emre; Schmitt, Dieter; Schüssler, Manfred
Bibcode: 2013IAUS..294..431I
Altcode: 2012arXiv1211.1414I
  We investigate the relations between tachocline-based dynamos and
  the surface flux transport mechanisms in stars with outer convection
  zones. Using our combined models of flux generation and transport,
  we demonstrate the importance of the buoyant rise of magnetic flux,
  which physically determines the emergence latitudes and tilt angles of
  bipolar magnetic regions. The combined effects of the dynamo strength,
  flux rise, and surface transport lead to various cyclic and non-cyclic
  time series of total unsigned surface magnetic flux.

Title: Solar magneto-convection
Authors: Schüssler, Manfred
Bibcode: 2013IAUS..294...95S
Altcode: 2012arXiv1211.6253S
  An overview is given about recent developments and results of
  comprehensive simulations of magneto-convective processes in the
  near-surface layers and photosphere of the Sun. Simulations now cover
  a wide range of phenomena, from whole active regions, over individual
  sunspots and pores, magnetic flux concentrations and vortices in
  intergranular lanes, down to the intricate mixed-polarity structure
  of the magnetic field generated by small-scale dynamo action. The
  simulations in concert with high-resolution observations have provided
  breakthroughs in our understanding of the structure and dynamics of
  the magnetic fields in the solar photosphere.

Title: Can Surface Flux Transport Account for the Weak Polar Field
    in Cycle 23?
Authors: Jiang, Jie; Cameron, Robert H.; Schmitt, Dieter; Schüssler,
   Manfred
Bibcode: 2013SSRv..176..289J
Altcode: 2011SSRv..tmp..212J; 2011SSRv..tmp...69J; 2011arXiv1104.4183J;
   2011SSRv..tmp..136J; 2011SSRv..tmp..368J
  To reproduce the weak magnetic field on the polar caps of the Sun
  observed during the declining phase of cycle 23 poses a challenge to
  surface flux transport models since this cycle has not been particularly
  weak. We use a well-calibrated model to evaluate the parameter changes
  required to obtain simulated polar fields and open flux that are
  consistent with the observations. We find that the low polar field
  of cycle 23 could be reproduced by an increase of the meridional flow
  by 55% in the last cycle. Alternatively, a decrease of the mean tilt
  angle of sunspot groups by 28% would also lead to a similarly low polar
  field, but cause a delay of the polar field reversals by 1.5 years in
  comparison to the observations.

Title: Coupled model for the formation of an active region corona
Authors: Chen, Feng; Bingert, Sven; Peter, Hardi; Cameron, Robert;
   Schüssler; , Manfred; Cheung, Mark C. M.
Bibcode: 2013enss.confE..21C
Altcode:
  We will present the first model that couples the formation of an active
  region corona to a model of the emergence. This allows us to study
  when, where, and why active region loops form, and how they evolve. For
  this we use an existing 3D radiation MHD model of the emergence of an
  active region through the upper convection zone and the photosphere
  as a lower boundary for a coronal model. Our 3D MHD coronal model
  accounts for the braiding of the magnetic field lines that induces
  currents in the corona that is getting filled with the emerging magnetic
  field. Starting with a basically field-free atmosphere we follow the
  flux emergence until numerous individually identifiable hot coronal
  loops have been formed. The temperatures in the coronal loops of well
  above 1 MK are reached at densities corresponding to actually observed
  active region loops. The loops develop over a very short time period
  of the order of several minutes through the evaporation of material
  from the chromosphere. Because we have full access to the heating
  rate as a function of time and space in our computational domain we
  can determine the conditions under which these loops form.

Title: On the relation between continuum brightness and magnetic
    field in solar active regions
Authors: Danilovic, S.; Röhrbein, D.; Cameron, R. H.; Schüssler, M.
Bibcode: 2013A&A...550A.118D
Altcode:
  Context. Variations of solar irradiance are mainly determined
  by the changing coverage of the visible solar disk with magnetic
  flux concentrations. The relationship between brightness and field
  strength is an important ingredient for models and reconstructions of
  irradiance variations. <BR /> Aims: We assess the effect of limited
  observational resolution on the relationship between brightness
  and magnetic field by comparing comprehensive MHD simulations with
  observational results. <BR /> Methods: Simulations of magnetoconvection
  representing the near-surface layers of a plage region were used to
  determine maps of the continuum brightness and Stokes profiles for the
  Fe i line at 630.22 nm. After convolving with instrumental profiles,
  synthetic observations of the magnetic field were generated by applying
  a Stokes inversion code. We compare the resulting relation between
  brightness and apparent vertical magnetic field to the corresponding
  outcome derived from real observations of a plage region with the
  Hinode satellite. <BR /> Results: Consideration of the image smearing
  effects due to the limited resolution of the observations transform the
  largely monotonic relation between brightness and field strength at the
  original resolution of the simulations into a profile with a maximum
  at intermediate field strength, which is in good agreement with the
  observations. <BR /> Conclusions: Considering the effect of limited
  observational resolution renders the relation between brightness and
  magnetic field from comprehensive MHD simulations consistent with
  observational results. This is a necessary prerequisite for the
  utilization of simulations for models and reconstruction of solar
  irradiance variations.

Title: Multiwavelength Study of the Region Around the ANTARES
    Neutrino Excess
Authors: Schüssler; , Fabian; Brun, P.; Chaves, R. C. G.;
   Glicenstein, J. -F.; Kosack, K.; Moulin, E.; Peyaud, B.; Wouters,
   D.; H. E. S. S. Collaboration; Stolarczyk, T.; Vallage, B.
Bibcode: 2013ICRC...33.2686S
Altcode: 2013arXiv1307.6074S
  The ANTARES collaboration reported the results of a search for
  point-like neutrino sources using data taken in the period 2007-2010. An
  unbinned maximum likelihood based all-sky search yielded a cluster
  of 9 (5) events within a cone of 3 (1) degrees around (R.A., Dec) =
  (-46.5deg, -65.0deg). The trial factor corrected p-value of 2.6%
  (2.2 sigma) is not significant enough to claim the observation of
  an astrophysical point source. However, it currently constitutes the
  most significant localized neutrino excess observed by ANTARES. Here
  we present a multi-wavelength analysis including optical to X-ray
  archival data and a dedicated analysis of gamma-ray data from
  Fermi-LAT. In order to cover the TeV domain, dedicated observations
  with the H.E.S.S. telescope array were carried out. We present these
  data and discuss implications of the results in terms of signatures
  for a cosmic-ray acceleration site.

Title: Are the strengths of solar cycles determined by converging
    flows towards the activity belts?
Authors: Cameron, R. H.; Schüssler, M.
Bibcode: 2012A&A...548A..57C
Altcode: 2012arXiv1210.7644C
  It is proposed that the observed near-surface inflows towards the
  active regions and sunspot zones provide a nonlinear feedback mechanism
  that limits the amplitude of a Babcock-Leighton-type solar dynamo and
  determines the variation of the cycle strength. This hypothesis is
  tested with surface flux transport simulations including converging
  latitudinal flows that depend on the surface distribution of magnetic
  flux. The inflows modulate the build-up of polar fields (represented
  by the axial dipole) by reducing the tilt angles of bipolar magnetic
  regions and by affecting the cross-equator transport of leading-polarity
  magnetic flux. With flux input derived from the observed record of
  sunspot groups, the simulations cover the period between 1874 and 1980
  (corresponding to solar cycles 11 to 20). The inclusion of the inflows
  leads to a strong correlation of the simulated axial dipole strength
  during activity minimum with the observed amplitude of the subsequent
  cycle. This in agreement with empirical correlations and in line with
  what is expected from a Babcock-Leighton-type dynamo. The results
  provide evidence that the latitudinal inflows are a key ingredient in
  determining the amplitude of solar cycles.

Title: First Results from the SUNRISE Mission
Authors: Solanki, S. K.; Barthol, P.; Danilovic, S.; Feller, A.;
   Gandorfer, A.; Hirzberger, J.; Jafarzadeh, S.; Lagg, A.; Riethmüller,
   T. L.; Schüssler, M.; Wiegelmann, T.; Bonet, J. A.; González,
   M. J. M.; Pillet, V. M.; Khomenko, E.; Yelles Chaouche, L.; Iniesta,
   J. C. d. T.; Domingo, V.; Palacios, J.; Knölker, M.; González,
   N. B.; Borrero, J. M.; Berkefeld, T.; Franz, M.; Roth, M.; Schmidt,
   W.; Steiner, O.; Title, A. M.
Bibcode: 2012ASPC..455..143S
Altcode:
  The SUNRISE balloon-borne solar observatory consists of a 1m aperture
  Gregory telescope, a UV filter imager, an imaging vector polarimeter,
  an image stabilization system, and further infrastructure. The first
  science flight of SUNRISE yielded high-quality data that reveal the
  structure, dynamics, and evolution of solar convection, oscillations,
  and magnetic fields at a resolution of around 100 km in the quiet
  Sun. Here we describe very briefly the mission and the first results
  obtained from the SUNRISE data, which include a number of discoveries.

Title: Vortices, shocks, and heating in the solar photosphere:
    effect of a magnetic field
Authors: Moll, R.; Cameron, R. H.; Schüssler, M.
Bibcode: 2012A&A...541A..68M
Altcode: 2012arXiv1201.5981M
  <BR /> Aims: We study the differences between non-magnetic and
  magnetic regions in the flow and thermal structure of the upper solar
  photosphere. <BR /> Methods: Radiative MHD simulations representing
  a quiet region and a plage region, respectively, which extend into
  the layers around the temperature minimum, are analyzed. <BR />
  Results: The flow structure in the upper photospheric layers of the
  two simulations is considerably different: the non-magnetic simulation
  is dominated by a pattern of moving shock fronts while the magnetic
  simulation shows vertically extended vortices associated with magnetic
  flux concentrations. Both kinds of structures induce substantial local
  heating. The resulting average temperature profiles are characterized by
  a steep rise above the temperature minimum due to shock heating in the
  non-magnetic case and by a flat photospheric temperature gradient mainly
  caused by Ohmic dissipation in the magnetic run. <BR /> Conclusions:
  Shocks in the quiet Sun and vortices in the strongly magnetized
  regions represent the dominant flow structures in the layers around
  the temperature minimum. They are closely connected with dissipation
  processes providing localized heating.

Title: Joint Response of the Helium Lines to Chromospheric Heating
    in Solar-type Stars
Authors: Giampapa, Mark S.; Andretta, V.; Beeck, B.; Reiners, A.;
   Schussler, M.
Bibcode: 2012AAS...22020314G
Altcode:
  We present a preliminary report on simultaneous observations of
  the He I 5876 and 10830 triplet lines, respectively, in a sample of
  solar-type stars. The near-IR spectra were obtained with the VLT and
  CRIRES instrument while the visible spectra were acquired with the
  MPG/ESO 2.2-m telescope in conjunction with the FEROS spectrograph. The
  correlation of the observed strengths of these lines will be examined
  and their potential as diagnostics of active region area coverage in
  solar-type stars will be discussed. <P />The NSO is operated by AURA
  under a cooperative agreement with the National Science Foundation.

Title: Simulations of the solar near-surface layers with the CO5BOLD,
    MURaM, and Stagger codes
Authors: Beeck, B.; Collet, R.; Steffen, M.; Asplund, M.; Cameron,
   R. H.; Freytag, B.; Hayek, W.; Ludwig, H. -G.; Schüssler, M.
Bibcode: 2012A&A...539A.121B
Altcode: 2012arXiv1201.1103B
  Context. Radiative hydrodynamic simulations of solar and stellar surface
  convection have become an important tool for exploring the structure and
  gas dynamics in the envelopes and atmospheres of late-type stars and for
  improving our understanding of the formation of stellar spectra. <BR
  /> Aims: We quantitatively compare results from three-dimensional,
  radiative hydrodynamic simulations of convection near the solar surface
  generated with three numerical codes (CO<SUP>5</SUP>BOLD, MURaM,
  and Stagger) and different simulation setups in order to investigate
  the level of similarity and to cross-validate the simulations. <BR
  /> Methods: For all three simulations, we considered the average
  stratifications of various quantities (temperature, pressure, flow
  velocity, etc.) on surfaces of constant geometrical or optical depth,
  as well as their temporal and spatial fluctuations. We also compared
  observables, such as the spatially resolved patterns of the emerging
  intensity and of the vertical velocity at the solar optical surface
  as well as the center-to-limb variation of the continuum intensity
  at various wavelengths. <BR /> Results: The depth profiles of the
  thermodynamical quantities and of the convective velocities as well as
  their spatial fluctuations agree quite well. Slight deviations can be
  understood in terms of differences in box size, spatial resolution
  and in the treatment of non-gray radiative transfer between the
  simulations. <BR /> Conclusions: The results give confidence in the
  reliability of the results from comprehensive radiative hydrodynamic
  simulations.

Title: MHD Simulations Reveal Crucial Differences Between Solar and
    Very Cool Star Magnetic Structures
Authors: Beeck, B.; Schüssler, M.; Reiners, A.
Bibcode: 2011ASPC..448.1071B
Altcode: 2011arXiv1101.3848B; 2011csss...16.1071B
  We carried out 3D radiative magnetohydrodynamic simulations of the
  convective and magnetic structure in the surface layers (uppermost
  part of the convection zone and photosphere) of main-sequence stars
  of spectral types F3 to M2. The simulation results were analyzed in
  terms of sizes and properties of the convection cells (granules) and
  magnetic flux concentrations as well as velocity, pressure, density, and
  temperature profiles. Our numerical simulations show for the first time
  a qualitative difference in the magneto-convection between solar-like
  stars and M dwarfs. Owing to higher surface gravity, lower opacity
  (resulting in higher density at optical depth unity), and more stable
  downflows, small-scale magnetic structures concentrate into pore-like
  configurations of reduced intensity. This implies that in very cool
  stars magnetic surface structures like plage regions and starspots
  significantly differ from the solar example. Such a difference would
  have major impact on the interpretation of Doppler imaging data and
  the analysis of M dwarf spectra.

Title: Can Overturning Motions in Penumbral Filaments BE Detected?
Authors: Bharti, Lokesh; Schuessler, Manfred; Rempel, Matthias
Bibcode: 2011sdmi.confE..79B
Altcode:
  Numerical simulations indicate that the filamentation of sunspot
  penumbrae and the associated systematic outflow (the Evershed effect)
  are due to convectively driven fluid motions constrained by the inclined
  magnetic field. We investigate whether these motions, in particular the
  upflows in the bright filaments and the downflows at their edges, can be
  reliably observed with existing instrumentation. We use a snapshot from
  a sunspot simulation to calculate two-dimensional maps of synthetic
  line profiles for the spectral lines Fe I 7090.4 Å and C I 5380.34
  Å. The maps are spatially and spectrally degraded according to typical
  instrument properties. Line-of-sight velocities are determined from
  line bisector shifts. We find that the detectability of the convective
  flows is strongly affected by spatial smearing, particularly so for
  the downflows. Furthermore, the line-of-sight velocities are dominated
  by the Evershed flow unless the observation is made very near the disk
  center. These problems may have compromised recent attempts to detect
  overturning penumbral convection. Lines with a low formation height
  are best suited for detecting the convective flows.

Title: Can Overturning Motions in Penumbral Filaments Be Detected?
Authors: Bharti, Lokesh; Schüssler, Manfred; Rempel, Matthias
Bibcode: 2011ApJ...739...35B
Altcode: 2011arXiv1107.0398B
  Numerical simulations indicate that the filamentation of sunspot
  penumbrae and the associated systematic outflow (the Evershed effect)
  are due to convectively driven fluid motions constrained by the inclined
  magnetic field. We investigate whether these motions, in particular the
  upflows in the bright filaments and the downflows at their edges, can be
  reliably observed with existing instrumentation. We use a snapshot from
  a sunspot simulation to calculate two-dimensional maps of synthetic
  line profiles for the spectral lines Fe I 7090.4 Å and C I 5380.34
  Å. The maps are spatially and spectrally degraded according to typical
  instrument properties. Line-of-sight velocities are determined from
  line bisector shifts. We find that the detectability of the convective
  flows is strongly affected by spatial smearing, particularly so for
  the downflows. Furthermore, the line-of-sight velocities are dominated
  by the Evershed flow unless the observation is made very near the disk
  center. These problems may have compromised recent attempts to detect
  overturning penumbral convection. Lines with a low formation height
  are best suited for detecting the convective flows.

Title: Vortices in simulations of solar surface convection
Authors: Moll, R.; Cameron, R. H.; Schüssler, M.
Bibcode: 2011A&A...533A.126M
Altcode: 2011arXiv1108.0800M
  We report on the occurrence of small-scale vortices in simulations of
  the convective solar surface. Using an eigenanalysis of the velocity
  gradient tensor, we find the subset of high-vorticity regions in which
  the plasma is swirling. The swirling regions form an unsteady, tangled
  network of filaments in the turbulent downflow lanes. Near-surface
  vertical vortices are underdense and cause a local depression of the
  optical surface. They are potentially observable as bright points in
  the dark intergranular lanes. Vortex features typically exist for a
  few minutes, during which they are moved and twisted by the motion
  of the ambient plasma. The bigger vortices found in the simulations
  are possibly, but not necessarily, related to observations of
  granular-scale spiraling pathlines in "cork animations" or feature
  tracking. <P />Three movies are available in electronic form at <A
  href="http://www.aanda.org">http://www.aanda.org</A>

Title: Decay of a simulated mixed-polarity magnetic field in the
    solar surface layers
Authors: Cameron, R.; Vögler, A.; Schüssler, M.
Bibcode: 2011A&A...533A..86C
Altcode: 2011arXiv1108.1155C
  Magnetic flux is continuously being removed and replenished on the
  solar surface. To understand the removal process we carried out 3D
  radiative MHD simulations of the evolution of patches of photospheric
  magnetic field with equal amounts of positive and negative flux. We
  find that the flux is removed at a rate corresponding to an effective
  turbulent diffusivity, η<SUB>eff</SUB>, of 100-340 km<SUP>2</SUP>
  s<SUP>-1</SUP>, depending on the boundary conditions. For average
  unsigned flux densities above about 70 Gauss, the percentage of surface
  magnetic energy coming from different field strengths is almost
  invariant. The overall process is then one where magnetic elements
  are advected by the horizontal granular motions and occasionally come
  into contact with opposite-polarity elements. These reconnect above
  the photosphere on a comparatively short time scale after which the
  U loops produced rapidly escape through the upper surface while the
  downward retraction of inverse-U loops is significantly slower, because
  of the higher inertia and lower plasma beta in the deeper layers.

Title: The Sun at high resolution: first results from the Sunrise
    mission
Authors: Solanki, S. K.; Barthol, P.; Danilovic, S.; Feller,
   A.; Gandorfer, A.; Hirzberger, J.; Lagg, A.; Riethmüller, T. L.;
   Schüssler, M.; Wiegelmann, T.; Bonet, J. A.; Pillet, V. Martínez;
   Khomenko, E.; del Toro Iniesta, J. C.; Domingo, V.; Palacios, J.;
   Knölker, M.; González, N. Bello; Borrero, J. M.; Berkefeld, T.;
   Franz, M.; Roth, M.; Schmidt, W.; Steiner, O.; Title, A. M.
Bibcode: 2011IAUS..273..226S
Altcode:
  The Sunrise balloon-borne solar observatory consists of a 1m aperture
  Gregory telescope, a UV filter imager, an imaging vector polarimeter,
  an image stabilization system and further infrastructure. The first
  science flight of Sunrise yielded high-quality data that reveal the
  structure, dynamics and evolution of solar convection, oscillations
  and magnetic fields at a resolution of around 100 km in the quiet
  Sun. Here we describe very briefly the mission and the first results
  obtained from the Sunrise data, which include a number of discoveries.

Title: Is there a non-monotonic relation between photospheric
    brightness and magnetic field strength in solar plage regions?
Authors: Röhrbein, D.; Cameron, R.; Schüssler, M.
Bibcode: 2011A&A...532A.140R
Altcode:
  Context. The relationship between the brightness and field strength
  of small-scale solar magnetic features is an important factor for
  solar irradiance variations and a constraint for simulations of solar
  magneto-convection. <BR /> Aims: We wish to clarify the origin of
  the apparent discrepancy between observational results and radiative
  MHD simulations. <BR /> Methods: Maps of (bolometric) brightness and
  magnetic field strength from the simulation of a plage region were
  convolved and rebinned to mimic observations obtained with telescopes
  with finite aperture. <BR /> Results: Image smearing changes the
  monotonic relation between brightness and field strength obtained at
  the original resolution of the simulation into a profile with a maximum
  at intermediate field strength, which is in qualitative agreement with
  the observations. This result is mainly due to the smearing of strong
  magnetic fields at the bright edges of magnetic structures into the
  weakly magnetized adjacent areas. <BR /> Conclusions: Observational
  and simulation results are qualitatively consistent with each other if
  the finite spatial resolution of the observations is taken into account.

Title: Universality of the Small-scale Dynamo Mechanism
Authors: Moll, R.; Pietarila Graham, J.; Pratt, J.; Cameron, R. H.;
   Müller, W. -C.; Schüssler, M.
Bibcode: 2011ApJ...736...36M
Altcode: 2011arXiv1105.0546M
  We quantify possible differences between turbulent dynamo action in
  the Sun and the dynamo action studied in idealized simulations. For
  this purpose, we compare Fourier-space shell-to-shell energy transfer
  rates of three incrementally more complex dynamo simulations: an
  incompressible, periodic simulation driven by random flow, a simulation
  of Boussinesq convection, and a simulation of fully compressible
  convection that includes physics relevant to the near-surface layers
  of the Sun. For each of the simulations studied, we find that the
  dynamo mechanism is universal in the kinematic regime because energy
  is transferred from the turbulent flow to the magnetic field from
  wavenumbers in the inertial range of the energy spectrum. The addition
  of physical effects relevant to the solar near-surface layers, including
  stratification, compressibility, partial ionization, and radiative
  energy transport, does not appear to affect the nature of the dynamo
  mechanism. The role of inertial-range shear stresses in magnetic
  field amplification is independent from outer-scale circumstances,
  including forcing and stratification. Although the shell-to-shell energy
  transfer functions have similar properties to those seen in mean-flow
  driven dynamos in each simulation studied, the saturated states of
  these simulations are not universal because the flow at the driving
  wavenumbers is a significant source of energy for the magnetic field.

Title: Universality of the Small-Scale Dynamo Mechanism
Authors: Pietarila Graham, Jonathan; Moll, R.; Pratt, J.; Cameron,
   R.; Mueller, W.; Schuessler, M.
Bibcode: 2011SPD....42.1621P
Altcode: 2011BAAS..43S.1621P
  We quantify possible differences between turbulent dynamo action in
  the Sun and the dynamo action studied in idealized simulation. For this
  purpose we compare Fourier-space shell-to-shell energy transfer rates of
  three incrementally more complex dynamo simulations: an incompressible,
  periodic simulation driven by random flow, a simulation of Boussinesq
  convection, and a simulation of fully compressible convection that
  includes physics relevant to the near-surface layers of the Sun. For
  each of the simulations studied, we find that energy is transferred
  from the turbulent flow to the magnetic field from length-scales in the
  inertial range of the energy spectrum. The addition of physical effects
  relevant to the solar near-surface layers, including stratification,
  compressibility, partial ionization, and radiative energy transport,
  does not appear to affect the nature of the dynamo mechanism. The role
  of inertial-range shear stresses in magnetic field amplification is
  independent from outer-scale circumstances, including forcing and
  stratification. Although shell-to-shell energy transfer functions
  have similar properties in each simulation studied, the saturated
  states of these simulations are not universal; the flow at the driving
  scales is a significant source of energy for the magnetic field. The
  mechanism of energy-transfer in kinematic small-scale dynamo simulations
  exhibits universal properties. <P />This work has been supported by
  the Max-Planck Society in the framework of the Interinstitutional
  Research Initiative Turbulent transport and ion heating, reconnection
  and electron acceleration in solar and fusion plasmas&lt;/u&gt; of the
  MPI for Solar System Research, Katlenburg-Lindau, and the Institute
  for Plasma Physics, Garching (project MIF-IF-A-AERO8047).

Title: The solar magnetic field since 1700. II. Physical
    reconstruction of total, polar and open flux
Authors: Jiang, J.; Cameron, R. H.; Schmitt, D.; Schüssler, M.
Bibcode: 2011A&A...528A..83J
Altcode: 2011arXiv1102.1270J
  We have used semi-synthetic records of emerging sunspot groups based
  on sunspot number data as input for a surface flux transport model to
  reconstruct the evolution of the large-scale solar magnetic field and
  the open heliospheric flux from the year 1700 onward. The statistical
  properties of the semi-synthetic sunspot group records reflect
  those of the observed Royal Greenwich Observatory photoheliographic
  results. These include correlations between the sunspot numbers
  and sunspot group latitudes, longitudes, areas and tilt angles. The
  reconstruction results for the total surface flux, the polar field,
  and the heliospheric open flux (determined by a current sheet source
  surface extrapolation) agree well with the available observational or
  empirically derived data and reconstructions. We confirm a significant
  positive correlation between the polar field during activity minimum
  periods and the strength of the subsequent sunspot cycle, which has
  implications for flux transport dynamo models for the solar cycle. Just
  prior to the Dalton minimum, at the end of the 18th century, a long
  cycle was followed by a weak cycle. We find that introducing a possibly
  "lost" cycle between 1793 and 1800 leads to a shift of the minimum of
  the open flux by 15 years which is inconsistent with the cosmogenic
  isotope record.

Title: The solar magnetic field since 1700. I. Characteristics of
    sunspot group emergence and reconstruction of the butterfly diagram
Authors: Jiang, J.; Cameron, R. H.; Schmitt, D.; Schüssler, M.
Bibcode: 2011A&A...528A..82J
Altcode: 2011arXiv1102.1266J
  We use the historic record of sunspot groups compiled by the Royal
  Greenwich Observatory together with the sunspot number to derive
  the dependence of the statistical properties of sunspot emergence on
  the cycle phase and strength. In particular we discuss the latitude,
  longitude, area and tilt angle of sunspot groups as functions of the
  cycle strength and of time during the solar cycle. Using these empirical
  characteristics the time-latitude diagram of sunspot group emergence
  (butterfly diagram) is reconstructed from 1700 onward on the basis of
  the Wolf and group sunspot numbers. This reconstruction will be useful
  in studies of the long-term evolution of the Sun's magnetic field.

Title: Magnetic flux generation and transport in cool stars
Authors: Işık, E.; Schmitt, D.; Schüssler, M.
Bibcode: 2011A&A...528A.135I
Altcode: 2011arXiv1102.0569I
  Context. The Sun and other cool stars harbouring outer convection zones
  manifest magnetic activity in their atmospheres. The connection between
  this activity and the properties of a deep-seated dynamo generating
  the magnetic flux is not well understood. <BR /> Aims: By employing
  physical models, we study the spatial and temporal characteristics of
  the observable surface field for various stellar parameters. <BR />
  Methods: We combine models for magnetic flux generation, buoyancy
  instability, and transport, which encompass the entire convection
  zone. The model components are: (i) a thin-layer αΩ dynamo at the
  base of the convection zone; (ii) buoyancy instabilities and the rise
  of flux tubes through the convection zone in 3D, which provides a
  physically consistent determination of emergence latitudes and tilt
  angles; and (iii) horizontal flux transport at the surface. <BR />
  Results: For solar-type stars and rotation periods longer than about
  10 days, the latitudinal dynamo waves generated by the deep-seated
  αΩ dynamo are faithfully reflected by the surface distribution
  of magnetic flux. For rotation periods of the order of two days,
  however, Coriolis acceleration of rising flux loops leads to surface
  flux emergence at much higher latitudes than the dynamo waves at
  the bottom of the convection zone reach. A similar result is found
  for a K0V star with a rotation period of two days. In the case of a
  rapidly rotating K1 subgiant, overlapping dynamo waves lead to noisy
  activity cycles and mixed-polarity fields at high latitudes. <BR />
  Conclusions: The combined model reproduces the basic observed features
  of the solar cycle. The differences between the latitude distributions
  of the magnetic field at the bottom of the convection zone and the
  emerging surface flux grow with increasing rotation rate and convection
  zone depth, becoming quite substantial for rapidly rotating dwarfs and
  subgiants. The dynamical evolution of buoyantly rising magnetic flux
  should be considered as an essential ingredient in stellar dynamo
  models. <P />3 movies are only available in electronic form at <A
  href="http://www.aanda.org">http://www.aanda.org</A>

Title: Intensity contrast from MHD simulations and HINODE observations
Authors: Afram, N.; Unruh, Y. C.; Solanki, S. K.; Schüssler, M.;
   Lagg, A.; Vögler, A.
Bibcode: 2011A&A...526A.120A
Altcode: 2010arXiv1011.6102A
  Context. Changes in the solar surface area, which is covered by
  small-scale magnetic elements, are thought to cause long-term changes
  in the solar spectral irradiance, which are important for determining
  the impact on Earth's climate. <BR /> Aims: To study the effect of
  small-scale magnetic elements on the total and spectral irradiance,
  we derive their contrasts from 3-D MHD simulations of the solar
  atmosphere. These calculations are necessary because measurements of
  small-scale flux tube contrasts are confined to a few wavelengths and
  affected by scattered light and instrument defocus, even for space
  observations. <BR /> Methods: To test the contrast calculations,
  we compare rms contrasts from simulations with those obtained with
  the broad-band filter imager mounted on the Solar Optical Telescope
  (SOT) onboard the Hinode satellite and also analyse centre-to-limb
  variations (CLV). The 3-D MHD simulations include the interaction
  between convection and magnetic flux tubes. They are performed by
  assuming non-grey radiative transfer and using the MURaM code. The
  simulations have an average vertical magnetic field of 0 G, 50 G, and
  200 G. Emergent intensities are calculated with the spectral synthesis
  code ATLAS9 and are convolved with a theoretical point-spread function
  to account for the properties of the observations' optical system. <BR
  /> Results: We find reasonable agreement between simulated and observed
  intensity distributions in the visible continuum bands. Agreement is
  poorer for the CN and G-bands. The analysis of the simulations uncovers
  a potentially more realistic centre-to-limb behaviour than calculations
  based on 1-D model atmospheres. <BR /> Conclusions: We conclude that
  starting from 3-D MHD simulations represents a powerful approach
  to obtaining intensity contrasts for a wide wavelength coverage and
  different positions across on the solar disk. This also paves the way
  for future calculations of facular and network contrast as a function
  of magnetic fluxes.

Title: The Sunrise Mission
Authors: Barthol, P.; Gandorfer, A.; Solanki, S. K.; Schüssler,
   M.; Chares, B.; Curdt, W.; Deutsch, W.; Feller, A.; Germerott, D.;
   Grauf, B.; Heerlein, K.; Hirzberger, J.; Kolleck, M.; Meller, R.;
   Müller, R.; Riethmüller, T. L.; Tomasch, G.; Knölker, M.; Lites,
   B. W.; Card, G.; Elmore, D.; Fox, J.; Lecinski, A.; Nelson, P.;
   Summers, R.; Watt, A.; Martínez Pillet, V.; Bonet, J. A.; Schmidt,
   W.; Berkefeld, T.; Title, A. M.; Domingo, V.; Gasent Blesa, J. L.;
   del Toro Iniesta, J. C.; López Jiménez, A.; Álvarez-Herrero, A.;
   Sabau-Graziati, L.; Widani, C.; Haberler, P.; Härtel, K.; Kampf,
   D.; Levin, T.; Pérez Grande, I.; Sanz-Andrés, A.; Schmidt, E.
Bibcode: 2011SoPh..268....1B
Altcode: 2010arXiv1009.2689B; 2010SoPh..tmp..224B
  The first science flight of the balloon-borne Sunrise telescope took
  place in June 2009 from ESRANGE (near Kiruna/Sweden) to Somerset
  Island in northern Canada. We describe the scientific aims and
  mission concept of the project and give an overview and a description
  of the various hardware components: the 1-m main telescope with its
  postfocus science instruments (the UV filter imager SuFI and the imaging
  vector magnetograph IMaX) and support instruments (image stabilizing
  and light distribution system ISLiD and correlating wavefront sensor
  CWS), the optomechanical support structure and the instrument mounting
  concept, the gondola structure and the power, pointing, and telemetry
  systems, and the general electronics architecture. We also explain
  the optimization of the structural and thermal design of the complete
  payload. The preparations for the science flight are described,
  including AIV and ground calibration of the instruments. The course
  of events during the science flight is outlined, up to the recovery
  activities. Finally, the in-flight performance of the instrumentation
  is discussed.

Title: Small-scale dynamo in solar surface simulations
Authors: Graham, J. P.; Moll, R.; Cameron, R.; Schüssler, M.
Bibcode: 2010AGUFMNG51C..01G
Altcode:
  A magneto-convection simulation incorporating essential physical
  processes governing solar surface convection exhibits turbulent
  small-scale dynamo action. By presenting a derivation of the
  energy balance equation and transfer functions for compressible
  magnetohydrodynamics (MHD), we quantify the source of magnetic energy
  on a scale-by-scale basis. We rule out the two alternative mechanisms
  for the generation of small-scale magnetic field in the simulations:
  tangling of magnetic field lines associated with the turbulent cascade
  and Alfvenization of small-scale velocity fluctuations ("turbulent
  induction"). Instead, we find the dominant source of small-scale
  magnetic energy is stretching by inertial-range fluid motions of
  small-scale magnetic field lines against the magnetic tension force to
  produce (against Ohmic dissipation) more small-scale magnetic field. The
  scales involved become smaller with increasing Reynolds number, which
  identifies the dynamo as a small-scale turbulent dynamo. Comparisons
  are made between the details of the dynamo mechanism in compressible
  magneto-convection, Boussinesq convection, and randomly-forced
  incompressible turbulence. Net energy transfers (kinematic phase):
  work against magnetic tension (stretching) is 95% of magnetic energy
  generated; work against magnetic pressure (compression) is 5%. The
  latter is involved in the breaking down larger-scale field (25%) into
  smaller-scale field (30%) as part of the cascade. The dominant producer
  of magnetic energy is the stretching of magnetic field lines against the
  magnetic tension force (turbulent dynamo action). <P />Fluid motions
  at a scale of ~140km create magnetic energy predominately at a scale
  of ~65km. As the three wave-vectors must form a triad, the scale of
  the magnetic field being stretched must is 80+/-40km. All 3 scales
  are in the inertial range: this is turbulent small-scale dynamo.

Title: SUNRISE: Instrument, Mission, Data, and First Results
Authors: Solanki, S. K.; Barthol, P.; Danilovic, S.; Feller, A.;
   Gandorfer, A.; Hirzberger, J.; Riethmüller, T. L.; Schüssler, M.;
   Bonet, J. A.; Martínez Pillet, V.; del Toro Iniesta, J. C.; Domingo,
   V.; Palacios, J.; Knölker, M.; Bello González, N.; Berkefeld, T.;
   Franz, M.; Schmidt, W.; Title, A. M.
Bibcode: 2010ApJ...723L.127S
Altcode: 2010arXiv1008.3460S
  The SUNRISE balloon-borne solar observatory consists of a 1 m aperture
  Gregory telescope, a UV filter imager, an imaging vector polarimeter,
  an image stabilization system, and further infrastructure. The first
  science flight of SUNRISE yielded high-quality data that revealed the
  structure, dynamics, and evolution of solar convection, oscillations,
  and magnetic fields at a resolution of around 100 km in the quiet
  Sun. After a brief description of instruments and data, the first
  qualitative results are presented. In contrast to earlier observations,
  we clearly see granulation at 214 nm. Images in Ca II H display narrow,
  short-lived dark intergranular lanes between the bright edges of
  granules. The very small-scale, mixed-polarity internetwork fields
  are found to be highly dynamic. A significant increase in detectable
  magnetic flux is found after phase-diversity-related reconstruction
  of polarization maps, indicating that the polarities are mixed right
  down to the spatial resolution limit and probably beyond.

Title: Bright Points in the Quiet Sun as Observed in the Visible
    and Near-UV by the Balloon-borne Observatory SUNRISE
Authors: Riethmüller, T. L.; Solanki, S. K.; Martínez Pillet, V.;
   Hirzberger, J.; Feller, A.; Bonet, J. A.; Bello González, N.; Franz,
   M.; Schüssler, M.; Barthol, P.; Berkefeld, T.; del Toro Iniesta,
   J. C.; Domingo, V.; Gandorfer, A.; Knölker, M.; Schmidt, W.
Bibcode: 2010ApJ...723L.169R
Altcode: 2010arXiv1009.1693R
  Bright points (BPs) are manifestations of small magnetic elements
  in the solar photosphere. Their brightness contrast not only gives
  insight into the thermal state of the photosphere (and chromosphere) in
  magnetic elements, but also plays an important role in modulating the
  solar total and spectral irradiance. Here, we report on simultaneous
  high-resolution imaging and spectropolarimetric observations of
  BPs using SUNRISE balloon-borne observatory data of the quiet Sun
  at the disk center. BP contrasts have been measured between 214 nm
  and 525 nm, including the first measurements at wavelengths below
  388 nm. The histograms of the BP peak brightness show a clear trend
  toward broader contrast distributions and higher mean contrasts at
  shorter wavelengths. At 214 nm, we observe a peak brightness of up to
  five times the mean quiet-Sun value, the highest BP contrast so far
  observed. All BPs are associated with a magnetic signal, although in
  a number of cases it is surprisingly weak. Most of the BPs show only
  weak downflows, the mean value being 240 m s<SUP>-1</SUP>, but some
  display strong down- or upflows reaching a few km s<SUP>-1</SUP>.

Title: Transverse Component of the Magnetic Field in the Solar
    Photosphere Observed by SUNRISE
Authors: Danilovic, S.; Beeck, B.; Pietarila, A.; Schüssler, M.;
   Solanki, S. K.; Martínez Pillet, V.; Bonet, J. A.; del Toro Iniesta,
   J. C.; Domingo, V.; Barthol, P.; Berkefeld, T.; Gandorfer, A.;
   Knölker, M.; Schmidt, W.; Title, A. M.
Bibcode: 2010ApJ...723L.149D
Altcode: 2010arXiv1008.1535D
  We present the first observations of the transverse component of
  a photospheric magnetic field acquired by the imaging magnetograph
  SUNRISE/IMaX. Using an automated detection method, we obtain statistical
  properties of 4536 features with significant linear polarization
  signal. We obtain a rate of occurrence of 7 × 10<SUP>-4</SUP>
  s<SUP>-1</SUP> arcsec<SUP>-2</SUP>, which is 1-2 orders of magnitude
  larger than the values reported by previous studies. We show that
  these features have no characteristic size or lifetime. They appear
  preferentially at granule boundaries with most of them being caught
  in downflow lanes at some point. Only a small percentage are entirely
  and constantly embedded in upflows (16%) or downflows (8%).

Title: Fully Resolved Quiet-Sun Magnetic flux Tube Observed with
    the SUNRISE/IMAX Instrument
Authors: Lagg, A.; Solanki, S. K.; Riethmüller, T. L.; Martínez
   Pillet, V.; Schüssler, M.; Hirzberger, J.; Feller, A.; Borrero,
   J. M.; Schmidt, W.; del Toro Iniesta, J. C.; Bonet, J. A.; Barthol, P.;
   Berkefeld, T.; Domingo, V.; Gandorfer, A.; Knölker, M.; Title, A. M.
Bibcode: 2010ApJ...723L.164L
Altcode: 2010arXiv1009.0996L
  Until today, the small size of magnetic elements in quiet-Sun areas has
  required the application of indirect methods, such as the line-ratio
  technique or multi-component inversions, to infer their physical
  properties. A consistent match to the observed Stokes profiles could
  only be obtained by introducing a magnetic filling factor that specifies
  the fraction of the observed pixel filled with magnetic field. Here,
  we investigate the properties of a small magnetic patch in the quiet
  Sun observed with the IMaX magnetograph on board the balloon-borne
  telescope SUNRISE with unprecedented spatial resolution and low
  instrumental stray light. We apply an inversion technique based on
  the numerical solution of the radiative transfer equation to retrieve
  the temperature stratification and the field strength in the magnetic
  patch. The observations can be well reproduced with a one-component,
  fully magnetized atmosphere with a field strength exceeding 1 kG and
  a significantly enhanced temperature in the mid to upper photosphere
  with respect to its surroundings, consistent with semi-empirical flux
  tube models for plage regions. We therefore conclude that, within the
  framework of a simple atmospheric model, the IMaX measurements resolve
  the observed quiet-Sun flux tube.

Title: Quiet-sun Intensity Contrasts in the Near-ultraviolet as
    Measured from SUNRISE
Authors: Hirzberger, J.; Feller, A.; Riethmüller, T. L.; Schüssler,
   M.; Borrero, J. M.; Afram, N.; Unruh, Y. C.; Berdyugina, S. V.;
   Gandorfer, A.; Solanki, S. K.; Barthol, P.; Bonet, J. A.; Martínez
   Pillet, V.; Berkefeld, T.; Knölker, M.; Schmidt, W.; Title, A. M.
Bibcode: 2010ApJ...723L.154H
Altcode:
  We present high-resolution images of the Sun in the near-ultraviolet
  spectral range between 214 nm and 397 nm as obtained from the first
  science flight of the 1 m SUNRISE balloon-borne solar telescope. The
  quiet-Sun rms intensity contrasts found in this wavelength range are
  among the highest values ever obtained for quiet-Sun solar surface
  structures—up to 32.8% at a wavelength of 214 nm. We compare the
  rms contrasts obtained from the observational data with theoretical
  intensity contrasts obtained from numerical magnetohydrodynamic
  simulations. For 388 nm and 312 nm the observations agree well with
  the numerical simulations whereas at shorter wavelengths discrepancies
  between observed and simulated contrasts remain.

Title: Mesogranular structure in a hydrodynamical simulation
Authors: Matloch, Ł.; Cameron, R.; Shelyag, S.; Schmitt, D.;
   Schüssler, M.
Bibcode: 2010A&A...519A..52M
Altcode: 2010arXiv1007.0387M
  <BR /> Aims: We analyse mesogranular flow patterns in
  a three-dimensional hydrodynamical simulation of solar surface
  convection in order to determine its characteristics. <BR /> Methods:
  We calculate divergence maps from horizontal velocities obtained with
  the local correlation tracking (LCT) method. Mesogranules are identified
  as patches of positive velocity divergence. We track the mesogranules
  to obtain their size and lifetime distributions. We vary the analysis
  parameters to verify if the pattern has characteristic scales. <BR />
  Results: The characteristics of the resulting flow patterns depend on
  the averaging time and length used in the analysis. <BR /> Conclusions:
  We conclude that the mesogranular patterns do not exhibit intrinsic
  length and time scales.

Title: Simulation of the Formation of a Solar Active Region
Authors: Cheung, M. C. M.; Rempel, M.; Title, A. M.; Schüssler, M.
Bibcode: 2010ApJ...720..233C
Altcode: 2010arXiv1006.4117C
  We present a radiative magnetohydrodynamics simulation of the formation
  of an active region (AR) on the solar surface. The simulation models
  the rise of a buoyant magnetic flux bundle from a depth of 7.5 Mm in
  the convection zone up into the solar photosphere. The rise of the
  magnetic plasma in the convection zone is accompanied by predominantly
  horizontal expansion. Such an expansion leads to a scaling relation
  between the plasma density and the magnetic field strength such that
  B vprop rhov<SUP>1/2</SUP>. The emergence of magnetic flux into the
  photosphere appears as a complex magnetic pattern, which results
  from the interaction of the rising magnetic field with the turbulent
  convective flows. Small-scale magnetic elements at the surface
  first appear, followed by their gradual coalescence into larger
  magnetic concentrations, which eventually results in the formation
  of a pair of opposite polarity spots. Although the mean flow pattern
  in the vicinity of the developing spots is directed radially outward,
  correlations between the magnetic field and velocity field fluctuations
  allow the spots to accumulate flux. Such correlations result from
  the Lorentz-force-driven, counterstreaming motion of opposite polarity
  fragments. The formation of the simulated AR is accompanied by transient
  light bridges between umbrae and umbral dots. Together with recent
  sunspot modeling, this work highlights the common magnetoconvective
  origin of umbral dots, light bridges, and penumbral filaments.

Title: Changes of the Solar Meridional Velocity Profile During Cycle
    23 Explained by Flows Toward the Activity Belts
Authors: Cameron, R. H.; Schüssler, M.
Bibcode: 2010ApJ...720.1030C
Altcode: 2010arXiv1007.2548C
  The solar meridional flow is an important ingredient in Babcock-Leighton
  type models of the solar dynamo. Global variations of this flow
  have been suggested to explain the variations in the amplitudes and
  lengths of the activity cycles. Recently, cycle-related variations in
  the amplitude of the P <SUP>1</SUP> <SUB>2</SUB> term in the Legendre
  decomposition of the observed meridional flow have been reported. The
  result is often interpreted in terms of an overall variation in the
  flow amplitude during the activity cycle. Using a semi-empirical model
  based upon the observed distribution of magnetic flux on the solar
  surface, we show that the reported variations of the P <SUP>1</SUP>
  <SUB>2</SUB> term can be explained by the observed localized inflows
  into the active region belts. No variation of the overall meridional
  flow amplitude is required.

Title: Quiet-Sun intensity contrasts in the near ultraviolet
Authors: Hirzberger, Johann; Feller, Alex; Riethmüller, Tino L.;
   Schüssler, Manfred; Borrero, Juan M.; Afram, Nadine; Unruh, Yvonne C.;
   Berdyugina, Svetlana V.; Gandorfer, Achim; Solanki, Sami K.; Barthol,
   Peter; Bonet, Jose A.; Martínez Pillet, Valentin; Berkefeld, Thomas;
   Knölker, Michael; Schmidt, Wolfgang; Title, Alan M.
Bibcode: 2010arXiv1009.1050H
Altcode:
  We present high-resolution images of the Sun in the near ultraviolet
  spectral range between 214 nm and 397 nm as obtained from the first
  science flight of the 1-m Sunrise balloon-borne solar telescope. The
  quiet-Sun rms intensity contrasts found in this wavelength range
  are among the highest values ever obtained for quiet-Sun solar
  surface structures - up to 32.8% at a wavelength of 214 nm. We
  compare with theoretical intensity contrasts obtained from numerical
  magneto-hydrodynamic simulations. For 388 nm and 312 nm the observations
  agree well with the numerical simulations whereas at shorter wavelengths
  discrepancies between observed and simulated contrasts remain.

Title: Surface Flux Transport Modeling for Solar Cycles 15-21:
    Effects of Cycle-Dependent Tilt Angles of Sunspot Groups
Authors: Cameron, R. H.; Jiang, J.; Schmitt, D.; Schüssler, M.
Bibcode: 2010ApJ...719..264C
Altcode: 2010arXiv1006.3061C
  We model the surface magnetic field and open flux of the Sun from
  1913 to 1986 using a surface flux transport model, which includes the
  observed cycle-to-cycle variation of sunspot group tilts. The model
  reproduces the empirically derived time evolution of the solar open
  magnetic flux and the reversal times of the polar fields. We find
  that both the polar field and the axial dipole moment resulting from
  this model around cycle minimum correlate with the strength of the
  following cycle.

Title: The Effect of Activity-related Meridional Flow Modulation on
    the Strength of the Solar Polar Magnetic Field
Authors: Jiang, J.; Işik, E.; Cameron, R. H.; Schmitt, D.;
   Schüssler, M.
Bibcode: 2010ApJ...717..597J
Altcode: 2010arXiv1005.5317J
  We studied the effect of the perturbation of the meridional flow in the
  activity belts detected by local helioseismology on the development and
  strength of the surface magnetic field at the polar caps. We carried
  out simulations of synthetic solar cycles with a flux transport model,
  which follows the cyclic evolution of the surface field determined
  by flux emergence and advective transport by near-surface flows. In
  each hemisphere, an axisymmetric band of latitudinal flows converging
  toward the central latitude of the activity belt was superposed
  onto the background poleward meridional flow. The overall effect of
  the flow perturbation is to reduce the latitudinal separation of the
  magnetic polarities of a bipolar magnetic region and thus diminish its
  contribution to the polar field. As a result, the polar field maximum
  reached around cycle activity minimum is weakened by the presence of
  the meridional flow perturbation. For a flow perturbation consistent
  with helioseismic observations, the polar field is reduced by about 18%
  compared to the case without inflows. If the amplitude of the flow
  perturbation depends on the cycle strength, its effect on the polar
  field provides a nonlinearity that could contribute to limiting the
  amplitude of a Babcock-Leighton type dynamo.

Title: SUNRISE Impressions from a successful science flight
Authors: Schmidt, W.; Solanki, S. K.; Barthol, P.; Berkefeld, T.;
   Gandorfer, A.; Knölker, M.; Martínez Pillet, V.; Schüssler, M.;
   Title, A.
Bibcode: 2010AN....331..601S
Altcode:
  SUNRISE is a balloon-borne telescope with an aperture of one meter. It
  is equipped with a filter imager for the UV wavelength range between
  214 nm and 400 nm (SUFI), and with a spectro-polarimeter that measures
  the magnetic field of the photosphere using the Fe I line at 525.02
  nm that has a Landé factor of 3. SUNRISE performed its first science
  flight from 8 to 14 June 2009. It was launched at the Swedish ESRANGE
  Space Center and cruised at an altitude of about 36 km and geographic
  latitudes between 70 and 74 degrees to Somerset Island in northern
  Canada. There, all data, the telescope and the gondola were successfully
  recovered. During its flight, Sunrise achieved high pointing stability
  during 33 hours, and recorded about 1.8 TB of science data. Already at
  this early stage of data processing it is clear that SUNRISE recorded
  UV images of the solar photosphere, and spectropolarimetric measurements
  of the quiet Sun's magnetic field of unprecedented quality.

Title: The solar cycle and the current solar minimum
Authors: Cameron, R.; Jiang, J.; Schmitt, D.; Schuessler, M.
Bibcode: 2010EGUGA..1215494C
Altcode:
  In this talk we discuss the evolution of the Sun's large-scale magnetic
  field, on timescales relevant to the solar cycle. This evolution can
  be modeled using the surface flux transport equations, and we will
  outline the ingredients which go into the model. Special attention
  will be paid to the term describing the emergence of new flux onto
  the solar surface. The results of the model will be compared against
  observations covering most of the twentieth century, and in particular
  we will discuss what determines the surface field during solar minima.

Title: Turbulent Small-Scale Dynamo Action in Solar Surface
    Simulations
Authors: Pietarila Graham, Jonathan; Cameron, Robert; Schüssler,
   Manfred
Bibcode: 2010ApJ...714.1606P
Altcode: 2010ApJ...714.1606G; 2010arXiv1002.2750P
  We demonstrate that a magneto-convection simulation incorporating
  essential physical processes governing solar surface convection exhibits
  turbulent small-scale dynamo action. By presenting a derivation of
  the energy balance equation and transfer functions for compressible
  magnetohydrodynamics, we quantify the source of magnetic energy on a
  scale-by-scale basis. We rule out the two alternative mechanisms for
  the generation of the small-scale magnetic field in the simulations:
  the tangling of magnetic field lines associated with the turbulent
  cascade and Alfvénization of small-scale velocity fluctuations
  ("turbulent induction"). Instead, we find that the dominant source
  of small-scale magnetic energy is stretching by inertial-range fluid
  motions of small-scale magnetic field lines against the magnetic tension
  force to produce (against Ohmic dissipation) more small-scale magnetic
  field. The scales involved become smaller with increasing Reynolds
  number, which identifies the dynamo as a small-scale turbulent dynamo.

Title: Probing quiet Sun magnetism using MURaM simulations and
Hinode/SP results: support for a local dynamo
Authors: Danilovic, S.; Schüssler, M.; Solanki, S. K.
Bibcode: 2010A&A...513A...1D
Altcode: 2010arXiv1001.2183D
  Context. Owing to the limited spatial resolution and the weak
  polarization signal coming from the quietest regions on the Sun, the
  organization of the magnetic field on the smallest scales is largely
  unknown. <BR /> Aims: We obtain information about the magnetic flux
  present in the quiet Sun by comparing radiative MHD simulations with
  observations, with particular emphasis on the role of surface dynamo
  action. <BR /> Methods: We synthesized Stokes profiles on the basis of
  the MHD simulation results. The profiles are degraded by taking the
  properties of the spectropolarimeter (SP) into account onboard the
  Hinode satellite. We used simulation runs with different magnetic
  Reynolds numbers (R<SUB>m</SUB>) and observations at different
  heliocentric angles with different levels of noise. <BR /> Results:
  Simulations with an imposed mixed-polarity field and R<SUB>m</SUB>
  below the threshold for dynamo action reproduce the observed vertical
  flux density, but do not display a high enough horizontal flux
  density. Surface dynamo simulations at the highest R<SUB>m</SUB>
  feasible at the moment yield a ratio of the horizontal and vertical
  flux density consistent with observational results, but the overall
  amplitudes are too low. Based on the properties of the local dynamo
  simulations, a tentative scaling of the magnetic field strength
  by a factor 2-3 reproduces the signal observed in the internetwork
  regions. <BR /> Conclusions: We find agreement with observations at
  different heliocentric angles. The mean field strength in internetwork
  implied by our analysis is roughly 170 G at the optical depth unity. Our
  study shows that surface dynamo could be responsible for most of the
  magnetic flux in the quiet Sun outside the network, given that the
  extrapolation to higher R<SUB>m</SUB> is valid.

Title: The small-scale solar surface dynamo
Authors: Pietarila Graham, Jonathan; Danilovic, Sanja; Schuessler,
   Manfred
Bibcode: 2010arXiv1003.0347P
Altcode:
  The existence of a turbulent small-scale solar surface dynamo is likely,
  considering existing numerical and laboratory experiments, as well as
  comparisons of a small-scale dynamo in MURaM simulations with Hinode
  observations. We find the observed peaked probability distribution
  function (PDF) from Stokes-V magnetograms is consistent with a monotonic
  PDF of the actual vertical field strength. The cancellation function
  of the vertical flux density from a Hinode SP observation is found to
  follow a self-similar power law over two decades in length scales down
  to the ~200 km resolution limit. This provides observational evidence
  that the scales of magnetic structuring in the photosphere extend
  at least down to 20 km. From the power law, we determine a lower
  bound for the true quiet-Sun mean vertical unsigned flux density of
  ~43 G, consistent with our numerically-based estimates that 80% or
  more of the vertical unsigned flux should be invisible to Stokes-V
  observations at a resolution of 200 km owing to cancellation. Our
  estimates significantly reduce the order-of-magnitude discrepancy
  between Zeeman- and Hanle-based estimates.

Title: A comparison of measured and simulated solar network contrast
Authors: Afram, N.; Unruh, Y. C.; Solanki, S. K.; Schüssler, M.;
   Mathew, S. K.
Bibcode: 2010IAUS..264...63A
Altcode: 2009arXiv0910.0976A
  Long-term trends in the solar spectral irradiance are important to
  determine the impact on Earth's climate. These long-term changes are
  thought to be caused mainly by changes in the surface area covered by
  small-scale magnetic elements. The direct measurement of the contrast
  to determine the impact of these small-scale magnetic elements
  is, however, limited to a few wavelengths, and is, even for space
  instruments, affected by scattered light and instrument defocus. In
  this work we calculate emergent intensities from 3-D simulations of
  solar magneto-convection and validate the outcome by comparing with
  observations from Hinode/SOT. In this manner we aim to construct the
  contrast at wavelengths ranging from the NUV to the FIR.

Title: Properties of simulated sunspot umbral dots
Authors: Bharti, L.; Beeck, B.; Schüssler, M.
Bibcode: 2010A&A...510A..12B
Altcode: 2009arXiv0911.5068B
  Realistic 3D radiative MHD simulations reveal the magneto-convective
  processes underlying the formation of the photospheric fine structure
  of sunspots, including penumbral filaments and umbral dots. Here we
  provide results from a statistical analysis of simulated umbral dots
  and compare them with reports from high-resolution observations. A
  multi-level segmentation and tracking algorithm has been used to isolate
  the bright structures in synthetic bolometric and continuum brightness
  images. Areas, brightness, and lifetimes of the resulting set of umbral
  dots are found to be correlated: larger umbral dots tend to be brighter
  and live longer. The magnetic field strength and velocity structure of
  umbral dots on surfaces of constant optical depth in the continuum at
  630 nm indicate that the strong field reduction and high velocities in
  the upper parts of the upflow plumes underlying umbral dots are largely
  hidden from spectro-polarimetric observations. The properties of the
  simulated umbral dots are generally consistent with the results of
  recent high-resolution observations. However, the observed population
  of small, short-lived umbral dots is not reproduced by the simulations,
  possibly owing to insufficient spatial resolution.

Title: High resolution imaging and polarimetry with SUNRISE, a
    balloon-borne stratospheric solar observatory
Authors: Barthol, Peter; Chares, Bernd; Deutsch, Werner; Feller, Alex;
   Gandorfer, Achim; Grauf, Bianca; Hirzberger, Johann; Meller, Reinhard;
   Riethmueller, Tino; Schuessler, Manfred; Solanki, Sami K.; Knoelker,
   Michael; Martinez Pillet, Valentin; Schmidt, Wolfgang; Title, Alan
Bibcode: 2010cosp...38.4063B
Altcode: 2010cosp.meet.4063B
  SUNRISE is an international collaboration for the development
  and operation of a meter-class balloon-borne stratospheric solar
  observatory. Prime science goal is the study of structure and dynamics
  of the magnetic field in the solar atmosphere and the interaction of
  the magnetic field with convective plasma flows. These processes are
  studied by high resolution imaging in the UV and polarimetry at visible
  wavelengths. The instrument has been successfully launched on June 8,
  2009 from ESRANGE, Kiruna, Northern Sweden. During the more than 5
  days flight about 1.5 TByte of scientific data were collected. The
  paper gives an overview of the instrument and mission, examples of
  the scientific output will also be presented. SUNRISE is a joint
  project of the Max-Planck-Institut fuer Sonnensystemforschung (MPS),
  Katlenburg-Lindau, with the Kiepenheuer-Institut fuer Sonnenphysik
  (KIS), Freiburg, the High-Altitude Observatory (HAO), Boulder, the
  Lockheed-Martin Solar and Astrophysics Lab. (LMSAL), Palo Alto, and
  the Spanish IMaX consortium.

Title: UV intensity distributions of the quiet Sun observed with
    Sunrise
Authors: Hirzberger, Johann; Feller, A.; Riethmueller, T.; Borrero,
   J. M.; Schüssler, M.; Barthol, P.; Berkefeld, T.; Gandorfer, A.;
   Knoelker, M.; Martínez Pillet, V.; Schmidt, W.; Solanki, S.; Title, A.
Bibcode: 2010cosp...38.1735H
Altcode: 2010cosp.meet.1735H
  High resolution solar images in the near UV have been obtained with
  the Solar UV Filtergraph (SUFI) onboard the Sunrise balloon borne
  observatory, amongst others in wavelength regions not accessible
  from the ground. We present intensity distributions of the quiet
  Sun at different heliocentric angles, from disk center to the solar
  limb. These results, obtained in spectral windows at 214 nm, 313 nm
  (OH band), 388 nm (CN band) and 396.7 nm (CaIIH), represent an important
  validation of numerical models of the solar photosphere and are, thus,
  fundamental ingredients for our understanding of the thermal processes
  in the solar surface region.

Title: Magnetic field intensification: comparison of 3D MHD
    simulations with Hinode/SP results
Authors: Danilovic, S.; Schüssler, M.; Solanki, S. K.
Bibcode: 2010A&A...509A..76D
Altcode: 2009arXiv0910.1211D
  Context. Recent spectro-polarimetric observations have provided detailed
  measurements of magnetic field, velocity and intensity during events of
  magnetic field intensification in the solar photosphere. <BR /> Aims:
  By comparing with synthetic observations derived from MHD simulations,
  we investigate the physical processes underlying the observations,
  as well as verify the simulations and the interpretation of the
  observations. <BR /> Methods: We consider the temporal evolution of
  the relevant physical quantities for three cases of magnetic field
  intensification in a numerical simulation. In order to compare with
  observations, we calculate Stokes profiles and take into account the
  spectral and spatial resolution of the spectropolarimeter (SP) on
  board Hinode. We determine the evolution of the intensity, magnetic
  flux density and zero-crossing velocity derived from the synthetic
  Stokes parameters, using the same methods as applied to the Hinode/SP
  observations to derive magnetic field and velocity information from
  the spectro-polarimetric data. <BR /> Results: The three events
  considered show a similar evolution: advection of magnetic flux to a
  granular vertex, development of a strong downflow, evacuation of the
  magnetic feature, increase of the field strength and the appearance
  of the bright point. The magnetic features formed have diameters of
  0.1-0.2´´. The downflow velocities reach maximum values of 5-10
  km s<SUP>-1</SUP> at τ = 1. In the largest feature, the downflow
  reaches supersonic speed in the lower photosphere. In the same case,
  a supersonic upflow develops approximately 200 s after the formation of
  the flux concentration. We find that synthetic and real observations
  are qualitatively consistent and, for one of the cases considered,
  also agree very well quantitatively. The effect of finite resolution
  (spatial smearing) is most pronounced in the case of small features,
  for which the synthetic Hinode/SP observations miss the bright point
  formation and also the high-velocity downflows during the formation
  of the smaller magnetic features. <BR /> Conclusions: The observed
  events are consistent with the process of field intensification by
  flux advection, radiative cooling, and evacuation by strong downflow
  found in MHD simulations. The quantitative agreement of synthetic and
  real observations indicates the validity of both the simulations and
  the interpretations of the spectro-polarimetric observations.

Title: Relation between the Sunrise photospheric magnetic field and
    the Ca II H bright features
Authors: Jafarzadeh, Shahin; Hirzberger, J.; Feller, A.; Lagg, A.;
   Solanki, S. K.; Pietarila, A.; Danilovic, S.; Riethmueller, T.;
   Barthol, P.; Berkefeld, T.; Gandorfer, A.; Knülker, M.; Martínez
   Pillet, V.; Schmidt, W.; Schüssler, M.; Title, A.
Bibcode: 2010cosp...38.2856J
Altcode: 2010cosp.meet.2856J
  Recent observations from the Sunrise balloon-borne solar telescope
  have enabled us to reach an unprecedented high spatial resolution
  on the solar surface with the near-ultraviolet photo-spheric and
  chromospheric images as well as the magnetograms. We use these high
  resolution observations to investigate the structure of the solar
  upper photosphere and lower chromosphere as well as their temporal
  evolutions. We study the relation between the inter-granular Ca II
  397 nm bright structures in images obtained by the Sunrise Filter
  Imager (SuFI) and their corresponding photospheric vector magnetic
  field computed from the Imaging Magnetogram eXperiment (IMaX)
  observations. The targets under study are in a quiet Sun region and
  close to disc-centre.

Title: Modeling the Sun's Open Magnetic Flux and the Heliospheric
    Current Sheet
Authors: Jiang, J.; Cameron, R.; Schmitt, D.; Schüssler, M.
Bibcode: 2010ApJ...709..301J
Altcode: 2009arXiv0912.0108J
  By coupling a solar surface flux transport model with an extrapolation
  of the heliospheric field, we simulate the evolution of the Sun's
  open magnetic flux and the heliospheric current sheet (HCS) based
  on observational data of sunspot groups since 1976. The results are
  consistent with measurements of the interplanetary magnetic field near
  Earth and with the tilt angle of the HCS as derived from extrapolation
  of the observed solar surface field. This opens the possibility for
  an improved reconstruction of the Sun's open flux and the HCS into
  the past on the basis of empirical sunspot data.

Title: Simulation of a flux emergence event and comparison with
    observations by Hinode
Authors: Yelles Chaouche, L.; Cheung, M. C. M.; Solanki, S. K.;
   Schüssler, M.; Lagg, A.
Bibcode: 2009A&A...507L..53Y
Altcode: 2009arXiv0910.5737Y
  Aims: We study the observational signature of flux emergence in
  the photosphere using synthetic data from a 3D MHD simulation of the
  emergence of a twisted flux tube. <BR />Methods: Several stages in the
  emergence process are considered. At every stage we compute synthetic
  Stokes spectra of the two iron lines Fe I 6301.5 Å and Fe I 6302.5
  Å and degrade the data to the spatial and spectral resolution of
  Hinode's SOT/SP. Then, following observational practice, we apply
  Milne-Eddington-type inversions to the synthetic spectra in order
  to retrieve various atmospheric parameters and compare the results
  with recent Hinode observations. <BR />Results: During the emergence
  sequence, the spectral lines sample different parts of the rising
  flux tube, revealing its twisted structure. The horizontal component
  of the magnetic field retrieved from the simulations is close to the
  observed values. The flattening of the flux tube in the photosphere is
  caused by radiative cooling, which slows down the ascent of the tube
  to the upper solar atmosphere. Consistent with the observations, the
  rising magnetized plasma produces a blue shift of the spectral lines
  during a large part of the emergence sequence. <P />Figure 3 is only
  available in electronic form at http://www.aanda.org

Title: The Small-Scale Solar Surface Dynamo (Keynote)
Authors: Pietarila Graham, J.; Danilovic, S.; Schüssler, M.
Bibcode: 2009ASPC..415...43P
Altcode:
  The existence of a turbulent small-scale solar surface dynamo is likely,
  considering existing numerical and laboratory experiments, as well as
  comparisons of a small-scale dynamo in MURaM simulations with Hinode
  observations. We find the observed peaked probability distribution
  function (PDF) from Stokes-V magnetograms is consistent with a monotonic
  PDF of the actual vertical field strength. The cancellation function
  of the vertical flux density from a Hinode SP observation is found
  to follow a self-similar power law over two decades in length scales
  down to the ≈200 km resolution limit. This provides observational
  evidence that the scales of magnetic structuring in the photosphere
  extend at least down to 20 km. From the power law, we determine a
  lower bound for the true quiet-Sun mean vertical unsigned flux density
  of ≈43 G, consistent with our numerically-based estimates that 80%
  or more of the vertical unsigned flux should be invisible to Stokes-V
  observations at a resolution of 200 km owing to cancellation. Our
  estimates significantly reduce the order-of-magnitude discrepancy
  between Zeeman- and Hanle-based estimates.

Title: Radiative MHD simulations of sunspot structure
Authors: Rempel, M.; Schuessler, M.; Cameron, R.; Knoelker, M.
Bibcode: 2009AGUFMSH53B..07R
Altcode:
  For a long time radiative MHD simulations of entire sunspots from
  first principles were out of reach due to insufficient computing
  resources. Over the past 4 years simulations have evolved from
  6x6x2 Mm size domains focusing on the details of umbral dots to
  simulations covering a pair of opposite polarity sunspots in a
  100x50x6 Mm domain. Numerical simulations point toward a common magneto
  convective origin of umbral dots and filaments in the inner and outer
  penumbra. Most recent simulations also capture the processes involved
  in the formation of an extended outer penumbra with strong horizontal
  outflows averaging around 5 km/s in the photosphere. In this talk I
  will briefly review the progress made in this field over the past 4
  years and discuss in detail the magneto convective origin of penumbral
  fine structure as well as the Evershed flow.

Title: Radiative MHD simulation of an Emerging Flux Region
Authors: Cheung, C.; Rempel, M.; Title, A. M.; Schuessler, M.
Bibcode: 2009AGUFMSH51A1267C
Altcode:
  We present a radiation magnetohydrodynamics (MHD) simulation of
  the birth of an active region. The simulation models the rise
  of a magnetic flux bundle from the convection zone into the solar
  photosphere. Observational properties of the simulation are consistent
  with recent, high-cadence and high spatial resolution observations of
  emerging flux regions taken by Hinode/SOT. Observational properties
  common to both simulation and observation include the hierarchical
  formation of progressively larger photospheric magnetic structures,
  the formation and disappearance of light bridges, umbral dots as well
  as penumbral filaments.

Title: Solar Surface Emerging Flux Regions: A Comparative Study of
    Radiative MHD Modeling and Hinode SOT Observations
Authors: Cheung, M.; Schüssler, M.; Tarbell, T. D.; Title, A. M.
Bibcode: 2009ASPC..415...79C
Altcode:
  We present results from three-dimensional radiative MHD simulations
  of the rise of buoyant magnetic flux tubes through the convection
  zone and into the photosphere. Due to the strong stratification
  of the convection zone, the rise results in a lateral expansion
  of the tube into a magnetic sheet, which acts as a reservoir for
  small-scale flux emergence events at the scale of granulation. The
  interaction of the convective downflows and the rising magnetic flux
  tube undulates it to form serpentine field lines that emerge into the
  photosphere. Observational characteristics of the simulated emerging
  flux regions are discussed in the context of new observations from
  Hinode SOT.

Title: Radiative MHD simulations of sunspot structure
Authors: Rempel, M.; Schüssler, M.; Cameron, R.; Knölker, M.
Bibcode: 2009iac..talk..192R
Altcode: 2009iac..talk..106R
  No abstract at ADS

Title: Modelling of solar mesogranulation
Authors: Matloch, L.; Cameron, R.; Schmitt, D.; Schüssler, M.
Bibcode: 2009A&A...504.1041M
Altcode:
  We study whether mesogranulation flow patterns at the solar surface
  can arise solely from the statistical properties of granules and
  intergranular lanes. We have developed one- and two-dimensional models
  with local interaction rules between the artificial “granules”
  mimicking the actual physical processes on the solar surface. Defining
  mesogranulation according to the age of intergranular (downflow) lanes
  corresponding to the often applied “cork method”, as well as the
  areas of divergence of the horizontal velocity (two-dimensional model),
  we find that mesogranular patterns are present in our models. Our study
  of the dependence of the properties of the mesogranular patterns on
  the model parameter and interaction rules reveals that the patterns
  do not possess intrinsic length and time scales. <P />Appendix is only
  available in electronic from at http://www.aanda.org

Title: Comparison of the thin flux tube approximation with 3D MHD
    simulations
Authors: Yelles Chaouche, L.; Solanki, S. K.; Schüssler, M.
Bibcode: 2009A&A...504..595Y
Altcode: 2009arXiv0910.1796Y
  Context: The structure and dynamics of small vertical photospheric
  magnetic flux concentrations has been often treated in the framework
  of an approximation based upon a low-order truncation of the Taylor
  expansions of all quantities in the horizontal direction, together
  with the assumption of instantaneous total pressure balance at
  the boundary to the non-magnetic external medium. Formally, such
  an approximation is justified if the diameter of the structure (a
  flux tube or a flux sheet) is small compared to all other relevant
  length scales (scale height, radius of curvature, wavelength,
  etc.). The advent of realistic 3D radiative MHD simulations opens
  the possibility of checking the consistency of the approximation with
  the properties of the flux concentrations that form in the course of
  a simulation. <BR />Aims: We carry out a comparative analysis between
  the thin flux tube/sheet models and flux concentrations formed in a 3D
  radiation-MHD simulation. <BR />Methods: We compare the distribution
  of the vertical and horizontal components of the magnetic field in a 3D
  MHD simulation with the field distribution in the case of the thin flux
  tube/sheet approximation. We also consider the total (gas plus magnetic)
  pressure in the MHD simulation box. <BR />Results: Flux concentrations
  with super-equipartition fields are reasonably well reproduced by
  the second-order thin flux tube/sheet approximation. The differences
  between approximation and simulation are due to the asymmetry and the
  dynamics of the simulated structures.

Title: Penumbral Structure and Outflows in Simulated Sunspots
Authors: Rempel, M.; Schüssler, M.; Cameron, R. H.; Knölker, M.
Bibcode: 2009Sci...325..171R
Altcode: 2009arXiv0907.2259R
  Sunspots are concentrations of magnetic field on the visible solar
  surface that strongly affect the convective energy transport in their
  interior and surroundings. The filamentary outer regions (penumbrae)
  of sunspots show systematic radial outward flows along channels of
  nearly horizontal magnetic field. These flows were discovered 100
  years ago and are present in all fully developed sunspots. By using
  a comprehensive numerical simulation of a sunspot pair, we show
  that penumbral structures with such outflows form when the average
  magnetic field inclination to the vertical exceeds about 45 degrees. The
  systematic outflows are a component of the convective flows that provide
  the upward energy transport and result from anisotropy introduced by
  the presence of the inclined magnetic field.

Title: Radiative MHD Simulations of Sunspot Structure
Authors: Rempel, Matthias D.; Schuessler, M.; Cameron, R.; Knoelker, M.
Bibcode: 2009SPD....40.0604R
Altcode:
  We summarize the recent progress made in magneto convection simulations
  of sunspot structure. Over the past 4 years simulations have evolved
  from local 6x6x2 Mm size domains focusing on the details of umbral
  dots to simulations covering a pair of opposite polarity spots in
  a 100x50x6 Mm domain. The simulations point out the common magneto
  convective origin of umbral dots and filaments in the inner penumbra
  and most recently also reveal the processes involved in the formation
  of an extended outer penumbra with strong horizontal outflows averaging
  around 5 km/s in the photosphere.

Title: Countercell Meridional Flow and Latitudinal Distribution of
    the Solar Polar Magnetic Field
Authors: Jiang, J.; Cameron, R.; Schmitt, D.; Schüssler, M.
Bibcode: 2009ApJ...693L..96J
Altcode:
  Recent observations indicate that the latitudinal profile of the
  magnetic flux shows a pronounced decrease close to the solar north
  pole during the minimum phase of solar cycle 23. Using a surface flux
  transport model, we have performed numerical experiments to study the
  conditions that could lead to such a latitudinal distribution. We find
  that a strong decrease of the magnetic field near the poles results
  if a weak countercell of the meridional flow at high latitudes with
  an equatorward speed of a few m s<SUP>-1</SUP> is present.

Title: Turbulent Magnetic Fields in the Quiet Sun: Implications of
    Hinode Observations and Small-Scale Dynamo Simulations
Authors: Pietarila Graham, Jonathan; Danilovic, Sanja; Schüssler,
   Manfred
Bibcode: 2009ApJ...693.1728P
Altcode: 2008arXiv0812.2125P
  Using turbulent MHD simulations (magnetic Reynolds numbers up to
  ≈8000) and Hinode observations, we study effects of turbulence on
  measuring the solar magnetic field outside active regions. First,
  from synthetic Stokes V profiles for the Fe I lines at 6301 and
  6302 Å, we show that a peaked probability distribution function
  (PDF) for observationally derived field estimates is consistent
  with a monotonic PDF for actual vertical field strengths. Hence, the
  prevalence of weak fields is greater than would be naively inferred from
  observations. Second, we employ the fractal self-similar geometry of the
  turbulent solar magnetic field to derive two estimates (numerical and
  observational) of the true mean vertical unsigned flux density. We also
  find observational evidence that the scales of magnetic structuring in
  the photosphere extend at least down to an order of magnitude smaller
  than 200 km: the self-similar power-law scaling in the signed measure
  from a Hinode magnetogram ranges (over two decades in length scales
  and including the granulation scale) down to the ≈200 km resolution
  limit. From the self-similar scaling, we determine a lower bound for
  the true quiet-Sun mean vertical unsigned flux density of ~50 G. This
  is consistent with our numerically based estimates that 80% or more of
  the vertical unsigned flux should be invisible to Stokes V observations
  at a resolution of 200 km owing to the cancellation of signal from
  opposite magnetic polarities. Our estimates significantly reduce
  the order-of-magnitude discrepancy between Zeeman- and Hanle-based
  estimates.

Title: Towards Long-Term Solar Irradiance Modelling: Network Contrasts
    from Magneto-Convection Simulations
Authors: Unruh, Y. C.; Solanki, S. K.; Schüssler, M.; Vögler, A.;
   Garcia-Alvarez, D.
Bibcode: 2009AIPC.1094..768U
Altcode: 2009csss...15..768U
  Solar irradiance changes on a wide range of time scales and is a key
  driver of the Earth's climate where secular variability in particular is
  relevant. This is, however, not well understood and our knowledge relies
  on reconstructions based on sunspot numbers and similar proxies. <P
  />The prime candidate to produce secular variability is a change
  in the surface coverage of small-scale magnetic elements. Direct
  observational determination of the flux emitted by these magnetic
  elements is difficult, especially as information covering a large
  spectral range is needed. Here we present a theoretical approach
  to this problem using intensity calculations from 3-D simulations
  of solar magneto-convection and compare these with the intensity
  calculations used in the successful semi-empirical S ATIRE models at
  disk centre. Eventually, such a comparison should lead to the removal
  of the last free parameter from S ATIRE-based irradiance reconstruction.

Title: Radiative Magnetohydrodynamic Simulation of Sunspot Structure
Authors: Rempel, M.; Schüssler, M.; Knölker, M.
Bibcode: 2009ApJ...691..640R
Altcode: 2008arXiv0808.3294R
  Results of a three-dimensional MHD simulation of a sunspot with
  a photospheric size of about 20 Mm are presented. The simulation
  has been carried out with the MURaM code, which includes a realistic
  equation of state with partial ionization and radiative transfer along
  many ray directions. The largely relaxed state of the sunspot shows
  a division in a central dark umbral region with bright dots and a
  penumbra showing bright filaments of about 2-3 Mm length with central
  dark lanes. By a process similar to the formation of umbral dots,
  the penumbral filaments result from magnetoconvection in the form of
  upflow plumes, which become elongated by the presence of an inclined
  magnetic field; the upflow is deflected in the outward direction while
  the magnetic field is weakened and becomes almost horizontal in the
  upper part of the plume near the level of optical depth unity. A dark
  lane forms owing to the piling up of matter near the cusp-shaped top
  of the rising plume that leads to an upward bulging of the surfaces of
  constant optical depth. The simulated penumbral structure corresponds
  well to the observationally inferred interlocking-comb structure of
  the magnetic field with Evershed outflows along dark-laned filaments
  with nearly horizontal magnetic field and overturning perpendicular
  ("twisting") motion, which are embedded in a background of stronger
  and less inclined field. Photospheric spectral lines are formed at the
  very top and somewhat above the upflow plumes, so that they do not
  fully sense the strong flow as well as the large field inclination
  and significant field strength reduction in the upper part of the
  plume structures.

Title: Solar Surface Emerging Flux Regions: A Comparative Study of
    Radiative MHD Modeling and Hinode SOT Observations
Authors: Cheung, M. C. M.; Schüssler, M.; Tarbell, T. D.; Title, A. M.
Bibcode: 2008ApJ...687.1373C
Altcode: 2008arXiv0810.5723C
  We present results from numerical modeling of emerging flux regions
  on the solar surface. The modeling was carried out by means of
  three-dimensional (3D) radiative MHD simulations of the rise of
  buoyant magnetic flux tubes through the convection zone and into the
  photosphere. Due to the strong stratification of the convection zone,
  the rise results in a lateral expansion of the tube into a magnetic
  sheet, which acts as a reservoir for small-scale flux emergence
  events at the scale of granulation. The interaction of the convective
  downflows and the rising magnetic flux tube undulates it to form
  serpentine field lines that emerge into the photosphere. Observational
  characteristics, including the pattern of the emerging flux regions,
  the cancellation of surface flux and associated high-speed downflows,
  the convective collapse of photospheric flux tubes, the appearance
  of anomalous darkenings, the formation of bright points, and the
  possible existence of transient kilogauss horizontal fields are
  discussed in the context of new observations from the Hinode Solar
  Optical Telescope. Implications for the local helioseismology of
  emerging flux regions are also discussed.

Title: A Robust Correlation between Growth Rate and Amplitude of
Solar Cycles: Consequences for Prediction Methods
Authors: Cameron, R.; Schüssler, M.
Bibcode: 2008ApJ...685.1291C
Altcode:
  We consider the statistical relationship between the growth rate
  of activity in the early phase of a solar cycle with its subsequent
  amplitude on the basis of four data sets of global activity indices
  (Wolf sunspot number, group sunspot number, sunspot area, and 10.7 cm
  radio flux). In all cases, a significant correlation is found: stronger
  cycles tend to rise faster. Owing to the overlapping of sunspot cycles,
  this correlation leads to an amplitude-dependent shift of the solar
  minimum epoch. We show that this effect explains the correlations
  underlying various so-called precursor methods for the prediction
  of solar cycle amplitudes and also affects the prediction tool of
  Dikpati et al. based on a dynamo model. Inferences as to the nature
  of the solar dynamo mechanism resulting from predictive schemes which
  (directly or indirectly) use the timing of solar minima should therefore
  be treated with caution.

Title: 3D MHD Simulations of Sunspot Structure
Authors: Rempel, M.; Schüssler, M.
Bibcode: 2008ESPM...12..3.9R
Altcode:
  We present results of a 3D MHD simulation of a sunspot with a
  photospheric size of about 20 Mm carried out with the MURaM MHD
  code. The simulation covers a time span of about 12 hours. The largely
  relaxed state of the sunspot shows a division in a central dark umbral
  region with bright dots and a penumbra showing bright filaments of
  about 3 to 4 Mm length with central dark lanes. By a process similar
  to the formation of umbral dots, the penumbral filaments result from
  magneto-convection in the form of upflow plumes, which become elongated
  by the presence of an inclined magnetic field: the upflow is deflected
  in the outward direction and bends down the magnetic field to become
  almost horizontal in the upper part of the plume near the level of
  optical depth unity. At the same time, roll-type motion leads to
  a flow perpendicular to the filament axis and to downflow near its
  edges. Expansion and flux expulsion leads to a strong reduction of
  the field strength in the upper part of the rising plume, where a dark
  lane forms owing to the piling up of matter near the cusp-shaped top
  and the upward bulging of the surfaces of constant optical depth. The
  simulated penumbral structure corresponds well to the observationally
  inferred interlocking-comb structure of the magnetic field with Evershed
  outflows along dark-laned filaments with nearly horizontal magnetic
  field and roll-type perpendicular motion, which are embedded in a
  background of stronger and less inclined field. Photospheric spectral
  lines are formed at the very top and somewhat above the upflow plumes,
  so that they do not fully sense the strong flow as well as the large
  field inclination and significant field strength reduction in the
  upper part of the plume structures.

Title: Storage of Magnetic Flux in the Solar Convective Overshoot
    Region
Authors: Isik, E.; Holzwarth, V.; Schüssler, M.
Bibcode: 2008ESPM...12..3.3I
Altcode:
  The stability of magnetic fields in the solar interior is of critical
  importance for dynamo models. Instabilities induced by external
  flows are relevant for the storage of magnetic field in the stably
  stratified lower convective overshoot region. We have investigated the
  effects of finite perturbations and external flows on the stability
  and dynamics of thin magnetic flux tubes in mechanical equilibrium
  in the overshoot region. We have used a model convection zone based
  on the non-local mixing length approximation. Numerical simulations
  have shown that the friction-induced flux tube instability occurs
  already for field strengths well below the critical value for the
  onset of Parker instability, though with significantly longer growth
  times. Above a certain amplitude of the radial perturbation, nonlinear
  effects introduced by the external stratification become sufficient
  to affect the further growth of the instability. We have carried out a
  systematic parameter study with flux tubes subject to external radial
  flows. We have obtained ranges for the maximum speed and the duration
  of the flow, which allow for the storage of magnetic flux tubes with
  near-equipartition field strengths in the overshoot region, for times
  comparable with the dynamo amplification time.

Title: How Well Do Zeeman Measurements Reflect the Turbulent Solar
    Magnetic Field?
Authors: Pietarila, J. Graham; Danilovic, S.; Schüssler, M.
Bibcode: 2008ESPM...12.3.13P
Altcode:
  We employ the turbulent nature of the magnetic field in the solar
  photosphere to constrain interpretations of Zeeman-polarimetry-based
  observations. Using higher Reynolds number, more turbulent, simulations
  of the solar convection zone than have previously been reported,
  we compare the distribution and cancellation statistics of the
  magnetic field itself with the statistics derived from simulated
  Stokes profiles. A favorable comparison between the cancellation
  statistics of the observables and the field allow for a prediction
  at the approximated magnetic Reynolds number of the sun, ReM = 1e6,
  and a comparison with Hinode observations. The difference between the
  probability distribution functions (PDFs) from simulations, Zeeman-based
  polarimetry, and interpretations of the Hanle effect are also examined
  and a possible explanation is suggested.

Title: MHD Simulation: From the Convection Zone to the Corona and
    Beyond (in 30 Minutes)
Authors: Schüssler, M.
Bibcode: 2008ESPM...12..1.1S
Altcode:
  The all-encompassing ab-initio simulation reaching from the tachocline
  out to the solar wind is not feasible within the foreseeable
  future. Therefore, we have to approach the problems of magnetic flux
  generation, transport, emergence, magneto-convective interaction,
  equilibrium, dynamics, and dissipation in a piecemeal fashion,
  focussing upon a realistic (as far as possible) description of
  the physics in more restricted domains. The last decade has seen
  significant progress along this line with large-scale simulations of
  the deep convection zone, of the near-surface layers and photosphere,
  and in the corona. It has been realized that the magnetic field is the
  coupling agent of all these domains and first attempts have been made
  to address the magnetic coupling in simulations. I will go through
  the various physical domains, and briefly review the respective state
  of the art, but mainly focus upon the open questions, problems and
  challenges ahead - of which there are plenty.

Title: Comparison of Magnetoconvection Simulations with the
    Approximation of Thin Flux Tubes
Authors: Yelles Chaouche, L.; Solanki, S.; Schuessler, M.
Bibcode: 2008ESPM...12..3.8Y
Altcode:
  The structure and dynamics of small vertical photospheric magnetic
  flux concentrations has been often treated in the framework of
  an approximation based upon a low-order truncation of the Taylor
  expansions of all quantities in the horizontal direction, together with
  the assumption of instantaneous total pressure balance at the boundary
  to the non-magnetic external medium. Formally, such an approximation
  is justified if the diameter of the structure (a flux tube or a
  flux sheet) is small compared to all other relevant length scales
  (scale height, radius of curvature, wavelength, etc.). The advent of
  realistic 3D radiative MHD simulations opens the possibility to check
  the consistency of the approximation with the properties of the flux
  concentrations that form in the course of the simulation. We make
  a comparative analysis between the thin flux tube/sheet model and
  flux concentrations existing in a 3D radiation-MHD simulation. We
  have found that for flux concentration well above the equipartition
  distribution, the MHD magnetic structures are reasonably well reproduced
  by the second-order thin flux tube/sheet approximation. The differences
  between approximation and simulation are due to the asymmetry and the
  dynamics of the simulated structures.

Title: The intensity contrast of solar granulation: comparing Hinode
    SP results with MHD simulations
Authors: Danilovic, S.; Gandorfer, A.; Lagg, A.; Schüssler, M.;
   Solanki, S. K.; Vögler, A.; Katsukawa, Y.; Tsuneta, S.
Bibcode: 2008A&A...484L..17D
Altcode: 2008arXiv0804.4230D
  Context: The contrast of granulation is an important quantity
  characterizing solar surface convection. <BR />Aims: We compare the
  intensity contrast at 630 nm, observed using the Spectro-Polarimeter
  (SP) aboard the Hinode satellite, with the 3D radiative MHD simulations
  of Vögler &amp; Schüssler (2007, A&amp;A, 465, L43). <BR />Methods:
  A synthetic image from the simulation is degraded using a theoretical
  point-spread function of the optical system, and by considering other
  important effects. <BR />Results: The telescope aperture and the
  obscuration by the secondary mirror and its attachment spider, reduce
  the simulated contrast from 14.4% to 8.5%. A slight effective defocus
  of the instrument brings the simulated contrast down to 7.5%, close to
  the observed value of 7.0%. <BR />Conclusions: A proper consideration
  of the effects of the optical system and a slight defocus, lead to
  sufficient degradation of the synthetic image from the MHD simulation,
  such that the contrast reaches almost the observed value. The remaining
  small discrepancy can be ascribed to straylight and slight imperfections
  of the instrument, which are difficult to model. Hence, Hinode SP data
  are consistent with a granulation contrast which is predicted by 3D
  radiation MHD simulations.

Title: Strong horizontal photospheric magnetic field in a surface
    dynamo simulation
Authors: Schüssler, M.; Vögler, A.
Bibcode: 2008A&A...481L...5S
Altcode: 2008arXiv0801.1250S
  Context: Observations with the Hinode spectro-polarimeter have
  revealed strong horizontal internetwork magnetic fields in the quiet
  solar photosphere. <BR />Aims: We aim to interpret the observations
  with results from numerical simulations. <BR />Methods: Radiative MHD
  simulations of dynamo action by near-surface convection are analyzed
  with respect to the relation between vertical and horizontal magnetic
  field components. <BR />Results: The dynamo-generated fields show
  a clear dominance of the horizontal field in the height range where
  the spectral lines used for the Hinode observations are formed. The
  ratio between the averaged horizontal and vertical field components is
  consistent with the values derived from the observations. This behavior
  results from the intermittent nature of the dynamo field with polarity
  mixing on small scales in the surface layers. <BR />Conclusions:
  Our results provide further evidence that local near-surface dynamo
  action contributes significantly to the solar internetwork fields.

Title: Magnetic Flux Emergence in the Solar Photosphere
Authors: Cheung, M. C. M.; Schüssler, M.; Moreno-Insertis, F.
Bibcode: 2008ASPC..384..181C
Altcode: 2008csss...14..181C
  The most prominent magnetic structures on the surface of the Sun are
  bipolar active regions. These magnetic complexes are comprised of
  a hierarchy of magnetic structures of different sizes, the largest
  of which are sunspots. Observations indicate that the appearance
  of active regions on the solar surface result from the emergence of
  bundles of magnetic flux from the underlying convection zone. We study
  the emergence process by means of 3D radiation MHD simulations. In the
  simulations, an initially buoyant magnetic flux tube is introduced into
  the near-surface layers of the convection zone. Subject to the buoyancy
  force, the flux tube rises towards the photosphere. Our simulations
  highlight the importance of magneto-convection on the evolution of
  the magnetic flux tube. The external convective flow field has an
  important influence on the emergence morphology of the emerging magnetic
  field. Depending on the initial properties of the magnetic flux tube
  (e.g. field strength, twist, entropy etc.), flux emergence may lead
  to a disturbance of the local granulation pattern. The observational
  signatures associated with emerging magnetic flux in our simulations
  are in qualitative and quantitative agreement with observational
  studies of emerging flux regions on the Sun.

Title: Are solar cycles predictable?
Authors: Schüssler, M.
Bibcode: 2007AN....328.1087S
Altcode: 2007arXiv0712.1917S
  Various methods (or recipes) have been proposed to predict future solar
  activity levels - with mixed success. Among these, some precursor
  methods based upon quantities determined around or a few years
  before solar minimum have provided rather high correlations with
  the strength of the following cycles. Recently, data assimilation
  with an advection-dominated (flux-transport) dynamo model has been
  proposed as a predictive tool, yielding remarkably high correlation
  coefficients. After discussing the potential implications of these
  results and the criticism that has been raised, we study the possible
  physical origin(s) of the predictive skill provided by precursor
  and other methods. It is found that the combination of the overlap
  of solar cycles and their amplitude-dependent rise time (Waldmeier's
  rule) introduces correlations in the sunspot number (or area) record,
  which account for the predictive skill of many precursor methods. This
  explanation requires no direct physical relation between the precursor
  quantity and the dynamo mechanism (in the sense of the Babcock-Leighton
  scheme or otherwise).

Title: A coupled model of magnetic flux generation and transport
    in stars
Authors: Işik, E.; Schmitt, D.; Schüssler, M.
Bibcode: 2007AN....328.1111I
Altcode: 2011arXiv1111.2453I
  We present a combined model for magnetic field generation and transport
  in cool stars with outer convection zones. The mean toroidal magnetic
  field, which is generated by a cyclic thin-layer \alpha\Omega dynamo at
  the bottom of the convection zone is taken to determine the emergence
  probability of magnetic flux tubes in the photosphere. Following the
  nonlinear rise of the unstable thin flux tubes, emergence latitudes and
  tilt angles of bipolar magnetic regions are determined. These quantities
  are put into a surface flux transport model, which simulates the surface
  evolution of magnetic flux under the effects of large-scale flows and
  turbulent diffusion. First results are discussed for the case of the
  Sun and for more rapidly rotating solar-type stars. <P />Movies are
  available via http://www.aip.de/AN/movies

Title: Photospheric Magnetic Flux Emergence: A comparative study
    between Hinode/SOT Observations and MHD simulations
Authors: Cheung, M. C.; Schüssler, M.; Moreno-Insertis, F.; Tarbell,
   T. D.
Bibcode: 2007AGUFMSH53A1073C
Altcode:
  With high angular resolution, high temporal cadence and a stable
  point spread function, the Solar Optical Telescope (SOT) onboard the
  Hinode satellite is the ideal instrument for the study of magnetic
  flux emergence and its manifestations on the solar surface. In this
  presentation, we focus on the development of ephemeral regions and
  small active regions. In many instances, SOT has been able to capture
  the entire emergence process from beginning to end: i.e. from the
  initial stages of flux appearance in granule interiors, through the
  intermediate stages of G-band bright point formation, and finally
  to the coalescence of small vertical flux elements to form pores. To
  investigate the physics of the flux emergence process, we performed
  3D numerical MHD simulations with the MURaM code. The models are able
  to reproduce, and help us explain, various observational signatures
  of magnetic flux emergence.

Title: Radiative magnetohydrodynamic simulations of solar pores
Authors: Cameron, R.; Schüssler, M.; Vögler, A.; Zakharov, V.
Bibcode: 2007A&A...474..261C
Altcode:
  Context: Solar pores represent a class of magnetic structures
  intermediate between small-scale magnetic flux concentrations in
  intergranular lanes and fully developed sunspots with penumbrae. <BR
  />Aims: We study the structure, energetics, and internal dynamics
  of pore-like magnetic structures by means of exploratory numerical
  simulations. <BR />Methods: The MURaM code has been used to carry
  out several 3D radiative MHD simulations for pores of various sizes
  and with different boundary conditions. <BR />Results: The general
  properties of the simulated pores (morphology, continuum intensity,
  magnetic field geometry, surrounding flow pattern, mean height
  profiles of temperature, pressure, and density) are consistent with
  observational results. No indications for the formation of penumbral
  structure are found. The simulated pores decay by gradually shedding
  magnetic flux into the surrounding pattern of intergranular downflows
  (“turbulent erosion”). When viewed under an angle (corresponding
  to observations outside solar disc center), granules behind the pore
  appear brightened. <BR />Conclusions: Radiative MHD simulations capture
  many observed properties of solar pores.

Title: Flow instabilities of magnetic flux tubes. II. Longitudinal
    flow
Authors: Holzwarth, V.; Schmitt, D.; Schüssler, M.
Bibcode: 2007A&A...469...11H
Altcode: 2007arXiv0704.3685H
  Context: Flow-induced instabilities are relevant for the storage and
  dynamics of magnetic fields in stellar convection zones and possibly
  also in other astrophysical contexts. <BR />Aims: We continue the
  study started in the first paper of this series by considering
  the stability properties of longitudinal flows along magnetic flux
  tubes. <BR />Methods: A linear stability analysis was carried out to
  determine criteria for the onset of instability in the framework of
  the approximation of thin magnetic flux tubes. <BR />Results: In the
  non-dissipative case, we find Kelvin-Helmholtz instability for flow
  velocities exceeding a critical speed that depends on the Alfvén speed
  and on the ratio of the internal and external densities. Inclusion of
  a friction term proportional to the relative transverse velocity leads
  to a friction-driven instability connected with backward (or negative
  energy) waves. We discuss the physical nature of this instability. In
  the case of a stratified external medium, the Kelvin-Helmholtz
  instability and the friction-driven instability can set in for flow
  speeds significantly lower than the Alfvén speed. <BR />Conclusions:
  Dissipative effects can excite flow-driven instability below the
  thresholds for the Kelvin-Helmholtz and the undulatory (Parker-type)
  instabilities. This may be important for magnetic flux storage in
  stellar convection zones and for the stability of astrophysical jets.

Title: Stokes diagnostics of simulated solar magneto-convection
Authors: Shelyag, S.; Schüssler, M.; Solanki, S. K.; Vögler, A.
Bibcode: 2007A&A...469..731S
Altcode: 2007astro.ph..3490S
  We present results of synthetic spectro-polarimetric diagnostics of
  radiative MHD simulations of solar surface convection with magnetic
  fields. Stokes profiles of Zeeman-sensitive lines of neutral iron in
  the visible and infrared spectral ranges emerging from the simulated
  atmosphere have been calculated in order to study their relation to the
  relevant physical quantities and compare with observational results. We
  have analyzed the dependence of the Stokes-I line strength and width as
  well as of the Stokes-V signal and asymmetries on the magnetic field
  strength. Furthermore, we have evaluated the correspondence between
  the actual velocities in the simulation with values determined from
  the Stokes-I (Doppler shift of the centre of gravity) and Stokes-V
  profiles (zero-crossing shift). We confirm that the line weakening in
  strong magnetic fields results from a higher temperature (at equal
  optical depth) in the magnetic flux concentrations. We also confirm
  that considerable Stokes-V asymmetries originate in the peripheral
  parts of strong magnetic flux concentrations, where the line of sight
  cuts through the magnetopause of the expanding flux concentration into
  the surrounding convective donwflow.

Title: Magnetic Flux Emergence In Granular Convection: Radiative
    MHD Simulations And Hinode SOT Observations
Authors: Cheung, Mark; Schüssler, M.; Moreno-Insertis, F.; Tarbell,
   T.; SOT Team
Bibcode: 2007AAS...210.9425C
Altcode: 2007BAAS...39..221C
  We model the emergence of buoyant magnetic flux from the convection
  zone into the photosphere by means of 3D radiative MHD simulations
  using the MURaM code. In a series of simulations, we study how
  an initially buoyant magnetic flux tube rises in the presence of
  granular convection. The simulations take into account the effects of
  radiative energy exchange, ionization effects in the equation of state
  and compressibility. An emphasis of this talk is the comparison of
  observational diagnostics from the simulations with recent observations
  from Hinode SOT.

Title: Magnetic flux emergence in granular convection: radiative
    MHD simulations and observational signatures
Authors: Cheung, M. C. M.; Schüssler, M.; Moreno-Insertis, F.
Bibcode: 2007A&A...467..703C
Altcode: 2007astro.ph..2666C
  Aims:We study the emergence of magnetic flux from the near-surface
  layers of the solar convection zone into the photosphere. <BR />Methods:
  To model magnetic flux emergence, we carried out a set of numerical
  radiative magnetohydrodynamics simulations. Our simulations take into
  account the effects of compressibility, energy exchange via radiative
  transfer, and partial ionization in the equation of state. All these
  physical ingredients are essential for a proper treatment of the
  problem. Furthermore, the inclusion of radiative transfer allows us
  to directly compare the simulation results with actual observations
  of emerging flux. <BR />Results: We find that the interaction between
  the magnetic flux tube and the external flow field has an important
  influence on the emergent morphology of the magnetic field. Depending
  on the initial properties of the flux tube (e.g. field strength,
  twist, entropy etc.), the emergence process can also modify the local
  granulation pattern. The emergence of magnetic flux tubes with a
  flux of 10<SUP>19</SUP> Mx disturbs the granulation and leads to the
  transient appearance of a dark lane, which is coincident with upflowing
  material. These results are consistent with observed properties of
  emerging magnetic flux. <P />Movies are only available in electronic
  form at http://www.aanda.org

Title: Photospheric magnetoconvection
Authors: Cameron, Robert; Vögler, Alexander; Schüssler, Manfred
Bibcode: 2007IAUS..239..475C
Altcode:
  No abstract at ADS

Title: A solar surface dynamo
Authors: Vögler, A.; Schüssler, M.
Bibcode: 2007A&A...465L..43V
Altcode: 2007astro.ph..2681V
  Context: Observations indicate that the "quiet" solar photosphere
  outside active regions contains considerable amounts of magnetic energy
  and magnetic flux, with mixed polarity on small scales. The origin of
  this flux is unclear. <BR />Aims: We test whether local dynamo action
  of the near-surface convection (granulation) can generate a significant
  contribution to the observed magnetic flux. <BR />Methods: We have
  carried out MHD simulations of solar surface convection, including the
  effects of strong stratification, compressibility, partial ionization,
  radiative transfer, as well as an open lower boundary. <BR />Results:
  Exponential growth of a weak magnetic seed field (with vanishing net
  flux through the computational box) is found in a simulation run with a
  magnetic Reynolds number of about 2600. The magnetic energy approaches
  saturation at a level of a few percent of the total kinetic energy of
  the convective motions. Near the visible solar surface, the (unsigned)
  magnetic flux density reaches at least a value of about 25 G. <BR
  />Conclusions: .A realistic flow topology of stratified, compressible,
  non-helical surface convection without enforced recirculation is
  capable of turbulent local dynamo action near the solar surface.

Title: Solar Cycle Prediction Using Precursors and Flux Transport
    Models
Authors: Cameron, R.; Schüssler, M.
Bibcode: 2007ApJ...659..801C
Altcode: 2006astro.ph.12693C
  We study the origin of the predictive skill of some methods to
  forecast the strength of solar activity cycles. A simple flux
  transport model for the azimuthally averaged radial magnetic field at
  the solar surface is used, which contains a source term describing
  the emergence of new flux based on observational sunspot data. We
  consider the magnetic flux diffusing over the equator as a predictor,
  since this quantity is directly related to the global dipole field from
  which a Babcock-Leighton dynamo generates the toroidal field for the
  next activity cycle. If the source is represented schematically by a
  narrow activity belt drifting with constant speed over a fixed range
  of latitudes between activity minima, our predictor shows considerable
  predictive skill, with correlation coefficients up to 0.95 for past
  cycles. However, the predictive skill is completely lost when the
  actually observed emergence latitudes are used. This result originates
  from the fact that the precursor amplitude is determined by the sunspot
  activity a few years before solar minimum. Since stronger cycles tend to
  rise faster to their maximum activity (known as the Waldmeier effect),
  the temporal overlapping of cycles leads to a shift of the minimum
  epochs that depends on the strength of the following cycle. This
  information is picked up by precursor methods and also by our flux
  transport model with a schematic source. Therefore, their predictive
  skill does not require a memory, i.e., a physical connection between
  the surface manifestations of subsequent activity cycles.

Title: Magnetic flux transport on active cool stars and starspot
    lifetimes
Authors: Işik, E.; Schüssler, M.; Solanki, S. K.
Bibcode: 2007A&A...464.1049I
Altcode: 2006astro.ph.12399I
  Context: Many rapidly rotating cool stars show signatures of large
  magnetic regions at all latitudes. Mid-latitude starspots and magnetic
  regions have characteristic lifetimes of 1 month or less, as indicated
  by observations using (Zeeman-) Doppler imaging techniques. <BR
  />Aims: We aim to estimate the lifetimes of bipolar magnetic regions
  and starspots on the surfaces of cool stars. We consider different
  possible configurations for starspots and compare their flux variations
  and lifetimes based on a magnetic flux transport model. <BR />Methods:
  We carry out numerical simulations of the surface evolution of bipolar
  magnetic regions (BMRs) and magnetic spots on stars, which have radii
  and surface rotational shears of <ASTROBJ>AB Doradus</ASTROBJ>, the Sun,
  and the <ASTROBJ>HR 1099</ASTROBJ> primary. The surface flux transport
  model is based on the magnetic induction equation for radial fields
  under the effects of surface differential rotation, meridional flow,
  and turbulent diffusion due to convective flow patterns. We calculate
  the flux evolution and the lifetimes of BMRs and unipolar starspots,
  varying the emergence latitude, surface shear rate, and tilt angle. <BR
  />Results: For BMRs comparable to the largest observed on the Sun, we
  find that varying the surface flows and the tilt angle modifies the
  lifetimes over a range of one month. For very large BMRs (area ~10%
  of the stellar surface) the assumption of a tilt angle increasing with
  latitude leads to a significant increase of lifetime, as compared to
  the case without tilt. Such regions can evolve to polar spots that live
  more than a year. Adopting the observed weak latitudinal shear and the
  radius of the active subgiant component of HR 1099, we find longer
  BMR lifetimes as compared to the more strongly sheared <ASTROBJ>AB
  Dor</ASTROBJ> case. Random emergence of six additional tilted bipoles
  in an activity belt at 60° latitude enhanced the lifetimes of polar
  caps up to 7 years. We have also compared the evolution and lifetime
  of monolithic starspots with those of conglomerates of smaller spots
  of similar total area. We find similar decay patterns and lifetimes
  for both configurations.

Title: Flow instabilities of magnetic flux tubes. I. Perpendicular
    flow
Authors: Schüssler, M.; Ferriz Mas, A.
Bibcode: 2007A&A...463...23S
Altcode:
  Context: The stability properties of filamentary magnetic structures
  are relevant for the storage and dynamics of magnetic fields in
  stellar convection zones and possibly also in other astrophysical
  contexts. <BR />Aims: In a series of papers we study the effect
  of external and internal flows on the stability of magnetic flux
  tubes. In this paper we consider the effect of a flow perpendicular
  to a straight, horizontal flux tube embedded in a gravitationally
  stratified fluid. The flow acts on the flux tube by exerting an
  aerodynamic drag force and by modifying the pressure stratification
  in the background medium. <BR />Methods: We carry out a Lagrangian
  linear stability analysis in the framework of the approximation of
  thin magnetic flux tubes. <BR />Results: The external flow can drive
  monotonic and oscillatory instability (overstability). The stability
  condition depends on direction and magnitude of the external velocity as
  well as on its first and second derivatives with respect to depth. The
  range of the flow-driven instabilities typically extends to modes with
  much shorter wavelengths than for the buoyancy-driven undulatory Parker
  instability. <BR />Conclusions: .Perpendicular flows with Alfvénic
  Mach number of order unity can drive monotonic as well as oscillatory
  instability of thin magnetic flux tubes. Such instability can affect
  the storage of magnetic flux in stellar interiors.

Title: Solar mesogranulation as a cellular automaton effect
Authors: Matloch, L.; Cameron, R.; Schmitt, D.; Schüssler, M.
Bibcode: 2007msfa.conf..339M
Altcode:
  We present a simple cellular automaton model of solar granulation
  that captures the granular cell characteristics in terms of lifetime
  and size distributions. We show that mesogranulation, as defined in
  observational data, is an intrinsic feature of such a cell system.

Title: The origin of the reversed granulation in the solar photosphere
Authors: Cheung, M. C. M.; Schüssler, M.; Moreno-Insertis, F.
Bibcode: 2007A&A...461.1163C
Altcode: 2006astro.ph.12464C
  Aims:We study the structure and reveal the physical nature of the
  reversed granulation pattern in the solar photosphere by means of
  3-dimensional radiative hydrodynamics simulations. <BR />Methods: We
  used the MURaM code to obtain a realistic model of the near-surface
  layers of the convection zone and the photosphere. <BR />Results:
  The pattern of horizontal temperature fluctuations at the base of
  the photosphere consists of relatively hot granular cells bounded by
  the cooler intergranular downflow network. With increasing height
  in the photosphere, the amplitude of the temperature fluctuations
  diminishes. At a height of z=130-140 km in the photosphere, the pattern
  of horizontal temperature fluctuations reverses so that granular regions
  become relatively cool compared to the intergranular network. Detailed
  analysis of the trajectories of fluid elements through the photosphere
  reveal that the motion of the fluid is non-adiabatic, owing to strong
  radiative cooling when approaching the surface of optical depth
  unity followed by reheating by the radiation field from below. The
  temperature structure of the photosphere results from the competition
  between expansion of rising fluid elements and radiative heating. The
  former acts to lower the temperature of the fluid whereas the latter
  acts to increase it towards the radiative equilibrium temperature with
  a net entropy gain. After the fluid overturns and descends towards the
  convection zone, radiative energy loss again decreases the entropy
  of the fluid. Radiative heating and cooling of fluid elements that
  penetrate into the photosphere and overturn do not occur in equal
  amounts. The imbalance in the cumulative heating and cooling of
  these fluid elements is responsible for the reversal of temperature
  fluctuations with respect to height in the photosphere.

Title: Magnetic flux transport and the lifetimes of spots on active
    cool stars
Authors: Işik, E.; Schüssler, M.; Solanki, S. K.
Bibcode: 2007msfa.conf..367I
Altcode:
  We present results of numerical simulations of magnetic flux transport
  on the surfaces of cool stars with radii of 1 Rm and 3.3 Rm. The
  effects of differential rotation and the tilt angle on the lifetimes
  of stellar bipolar magnetic regions are discussed. The existence of
  long-lasting polar spots can be explained by high-latitude persistent
  emergence of bipolar regions.

Title: Modeling the Sun's open magnetic flux
Authors: Schüssler, M.; Baumann, I.
Bibcode: 2006A&A...459..945S
Altcode:
  Context: .The heliospheric magnetic field can be extrapolated from
  the photospheric field distribution using models based upon various
  approximations. Such models are required for the reconstruction of
  the open solar magnetic flux prior to the time of direct measurements
  on the basis of surface flux transport simulations.<BR /> Aims:
  .We evaluate the consistency of extrapolation models with direct
  measurements of the heliospheric magnetic field. Furthermore, we study
  whether extrapolations on the basis of a surface flux transport model
  for the photospheric magnetic field reproduce the temporal evolution
  of the measured near-Earth magnetic field.<BR /> Methods: .We use the
  potential field source surface (PFSS) model and the current sheet
  source surface (CSSS) model to extrapolate the heliospheric field
  on the basis of the Wilcox Solar Observatory (WSO) synoptic maps of
  the solar surface field from 1976-2005. The results are compared with
  the near-Earth measurements of the radial heliospheric field and its
  independence of latitude found with Ulysses. Furthermore, we determine
  extrapolations on the basis of photospheric flux distributions computed
  with a surface flux transport code, using as input sunspot group
  areas from the SOON database.<BR /> Results: .The CSSS model based
  upon WSO data, with a source surface located at ≥10~R<SUB>⊙</SUB>
  and cusp surface at 1.7~R<SUB>⊙</SUB>, yields the best agreement
  with the measurements. The flux transport simulations reproduce the
  observed surface flux together with the open flux if the tilt angle
  of emerging bipolar magnetic regions is smaller than commonly assumed,
  but consistent with sunspot observations.<BR /> Conclusions: .The CSSS
  model with a source surface in the vicinity of the Alfvénic point
  of the solar wind leads to a good extrapolation of the heliospheric
  field from solar surface data. A surface flux transport model based
  upon sunspot data with consistently calibrated tilt angles reproduces
  the observed evolution of the solar total open flux.

Title: Flux Emergence at the Photosphere
Authors: Cheung, M. C. M.; Schüssler, M.; Moreno-Insertis, F.
Bibcode: 2006ASPC..354...97C
Altcode:
  To model the emergence of magnetic fields at the photosphere, we
  carried out 3D magneto-hydrodynamics (MHD) simulations using the MURaM
  code. Our simulations take into account the effects of compressibility,
  energy exchange via radiative transfer and partial ionization in the
  equation of state. All these physical ingredients are essential for
  a proper treatment of the problem. In the simulations, an initially
  buoyant magnetic flux tube is embedded in the upper layers of the
  convection zone. We find that the interaction between the flux tube
  and the external flow field has an important influence on the emergent
  morphology of the magnetic field. Depending on the initial properties of
  the flux tube (e.g. field strength, twist, entropy etc.), the emergence
  process can also modify the local granulation pattern. The inclusion
  of radiative transfer allows us to directly compare the simulation
  results with real observations of emerging flux.

Title: The Dynamical Disconnection of Sunspots from their Magnetic
    Roots
Authors: Rempel, M.; Schüssler, M.
Bibcode: 2006ASPC..354..148R
Altcode:
  After a dynamically active emergence phase, magnetic flux at the
  solar surface soon ceases to show strong signs of the subsurface
  dynamics of its parent magnetic structure. This indicates that some
  kind of disconnection of the emerged flux from its roots in the deep
  convection zone should take place. We propose a mechanism for the
  dynamical disconnection of the surface flux based upon the buoyant
  upflow of plasma along the field lines. Such flows arise in the upper
  part of a rising flux loop during the final phases of its buoyant
  ascent towards the surface. The combination of the pressure buildup
  by the upflow and the cooling of the upper layers of an emerged flux
  tube by radiative losses at the surface lead to a progressive weakening
  of the magnetic field in several Mm depth. When the field strength has
  become sufficiently low, convective motions and the fluting instability
  disrupt the flux tube into thin, passively advected flux fragments,
  thus providing a dynamical disconnection of the emerged part from its
  roots. We substantiate this scenario by considering the quasi-static
  evolution of a sunspot model under the effects of radiative cooling,
  convective energy transport, and pressure buildup by a prescribed inflow
  at the bottom of the model. For inflow speeds in the range shown by
  simulations of thin flux tubes, we find that the disconnection takes
  place in a depth between two and six Mm for disconnection times up to
  three days.

Title: Magnetic Flux Transport on Active Cool Stars and Starspot
    Lifetimes
Authors: Isik, E.; Schüssler, M.; Solanki, S. K.
Bibcode: 2006IAUJD...8E..21I
Altcode:
  Rapidly rotating cool stars are known to have large magnetic regions
  at mid- to high-latitudes. Mid-latitude starspots and magnetic regions
  have characteristic lifetimes on the order of one month as observed
  using (Zeeman-) Doppler imaging techniques. The structure and detailed
  morphology of starspots are not observable at present. In this study,
  we present numerical simulations of the surface transport of bipolar
  magnetic regions (BMRs) and magnetic spots on stars which have radii
  and surface rotational shears of AB Doradus, the Sun, and the HR 1099
  primary. The surface flux transport model is based on the magnetic
  induction equation for radial fields under the effects of surface
  differential rotation, meridional flow, and turbulent diffusion due to
  supergranulation. We calculate flux evolution and lifetimes of BMRs with
  different emergence latitudes, surface shear rates, and tilt angles. For
  BMRs comparable to the largest ones on the Sun, we find that varying
  the surface flows and tilt angle modifies the lifetimes over a range
  of a month. For very large BMRs (area fraction ~ 0.1) the assumption
  of Joy's law for the tilt angle - as compared to the case with zero
  tilt - leads to a significant increase of lifetime. Such regions can
  evolve to form circumpolar spots that live more than a year. Taking the
  observed weak latitudinal shear and the radius of the active subgiant
  component of HR 1099, we find longer BMR lifetimes as compared to
  the more strongly sheared AB Dor case. We have also considered the
  effect on decay and lifetimes of starspots if they are monolithic or
  a conglomerate of smaller spots of similar total size. We find these
  different configurations differ neither in their decay patterns,
  nor in their lifetimes. We also give an analytical explanation for
  the linear decay of magnetic flux in the monolithic-spot simulation.

Title: Catalysis of candidate TPS Materials for EXPERT - a Basis
    for TPS Design and Catalysis based in-flight Instrumentations
Authors: Herdrich, G.; Auweter-Kurtz, M.; Fertig, M.; Fischer, W.;
   Muylaert, J. -M.; Pidan, S.; Schüssler, M.; Trabandt, U.
Bibcode: 2006ESASP.631E..42H
Altcode: 2006tpsh.confE..42H
  No abstract at ADS

Title: Moving magnetic tubes: fragmentation, vortex streets and the
    limit of the approximation of thin flux tubes
Authors: Cheung, M. C. M.; Moreno-Insertis, F.; Schüssler, M.
Bibcode: 2006A&A...451..303C
Altcode:
  Aims.We study the buoyant rise of magnetic flux tubes in a stratified
  layer over a range of Reynolds numbers (25 ⪉ Re ⪉ 2600) by means
  of numerical simulations. Special emphasis is placed on studying the
  fragmentation of the rising tube, its trailing wake and the formation
  of a vortex street in the high-Reynolds number regime. Furthermore,
  we evaluate the relevance of the thin flux tube approximation
  with regard to describing the evolution of magnetic flux tubes
  in the simulations.<BR /> Methods: .We used the FLASH code, which
  has an adaptive mesh refinement (AMR) algorithm, thus allowing the
  simulations to be carried out at high Reynolds numbers.<BR /> Results:
  .The evolution of the magnetic flux tube and its wake depends on the
  Reynolds number. At Re up to a few hundred, the wake consists of two
  counter-rotating vortex rolls. At higher Re, the vortex rolls break up
  and the shedding of flux into the wake occurs in a more intermittent
  fashion. The amount of flux retained by the central portion of the
  tube increases with the field line twist (in agreement with previous
  literature) and with Re. The time evolution of the twist is compatible
  with a homologous expansion of the tube. The motion of the central
  portion of the tube in the simulations is very well described by the
  thin flux tube model whenever the effects of flux loss or vortex forces
  can be neglected. If the flux tube has an initial net vorticity, it
  undergoes asymmetric vortex shedding. In this case, the lift force
  accelerates the tube in such a way that an oscillatory horizontal
  motion is super-imposed on the vertical rise of the tube, which leaves
  behind a vortex street. This last result is in accordance with previous
  simulations reported in the literature, which were carried out at
  lower Reynolds number.<BR />

Title: Magnetoconvection in a Sunspot Umbra
Authors: Schüssler, M.; Vögler, A.
Bibcode: 2006ApJ...641L..73S
Altcode: 2006astro.ph..3078S
  Results from a realistic simulation of three-dimensional radiative
  magnetoconvection in a strong background magnetic field corresponding
  to the conditions in sunspot umbrae are shown. The convective energy
  transport is dominated by narrow upflow plumes with adjacent downflows,
  which become almost field-free near the surface layers. The strong
  external magnetic field forces the plumes to assume a cusplike shape
  in their top parts, where the upflowing plasma loses its buoyancy. The
  resulting bright features in intensity images correspond well (in terms
  of brightness, size, and lifetime) to the observed umbral dots in the
  central parts of sunspot umbrae. Most of the simulated umbral dots
  have a horizontally elongated form with a central dark lane. Above
  the cusp, most plumes show narrow upflow jets, which are driven
  by the pressure of the piled-up plasma below. The large velocities
  and low field strengths in the plumes are effectively screened from
  spectroscopic observation because the surfaces of equal optical depth
  are locally elevated, so that spectral lines are largely formed above
  the cusp. Our simulations demonstrate that nearly field-free upflow
  plumes and umbral dots are a natural result of convection in a strong,
  initially monolithic magnetic field.

Title: The solar magnetic field
Authors: Solanki, Sami K.; Inhester, Bernd; Schüssler, Manfred
Bibcode: 2006RPPh...69..563S
Altcode: 2010arXiv1008.0771S
  The magnetic field of the Sun is the underlying cause of the many
  diverse phenomena combined under the heading of solar activity. Here
  we describe the magnetic field as it threads its way from the bottom
  of the convection zone, where it is built up by the solar dynamo, to
  the solar surface, where it manifests itself in the form of sunspots
  and faculae, and beyond into the outer solar atmosphere and, finally,
  into the heliosphere. On the way it transports energy from the surface
  and the subsurface layers into the solar corona, where it heats the
  gas and accelerates the solar wind.

Title: SUNRISE: high-resolution UV/VIS observations of the Sun from
    the stratosphere
Authors: Solanki, S. K.; Barthol, P.; Gandorfer, A.; Schüssler, M.;
   Lites, B. W.; Martinez Pillet, V.; Schmidt, W.; Title, A. M.
Bibcode: 2006cosp...36.2416S
Altcode: 2006cosp.meet.2416S
  SUNRISE is a balloon-borne solar telescope with an aperture of 1m
  working in the UV VIS optical domain The main scientific goal of
  SUNRISE is to study the structure and dynamics of the magnetic field
  in the atmosphere of the Sun at high spatial resolution SUNRISE will
  provide diffraction-limited images of the photosphere and chromosphere
  with an unprecedented resolution down to 35km at wavelengths around
  220nm Focal-plane instruments are a UV filter imager a Fabry-Perot
  filter magnetograph and a spectrograph polarimeter Stratospheric
  long-duration balloon flights of SUNRISE over the North Atlantic
  and or Antarctica are planned SUNRISE is a joint project of the
  Max-Planck-Institut fuer Sonnensystemforschung MPS Katlenburg-Lindau
  with the Kiepenheuer-Institut fuer Sonnenphysik KIS Freiburg the
  High-Altitude Observatory HAO Boulder the Lockheed-Martin Solar and
  Astrophysics Lab LMSAL Palo Alto and the spanish IMaX consortium The
  presentation will give an overview about the mission and a description
  of the instrumentation now at the beginning of the hardware construction
  phase

Title: A necessary extension of the surface flux transport model
Authors: Baumann, I.; Schmitt, D.; Schüssler, M.
Bibcode: 2006A&A...446..307B
Altcode:
  Customary two-dimensional flux transport models for the evolution
  of the magnetic field at the solar surface do not account for the
  radial structure and the volume diffusion of the magnetic field. When
  considering the long-term evolution of magnetic flux, this omission
  can lead to an unrealistic long-term memory of the system and to
  the suppression of polar field reversals. In order to avoid such
  effects, we propose an extension of the flux transport model by a
  linear decay term derived consistently on the basis of the eigenmodes
  of the diffusion operator in a spherical shell. A decay rate for each
  eigenmode of the system is determined and applied to the corresponding
  surface part of the mode evolved in the flux transport model. The
  value of the volume diffusivity associated with this decay term can
  be estimated to be in the range 50-100 km<SUP>2</SUP> s<SUP>-1</SUP>
  by considering the reversals of the polar fields in comparison of flux
  transport simulations with observations. We show that the decay term
  prohibits a secular drift of the polar field in the case of cycles of
  varying strength, like those exhibited by the historical sunspot record.

Title: Simulations of Solar Pores
Authors: Cameron, R.; Vögler, A.; Schüssler, M.; Zakharov, V.
Bibcode: 2005ESASP.600E..11C
Altcode: 2005ESPM...11...11C; 2005dysu.confE..11C
  No abstract at ADS

Title: D Magneto-Convection and Flux Emergence in the Photosphere
Authors: Cheung, M.; Schüssler, M.; Moreno-Insertis, F.
Bibcode: 2005ESASP.596E..54C
Altcode: 2005ccmf.confE..54C
  No abstract at ADS

Title: Solar activity, cosmic rays, and Earth's temperature: A
    millennium-scale comparison
Authors: Usoskin, I. G.; Schüssler, M.; Solanki, S. K.; Mursula, K.
Bibcode: 2005JGRA..11010102U
Altcode:
  Previous studies of a solar influence on climate variations have often
  suffered from the relatively short length of continuous direct solar
  observations of less than 400 years. We use two recently reconstructed
  series of the sunspot number and the cosmic ray flux to study this
  question over time intervals of up to nearly 1800 years. Comparison of
  the Sun-related data sets with various reconstructions of terrestrial
  Northern Hemisphere mean surface temperatures reveals consistently
  positive correlation coefficients for the sunspot numbers and
  consistently negative correlation coefficients for the cosmic rays. The
  significance levels reach up to 99% but vary strongly for the different
  data sets. The major part of the correlation is due to the similarity
  of the long-term trends in the data sets. The trend of the cosmic ray
  flux correlates somewhat better with the terrestrial temperature than
  the sunspot numbers derived from the same cosmogenic isotope data.

Title: The dynamical disconnection of sunspots from their magnetic
    roots
Authors: Schüssler, M.; Rempel, M.
Bibcode: 2005A&A...441..337S
Altcode: 2005astro.ph..6654S
  After a dynamically active emergence phase, magnetic flux at the
  solar surface soon ceases to show strong signs of the subsurface
  dynamics of its parent magnetic structure. This indicates that some
  kind of disconnection of the emerged flux from its roots in the deep
  convection zone should take place. We propose a mechanism for the
  dynamical disconnection of the surface flux based upon the buoyant
  upflow of plasma along the field lines. Such flows arise in the upper
  part of a rising flux loop during the final phases of its buoyant
  ascent towards the surface. The combination of the pressure buildup
  by the upflow and the cooling of the upper layers of an emerged flux
  tube by radiative losses at the surface lead to a progressive weakening
  of the magnetic field in several Mm depth. When the field strength has
  become sufficiently low, convective motions and the fluting instability
  disrupt the flux tube into thin, passively advected flux fragments,
  thus providing a dynamical disconnection of the emerged part from its
  roots. We substantiate this scenario by considering the quasi-static
  evolution of a sunspot model under the effects of radiative cooling,
  convective energy transport, and pressure buildup by a prescribed inflow
  at the bottom of the model. For inflow speeds in the range shown by
  simulations of thin flux tubes, we find that the disconnection takes
  place in a depth between 2 and 6 Mm for disconnection times up to
  3 days.

Title: Is there a phase constraint for solar dynamo models?
Authors: Schüssler, M.
Bibcode: 2005A&A...439..749S
Altcode: 2005astro.ph..5596S
  The spatio-temporal relationship between the sign of the observed
  radial component of the magnetic field at the solar surface and the
  sign of the toroidal field as inferred from Hale's polarity rules
  for sunspots is usually interpreted as signifying the phase relation
  between the poloidal and the toroidal magnetic field components involved
  in the solar dynamo process. This has been taken as a constraint
  for models of the solar dynamo. This note draws attention to the
  fact that the observed phase relation is naturally and inevitably
  produced by the emergence of tilted bipolar regions and flux transport
  through surface flows, without any necessity of recourse to the dynamo
  process. Consequently, there is no constraint on dynamo models resulting
  from the observed phase relation.

Title: Climate:  How unusual is today's solar activity? (reply)
Authors: Solanki, S. K.; Usoskin, I. G.; Kromer, B.; Schüssler, M.;
   Beer, J.
Bibcode: 2005Natur.436E...4S
Altcode:
  Muscheler et al. claim that the solar activity affecting cosmic rays
  was much higher in the past than we deduced from <SUP>14</SUP>C
  measurements. However, this claim is based on a problematic
  normalization and is in conflict with independent results, such as
  the <SUP>44</SUP>Ti activity in meteorites and the <SUP>10</SUP>Be
  concentration in ice cores.

Title: Comments on the structure and dynamics of magnetic fields in
    stellar convection zones
Authors: Schuessler, Manfred
Bibcode: 2005astro.ph..6050S
Altcode:
  This paper is a ``Habilitationsschrift'', a second thesis required
  until recently by universities in Germany and in a few other countries
  to obtain the right to lecture. It was accepted by the University of
  Goettingen in 1990 after review by a number of german and international
  experts. Although the introduction and the references represent the
  state of research in 1990, most of the remaining content is still
  relevant and has never been published elsewhere. The most important
  part is the derivation of a linear stability formalism for thin magnetic
  flux tubes following an arbitrary path in a gravitationally stratified
  medium with a stationary velocity. It was found later (Ferriz-Mas &amp;
  Schuessler, Geophys. Astrophys. Fluid Dyn. vol. 72, 209; 1995) that,
  for consistency, the inertial term in the equation of motion for the
  external medium should be included in Eq. (3.24), which leads to an
  additional term in the stability equations in the case of a spatially
  varying external velocity. This term is missing in the present text,
  but can be easily introduced into the formalism. The full abstract
  can be found at the beginning of the paper.

Title: Flux tubes, surface magnetism, and the solar dynamo:
    constraints and open problems
Authors: Schüssler, M.
Bibcode: 2005AN....326..194S
Altcode:
  The flux-tube paradigm has proven to be a remarkably useful tool
  to understand the connection between the dynamo process in the solar
  interior and its observable manifestations at the surface. After a brief
  review of the justification of this approach and of its successes, we
  discuss in some detail its loose ends and the remaining open questions -
  and attempt to provide some tentative answers. This includes the origin
  of fields much stronger than the dynamical equipartition value, the
  structure of the stored magnetic flux (flux tubes versus a magnetic
  layer) and the importance of convective pumping, as well as processes
  connected with flux emergence and the subsequent development of the
  magnetic flux. It is argued that the observations of the surface field
  indicate a dynamical disconnection of the emerged flux from its roots
  in the deep convection zone. Based on the `explosion' of magnetic
  flux tubes, a scenario and an illustrative model for the disconnection
  process are suggested. In the last part of the paper, the significance
  of observed properties of the surface magnetism for constraining
  solar dynamo models a critically discussed. It is shown that some
  properties of the butterfly diagram do neither confirm nor refute
  specific dynamo models. Furthermore, the observed phase relationship
  between the average toroidal and poloidal magnetic field components is
  shown to result from the tilt angle of bipolar magnetic regions, so that
  it imposes no constraint for models of the deep-seated solar dynamo.

Title: Solar activity over the last 1150 years: does it correlate
    with climate?
Authors: Usoskin, I. G.; Schüssler, M.; Solanki, S. K.; Mursula, K.
Bibcode: 2005ESASP.560...19U
Altcode: 2005csss...13...19U
  No abstract at ADS

Title: Simulations of magneto-convection in the solar photosphere.
    Equations, methods, and results of the MURaM code
Authors: Vögler, A.; Shelyag, S.; Schüssler, M.; Cattaneo, F.;
   Emonet, T.; Linde, T.
Bibcode: 2005A&A...429..335V
Altcode:
  We have developed a 3D magnetohydrodynamics simulation code for
  applications in the solar convection zone and photosphere. The code
  includes a non-local and non-grey radiative transfer module and takes
  into account the effects of partial ionization. Its parallel design
  is based on domain decomposition, which makes it suited for use on
  parallel computers with distributed memory architecture. We give a
  description of the equations and numerical methods and present the
  results of the simulation of a solar plage region. Starting with
  a uniform vertical field of 200 G, the processes of flux expulsion
  and convective field amplification lead to a dichotomy of strong,
  mainly vertical fields embedded in the granular downflow network and
  weak, randomly oriented fields filling the hot granular upflows. The
  strong fields form a magnetic network with thin, sheet-like structures
  extending along downflow lanes and micropores with diameters of up to
  1000 km which form occasionally at vertices where several downflow
  lanes merge. At the visible surface around optical depth unity,
  the strong field concentrations are in pressure balance with their
  weakly magnetized surroundings and reach field strengths of up to 2
  kG, strongly exceeding the values corresponding to equipartition with
  the kinetic energy density of the convective motions. As a result of
  the channelling of radiation, small flux concentrations stand out as
  bright features, while the larger micropores appear dark in brightness
  maps owing to the suppression of the convective energy transport. The
  overall shape of the magnetic network changes slowly on a timescale
  much larger than the convective turnover time, while the magnetic flux
  is constantly redistributed within the network leading to continuous
  formation and dissolution of flux concentrations. <P />Appendices A-D
  are only available in electronic form at http://www.edpsciences.org

Title: A necessary extension of the flux transport model
Authors: Baumann, I.; Schmitt, D.; Schüssler, M.
Bibcode: 2005MmSAI..76..933B
Altcode: 2005astro.ph.10322B
  Customary two-dimensional flux-transport models for the evolution
  of the magnetic field at the solar surface do not account for the
  radial structure and the volume diffusion of the magnetic field
  \cite{Schrijver2002}. When considering the long-term evolution of
  magnetic flux, this omission can lead to an unrealistic long-term memory
  of the system and to the suppression of polar field reversals. In order
  to avoid such effects, we propose an extension of the flux-transport
  model by a linear decay term derived consistently on the basis of the
  eigenmodes of the diffusion operator in a spherical shell. The value of
  the volume diffusivity eta associated with this term can be estimated
  to be in the range 50-100 km<SUP>2</SUP> s<SUP>-1</SUP> by considering
  the reversals of the polar fields in comparison of flux-transport
  simulations with observations. We show that the decay term prohibits
  a secular drift of the polar field in the case of cycles of varying
  strength, like those exhibited by the historical sunspot record.

Title: Mechanisms of secular magnetic field variations.
Authors: Solanki, S. K.; Schüssler, M.
Bibcode: 2005MmSAI..76..781S
Altcode:
  The variability of the solar magnetic field on time scales of decades
  and longer lies at the root of the various mechanisms by which the
  changing Sun could affect Earth's climate. We discuss the origin of
  the secular variability of both the open heliospheric flux and the
  total unsigned solar surface flux and review models that have been put
  forward to describe these variations. We propose that a combination of
  the effects of overlapping activity cycles and the long decay time of
  large-scale magnetic patterns is responsible for the secular variability
  of the solar magnetic field.

Title: The Decay of a Simulated Pore
Authors: Cameron, R.; Vögler, A.; Shelyag, S.; Schüssler, M.
Bibcode: 2004ASPC..325...57C
Altcode:
  Using MURaM -- Max-Planck Institut für Aeronomie University of
  Chicago Radiative Magnetohydrodynamics -- an MHD code which includes
  radiative transfer and partial ionization, we have studied the decay
  phase of a solar pore. The simulations are sufficiently realistic
  in their treatment of the photosphere to allow a direct comparison
  with observations, both current and those of upcoming missions such
  as SolarB. As well as discussing the structure and decay of pores,
  we show the formation of shallow, field aligned, convective rolls
  which are an important feature of our solutions.

Title: Small-Scale Solar Magnetic Elements: Simulations and
    Observations
Authors: Solanki, S. K.; Schüssler, M.
Bibcode: 2004ASPC..325..105S
Altcode:
  Both the small-scale and large-scale properties of solar features,
  such as sunspots and the solar corona, are influenced strongly by
  the small-scale structure of the underlying magnetic field. Even
  some global properties of the Sun, such as variations of the Sun's
  irradiance, depend on the local properties of small-scale magnetic
  features. We briefly describe these dependences, as well as recent
  results concerning the small-scale magnetic elements deduced from
  radiation MHD simulations and spectropolarimetric observations. The
  simulations reproduce a number of sensitive observational tests and
  explain, e.g. why G-band images allow only a part of the magnetic flux
  to be identified.

Title: G-band spectral synthesis and diagnostics of simulated solar
    magneto-convection
Authors: Shelyag, S.; Schüssler, M.; Solanki, S. K.; Berdyugina,
   S. V.; Vögler, A.
Bibcode: 2004A&A...427..335S
Altcode:
  Realistic simulations of radiative magneto-convection in the solar
  (sub)photosphere are used for a spectral synthesis of Fraunhofer's G
  band, which is dominated by spectral lines from the CH molecule. It
  is found that the spatial pattern of integrated G-band brightness
  closely matches the spatial structure of magnetic flux concentrations
  in the convective downflow regions. The brightness contrast is mainly
  caused by the weakening of CH lines due to the reduced CH abundance
  and the resulting shift of the optical depth scale in the hot and
  tenuous magnetic flux concentrations. Various properties of the
  synthetic brightness images agree well with G-band observations. These
  results lends credit to the observational usage of G-band bright
  features as proxies for magnetic flux concentrations in the solar
  photosphere. However, the converse is only correct in a limited sense:
  only a fraction of the magnetic flux concentrations turn out to be
  bright in the G band.

Title: Evolution of the large-scale magnetic field on the solar
surface: A parameter study
Authors: Baumann, I.; Schmitt, D.; Schüssler, M.; Solanki, S. K.
Bibcode: 2004A&A...426.1075B
Altcode:
  Magnetic flux emerging on the Sun's surface in the form of bipolar
  magnetic regions is redistributed by supergranular diffusion, a poleward
  meridional flow and differential rotation. We perform a systematic and
  extensive parameter study of the influence of various parameters on the
  large-scale field, in particular the total unsigned surface flux and the
  flux in the polar caps, using a flux transport model. We investigate
  both, model parameters and source term properties. We identify the
  average tilt angle of the emerging bipolar regions, the diffusion
  coefficient (below a critical value), the total emergent flux and,
  for the polar field, the meridional flow velocity and the cycle length
  as parameters with a particularly large effect. Of special interest is
  the influence of the overlap between successive cycles. With increasing
  overlap, an increasing background field (minimum flux at cycle minimum)
  is built up, which is of potential relevance for secular trends of
  solar activity and total irradiance.

Title: SUNRISE: high-resolution UV/VIS observations of the Sun from
    the stratosphere
Authors: Gandorfer, Achim M.; Solanki, Sami K.; Schüssler, Manfred;
   Curdt, Werner; Lites, Bruce W.; Martínez Pillet, Valentin; Schmidt,
   Wolfgang; Title, Alan M.
Bibcode: 2004SPIE.5489..732G
Altcode:
  SUNRISE is a balloon-borne solar telescope with an aperture of 1m,
  working in the UV/VIS optical domain. The main scientific goal
  of SUNRISE is to understand the structure and dynamics of the
  magnetic field in the atmosphere of the Sun. SUNRISE will provide
  diffraction-limited images of the photosphere and chromosphere with
  an unpredecented resolution down to 35km at wavelengths around
  220nm. Focal-plane instruments are a spectrograph/polarimeter,
  a Fabry-Perot filter magnetograph, and a filter imager. The first
  stratospheric long-duration balloon flight of SUNRISE over Antarctica
  is planned in winter 2006/2007. SUNRISE is a joint project of the
  Max-Planck-Institut fur Sonnensystemforschung (MPS), Katlenburg-Lindau,
  with the Kiepenheuer-Institut für Sonnenphysik (KIS), Freiburg, the
  High-Altitude Observatory (HAO), Boulder, the Lockheed-Martin Solar and
  Astrophysics Lab. (LMSAL), Palo Alto, and the Instituto de Astrofisica
  de Canarias, La Laguna, Tenerife. In this paper we will present an
  overview on the mission and give a description of the instrumentation,
  now, at the beginning of the hardware construction phase.

Title: Unusual activity of the Sun during recent decades compared
    to the previous 11,000 years
Authors: Solanki, S. K.; Usoskin, I. G.; Kromer, B.; Schüssler, M.;
   Beer, J.
Bibcode: 2004Natur.431.1084S
Altcode:
  Direct observations of sunspot numbers are available for the past
  four centuries, but longer time series are required, for example,
  for the identification of a possible solar influence on climate
  and for testing models of the solar dynamo. Here we report a
  reconstruction of the sunspot number covering the past 11,400 years,
  based on dendrochronologically dated radiocarbon concentrations. We
  combine physics-based models for each of the processes connecting
  the radiocarbon concentration with sunspot number. According to our
  reconstruction, the level of solar activity during the past 70 years is
  exceptional, and the previous period of equally high activity occurred
  more than 8,000 years ago. We find that during the past 11,400 years
  the Sun spent only of the order of 10% of the time at a similarly high
  level of magnetic activity and almost all of the earlier high-activity
  periods were shorter than the present episode. Although the rarity
  of the current episode of high average sunspot numbers may indicate
  that the Sun has contributed to the unusual climate change during the
  twentieth century, we point out that solar variability is unlikely
  to have been the dominant cause of the strong warming during the past
  three decades.

Title: Does the butterfly diagram indicate a solar flux-transport
    dynamo?
Authors: Schüssler, M.; Schmitt, D.
Bibcode: 2004A&A...421..349S
Altcode: 2004astro.ph..3570S
  We address the question whether the properties of the observed
  latitude-time diagram of sunspot occurrence (the butterfly diagram)
  provide evidence for the operation of a flux-transport dynamo, which
  explains the migration of the sunspot zones and the period of the solar
  cycle in terms of a deep equatorward meridional flow. We show that the
  properties of the butterfly diagram are equally well reproduced by a
  conventional dynamo model with migrating dynamo waves, but without
  transport of magnetic flux by a flow. These properties seem to be
  generic for an oscillatory and migratory field of dipole parity and
  thus do not permit an observational distinction between different
  dynamo approaches.

Title: Approximations for non-grey radiative transfer in numerical
    simulations of the solar photosphere
Authors: Vögler, A.; Bruls, J. H. M. J.; Schüssler, M.
Bibcode: 2004A&A...421..741V
Altcode:
  Realistic simulations of solar (magneto-)convection require an
  accurate treatment of the non-grey character of the radiative energy
  transport. Owing to the large number of spectral lines in the solar
  atmosphere, statistical representations of the line opacities have to
  be used in order to keep the problem numerically tractable. We consider
  two statistical approaches, the opacity distribution function (ODF)
  concept and the multigroup (or opacity binning) method and provide a
  quantitative assessment of the errors that arise from the application
  of these methods in the context of 2D/3D simulations. In a first step,
  the ODF- and multigroup methods are applied to a 1D model-atmosphere
  and the resulting radiative heating rates are compared. A number of
  4-6 frequency bins is found to warrant a satisfactory modeling of the
  radiative energy exchange. Further tests in 2D model-atmospheres show
  the applicability of the multigroup method in realistic situations
  and underline the importance of a non-grey treatment. Furthermore, we
  address the question of an appropriate opacity average in multigroup
  calculations and discuss the significance of velocity gradients for
  the radiative heating rates.

Title: Kelvin-Helmholtz and shear instability of a helical flow
    around a magnetic flux tube
Authors: Kolesnikov, F.; Bünte, M.; Schmitt, D.; Schüssler, M.
Bibcode: 2004A&A...420..737K
Altcode:
  Magnetic flux concentrations in the solar (sub)photosphere are
  surrounded by strong downflows, which come into swirling motion owing
  to the conservation of angular momentum. While such a whirl flow can
  stabilize a magnetic flux tube against the MHD fluting instability, it
  potentially becomes subject to Kelvin-Helmholtz and shear instability
  near the edge of the flux tube, which may lead to twisting of the
  magnetic field and perhaps even to the disruption of the magnetic
  structure. As a first step towards studying the relevance of such
  instabilities, we investigate the stability of an incompressible
  flow with longitudinal and azimuthal (whirl) components surrounding a
  cylinder with a uniform longitudinal magnetic field. We find that a
  sharp jump of the azimuthal flow component at the cylinder boundary
  always leads to Kelvin-Helmholtz-type instability for sufficiently
  small wavelength of the perturbation. On the other hand, a smooth and
  wide enough transition of the azimuthal velocity towards the surface of
  the cylinder leads to stable configurations, even for a discontinuous
  profile of the longitudinal flow.

Title: On the Origin of Solar Faculae
Authors: Keller, C. U.; Schüssler, M.; Vögler, A.; Zakharov, V.
Bibcode: 2004ApJ...607L..59K
Altcode:
  Solar faculae appear as bright small features close to the solar
  limb. Recent high-resolution images show these brightenings in
  unprecedented detail. Our analysis of numerical MHD simulations
  reproduces the observed small-scale features. The simulations reveal
  that faculae originate from a thin layer within granules just below
  largely transparent magnetic flux concentrations. This is basically
  the ``bright wall'' model of Spruit. The dark, narrow lanes often
  associated with faculae occur at the opposite side of the magnetic flux
  concentration and are due to an extended layer with lower-than-average
  temperature.

Title: Usoskin et al. Reply:
Authors: Usoskin, Ilya G.; Solanki, Sami K.; Schüssler, Manfred;
   Mursula, Kalevi
Bibcode: 2004PhRvL..92s9002U
Altcode:
  A Reply to the Comment by G. M. Raisbeck and F. Yiou.

Title: Distribution of magnetically confined circumstellar matter
    in oblique rotators
Authors: Preuss, O.; Schüssler, M.; Holzwarth, V.; Solanki, S. K.
Bibcode: 2004A&A...417..987P
Altcode: 2004astro.ph..1599P
  We consider the mechanical equilibrium and stability of matter trapped
  in the magnetosphere of a rapidly rotating star. Assuming a dipolar
  magnetic field and arbitrary inclination of the magnetic axis with
  respect to the axis of rotation we find stable equilibrium positions a)
  in a (warped) disk roughly aligned with the magnetic equatorial plane
  and b) at two locations above and below the disk, whose distance from
  the star increases with decreasing inclination angle between dipole and
  rotation axis. The distribution of matter is not strongly affected by
  allowing for a spatial offset of the magnetic dipole. These results
  provide a possible explanation for some observations of corotating
  localized mass concentrations in hot magnetic stars.

Title: Stokes diagnostics of magneto-convection. Profile shapes
    and asymmetries
Authors: Khomenko, E. V.; Shelyag, S.; Solanki, S. K.; Vögler, A.;
   Schüssler, M.
Bibcode: 2004IAUS..223..635K
Altcode: 2005IAUS..223..635K
  We discuss the polarization signals produced in recent realistic 3D
  simulations of solar magnetoconvection. The Stokes profiles of the
  Fe I 6301.5, 6302.5, 15648 and 15652 mathrm{Å} Zeeman-sensitive
  spectral lines are synthesised and smeared to simulate the image
  degradation caused by the Earth's atmosphere and finite telescope
  resolution. A Principal Component Analysis approach is applied to
  classify the profiles. We find that the classes of Stokes V profiles
  as well as their amplitude and area asymmetries are very close to the
  observations in the network and inter-network regions.

Title: Stokes Diagnostics of Magnetoconvection. Profile shapes
    and asymmetries.
Authors: Khomenko, E. V.; Shelyag, S.; Solanki, S. K.; Vogler, A.;
   Schussler, M.
Bibcode: 2004cosp...35.2131K
Altcode: 2004cosp.meet.2131K
  Stokes profiles observed in the quiet Sun have a broad range of
  asymmetries and show a variety of shapes. These asymmetries are the
  result of the velocity and magnetic field gradients both in horizontal
  and vertical directions. We use the most recent realistic 3D simulations
  of magnetoconvection at the solar surface to synthesize Stokes profiles
  of some photospheric lines and to compare them with observations. Such
  comparison provides an important constrains on the MHD models allowing
  to conclude about their realism and, thus, to understand the nature of
  solar magnetoconvection. The following Zeeman-sensitive spectral lines
  are considered: Fe I 6301.5, 6302.5, 15648 and 15652 Å. These lines are
  extensively used in observations. The computed Stokes profiles of these
  lines were spatially smeared to simulate the effects of a telescope and
  atmospheric seeing. A Principal Component Analysis approach is applied
  to classify the profiles. The effects of spatial resolution and the
  amount of the magnetic flux in the MHD model on the profile shapes are
  discussed. The profiles of different classes are clustered together and
  form patches on the surface. The size of these patches decreases with
  increasing spatial resolution. The distributions of the amplitude and
  area asymmetries of Stokes V profiles are very close to the observations
  in network and inter-network regions. Some 15% of the profiles smeared
  with a 0.''5 seeing have irregular shape with 3 or more lobes. Finally,
  we show that simultaneous observations of the same area of the solar
  disc using infrared Fe I 15648, 15652 Å and the visible Fe I 6301.5,
  6302.5 Å lines done under different seeing conditions (for example
  in the case of simultaneous observations at different telescopes)
  may result in different asymmetries and even different polarities
  of the profiles in two spectral regions observed at the same spatial
  point. This work was partially supported by INTAS grant 00-00084.

Title: Reconstruction of solar activity for the last millennium
    using <SUP>10</SUP>Be data
Authors: Usoskin, I. G.; Mursula, K.; Solanki, S.; Schüssler, M.;
   Alanko, K.
Bibcode: 2004A&A...413..745U
Altcode: 2003astro.ph..9556U
  In a recent paper (Usoskin et al. 2002a), we have reconstructed
  the concentration of the cosmogenic <SUP>10</SUP>Be isotope in ice
  cores from the measured sunspot numbers by using physical models
  for <SUP>10</SUP>Be production in the Earth's atmosphere, cosmic ray
  transport in the heliosphere, and evolution of the Sun's open magnetic
  flux. Here we take the opposite route: starting from the <SUP>10</SUP>Be
  concentration measured in ice cores from Antarctica and Greenland, we
  invert the models in order to reconstruct the 11-year averaged sunspot
  numbers since 850 AD. The inversion method is validated by comparing
  the reconstructed sunspot numbers with the directly observed sunspot
  record since 1610. The reconstructed sunspot record exhibits a prominent
  period of about 600 years, in agreement with earlier observations
  based on cosmogenic isotopes. Also, there is evidence for the century
  scale Gleissberg cycle and a number of shorter quasi-periodicities
  whose periods seem to fluctuate in the millennium time scale. This
  invalidates the earlier extrapolation of multi-harmonic representation
  of sunspot activity over extended time intervals.

Title: Theoretical Models of Solar Magnetic Variability
Authors: Schüssler, Manfred; Schmitt, Dieter
Bibcode: 2004GMS...141...33S
Altcode:
  Solar variability on all observationally accessible temporal and
  spatial scales is intimately connected with the variations of the
  solar magnetic field and its interaction with the non-stationary flow
  patterns in the convection zone. We briefly review the current status of
  theoretical models (of conceptual, analytical, and numerical kind) for
  the various manifestations of granulation to the long-term variability
  of the global solar cycle and its underlying dynamo mechanism. A model
  aiming at reconstructing the evolution of the total (unsigned) solar
  magnetic flux since 1610 on the basis of the group sunspot number
  record is discussed in the final section.

Title: Solar activity and climate during the last millennium
Authors: Solanki, S. K.; Usoskin, I.; Schüssler, M.
Bibcode: 2004cosp...35.2535S
Altcode: 2004cosp.meet.2535S
  The sunspot number is the longest running direct index of solar
  activity, with direct measurements starting in 1610. For many purposes,
  e.g., for comparisons with climate indices, it is still too short. We
  present a reconstruction of the cycle-averaged sunspot number over the
  last millennium based on <SUP>10</SUP>Be concentrations in Greenland
  and Antarctic ice cores. As intermediate steps of the method, we also
  reconstruct the cosmic ray flux at Earth and the Sun's open magnetic
  flux. The reconstructions are validated by comparison with direct
  measurements or independent reconstructions. We also compare with
  records of global climate, in particular with the global temperature
  ("hockey stick") curve of Mann et al (1998). A reasonable agreement
  is found for the entire millennium, excluding only the last decades,
  when the two curves start diverging from each other.

Title: Why Solar Magnetic Flux Concentrations Are Bright in Molecular
    Bands
Authors: Schüssler, M.; Shelyag, S.; Berdyugina, S.; Vögler, A.;
   Solanki, S. K.
Bibcode: 2003ApJ...597L.173S
Altcode:
  Using realistic ab initio simulations of radiative magnetoconvection,
  we show that the bright structures in images taken in the ``G band,''
  a spectral band dominated by lines of the CH molecule, precisely
  outline small-scale concentrations of strong magnetic fields on the
  visible solar surface. The brightening is caused by a depletion of CH
  molecules in the hot and tenuous magnetic structures, thus confirming
  the model of radiatively heated magnetic flux concentrations. These
  results provide a firm basis for observational studies of the evolution
  and dynamics of the small-scale solar magnetic field derived through
  ``proxy magnetometry'' with G-band images.

Title: Millennium-Scale Sunspot Number Reconstruction: Evidence for
    an Unusually Active Sun since the 1940s
Authors: Usoskin, Ilya G.; Solanki, Sami K.; Schüssler, Manfred;
   Mursula, Kalevi; Alanko, Katja
Bibcode: 2003PhRvL..91u1101U
Altcode: 2003astro.ph.10823U
  The extension of the sunspot number series backward in time is of
  considerable interest for dynamo theory, solar, stellar, and climate
  research. We have used records of the <SUP>10</SUP>Be concentration in
  polar ice to reconstruct the average sunspot activity level for the
  period between the year 850 to the present. Our method uses physical
  models for processes connecting the <SUP>10</SUP>Be concentration with
  the sunspot number. The reconstruction shows reliably that the period
  of high solar activity during the last 60years is unique throughout
  the past 1150years. This nearly triples the time interval for which
  such a statement could be made previously.

Title: Sunrise: balloon-borne high-resolution observation of the Sun
Authors: Schüssler, M.; Sunrise Team
Bibcode: 2003ESASP.530..279S
Altcode: 2003erbp.conf..279S
  Sunrise is a balloon-borne solar telescope with an aperture of 1 m, a
  project developed in cooperation between institutes in Germany, Spain,
  and the USA. The main scientific goal of Sunrise is to understand
  the structure and dynamics of the magnetic field in the atmosphere
  of the Sun. Sunrise will provide diffraction-limited images of the
  photosphere and chromosphere with an unpredecented resolution down
  to 35 km at wavelengths around 200 nm. Focal-plane instruments are
  a spectrograph/polarimeter, a Fabry-Perot filter magnetograph, and a
  filter imager. The first long-duration flight of Sunrise over Antarctica
  is planned in winter 2006/2007.

Title: Dynamics of magnetic flux tubes in close binary
    stars. I. Equilibrium and stability properties
Authors: Holzwarth, V.; Schüssler, M.
Bibcode: 2003A&A...405..291H
Altcode: 2003astro.ph..4496H
  Surface reconstructions of active close binary stars based on
  photometric and spectroscopic observations reveal non-uniform starspot
  distributions, which indicate the existence of preferred spot longitudes
  (with respect to the companion star). We consider the equilibrium and
  linear stability of toroidal magnetic flux tubes in close binaries to
  examine whether tidal effects are capable to initiate the formation
  of rising flux loops at preferred longitudes near the bottom of
  the stellar convection zone. The tidal force and the deviation
  of the stellar structure from spherical symmetry are treated in
  lowest-order perturbation theory assuming synchronised close binaries
  with orbital periods of a few days. The frequency, growth time, and
  spatial structure of linear eigenmodes are determined by a stability
  analysis. We find that, despite their small magnitude, tidal effects
  can lead to a considerable longitudinal asymmetry in the formation
  probability of flux loops, since the breaking of the axial symmetry due
  to the presence of the companion star is reinforced by the sensitive
  dependence of the stability properties on the stellar stratification
  and by resonance effects. The orientation of preferred longitudes of
  loop formation depends on the equilibrium configuration and the wave
  number of the dominating eigenmode. The change of the growth times of
  unstable modes with respect to the case of a single star is very small.

Title: The Kelvin-Helmholz and Shear Instabilities of a Vortex Flow
    Around a Magnetic Flux Tube
Authors: Kolesnikov, Fedor; Schüssler, Manfred
Bibcode: 2003ANS...324R..64K
Altcode: 2003ANS...324..I05K
  No abstract at ADS

Title: Long-Term Cosmic Ray Intensities: Physical Reconstruction
Authors: Usoskin, I. G.; Mursula, K.; Solanki, S. K.; Schuessler,
   M.; Kovaltsov, G. A.
Bibcode: 2003ICRC....7.4041U
Altcode: 2003ICRC...28.4041U
  Solanki et al. (2000) have recently calculated the open solar magnetic
  flux for the last 400 years from sunspot data. Using this reconstructed
  magnetic flux as an input to a simple spherically symmetric quasi-steady
  state model of the heliosphere, we calculate the expected differential
  spectra and integral intensity of galactic cosmic rays at the Earth's
  orbit since 1610. The calculated cosmic ray integral intensity is in
  good agreement with the neutron monitor measurements during the last
  50 years. Moreover, using the specific yield function of cosmogenic
  10 Be radionuclide production in the atmosphere, we also calculate
  the expected 10 Be production rate which exhibits an excellent
  agreement with the actual 10 Be abundance in polar ice over the
  last 400 years. Here we present a physical model for the long-term
  reconstruction of cosmic ray intensity at 1 AU. The reconstruction
  is based on a combination of the solar magnetic flux model and a
  heliospheric model. This model allows us to calculate the expected
  intensity of galactic cosmic rays (GCR) at the Earth's orbit for the
  last 400 years. Details can be found in [25]. Using the numerical recip
  e of Solanki et al. [21] and the group sunspot number series (Fig. 1.a)
  [11] we have calculated the open solar magnetic flux Fo since 1610 as
  shown in Fig. 1.b. In order to calculate galactic cosmic ray (GCR)
  spectra we use a spherically symmetric quasi-steady sto chastic
  simulation model described in detail elsewhere [24], which reliably
  describes the long-term GCR modulation during the last 50 years. In this
  model, the most important parameter of the heliospheric modulation of
  GCR is the modulation strength [10]: Φ = (D - rE )V /(3κo), where D
  = 100 AU is the heliospheric boundary and rE = 1 AU, V = 400 km/s is
  the constant solar wind velocity and κo is the rigidity indep endent
  part of the diffusion coefficient. Thus, all changes in the modulation
  strength Φ in our model are related to the changing diffusion

Title: Dynamics of magnetic flux tubes in close binary
    stars. II. Nonlinear evolution and surface distributions
Authors: Holzwarth, V.; Schüssler, M.
Bibcode: 2003A&A...405..303H
Altcode: 2003astro.ph..4498H
  Observations of magnetically active close binaries with orbital
  periods of a few days reveal the existence of starspots at preferred
  longitudes (with respect to the direction of the companion star). We
  numerically investigate the non-linear dynamics and evolution of
  magnetic flux tubes in the convection zone of a fast-rotating component
  of a close binary system and explore whether the tidal effects are able
  to generate non-uniformities in the surface distribution of erupting
  flux tubes. Assuming a synchronised system with a rotation period of
  two days and consisting of two solar-type components, both the tidal
  force and the deviation of the stellar structure from spherical shape
  are considered in lowest-order perturbation theory. The magnetic field
  is initially stored in the form of toroidal magnetic flux rings within
  the stably stratified overshoot region beneath the convection zone. Once
  the field has grown sufficiently strong, instabilities initiate the
  formation of rising flux loops, which rise through the convection
  zone and emerge at the stellar surface. We find that although the
  magnitude of tidal effects is rather small, they nevertheless lead
  to the formation of clusters of flux tube eruptions at preferred
  longitudes on opposite sides of the star, which result from the
  cumulative and resonant character of the action of tidal effects on
  rising flux tubes. The longitude distribution of the clusters depends
  on the initial parameters of flux tubes in the overshoot region like
  magnetic field strength and latitude, implying that there is no globally
  unique preferred longitude along a fixed direction.

Title: SUNRISE: Balloon-borne High-Resolution Observation of the Sun
Authors: Solanki, S. K.; Curdt, W.; Gandorfer, A.; Schüssler,
   M.; Lites, B. W.; Martinez Pillet, V.; Schmidt, W.; Title, A. M.;
   Sunrise Team
Bibcode: 2003ANS...324..113S
Altcode: 2003ANS...324..P20S
  No abstract at ADS

Title: Magnetic flux tubes and the dynamo problem
Authors: Schüssler, Manfred; Ferriz-Mas, Antonio
Bibcode: 2003and..book..123S
Altcode: 2003eclm.book..123S
  The observed properties of the magnetic field in the solar photosphere
  and theoretical studies of magneto-convection in electrically
  well-conducting fluids suggest that the magnetic field in stellar
  convection zones is quite inhomogeneous: magnetic flux is concentrated
  into magnetic flux tubes embedded in significantly less magnetized
  plasma. Such a state of the magnetic field potentially has strong
  implications for stellar dynamo theory since the dynamics of an ensemble
  of flux tubes is rather different from that of a more uniform field
  and new phenomena like magnetic buoyancy appear. <P />If the diameter
  of a magnetic flux tube is much smaller than any other relevant length
  scale, the MHD equations governing its evolution can be considerably
  simplified in terms of the thin-flux-tube approximation. Studies of
  thin flux tubes in comparison with observed properties of sunspot
  groups have led to far-reaching conclusions about the nature of the
  dynamo-generated magnetic field in the solar interior. The storage
  of magnetic flux for periods comparable to the amplification time of
  the dynamo requires the compensation of magnetic buoyancy by a stably
  stratified medium, a situation realized in a layer of overshooting
  convection at the bottom of the convection zone. Flux tubes stored
  in mechanical force equilibrium in this layer become unstable with
  respect to an undular instability once a critical field strength is
  exceeded, flux loops rise through the convection zone and erupt as
  bipolar magnetic regions at the surface. For parameter values relevant
  for the solar case, the critical field strength is of the order of
  10<SUP>5</SUP> G. A field of similar strength is also required to
  prevent the rising unstable flux loops from being strongly deflected
  poleward by the action of the Coriolis force and also from `exploding'
  in the middle of the convection zone. The latter process is caused by
  the superadiabatic stratification. <P />The magnetic energy density of
  a field of 10<SUP>5</SUP> G is two orders of magnitude larger than the
  kinetic energy density of the convective motions in the lower solar
  convection zone. This raises serious doubts whether the conventional
  turbulent dynamo process based upon cyclonic convection can work on the
  basis of such a strong field. Moreover, it is unclear whether solar
  differential rotation is capable of generating a toroidal magnetic
  field of 10<SUP>5</SUP> G; it is conceivable that thermal processes like
  an entropy-driven outflow from exploded flux tubes leads to the large
  field strength required. <P />The instability of magnetic flux tubes
  stored in the overshoot region suggests an alternative dynamo mechanism
  based upon growing helical waves propagating along the tubes. Since
  this process operates only for field strengths exceeding a critical
  value, such a dynamo can fall into a `grand minimum' once the field
  strength is globally driven below this value, for instance by magnetic
  flux pumped at random from the convection zone into the dynamo region
  in the overshoot layer. The same process may act as a (re-)starter of
  the dynamo operation. Other non-conventional dynamo mechanisms based
  upon the dynamics of magnetic flux tubes are also conceivable.

Title: SUNRISE: a balloon-borne telescope for high resolution solar
    observations in the visible and UV
Authors: Solanki, Sami K.; Gandorfer, Achim M.; Schuessler, Manfred;
   Curdt, W.; Lites, Bruce W.; Martinez-Pillet, Valentin; Schmidt,
   Wolfgang; Title, Alan M.
Bibcode: 2003SPIE.4853..129S
Altcode:
  Sunrise is a light-weight solar telescope with a 1 m aperture for
  spectro-polarimetric observations of the solar atmosphere. The telescope
  is planned to be operated during a series of long-duration balloon
  flights in order to obtain time series of spectra and images at the
  diffraction-limit and to study the UV spectral region down to ~200 nm,
  which is not accessible from the ground. The central aim of Sunrise
  is to understand the structure and dynamics of the magnetic field in
  the solar atmosphere. Through its interaction with the convective flow
  field, the magnetic field in the solar photosphere develops intense
  field concentrations on scales below 100 km, which are crucial for the
  dynamics and energetics of the whole solar atmosphere. In addition,
  Sunrise aims to provide information on the structure and dynamics
  of the solar chromosphere and on the physics of solar irradiance
  changes. Sunrise is a joint project of the Max-Planck-Institut fuer
  Aeronomie (MPAe), Katlenburg-Lindau, with the Kiepenheuer-Institut fuer
  Sonnenphysik (KIS), Freiburg, the High-Altitude Observatory (HAO),
  Boulder, the Lockheed-Martin Solar and Astrophysics Lab. (LMSAL),
  Palo Alto, and the Instituto de Astrofi sica de Canarias, La Laguna,
  Tenerife. In addition, there are close contacts with associated
  scientists from a variety of institutes.

Title: Studying magneto-convection by numerical simulation
Authors: Vögler, A.; Schüssler, M.
Bibcode: 2003AN....324..399V
Altcode:
  Following a brief overview of the two main approaches to investigate
  the interaction between magnetic fields and convective flows near the
  solar surface layers by numerical simulation, namely idealized model
  problems and `realistic' large-eddy simulations, we present first
  results obtained with a newly developed MHD code. The first example
  concerns the realistic simulation of the magnetic field dynamics in a
  solar plage region while the second example demonstrates small-scale
  dynamo action in idealized compressible convection.

Title: Simulation of Solar Magnetoconvection
Authors: Vögler, A.; Shelyag, S.; Schüssler, M.; Cattaneo, F.;
   Emonet, T.; Linde, T.
Bibcode: 2003IAUS..210..157V
Altcode:
  No abstract at ADS

Title: MHD Simulations: What's Next?
Authors: Schüssler, M.
Bibcode: 2003ASPC..307..601S
Altcode:
  No abstract at ADS

Title: Search for a relationship between solar cycle amplitude
    and length
Authors: Solanki, S. K.; Krivova, N. A.; Schüssler, M.; Fligge, M.
Bibcode: 2002A&A...396.1029S
Altcode:
  The cross-correlation between time series of solar cycle length
  and amplitude suggests that the length precedes the amplitude. The
  relationship between the two is found to be more complex than a simple
  lag or phase shift, however. A simple empirical model is constructed
  which allows the amplitude of a given cycle to be predicted with
  relatively high accuracy from the lengths of earlier cycles. This
  result not only adds to the means at our disposal for predicting the
  amplitudes of future cycles, but also implies that the solar dynamo
  carries a memory of the length of one cycle over into the next. It may
  also have a bearing on why solar cycle length correlates better with
  the Earth's temperature record than cycle amplitude (Friis-Christensen
  &amp; Lassen \cite{Friis-Christensen:Lassen:1991}). Thoughts on possible
  physical causes are presented.

Title: A physical reconstruction of cosmic ray intensity since 1610
Authors: Usoskin, Ilya G.; Mursula, Kalevi; Solanki, Sami K.;
   Schüssler, Manfred; Kovaltsov, Gennady A.
Bibcode: 2002JGRA..107.1374U
Altcode:
  The open solar magnetic flux has been recently reconstructed by [2000,
  2002] for the last 400 years from sunspot data. Using this reconstructed
  magnetic flux as an input to a spherically symmetric quasi-steady
  state model of the heliosphere, we calculate the expected intensity
  of galactic cosmic rays at the Earth's orbit since 1610. This new,
  physical reconstruction of the long-term cosmic ray intensity is
  in good agreement with the neutron monitor measurements during the
  last 50 years. Moreover, it resolves the problems related to previous
  reconstruction for the last 140 years based on linear correlations. We
  also calculate the flux of 2 GeV galactic protons and compare it to
  the cosmogenic <SUP>10</SUP>Be level in polar ice in Greenland and
  Antarctica. An excellent agreement between the calculated and measured
  levels is found over the last 400 years.

Title: Sunrise: a 1-m balloon borne solar telescope
Authors: Solanki, S. K.; Schüssler, M.; Curdt, W.; Lites, B. W.;
   Martinez Pillet, V.; Schmidt, W.; Title, A. M.; Sunrise Team
Bibcode: 2002ESASP.505...27S
Altcode: 2002solm.conf...27S; 2002IAUCo.188...27S
  Sunrise is a light-weight solar telescope with a 1 m aperture
  for spectro-polarimetric observations of the solar atmosphere. The
  telescope is planned to be operated during a series of long-duration
  balloon flights in order to obtain time series of spectra and images
  at the diffraction-limit and to study the UV spectral region down to
  ≅200 nm, which is not accessible from the ground. The central aim of
  Sunrise is to understand the structure and dynamics of the magnetic
  field in the solar atmosphere. Interacting with the convective flow
  field, the magnetic field in the solar photosphere develops intense
  field concentrations on scales below 100 km, which are crucial for the
  dynamics and energetics of the whole solar atmosphere. In addition,
  Sunrise aims to provide information on the structure and dynamics of
  the solar chromosphere and on the physics of solar irradiance changes.

Title: The formation of sunspots and starspots
Authors: Schüssler, M.
Bibcode: 2002AN....323..377S
Altcode:
  The spatial scale of large solar active regions and many of their
  observed features indicate that they originate from the emergence of a
  coherent magnetic structure of rather well-ordered toroidal magnetic
  flux in the solar interior. Numerical simulations of the instability
  of magnetic flux tubes and their rise through the convection zone are
  consistent with the basic observed properties of sunspot groups like
  low emergence latitudes, tilt angles with respect to the east-west
  direction, and proper motions. The success of this approach in the
  case of the Sun motivates its application to other magnetically active
  stars. The effect of the Coriolis force on rising magnetic flux tubes
  provides an explanation for the existence of high-latitude spots on
  rapidly rotating active stars. Detailed models have been developed for
  the distribution of flux emergence latitudes depending on the rotation
  rate and internal structure of cool stars in various evolutionary
  stages. The models for very young (T-Tauri-like) stars exhibit extended
  latitude ranges of flux emergence, including the poles. In the case of
  rapidly rotating main-sequence stars, the flux tubes appear in mid to
  high latitudes; neither equatorial nor truly polar flux emergence is
  found. The trapping of flux tubes in giants with a sufficiently small
  (relative) core size suggests an explanation for the strong decline
  of X-ray emission of cool giants across the `coronal dividing line'
  in the Hertzsprung-Russell diagram. We give an overview of the solar
  and stellar results and discuss the benefits and the limitations of
  applying the solar paradigm to active stars.

Title: Do tidal effects determine the spot distribution on active
    binaries?
Authors: Holzwarth, V.; Schüssler, M.
Bibcode: 2002AN....323..399H
Altcode:
  Observations of spot distributions on active binaries indicate the
  existence of preferred longitudes, which are more pronounced if the
  system period is shorter. This suggests that this effect is caused
  by the proximity of the companion star. We explore whether preferred
  longitudes can be explained by tidal effects on the dynamics of
  magnetic flux tubes, which are thought to give rise to starspots
  upon emergence at the stellar surface. We find that in the case of
  fast-rotating binaries flux tubes erupting at mid latitudes show indeed
  a considerably non-uniform longitudinal distribution.

Title: Thermal properties of magnetic flux tubes. I. Solution of
    the diffusion problem
Authors: Moreno-Insertis, F.; Schüssler, M.; Glampedakis, K.
Bibcode: 2002A&A...388.1022M
Altcode:
  The heat flow and temperature structure within and surrounding a
  magnetic flux tube stored in mechanical equilibrium in a stellar
  convection zone are considered. The stationary thermal equilibrium
  state is determined through the analytical solution of a two-dimensional
  heat diffusion problem for an infinitely long cylinder with different
  thermal conductivities inside and outside the cylinder, both spatially
  variable. In the exterior of the cylinder, convective heat transport
  is approximated in terms of a linear diffusive process, while in its
  interior convection is assumed to be suppressed and only the much
  smaller radiative conductivity remains. The results show that, under
  the conditions prevailing near the bottom of the solar convection zone
  and in the limit of small cylinder radius, the temperature disturbance
  (thermal shadow) in the exterior of the insulating cylinder is
  almost negligible due to the large effiency of convective energy
  transport. The spatial dependence of the conductivities and the
  curvature of the external temperature profile lead to a temperature
  excess in the interior with respect to the undisturbed temperature
  profile far away from the cylinder. We show that, within the framework
  of the thin magnetic flux tube approximation, this temperature excess
  is due to a heating term equal to the negative divergence of the
  undisturbed radiative heat flow, as suggested earlier by Fan &amp;
  Fisher (\cite{Fan:Fisher:1996}). These results are independent of the
  treatment of the convective transport in the exterior as long as the
  stratification is almost adiabatic. The consequences for the storage
  of magnetic flux in the solar convection zone, brought about by the
  enhanced buoyancy and caused by the heating effect, are discussed.

Title: Structure of the magnetic field in the lower convection zone
Authors: Schüssler, Manfred; Rempel, Matthias
Bibcode: 2002ESASP.508..499S
Altcode: 2002soho...11..499S
  The properties of the magnetic field and the convective flows near
  the base of the solar convection zone are crucial for understanding
  the working of the solar dynamo. We consider three aspects of
  this complex problem. (I) Magnetic flux needs to be stored against
  buoyant loss for a sufficiently long time in order to be amplified
  by the dynamo process. Convective pumping in strongly stratified
  convection is probably not sufficient for the strong fields (of order
  10<SUP>5</SUP>G) which have been inferred from the simulations of
  rising flux tubes. The required subadiabatically stratified storage
  region is likely to be generated by the asymmetric flow field (strong
  coherent downflows, weak upflows) characteristic for compressible
  convection in a stratified medium. (II) In a weakly subadiabatic region
  or a convective overshoot layer, the force equilibrium of a magnetic
  layer is very similar to that of an isolated flux tube: zero buoyancy
  and balance between the magnetic curvature (tension) force and the
  Coriolis force induced by a longitudinal flow along the field lines
  in a rotating system. In a strongly subadiabatic radiative region,
  a magnetic layer develops a different kind of force equilibrium,
  which involves buoyancy and a latitudinal pressure gradient. (III)
  A field of 10<SUP>5</SUP>G is difficult to generate by convection or
  differential rotation. The outflow of plasma from an "exploded" flux
  tube provides an intensification mechanisms which is not limited by
  the Lorentz force and converts potential energy of a superadiabatic
  stratification into magnetic energy.

Title: Magnetic variability of the Sun
Authors: Schüssler, Manfred
Bibcode: 2002ESASP.477....3S
Altcode: 2002scsw.conf....3S
  The solar magnetic field is variable on all temporal and spatial
  scales which are accessible to observation. Convective motions and
  magnetic flux interact on timescales of minutes (granulation) to days
  (supergranulation) while the evolution of active regions and the
  global transport of magnetic flux on the solar surface takes place on
  timescales ranging from days to years. The solar activity cycle and
  its long-term modulation vary on scales of decades to centuries. This
  contribution gives a brief overview about what is known about solar
  magnetic variability and what is our present understanding of the
  underlying physics. In addition, a simple model for the secular
  variation of the magnetic flux on the solar surface since the Maunder
  minimum is presented.

Title: Secular variation of the Sun's magnetic flux
Authors: Solanki, S. K.; Schüssler, M.; Fligge, M.
Bibcode: 2002A&A...383..706S
Altcode:
  We present an extension of the model of \citet{Solanki:etal:2000}
  that allows us to reconstruct the time evolution of both the total
  and the open magnetic flux at the solar surface since 1700. The flux
  emerging in large active regions is determined using the sunspot number
  as a proxy, while the flux emergence in small ephemeral regions is
  described by an extended cycle whose amplitude and length are related
  to the corresponding sunspot cycle. Both types of regions contribute
  to the open flux, which is the source of the heliospheric field. The
  overlap of the activity cycles of ephemeral regions leads to a secular
  variation of the total cycle-related magnetic flux (active region
  flux + ephemeral region flux + open flux). The model results indicate
  that the total surface flux has doubled in the first half of the last
  century. The evolution of the open flux is in good agreement with the
  reconstruction by \citet{Lockwood:etal:1999}.

Title: Reconstruction of Cosmic Ray Intensity Since 1610
Authors: Usoskin, I. G.; Mursula, K.; Solanki, S. K.; Schüssler,
   M.; Kovaltsov, G. A.
Bibcode: 2002EGSGA..27.5173U
Altcode:
  Open solar magnetic flux has been recently reconstructed by Solanki
  et al. (2000, 2002) for the last 400 years from sunspot data. Using
  this reconstructed magnetic flux as an input to a spherically symmetric
  quasi-steady state model of the heliosphere, we calculate the expected
  intensity of galactic cosmic rays at the Earth's orbit since 1610. This
  calculated cosmic ray intensity is in good agreement with the neutron
  monitor measurements during the last 50 years. Moreover, we calculate
  the flux of 2 GeV galactic protons and compare it to the cosmogenic
  10Be level in polar ice in Greenland and Antarctica. An excellent
  agreement between the calculated and actual levels is found over the
  last 400 years.

Title: Physical reconstruction of long-term solar activity
Authors: Usoskin, I.; Solanki, S.; Schuessler, M.; Mursula, K.;
   Kovaltsov, G.
Bibcode: 2002cosp...34E.901U
Altcode: 2002cosp.meetE.901U
  For many applications in dynamo theory and solar-terrestrial research
  it is important to know the evolution of solar activity on long time
  scales (centuries to millennia). Previous reconstructions were based
  upon either multi-harmonic backward extrapolation of the known sunspot
  records or on the assumption of a linear relation between terrestrial
  proxies (e.g., cosmogenic isotope abundance) and solar activity. Here
  we present, for the first time, a physical reconstruction of sunspot
  activity on long time scales from the cosmogenic 10 Be records. We
  use a numerical inversion of a combined physical solar-heliospheric
  model (Usoskin et al., 2002), which is essentially non-linear. Using
  physical rather than empirical relations on all steps, we present a
  reconstruction of sunspot activity since the 15th century. Uncertainties
  of the reconstruction are discussed in details. It is important that
  the current high level of sunspot activity is unique on the millennium
  time scale.

Title: Buried magnetic flux tubes in giant stars near the ``Coronal
    Dividing Line''
Authors: Holzwarth, V.; Schüssler, M.
Bibcode: 2001A&A...377..251H
Altcode:
  We apply the ``solar paradigm'' for stellar magnetic activity
  to the post-main-sequence evolution of stars in the mass range 1
  M<SUB>sun</SUB> &lt;= M<SUB>star </SUB> &lt;= 3 M<SUB>sun</SUB>. The
  model starts from a strong toroidal magnetic field generated by a dynamo
  working in the overshoot layer below the convection envelope. Once
  a critical field strength is exceeded, an undulatory (Parker-type)
  instability leads to rising flux loops. Upon emergence at the stellar
  surface, they form bipolar magnetic regions and large-scale coronal
  loops. By considering the stability, dynamics, and rise of magnetic
  flux tubes along evolutionary sequences of stellar models, we find
  that the flux loops become trapped in the stellar interior when
  the depth of convective envelope exceeds about 80% of the stellar
  radius. Trapping is caused by an increase of field line curvature at the
  loop summit, so that eventually the magnetic tension force dominates
  over the buoyancy force. The magnetic loops find a stable equilibrium
  configuration within the convection zone and do not emerge at the
  stellar surface. The transition from emerging to trapped flux tubes
  falls in the range of spectral types G7 to K0 for luminosity class
  III giants, which is close to the observed ``coronal dividing line''
  in the Hertzsprung-Russell diagram. This result is remarkably stable
  within large ranges of stellar parameters (mass, rotation) and flux
  tube parameters (field strength, magnetic flux) and depends practically
  exclusively on the fractional radius of the stellar radiative core. We
  suggest that flux tube trapping is the cause for the strong decline
  of stellar X-ray emission across the ``coronal dividing line''.

Title: Intensification of Magnetic Fields by Conversion of Potential
    Energy
Authors: Rempel, M.; Schüssler, M.
Bibcode: 2001ApJ...552L.171R
Altcode:
  A strong superequipartition magnetic field strength on the order of
  10 T (10<SUP>5</SUP> G) has been inferred at the bottom of the solar
  convection zone. We show that the ``explosion'' of weak magnetic flux
  tubes, which is caused by a sudden loss of pressure equilibrium in the
  flux loop rising through the superadiabatically stratified convection
  zone, provides a mechanism that leads to a strong field: the flow of
  high-entropy material out of the exploded loop leads to a significant
  intensification of the magnetic field in the underlying flux sheet at
  the bottom. In contrast to the amplification by differential rotation,
  this process converts the potential energy of the stratification into
  magnetic energy and thus is not dynamically limited by the back-reaction
  on the flow field via the Lorentz force.

Title: The Formation of One-Lobed Stokes V Profiles in an
    Inhomogeneous Atmosphere
Authors: Ploner, S. R. O.; Schussler, M.; Solanki, S. K.; Sheminova,
   V. A.; Gadun, A. S.; Frutiger, C.
Bibcode: 2001ASPC..236..371P
Altcode: 2001aspt.conf..371P
  We assess the diagnostic potential of the observed pathological Stokes V
  profiles that differ strongly from the customary, nearly antisymmetric
  two-lobed shape. In particular, we consider the formation of one-lobed
  Stokes V profiles using the results of an MHD simulation. We find
  that the majority of one-lobed profiles is produced in regions of
  weak horizontal field with significant cancellation caused by mixed
  polarity along the line of sight. A minority of one-lobed profiles
  originates close to strong magnetic field concentrations with strong
  gradients of velocity and magnetic field strength.

Title: Magneto-Convection
Authors: Schüssler, M.; Knölker, M.
Bibcode: 2001ASPC..248..115S
Altcode: 2001mfah.conf..115S
  No abstract at ADS

Title: A Model for the Decline of Coronal X-ray Emission of Cool
    Giant Stars
Authors: Holzwarth, V.; Schüssler, M.; Solanki, S. K.
Bibcode: 2001ASPC..248..259H
Altcode: 2001mfah.conf..259H
  No abstract at ADS

Title: Numerical Simulation of Solar Magneto-Convection
Authors: Schüssler, M.
Bibcode: 2001ASPC..236..343S
Altcode: 2001aspt.conf..343S
  No abstract at ADS

Title: An Example of Reconnection and Magnetic Flux Recycling near
    the Solar Surface
Authors: Ploner, S. R. O.; Schüssler, M.; Solanki, S. K.; Gadun, A. S.
Bibcode: 2001ASPC..236..363P
Altcode: 2001aspt.conf..363P
  No abstract at ADS

Title: Preferred Longitudes of Starspots on Magnetically Active
    Close Binaries
Authors: Holzwarth, V.; Schüssler, M.
Bibcode: 2001ASPC..248..247H
Altcode: 2001mfah.conf..247H
  No abstract at ADS

Title: Intensification of Magnetic Field in a Stellar Convection
    Zone by Conversion of Potential Energy
Authors: Rempel, M.; Schüssler, M.
Bibcode: 2001ASPC..248..165R
Altcode: 2001mfah.conf..165R
  No abstract at ADS

Title: Buried Flux Tubes in the Coronal Graveyard (CD-ROM Directory:
    contribs/schussle)
Authors: Schüssler, M.; Holzwarth, V.; Solanki, S. K.; Charbonnel, C.
Bibcode: 2001ASPC..223.1114S
Altcode: 2001csss...11.1114S
  No abstract at ADS

Title: Storage of a Strong Magnetic Field Below the Solar Convection
Zone (CD-ROM Directory: contribs/rempel)
Authors: Rempel, M.; Schüssler, M.; Moreno-Insertis, F.; Tóth, G.
Bibcode: 2001ASPC..223..738R
Altcode: 2001csss...11..738R
  No abstract at ADS

Title: High-resolution Solar Polarimetry with Sunrise
Authors: Schmidt, W.; Solanki, S. K.; Schüssler, M.; Curdt, W.;
   Lites, B. W.; Title, A. M.; Martinez Pillet, V.
Bibcode: 2001AGM....18S1001S
Altcode:
  Sunrise is a 1m balloon-borne solar telescope. It is equipped with
  a spectrograph polarimeter which combines vector-polarimetry in the
  visible with diagnostic spectroscopy in the visible and the UV, down
  to 200 nm. The instrumentation includes a filter-magnetograph and a
  medium-band filtergraph. The wavelength bands of the latter include
  the CH-band (430.6 nm) and a UV continuum at 205 nm. Diffraction
  limited resolution in the UV will be achieved by employing a phase
  diversity technique. The main telescope is based on a lightweight
  silicon-carbide mirror, developed within the Solar Lite program. During
  the long-duration flight at Antarctica, foreseen for late 2005, Sunrise
  will continuously observe the sun for a period of about ten days,
  with constant image quality across the full field of view. In-flight
  alignment of the telescope optics will be controlled by a wavefront
  sensor. The main goal of Sunrise is to understand the structure and
  dynamics of the magnetic field in the atmosphere of the sun. To this
  end, Sunrise will observe small magnetic flux concentrations with
  dimensions of less than 70 km with high polarimetric accuracy. At the
  same time, Sunrise will provide diffraction-limited filtergrams of
  the photosphere and chromosphere with a resolution down to 35 km at
  a wavelength of 200 nm.

Title: Star Spot Patterns on Young Stars: Theoretical Approach
(CD-ROM Directory: contribs/granzer2)
Authors: Granzer, Th.; Caligari, P.; Schüssler, M.; Strassmeier, K. G.
Bibcode: 2001ASPC..223.1232G
Altcode: 2001csss...11.1232G
  No abstract at ADS

Title: Evolution of the Sun's large-scale magnetic field since the
    Maunder minimum
Authors: Solanki, S. K.; Schüssler, M.; Fligge, M.
Bibcode: 2000Natur.408..445S
Altcode:
  The most striking feature of the Sun's magnetic field is its cyclic
  behaviour. The number of sunspots, which are dark regions of strong
  magnetic field on the Sun's surface, varies with a period of about
  11 years. Superposed on this cycle are secular changes that occur on
  timescales of centuries and events like the Maunder minimum in the
  second half of the seventeenth century, when there were very few
  sunspots. A part of the Sun's magnetic field reaches out from the
  surface into interplanetary space, and it was recently discovered
  that the average strength of this interplanetary field has doubled
  in the past 100 years. There has hitherto been no clear explanation
  for this doubling. Here we present a model describing the long-term
  evolution of the Sun's large-scale magnetic field, which reproduces
  the doubling of the interplanetary field. The model indicates that
  there is a direct connection between the length of the sunspot cycle
  and the secular variations.

Title: Physical Causes of Solar Variability - Discussion Session 1b
Authors: Kuhn, J. R.; Schüssler, M.
Bibcode: 2000SSRv...94..177K
Altcode:
  This report is divided into three parts: Section 1 gives a short
  introduction and a summary of the topics discussed. Section 2
  is a position statement by J. Kuhn on the interpretation of the
  irradiance measurements, while Section 3 gives a position statement
  by M. Schüssler discussing observations of stars that could be useful
  for understanding solar variability.

Title: Storage of magnetic flux at the bottom of the solar convection
    zone
Authors: Rempel, M.; Schüssler, M.; Tóth, G.
Bibcode: 2000A&A...363..789R
Altcode:
  We consider the mechanical equilibrium of a layer of axisymmetric
  toroidal magnetic field located in a subadiabatically stratified
  region near the bottom of the solar convection zone, with particular
  emphasis on the effects of spherical geometry. We determine equilibrium
  configurations and simulate numerically how these are reached from a
  non-equilibrium initial situation. While a subadiabatic stratification
  is essential for suppressing the buoyancy force, the latitudinal
  component of the magnetic curvature force is balanced by a latitudinal
  pressure gradient (in the case of a large subadiabaticity, as in the
  radiative interior) or by the Coriolis force due to a toroidal flow
  along the field lines (in the case of small subadiabaticity, as in
  a layer of convective overshoot). The latter case is found relevant
  for storing the magnetic flux generated by the solar dynamo. The
  corresponding equilibrium properties are similar to those of isolated
  magnetic flux tubes. Significant variations of the differential rotation
  at the bottom of the convection zone in the course of the solar cycle
  are expected for such a kind of equilibrium.

Title: Solar Magnetic Field
Authors: Schüssler, M.
Bibcode: 2000eaa..bookE1982S
Altcode:
  Electrical currents flowing in the solar plasma generate a magnetic
  field, which is detected in the SOLAR ATMOSPHERE by spectroscopic
  and polarization measurements (SOLAR MAGNETIC FIELD: INFERENCE
  BY POLARIMETRY). The SOLAR WIND carries the magnetic field into
  interplanetary space where it can be measured directly by instruments
  on space probes....

Title: Strong Stokes V asymmetries of photospheric spectral lines:
    What can they tell us about the magnetic field structure?
Authors: Grossmann-Doerth, U.; Schüssler, M.; Sigwarth, M.;
   Steiner, O.
Bibcode: 2000A&A...357..351G
Altcode:
  In an attempt to identify the mechanism responsible for the extremely
  asymmetric Stokes V profiles which were recently observed we analyzed
  several simple atmospheric configurations with separated layers of mass
  flow and magnetic field. We found that under appropriate conditions
  the models are capable of producing the observed one-lobe profiles.

Title: Distribution of starspots on cool stars. II. Pre-main-sequence
    and ZAMS stars between 0.4 M<SUB>sun</SUB> and 1.7 M<SUB>sun</SUB>
Authors: Granzer, Th.; Schüssler, M.; Caligari, P.; Strassmeier, K. G.
Bibcode: 2000A&A...355.1087G
Altcode:
  We study the dynamics of magnetic flux tubes in young stars with masses
  between 0.4 M<SUB>sun</SUB> and 1.7 M<SUB>sun</SUB> and for rotation
  rates between 0.25 Omega<SUB>sun</SUB> and 63 Omega<SUB>sun</SUB>
  . The resulting latitudinal emergence patterns at the stellar
  surface are compared with observed distributions of starspots in
  stellar latitude. The stellar models considered cover the range of
  evolutionary stages from shortly after the Hayashi phase down to
  the ZAMS, i.e. from the classical and weak-line T Tauri stars to
  the alpha -Persei stars. We use numerical simulations to follow the
  evolution of magnetic flux tubes from their origin at the bottom of the
  convection zone up to near-surface layers. We find a strong increase of
  emergence latitude with increasing rotation rate, a moderate decrease
  with increasing stellar mass, and a stronger decrease with stellar
  age. At very early evolutionary stages, when the central radiative
  zone is still quite small, we find magnetic flux emergence both in
  low latitudes as well as in the polar areas. High-latitude emergence
  is predicted in slightly more evolved pre-main-sequence and young
  main-sequence stars, but truly polar spots on these stars require an
  additional transport mechanism acting after magnetic flux emergence at
  the surface, probably meridional circulation or poleward slip of the
  `anchored' part of the erupted flux tubes. We discuss our findings in
  the light of the results obtained from Doppler-imaging studies.

Title: Solar Variability and Climate
Authors: Friis-Christensen, E.; Fröhlich, C.; Haigh, J. D.;
   Schüssler, M.; Von Steiger, R.
Bibcode: 2000svc..book.....F
Altcode:
  No abstract at ADS

Title: Dynamics of magnetic flux tubes in close binary stars
Authors: Holzwarth, Volkmar; Schüssler, Manfred
Bibcode: 2000IAUS..200P.217H
Altcode:
  Photometric and Doppler imaging observations of active binaries
  frequently show high latitude starspots and indicate in several
  cases the existence of spots at preferred longitudes (position angle
  with respect to the companion star). We investigate the dynamics of
  magnetic flux tubes in the convection zone of close, fast-rotating
  binary stars and explore whether the observed preferred longitudes
  could be caused by tidal forces and the deformation of the active
  star. We consider a close binary system in bound rotation with spin
  axes perpendicular to the orbital plane and a rotation rate of a few
  days. The resulting distance to the companion star is sufficiently
  large to consider the tidal force and the deviation from spherical
  structure in lowest order perturbation theory. The magnetic field
  is in the form of toroidal magnetic flux rings, which are stored in
  mechanical equilibrium within the stably stratified overshoot region
  beneath the convection zone until the undulatory instability initiates
  the rise and eruption of individual loops. Frequencies and geometry of
  stable as well as growth rates of unstable eigenmodes are determined
  by a linear stability analysis. Particular consideration is given to
  the question whether the effects of tidal forces and perturbations
  of the stellar structure can force an unstable flux tube to enter the
  convection zone at specific longitudes.

Title: Physical Causes of Solar Variability
Authors: Kuhn, J. R.; Schussler, M.
Bibcode: 2000svc..book..177K
Altcode:
  No abstract at ADS

Title: Stability of magnetic flux tubes in close binary stars
Authors: Holzwarth, V.; Schüssler, M.
Bibcode: 2000AN....321..175H
Altcode:
  Photometric and Doppler imaging observations of active binaries indicate
  the existence of starspots at preferred longitudes (position angles with
  respect to the companion star). We investigate the stability of magnetic
  flux tubes in the convection zone of close, fast-rotating binary stars
  and explore whether the observed preferred longitudes could be caused
  by tidal forces and the deformation of the active star. We assume a
  synchronized binary system with spin axes perpendicular to the orbital
  plane and a rotation period of a few days. The tidal force and the
  deviation from spherical structure are considered in lowest-order
  perturbation theory. The magnetic field is in the form of toroidal
  magnetic flux rings, which are stored in mechanical equilibrium within
  the stably stratified overshoot region beneath the convection zone until
  the field has grown sufficiently strong for the undulatory instability
  to initiate the formation of rising loops. Frequencies and geometry of
  stable as well as growth rates of unstable eigenmodes are determined
  by linear stability analysis. Particular consideration is given to the
  question whether the effects of tidal forces and perturbations of the
  stellar structure can force a rising flux loop to enter the convection
  zone at specific longitudes.

Title: Radiative cooling of a hot flux tube in the solar photosphere
Authors: Schlichenmaier, R.; Bruls, J. H. M. J.; Schüssler, M.
Bibcode: 1999A&A...349..961S
Altcode:
  Radiative energy transport is of key importance for the dynamics of
  slender magnetic flux tubes in the solar atmosphere, particularly
  so in connection with the filamentation of the sunspot penumbra. In
  investigations using the thin-flux-tube approximation of the MHD
  equations, the radiative exchange with the surrounding atmosphere has
  hitherto been described by the relaxation-time approach, also called
  `Newton's law of cooling'. The strongly nonlinear temperature-dependence
  of the radiative absorption coefficient and large temperature
  differences between the tube and its environment render this concept
  questionable. As a simple model of a bright penumbral filament we
  consider the cooling of a hot horizontal flux tube with a longitudinal
  flow, embedded in a non-stratified, homogeneous atmosphere at 4 800
  K. We compare the results of the relaxation-time approach and of
  a nonlinear diffusion approximation with the numerical solution of
  the equation of (grey) radiative transfer. We find that the cooling
  times given by the relaxation-time method compare well with the
  results from radiative transfer as long as the initial temperature
  of the tube is below 7 500 K and its lateral optical depth does not
  exceed unity. Under these conditions, the tube cools more or less
  homogeneously over its cross section. For hotter and optically thick
  tubes, the strong temperature-dependence of the absorption coefficient
  leads to the formation of a cooling front, which migrates radially
  inward at approximately constant speed. Such inhomogeneous cooling
  is well represented by the nonlinear diffusion approximation. The
  self-similar evolution of the cooling front permits an analytical
  estimate of the cooling time, which provides a reasonable approximation
  of the result of the radiative transfer calculation. This estimate can
  be used to derive an improved radiative cooling term in the framework
  of the thin-flux-tube approximation, so that both optically thin and
  optically thick flux tubes can be treated adequately. The results of
  the radiative transfer calculations are applied to obtain an estimate
  of the length and brightness of penumbral bright grains.

Title: Computing radiative heating on unstructured spatial grids
Authors: Bruls, J. H. M. J.; Vollmöller, P.; Schüssler, M.
Bibcode: 1999A&A...348..233B
Altcode:
  We discuss the basic problems and methods involved in the design of a
  radiative transfer module for a 2D/3D (magneto-)hydrodynamics simulation
  code aimed at applications in cool-star atmospheres. Attention
  is focused on the difficulties arising from the unstructured
  triangular/tetrahedral grid and the need to minimize the communication
  overhead, so that the code runs efficiently on parallel computers. In a
  first step, we use the gray approximation and ignore scattering effects,
  but even then the computation of the radiative heating rate, required
  as a source term in the energy equation, involves several integration
  steps that are discussed in detail. In particular, the details of the
  short-characteristics solver for the radiative transfer equation,
  the influence of the cell size, and the accuracy of the angular
  integrations of the specific intensity are considered. Theoretical
  estimates of possible errors are in general cumbersome to obtain;
  instead we use simple model problems for the accuracy estimates. A
  plane-parallel model for the quiet Sun serves as a testground for the
  basics while a schematic model of a magnetic flux sheet provides an
  acid test for the behavior of the computational methods under typical
  circumstances arising during simulations. Two alternative methods to
  compute the radiative heating rate are compared and their weaknesses
  are identified. The errors are minimized by a hybrid scheme that selects
  a method depending on the optical path length within a grid cell.

Title: Der Zyklus der Sonne.
Authors: Schüssler, M.; Wöhl, H.
Bibcode: 1999S&WSp...4...56S
Altcode:
  No abstract at ADS

Title: Klimaveränderung, Treibhauseffekt oder Sonnenzyklus
Authors: Schmitt, D.; Schüssler, M.
Bibcode: 1999S&WSp...4...64S
Altcode:
  No abstract at ADS

Title: Simulating solar MHD
Authors: Schüssler, M.
Bibcode: 1999AnGeo..17..578S
Altcode: 1999AnGeo..17..579S; 1999AnG....17..578S
  Two aspects of solar MHD are discussed in relation to the work of
  the MHD simulation group at KIS. Photospheric magneto-convection,
  the nonlinear interaction of magnetic field and convection in a
  strongly stratified, radiating fluid, is a key process of general
  astrophysical relevance. Comprehensive numerical simulations including
  radiative transfer have significantly improved our understanding of
  the processes and have become an important tool for the interpretation
  of observational data. Examples of field intensification in the solar
  photosphere ('convective collapse') are shown. The second line of
  research is concerned with the dynamics of flux tubes in the convection
  zone, which has far-reaching implications for our understanding of the
  solar dynamo. Simulations indicate that the field strength in the region
  where the flux is stored before erupting to form sunspot groups is of
  the order of 105 G, an order of magnitude larger than previous estimates
  based on equipartition with the kinetic energy of convective flows.

Title: Can Chromospheric Activity mimic a Polar Spot?
Authors: Bruls, J. H. M. J.; Schüssler, M.; Solanki, S. K.
Bibcode: 1999ASPC..158..182B
Altcode: 1999ssa..conf..182B
  No abstract at ADS

Title: Storage of toroidal magnetic field below the solar convection
    zone
Authors: Rempel, M.; Schüssler, M.; Moreno-Insertis, F.
Bibcode: 1999AGAb...15R..74R
Altcode: 1999AGM....15..J15R
  Simulations of erupting flux tubes in the thin flux tube approximation
  show that essential properties of sunspots can only be explained if the
  initial field strength of the flux tube at the base of the convection
  zone is about 10 T. Such strong magnetic field can only be stored below
  the solar convection zone in a subadiabatic stratification. We consider
  mechanical equilibria in form of magnetic flux tubes and magnetic sheets
  and discuss the influence of radiative and convective energy transport
  on these configurations. In the case of magnetic flux tubes, radiative
  inflow of heat leads to enhanced buoyancy which causes the flux tube
  to move upwards and leave the storage region. In the case of magnetic
  sheets, the compensation of the poleward directed magnetic tension
  force requires a deviation of the temperature from the hydrostatic
  background stratification. Convective energy transport disturbs the
  equilibrium and leads to thermal circulations.

Title: Radiative Transfer On Unstructured Triangular Grids
Authors: Bruls, J.; Vollmöller, P.; Schüssler, M.
Bibcode: 1999ASPC..183...44B
Altcode: 1999hrsp.conf...44B
  No abstract at ADS

Title: Radiative transfer for MHD simulations on unstructured grids
Authors: Bruls, J.; Vollmöller, P.; Schüssler, M.
Bibcode: 1999AGAb...15..141B
Altcode: 1999AGM....15.P108B
  We discuss the basic problems and methods involved in the design of a
  radiative transfer module for a 2D/3D (magneto-)hydrodynamics simulation
  code aimed at applications in cool-star atmospheres. Attention
  is focused on the difficulties arising from the unstructured
  triangular/tetrahedral grid and the need to minimize the communication
  overhead, so that the code runs efficiently on parallel computers. In a
  first step, we use the gray approximation and ignore scattering effects,
  but even then the computation of the radiative heating rate, required
  as a source term in the energy equation, involves several integration
  steps. In particular, the details of the short-characteristics solver
  for the radiative transfer equation, the influence of the cell size,
  and the accuracy of the angular integrations of the specific intensity
  are considered. Theoretical estimates of possible errors are in general
  cumbersome to obtain; instead we use simple model problems for the
  accuracy estimates. A plane-parallel model for the quiet Sun serves as
  a testground for the basics while a schematic model of a magnetic flux
  provides an acid test for the behavior of the computational methods
  under typical circumstances arising during simulations. Two alternative
  methods to compute the radiative heating rate are compared and their
  weaknesses are identified. The errors are minimized by a hybrid scheme
  that selects a method depending on the optical path length within a grid
  cell. We have implemented the radiative transfer model in a 2D Euler
  code and have performed test simulations of solar surface convection.

Title: Dynamics of magnetic flux tubes in evolved stars
Authors: Holzwarth, V.; Schüssler, M.
Bibcode: 1999AGAb...15R..71H
Altcode: 1999AGM....15..J08H
  We explore the limits of the `solar paradigm' by applying the magnetic
  flux tube model, which proved to be consistent with many observed
  properties of sunspot groups, to subgiant and giant stars. The model
  starts from a strong toroidal field generated by a dynamo working
  at the bottom of the stellar convection zone. The field is stored
  within the stably stratified convective overshoot layer in the form
  of magnetic flux tubes. Once a critical field strength is exceeded,
  the undulatory (Parker-type) instability leads to flux loops rising
  through the convection zone. Upon emergence at the stellar surface,
  the flux loops form bipolar magnetic regions and starspots. We apply
  this model to evolved stars using evolutionary sequences for stars
  between 1 M_odot and 2.5 M_odot. We determine the stability limit
  by linear analysis und follow the development of the unstable flux
  loops through numerical simulation. We find that the Coriolis force
  leads to flux emergence at high stellar latitudes for stars with
  deep convection zones and large rotation rate. On the other hand, if
  the relative size of the stellar core falls short of a certain limit,
  i.e., at some stage of the rapid expansion of a star during the giant
  phase, flux loops formed by the instability do not surface any longer
  but find a new equilibrium within the star. This transition could
  possibly be connected with the existence of a `coronal dividing line'
  in the Hertzsprung-Russell diagram.

Title: The formation of extremely asymmetric Stokes V profiles
Authors: Steiner, O.; Grossmann-Doerth, U.; Schüssler, M.; Sigwarth,
   M.
Bibcode: 1999AGAb...15R..10S
Altcode: 1999AGM....15..A11S
  Recent polarimetric observations at high spatial resolution (&lt;
  1 arcsec) and with high polarimetric accuracy (noise of Stokes V/I_c
  &lt; 3 cdot 10^{-4}) have revealed that about 10% of all Stokes V
  profiles of a quiet Sun region have an abnormal shape that strongly
  deviates from the more common, nearly antisymmetric profiles. 35% of the
  abnormal Stokes V profiles are of one-wing type, where the profile shows
  essentially one lobe only. We show, that a strongly asymmetric Stokes
  V profile can be obtained when the atmosphere, in which the profile
  is formed, is divided into two or more layers of different magnetic,
  flow, and thermal properties. A simple configuration of that kind
  (sometimes referred to as magnetic canopy) consists of a layer of
  plasma at rest with a magnetic field component parallel to the line
  of sight, located on top of a field-free layer with downdraft. The
  Stokes V asymmetry in this case sensitively depends on the position
  of the layers interface and may assume values up to about 80%. An
  asymmetry of nearly 100% (corresponding to a one-wing profile) is
  obtained when the plasma below the magnetic canopy is relatively cool,
  giving rise to the formation of a strong, redshifted spectral line in
  the field-free plasma beneath the canopy. If the core of this line is
  formed at lower temperature than the temperature of the canopy layer,
  the red wing of the resulting Stokes V profile is driven into emission,
  leading to a ``pathological'' V profile.

Title: Radiative cooling of a hot flux tube in the solar photosphere
Authors: Schlichenmaier, R.; Bruls, J. H. M. J.; Schüssler, M.
Bibcode: 1999AGAb...15Q..75S
Altcode: 1999AGM....15..J16S
  Radiative energy transport is of key importance for the dynamics of
  slender magnetic flux tubes in the solar atmosphere. In investigations
  using the thin-flux-tube approximation of the MHD equations, the
  radiative exchange with the surrounding atmosphere has hitherto been
  described by the relaxation-time approach, also called `Newton's law of
  cooling'. The strongly nonlinear temperature-dependence of the radiative
  absorption coefficient and large temperature differences between
  the tube and its environment render this concept questionable. As a
  simple model of a bright penumbral filament we consider the cooling
  of a hot horizontal flux tube with a longitudinal flow, embedded in
  a non-stratified, homogeneous atmosphere at 4 800 K. We compare the
  results of the relaxation-time approach and of a nonlinear diffusion
  approximation with the numerical solution of the equation of (grey)
  radiative transfer. We find that the cooling times given by the
  relaxation-time method compare well with the results from radiative
  transfer as long as the initial temperature of the tube is below 7
  500 K and its lateral optical depth does not exceed unity. Under these
  conditions, the tube cools homogeneously over its cross section. For
  hotter and optically thick tubes, the strong temperature-dependence of
  the absorption coefficient leads to the formation of a cooling front,
  which migrates radially inward at approximately constant speed. Such
  inhomogeneous cooling is well represented by the nonlinear diffusion
  approximation. The self-similar evolution of the cooling front permits
  an analytical estimate of the cooling time, which provides a reasonable
  approximation of the result of the radiative transfer calculation. This
  estimate can be used to derive an improved radiative cooling term in
  the framework of the thin-flux-tube approximation.

Title: Upwelling in a young sunspot
Authors: Sigwarth, M.; Schmidt, W.; Schuessler, M.
Bibcode: 1998A&A...339L..53S
Altcode:
  An upflow of with a velocity exceeding 0.5 km*s(-1) is found in the
  umbra of the preceding spot in a young active region with ongoing
  flux emergence. A weak downflow is indicated in the corresponding
  spot of follower polarity. Such a flow pattern is consistent with the
  counter-rotation flow along a rising magnetic flux loop driven by the
  Coriolis force as predicted by numerical simulations.

Title: Convective intensification of solar surface magnetic fields:
    results of numerical experiments
Authors: Grossmann-Doerth, U.; Schuessler, M.; Steiner, O.
Bibcode: 1998A&A...337..928G
Altcode:
  The concentration of magnetic flux by convective flows in the solar
  surface layers is studied by means of two-dimensional numerical
  simulations with radiative transfer. We follow the evolution of an
  initially homogeneous, vertical magnetic field, starting from an
  evolved state of simulated solar granulation. The results of three
  simulation runs with initial field strengths, B_0, of 100 G, 200 G,
  and 400 G, respectively, are shown. In all cases, horizontal convective
  flows rapidly sweep magnetic flux into the intergranular downflow
  channels. The field is further amplified up to kilogauss values by
  partial evacuation due to a strongly accelerated downflow within
  the magnetic structure. The value of the field strength reached at
  a given depth and the size of the flux concentrations grows with the
  initial field strength (i.e., the amount of magnetic flux within the
  computional box). In the case of B_0 = 400 G, the downflow within the
  flux concentration becomes so strong that it `bounces' off the high
  density plasma in the deeper layers; the resulting upflow leads to a
  strong, upward moving shock and to the dispersal of the flux sheet after
  a lifetime of about 200 s. In the cases with less magnetic flux (B_0 100
  G, 200 G), the downflow is less vigorous and the flux concentrations
  persist to the end of the simulation (about 5 minutes). Radiation
  diagnostics in the continuum and in spectral lines predicts observable
  signatures of the intensification process. The accelerated downflow
  leads to a conspicuous Doppler shift and a negative area asymmetry of
  Stokes V-profiles of spectral lines, while the intensification of the
  magnetic field may be detectable through the `magnetic line ratio'
  method in the visible and by direct Zeeman splitting of magnetically
  sensitive lines in the infrared wavelength ranges.

Title: Doppler imaging: the polar SPOT controversy
Authors: Bruls, J. H. M. J.; Solanki, S. K.; Schuessler, M.
Bibcode: 1998A&A...336..231B
Altcode:
  Doppler imaging studies have revealed that most rapidly rotating cool
  stars have high-latitude spots, which in many cases cover the stellar
  poles. The spectroscopic signature of polar spots is a filling in
  of the cores of spectral lines, which become flat-bottomed and may
  show bumps. Although the existence of polar spots is corroborated by
  spectroscopic and photometric measurements, and although theoretical
  models predict polar spots, they remain controversial. Most notably, it
  has been proposed that the line core filling in might also be caused by
  chromospheric activity. We present a NLTE radiative transfer analysis
  of 14 of the most-used Doppler-imaging lines which demonstrates that
  chromospheric activity can produce filling in of the observed line
  profiles only in a few of these lines. Moreover, such filling in is in
  general not of the type observed in the spectra of active stars. We
  are able to produce a flat-bottomed line core by concentrating the
  chromospheric activity near the poles, but only for two of the strongest
  lines, Fe i 5497 Angstroms and Fe i 6430 Angstroms. In the observations,
  however, also the weaker lines have flat-bottomed cores. Therefore, it
  is unlikely that polar spots are an artifact due to misinterpretation
  of the spectral signature of chromospheric activity. Nevertheless, we
  cannot exclude that chromospheric activity provides part of the filling
  in of the cores of some stronger lines; we present a diagnostic that may
  help to separate the contributions of chromospheric activity and spots.

Title: Emerging Flux Tubes in the Solar Convection Zone. II. The
    Influence of Initial Conditions
Authors: Caligari, P.; Schüssler, M.; Moreno-Insertis, F.
Bibcode: 1998ApJ...502..481C
Altcode:
  Numerical simulations of rising magnetic flux tubes in the solar
  convection zone have contributed significantly to our understanding
  of the basic properties of sunspot groups. They have provided an
  important clue to the operation of the solar dynamo by predicting
  strong (super-equipartition) magnetic fields near the bottom of the
  convection zone. We have investigated to what extent the simulation
  results (obtained on the basis of the thin flux tube approximation)
  depend on the assumptions made about the initial state of a magnetic
  flux tube at the start of the simulation. Two initial conditions used in
  the literature have been considered in detail: mechanical equilibrium
  (MEQ) and temperature balance (TBL). It turns out that the requirement
  of super-equipartition field strength is a robust feature of the
  simulations, largely independent of the choice of initial conditions:
  emergence of active regions at low latitudes and the correct dependence
  of their tilt angle (with respect to the east-west direction) as a
  function of heliographic latitude require an initial magnetic field
  strength on the order of 10<SUP>5</SUP> G. Other properties of rising
  flux tubes, such as the asymmetries of shape and field strength between
  the leading and following wings (with respect to the direction of
  rotation) of a rising loop, or the anchoring of part of the flux tube
  in the overshoot region, depend on the initial condition. Observed
  asymmetries in the magnetic flux distribution and of proper motions
  in emerging active regions favor MEQ over TBL as the proper initial
  condition. MEQ should also be preferred for other theoretical reasons:
  it allows for fewer free parameters, it requires no fine tuning for the
  tube geometry and background stratification in the overshoot region,
  and it can be easily made compatible with an encompassing model of the
  generation, storage, and eruption of the magnetic flux. We have also
  studied whether an external upflow (convective updraft) can trigger
  the eruption of an otherwise stably stored flux tube in the overshoot
  region. We find that a significant deformation and destabilization of
  a flux tube with equipartition field strength requires coherent upflow
  velocities of 20-50 m s<SUP>-1</SUP> in the overshoot layer, which is
  an order of magnitude larger than current estimates for such velocities.

Title: Dynamical Interaction of Solar Magnetic Elements and Granular
Convection: Results of a Numerical Simulation
Authors: Steiner, O.; Grossmann-Doerth, U.; Knölker, M.; Schüssler,
   M.
Bibcode: 1998ApJ...495..468S
Altcode:
  Nonstationary convection in the solar photosphere and its
  interaction with photospheric magnetic structures (flux sheets in
  intergranular lanes) have been simulated using a numerical code
  for two-dimensional MHD with radiative energy transfer. Dynamical
  phenomena are identified in the simulations, which may contribute to
  chromospheric and coronal heating. Among these are the bending and
  horizontal displacement of a flux sheet by convective flows and the
  excitation and propagation of shock waves both within and outside the
  magnetic structure. Observational signatures of these phenomena are
  derived from calculated Stokes profiles of Zeeman-sensitive spectral
  lines. We suggest that the extended red wings of the observed Stokes
  V profiles are due to downward coacceleration of magnetized material
  in a turbulent boundary layer between the flux sheet and the strong
  external downflow. Upward-propagating shocks in magnetic structures
  should be detectable if a time resolution of about 10 s is achieved,
  together with a spatial resolution that allows one to isolate individual
  magnetic structures. Determination of the complicated internal dynamics
  of magnetic elements requires observations with a spatial resolution
  better than 100 km in the solar photosphere.

Title: On the Asymmetry of Bipolar Active Regions
Authors: Ferriz-Mas, A.; Schüssler, M.
Bibcode: 1998ASPC..155...14F
Altcode: 1998sasp.conf...14F
  No abstract at ADS

Title: A Non-LTE Analysis of Doppler Imaging Lines
Authors: Bruls, J. H. M. J.; Solanki, S. K.; Schussler, M.
Bibcode: 1998ASPC..154.1959B
Altcode: 1998csss...10.1959B
  Doppler imaging studies have revealed that most stars with high activity
  levels have polar spots. Although their existence is corroborated by
  spectroscopic and photometric measurements, and although theoretical
  models have been produced that include polar spots, their existence
  remains controversial. Based on a NLTE radiative transfer analysis
  of the most-used Doppler-imaging lines we reject the claim that
  chromospheric activity might be responsible for the features in the
  spectral lines that are commonly interpreted as polar spots.

Title: The Solar Photosphere: Open Questions
Authors: Schüssler, M.
Bibcode: 1998ESASP.417....3S
Altcode: 1998cesh.conf....3S
  No abstract at ADS

Title: Variability of Solar and Stellar Activity by Two Interacting
    Hydromagnetic Dynamos
Authors: Schmitt, D.; Schussler, M.; Ferriz-Mas, A.
Bibcode: 1998ASPC..154.1324S
Altcode: 1998csss...10.1324S
  We propose a combination of two dynamos for solar and stellar magnetic
  activity. A strong-field dynamo operating in the overshoot layer at
  the base of the convection zone generating superequipartition fields
  concentrated in isolated flux tubes is responsible for cyclic activity
  (e.g., sunspots), while a turbulent weak-field dynamo in the convection
  zone produces a more irregular field. The combination of a threshold in
  field strength for dynamo action due to instability of magnetic flux
  tubes in the overshoot layer and random fluctuations due to magnetic
  fields from the turbulent convection zone leads, in the case of the
  Sun and solar-type stars, to activity cycles with strong amplitude
  variations and the occasional appearance of grand minima. Stronger
  fluctuations may destroy the cyclic behaviour of the overshoot layer
  dynamo and lead to increased but irregular activity. Such activity is
  observed in fast rotating cool stars. On the other hand, stars with
  low and non-variable magnetic activity may be in a state with only
  the turbulent convection zone dynamo active.

Title: Predicted Starspot Distributions on Pre-MS Stars
Authors: Granzer, Th.; Strassmeier, K. G.; Schussler, M.; Caligari, P.
Bibcode: 1998ASPC..154.1977G
Altcode: 1998csss...10.1977G
  We present an application of the magnetic flux-tube model of Caligari
  (1995, 1991) to pre-main-sequence stars. This model was originally
  designed as an explanation for sunspots. The primary goal is to
  derive the emerging latitude of flux tubes for a large parameter
  volume including stellar mass, age, and rotational period. Since the
  original model worked well in explaining certain sunspot features,
  the next step would be to compare our model predictions with observed
  starspot distributions obtained by Doppler imaging techniques.

Title: Distribution of sunspot groups from asymmetric rising flux
    loops.
Authors: Schuessler, M.; Woehl, H.
Bibcode: 1997A&A...327..361S
Altcode:
  Rising magnetic flux loops in the solar convection zone develop an
  asymmetric shape with a flat preceding part and a steeper following
  part with respect to the direction of solar rotation. By a statistical
  analysis of newly forming sunspot groups we test the conjecture that
  this geometrical asymmetry leads to an asymmetric distribution of
  secondary spot groups originating from the legs of the same rising flux
  loop as the primary group, which develops out of the loop summit. We
  find that ~10% of all sunspot groups actually develop secondary
  groups within +/-20° longitude distance from the primary group and
  within one day after its emergence. Those secondary groups related to
  small primary groups (area &lt;100 millionths of the solar hemisphere)
  are predominantly located on their eastern (following) side, while for
  larger primary groups the secondaries are more numerous on their western
  (preceding) side. Both results are consistent with the geometrical
  asymmetry developed by rising magnetic flux loops.

Title: Transmission Line Pulse Based Reliability Investigations of
    THz Schottky Diodes
Authors: Brandt, M.; Schüssler, M.; Lin, C. I.; Simon, A.; Hartnagel,
   H. L.
Bibcode: 1997ESASP.395...29B
Altcode: 1997esae.conf...29B
  No abstract at ADS

Title: (Erratum) The solar dynamo with meridional circulation.
Authors: Choudhuri, A. R.; Schuessler, M.; Dikpati, M.
Bibcode: 1997A&A...319..362C
Altcode:
  Erratum to Astron. Astrophys. 303, L29 (1995).

Title: Long-term Variation of Solar Activity by a Dynamo Based on
    Magnetic Flux Tubes
Authors: Schussler, M.; Schmitt, D.; Ferriz-Mas, A.
Bibcode: 1997ASPC..118...39S
Altcode: 1997fasp.conf...39S
  We show that cyclic activity and Maunder-type grand minima can be
  explained by a dynamo driven by the instability of magnetic flux tubes
  in the overshoot layer at the bottom of the solar convection zone. The
  combination of a threshold in field strength for dynamo action and
  random fluctuations due to magnetic fields from the turbulent convection
  zone leads to activity cycles with strong amplitude variations and
  the occasional appearance of grand minima.

Title: Numerical simulations of magnetic flux sheets.
Authors: Steiner, O.; Knölker, M.; Schüssler, M.
Bibcode: 1997smf..conf...31S
Altcode:
  Non-stationary convection in the solar photosphere and its interaction
  with photospheric magnetic structures (flux sheets in intergranular
  lanes) has been simulated using a numerical code for two-dimensional
  MHD with radiative transfer. Dynamical phenomena, which may contribute
  to chromospheric and coronal heating, like bending and horizontal
  displacement of a flux sheet by convective flows as well as the
  excitation and propagation of shock waves within and outside the
  magnetic structure are found. Observational signature of transversal
  displacement and shocks are derived. It is shown that upward propagating
  shocks in magnetic structures should be detectable. The evolution of
  an initially homogeneous vertical magnetic field is followed, starting
  from an evolved state of a two-dimensional numerical simulation of
  solar granulation.

Title: Origin of the Proper Motions of Emerging Bipolar Magnetic
    Regions
Authors: Caligari, P.; Schussler, M.; Moreno-Insertis, F.
Bibcode: 1997ASPC..118...76C
Altcode: 1997fasp.conf...76C
  We have performed numerical simulations of the rise of magnetic
  flux tubes through the convection zone. We find that the observed
  proper motions of pores and sunspots in young active regions can
  be understood as a consequence of the Coriolis force: conservation
  of angular momentum leads to a retardation of the rising flux loop
  with respect to those parts of the flux tube that remain anchored in
  the overshoot layer below the convection zone proper. The result is
  an asymmetric shape with the following flank of the loop being more
  vertical than the leading part. When emerging at the solar surface,
  the asymmetric shape of the tube leads to proper motions which are
  in qualitative agreement with the observations. By studying the
  dependence of the asymmetry on the initial state of the flux tube we
  find that the observed proper motions favor a mechanical equilibrium
  of the magnetic field in the overshoot layer. We also find that small
  active regions (emerging from flux tubes with little magnetic flux)
  are less asymmetric and should show weaker proper motions than large
  bipolar regions. This prediction can be put to an observational test.

Title: Observations of the quiet Sun's magnetic field.
Authors: Grossmann-Doerth, U.; Keller, C. U.; Schuessler, M.
Bibcode: 1996A&A...315..610G
Altcode:
  The profiles of Stokes I and V of FeI 5247A, CrI 5247A, FeI 5250.2A and
  Fe I 5250.6A have been measured in the quiet photosphere with ZIMPOL,
  a new polarimeter, with very low noise level. The spatial resolution
  element of about 1Mm was essentially determined by seeing. In about
  15% of the observed area, the signal-to-noise ratio was sufficiently
  large to determine the strength of the magnetic field and to derive
  various properties of the Stokes V profiles (asymmetries, zero-crossing
  shift and thermal line ratio). Our results show that, at least in this
  fraction of area, the magnetic field in the quiet Sun is dominated by
  strong flux concentrations which must be rather small. The data on the
  Stokes V properties provide observational constraints for theoretical
  models of flux concentrations.

Title: Dynamics of magnetic flux tubes in the solar convection zone.
Authors: Schüssler, M.
Bibcode: 1996NAWG.1996..234S
Altcode:
  The observed properties of sunspot groups on the surface of the
  Sun are consistent with the concept of magnetic flux tubes emerging
  from deep within the solar convection zone. Magnetic flux tubes are
  generated and storage in mechanical equilibrium in a subadiabatically
  stratified overshoot layer below the convection zone; they become
  unstable with respect to an undulatory (Parker-type) instability
  once the field strength exceeds a critical value; flux loops form,
  move through the convection zone and give rise to bipolar sunspot
  regions when they emerge at the surface. Both the stability criteria
  and the constraints set by the observed properties of sunspot groups
  (orientation, tilt angle, asymmetry, proper motions) require that
  the field strength at the bottom of the convection zone should be of
  the order of 10<SUP>5</SUP>G, an order of magnitude larger than the
  equipartition field strength with respect to the convective flows. This
  result has profound consequences for dynamo models for the solar cycle.

Title: Distribution of starspots on cool stars. I. Young and main
    sequence stars of 1M<SUB>sun</SUB>_.
Authors: Schuessler, M.; Caligari, P.; Ferriz-Mas, A.; Solanki, S. K.;
   Stix, M.
Bibcode: 1996A&A...314..503S
Altcode:
  Sunspots are restricted to a latitude band within 30degof the solar
  equator. In contrast, the latitudes of spots on the surfaces of
  rapidly rotating cool stars can range from their polar regions, for
  RS CVn systems and for T Tauri stars leaving the Hayashi track, to
  mid latitudes for stars close to or on the main sequence. In order to
  find an explanation for these observed spot latitudes we have applied
  the criteria for the undulatory instability (Parker instability) of
  a toroidal magnetic flux tube embedded in the convective overshoot
  layer below the outer convection zone and calculated the non-linear
  evolution of the rising magnetic loops formed by this instability. We
  describe the results for a star of one solar mass in different phases
  of its evolution before and on the main sequence. We find that there
  usually is a range of latitudes at which magnetic flux can emerge on
  the stellar surface. The mean latitude of emergence shifts towards
  the poles for increasingly rapid rotation. The internal structure
  of the star, however, plays an almost equally important role in
  determining the latitude of magnetic emergence. For stars of solar
  mass only the youngest objects, with extremely deep convection zones,
  should show spots emerging at the stellar poles. Pre-main sequence
  stars at an age of 10^7^ y (convection zone reaching down half-way to
  the centre) exhibit high latitude, but not truly polar spots, while
  a main sequence star of one solar mass, even at high rotation rates,
  only shows intermediate latitude spots. These results are found to be
  in good agreement with Doppler images of young rapid rotators.

Title: Enhanced inertia of thin magnetic flux tubes.
Authors: Moreno-Insertis, F.; Schuessler, M.; Ferriz-Mas, A.
Bibcode: 1996A&A...312..317M
Altcode:
  Accelerated bodies immersed in a fluid experience enhanced inertia
  due to the associated co-acceleration of a certain volume of fluid in
  their environment. We discuss the concept of enhanced inertia in the
  framework of the approximation of thin flux tubes, which is widely
  used to describe the dynamics of concentrated magnetic structures in
  astrophysical objects. Previous attempts to incorporate this effect
  have used a local approach, in which the reaction force of the external
  medium on a given tube mass element solely depends on the relative
  acceleration of tube and environment at that element. We show that
  those previous formulations are inconsistent (either on physical
  or geometrical grounds). We present here an alternative derivation
  of the enhanced inertia term by geometrical means, still within a
  local treatment of the problem but avoiding the pitfalls of previous
  works. Our formulation, on the other hand, reveals a basic problem: all
  local approaches are bound to give incorrect answers for the reaction
  force in as far as they disregard the variation of the external flow in
  the direction parallel to the flux tube: in doing so, they generally
  fail to provide for global momentum conservation. An exact solution
  and detailed analysis for an instance of this failure is given. The
  discussion of this paper may be of use also in the hydrodynamical
  framework of vortex tube dynamics.

Title: Intermittent solar activity by an on-off dynamo.
Authors: Schmitt, D.; Schuessler, M.; Ferriz-Mas, A.
Bibcode: 1996A&A...311L...1S
Altcode:
  We show that the alteration between intervals of cyclic activity and
  grand minima like the Maunder minimum in the 17th century, which is
  characteristic for the long-term variation of the solar activity,
  can be described as on-off intermittency arising in a dymano driven
  by the instability of magnetic flux tubes in the overshoot layer below
  the convection zone. The combination of a threshold in field strength
  for dynamo action and random fluctuations due to magnetic fields from a
  turbulent convection zone dynamo leads to activity cycles with strong
  amplitude variations and the occasional appearance of grand minima of
  very low activity. The resulting pattern is in qualitative agreement
  with the long-term records of solar activity.

Title: Polarized Radiation Diagnostics of Magnetohydrodynamic Models
    of the Solar Atmosphere
Authors: Steiner, O.; Grossmann-Doerth, U.; Schüssler, M.; Knölker,
   M.
Bibcode: 1996SoPh..164..223S
Altcode:
  Solar magnetic elements and their dynamical interaction with
  the convective surface layers of the Sun are numerically
  simulated. Radiation transfer in the photosphere is taken into
  account. A simulation run over 18.5 minutes real time shows that the
  granular flow is capable of moving and bending a magnetic flux sheet
  (the magnetic element). At times it becomes inclined by up to 30°
  with respect to the vertical around the level τ<SUB>5000</SUB> =
  1 and it moves horizontally with a maximal velocity of 4 km/s. Shock
  waves form outside and within the magnetic flux sheet. The latter
  cause a distinctive signature in a time series of synthetic Stokes
  V-profiles. Such shock events occur with a mean frequency of about
  2.5 minutes. A time resolution of at least 10 seconds in Stokes V
  recordings is needed to reveal an individual shock event by observation.

Title: Convective intensification of photospheric magnetic fields.
Authors: Schüssler, M.; Grossmann-Doerth, U.; Steiner, O.; Knölker,
   M.
Bibcode: 1996AGAb...12...89S
Altcode:
  No abstract at ADS

Title: Distribution of starspots on cool stars (review)
Authors: Schussler, M.
Bibcode: 1996IAUS..176..269S
Altcode:
  No abstract at ADS

Title: Magnetic flux tubes and the solar dynamo - storage, instability
    and eruption of magnetic flux.
Authors: Schüssler, M.
Bibcode: 1996ASIC..481...17S
Altcode:
  The observed properties of sunspot groups on the surface of the Sun
  are consistent with the concept of magnetic flux tubes emerging from
  deep within the solar convection zone. In order to maintain coherence
  and orientation during their rise in the turbulent convective flows,
  the magnetic field strength in these flux tubes should always exceed
  the equipartition value with respect to the kinetic energy density
  of the convective motions. Through linear stability analysis and
  nonlinear numerical simulations a consistent picture of the storage,
  instability, and rise of magnetic flux tubes has emerged in recent
  years. The author presents arguments which lead to this view and
  discusses a number of open questions in connection with the implied
  super-equipartition fields.

Title: Flux Tube Dynamics in Active Stars
Authors: Caligari, P.; Schüssler, M.; Solanki, S. K.; Schaerer, D.;
   Stix, M.
Bibcode: 1996ApL&C..34...17C
Altcode:
  No abstract at ADS

Title: Instabilities of Magnetic Flux Tubes in a Stellar Convection
    Zone
Authors: Ferriz-Mas, A.; Schüssler, M.
Bibcode: 1996ApL&C..34....1F
Altcode:
  No abstract at ADS

Title: The solar dynamo with meridional circulation.
Authors: Choudhuri, A. R.; Schussler, M.; Dikpati, M.
Bibcode: 1995A&A...303L..29C
Altcode:
  We show that meridional circulation can have a profound influence
  on dynamo models for the solar cycle. Motivated by the observed tilt
  angles of sunspot groups we assume that the generation of the poloidal
  field takes place near the surface, while a shear layer of radial
  differential rotation produces the toroidal field at the bottom of
  the convection zone. Both layers are coupled by a circulation with a
  poleward directed flow in the upper part and an equatorward flow in the
  deep layers of the convection zone. The circulation forces the toroidal
  field belts (which are responsible for the surface activity) to move
  equatorward. This leads to butterfly diagrams in qualitative agreement
  with the observations, even if the dynamo wave would propagate poleward
  in the absence of circulation. This result opens the possibility to
  construct models for the solar cycle which are based on observational
  data (tilt angles, differential rotation, and meridional circulation).

Title: ``Explosion'' and Intensification of Magnetic Flux Tubes
Authors: Moreno-Insertis, F.; Caligari, P.; Schuessler, M.
Bibcode: 1995ApJ...452..894M
Altcode:
  A magnetic flux tube anchored at the bottom of the solar convection
  zone and rising toward the surface as a result of an undulatory
  instability can be affected by a sudden catastrophic expansion and
  weakening of the magnetic field at its apex if the original field
  strength is below a few times 10<SUP>4</SUP> G. Such an "explosion"
  occurs if the flux tube evolves close enough to (adiabatic) hydrostatic
  equilibrium along the magnetic field lines in a super-adiabatically
  stratified environment. This condition is satisfied if the diameter of
  the tube is small enough for the drag force to dominate the dynamical
  evolution. For example, rising flux tubes with equipartition field
  (10<SUP>4</SUP> G) at their basis explode in the middle of the
  convection zone if their magnetic flux is below ≌10<SUP>21</SUP>
  Mx. <P />Apart from preventing flux tubes with equipartition field
  from reaching the surface, the explosion process may have other
  consequences for the evolution of magnetic fields in the convection
  zone: (a) it provides a source of weak field to be acted upon by
  the convective flows in the course of a turbulent dynamo process;
  (b) upflow of matter into the inflated top of a loop intensifies the
  magnetic field in the submerged part of the flux tube at the bottom of
  the convection zone. This might constitute a mechanism to produce the
  strong azimuthal fields in the overshoot region suggested by recent
  studies of the undular instability of magnetic flux tubes.

Title: Book-Review - Solar Magnetic Fields
Authors: Schussler, M.; Schmidt, W.; Roberts, B.
Bibcode: 1995Obs...115...97S
Altcode:
  No abstract at ADS

Title: Emerging Flux Tubes in the Solar Convection Zone. I. Asymmetry,
    Tilt, and Emergence Latitude
Authors: Caligari, P.; Moreno-Insertis, F.; Schussler, M.
Bibcode: 1995ApJ...441..886C
Altcode:
  The process of emergence of magnetic flux from the depths of the
  convection zone to the surface is presented in the framework of
  self-consistent model for the storage of field in the lower overshoot
  regions and as the mechanism responsible for some of the regularities
  observed in active regions. We have performed numerical simulations
  of the emergence of flux tubes in the solar convection zone including
  the effects of spherical geometry and rotation. The magnetic flux
  tubes can be stored in mechanical equilibrium in the overshoot region,
  which is the natural equilibrium of the flux rings in a subadiabatic
  layer. An undular instability leads to the formation of loops once
  a critical magnetic field strength of the order of 10<SUP>5</SUP>
  G is exceeded. In the nonlinear phase of their unstable evolution,
  the tubes move across the convection zone on a very fast time-scale,
  typically about one month. The geometry and dynamics of the flux tubes
  studied in these simulations permit prediction of some of the observed
  properties of the active regions. First, the wings of the tube show a
  marked asymmetry of inclination and velocity, which is compatible with
  the observed asymmetric proper motions of sunspots and with the position
  of the neutral line in emerging active regions. Second, upon emergence
  the flux tubes show a tilt angle with respect to the equator which
  fits reasonably well with the observed values. Third, the flux tubes
  rise roughly within a cone of radial directions in the Sun so that no
  outbreak at high latitudes takes place. The calculations lend further
  support to the possibility of superequipartition field strengths in the
  overshoot region. The implications of the present results for the dynamo
  mechanism are discussed and hints for observational work are also given.

Title: Solar Magnetic Fields.
Authors: Schuessler, M.
Bibcode: 1995RvMA....8...11S
Altcode:
  No abstract at ADS

Title: Book Review: Solar Magnetic Fields
Authors: Kundu, Mukul R.; Schüssler, M.; Schmidt, W.
Bibcode: 1995ComAp..18...36K
Altcode:
  No abstract at ADS

Title: Instabilities of magnetic flux tubes in a stellar convection
    zone II. Flux rings outside the equatorial plane
Authors: Ferriz-Mas, A.; Schüssler, M.
Bibcode: 1995GApFD..81..233F
Altcode:
  Motivated by the problems of magnetic flux storage and dynamo action in
  stars with convection zones, we study the equilibrium and stability of
  magnetic flux tubes under the influence of differential rotation and
  stratification. The formalism developed in the first paper in this
  series is applied to axisymmetric, toroidal flux tubes (flux rings)
  lying in planes parallel to the equator at an arbitrary latitude. We
  assume mechanical force equilibrium, which requires neutral buoyancy
  of the flux tube and a longitudinal internal flow in the direction
  of stellar rotation. Stability against isentropic perturbations is
  investigated by considering both axisymmetric and non-axisymmetric,
  three-dimensional displacements of the equilibrium configuration. For
  axisymmetric modes, we find qualitative differences between the
  stability criteria for flux tubes within and outside the equatorial
  plane, where instability is generally easier to excite and overstable
  modes appear. In the case of non-axisymmetric perturbations, the
  results of a numerical study with parameter values corresponding to
  the bottom of the solar convection zone are discussed. The stability
  properties depend in a complicated way on the various parameters (e.g.,
  latitude, magnetic field, superadiabaticity of the stratification,
  angular velocity and its gradient). While the magnetic field value for
  the onset of undulatory (Parker) instability with large growth rates
  is mainly determined by the stratification and the rotation rate,
  instabilities at somewhat lower field strengths with relatively small
  growth rates depend strongly on the sign and the value of the angular
  velocity gradient.

Title: Simulation of the Interaction of Convective Flow with Magnetic
    Elements in the Solar Atmosphere.
Authors: Steiner, O.; Grossmann-Doerth, U.; Knoelker, M.; Schuessler,
   M.
Bibcode: 1995RvMA....8...81S
Altcode:
  No abstract at ADS

Title: Observation of Surface Activity on Cool Giants with the VLT
    Interferometer
Authors: von der Lühe, O.; Schüssler, M.; Solanki, S. K.; Caligari,
   P.
Bibcode: 1995svlt.conf...94V
Altcode:
  No abstract at ADS

Title: Book-Review - Solar Magnetic Fields
Authors: Schussler, M.; Schmidt, W.; Priest, E. R.
Bibcode: 1994Ap&SS.222..266S
Altcode:
  No abstract at ADS

Title: Waves and Instabilities of a Toroidal Magnetic Flux Tube in
    a Rotating Star
Authors: Ferriz-Mas, A.; Schuessler, M.
Bibcode: 1994ApJ...433..852F
Altcode:
  The oscillation modes and instabilities of a toroidal flux tube
  lying in the equatorial plane of a differentially rotating star are
  investigated using the thin flux-tube approximation. The behavior
  of the frequencies as functions of the magnetic field strength
  and of the superadiabaticity is explored for both axisymmetric and
  nonaxisymmetric modes. In limiting cases, the modes can be identified
  with known normal modes of simple configurations (e.g., a plane-parallel
  atmosphere with constant gravity or a magnetic flux tube embedded in a
  homogeneous medium). The axisymmetric modes are essentially radial modes
  oscillating with a magnetically modified Brunt-Vaisala frequency. As
  to the nonaxisymmetric modes in the limit of rapid rotation (or of
  weak magnetic field) there is one pair of stable inertial waves, which
  are due to the Coriolis force, and one pair of magnetospheric modes,
  which are longitudinal slow modes modified by rotation. In the general
  case (intermediate rotation rates), the modes do not have a definite
  character, but their properties can be investigated analytically. The
  corresponding bifurcation diagrams are discussed.

Title: Books-Received - Solar Magnetic Fields
Authors: Schussler, M.; Schmidt, W.
Bibcode: 1994Sci...266..666S
Altcode:
  No abstract at ADS

Title: A dynamo effect due to instability of magnetic flux tubes.
Authors: Ferriz-Mas, A.; Schmitt, D.; Schuessler, M.
Bibcode: 1994A&A...289..949F
Altcode:
  We show that a non-axisymmetric instability of toroidal magnetic
  flux tubes in a rotating star provides a dynamo effect. The
  instability occurs in the form of propagating helical waves; their
  growth in amplitude causes a phase shift between the perturbations
  of magnetic field and velocity, which leads to an electric field
  (anti)parallel to the direction of the unperturbed field. Together with
  differential rotation, this effect is capable of driving a dynamo of the
  α{OMEGA}-type. In contrast to the conventional α-effect in cyclonic
  convection, this dynamo effect operates in strong (super-equipartition)
  magnetic fields which resist against distortion by convective flows. We
  calculate the induced electric field using results from linear stability
  analysis and a model of the solar convection zone which consistently
  includes an overshoot layer. We find that for growing magnetic field
  the dynamo effect occurs first in high latitudes (near the poles) in
  a region of weak instability which moves towards the equator as the
  field strength increases further. The dependence of the dynamo effect
  on the location of the flux tubes (in depth and latitude) and on the
  rotation rate is discussed.

Title: Active Region Asymmetry as a Result of the Rise of Magnetic
    Flux Tubes
Authors: Moreno-Insertis, F.; Caligari, P.; Schuessler, M.
Bibcode: 1994SoPh..153..449M
Altcode:
  The magnetic flux tubes that rise across the convection zone to produce
  active regions are shown to develop a difference in inclination between
  their preceding and follower sides. This asymmetry is such that the
  follower wing is more vertical (i.e., closer to the radial direction)
  than the preceding side. An asymmetry of this kind can be obtained as a
  natural consequence of the conservation of angular momentum along the
  rise. This process may explain a number of the observed asymmetries
  in morphology and behavior of the preceding and follower parts of the
  active regions. We present results of numerical simulations showing
  this effect and discuss possible observational consequences.

Title: Solar Magnetic Fields
Authors: Schüssler, Manfred; Schmidt, Wolfgang
Bibcode: 1994smf..conf.....S
Altcode: 1994QB539.M23S65...
  The study of solar magnetic fields includes such issues as how
  large-scale magnetic fields are generated in the Sun and how magnetic
  structures are spontaneously formed and then interact with convective
  flows. These are just a sample of the fundamental questions answered in
  this timely review of our understanding of solar magnetic fields. This
  volume collects together articles and research papers ranging from
  such topics as large-scale patterns and global dynamo action to tiny
  flux tubes, from the overshoot layer below the convection zone up to
  the corona, and from instrumental problems and theoretical methods
  to the latest ground-based and satellite observations. This volume
  provides an essential review of our knowledge to date for graduate
  students and researchers.

Title: The deep layers of solar magnetic elements
Authors: Grossmann-Doerth, U.; Knoelker, M.; Schuessler, M.; Solanki,
   S. K.
Bibcode: 1994A&A...285..648G
Altcode:
  We compare self-consistent theoretical models of solar magnetic
  flux sheets with spectropolarimetric observations of a solar plage
  and a network region. Our observational diagnostics mainly provide
  information on temperature and magnetic field of the deep photospheric
  layers. They are used to constrain the two free parameters of the
  models, viz. width and initial evacuation of the flux sheets. We find
  that the width of flux sheets in the network is approximately 200 km,
  while it is 300-350 km in an active plage. The flux sheets turn out
  to be less evacuated than previously thought, so they have continuum
  intensities close to unity. Since these are average values, however,
  our results do not exclude the presence of either smaller and brighter
  or larger and darker magnetic structures.

Title: Forces on Magnetic Flux Tubes Moving in Inhomogeneous Flows
Authors: Moreno-Insertis, F.; Ferriz-Mas, A.; Schussler, M.
Bibcode: 1994ApJ...422..652M
Altcode:
  The back-reaction of an incompressible two-dimensional flow of constant
  strain to the accelerated motion of a straight cylinder is calculated
  in the general case that the flow is inhomogeneous, nonstationary, and
  with nonvanishing (although constant) vorticity. The resulting enhanced
  inertia of the cylinder is basically given by its relative acceleration
  with respect to the background flow. Further force terms that appear
  because of the nonstationary and inhomogeneity of the background flow
  are the following: a force identical to that experienced by the mass
  elements of the unperturbed flow at the position of the axis of the
  cylinder; the customary lift force because of the circulation around
  the body; finally, a force term that appears only with nonvanishing
  relative speed between the body and the background flow and which
  is related to the energy which has to be imparted by the body to the
  surrounding fluid in order to adapt the perturbation to the new local
  velocity. The results have application to the dynamics of magnetic
  flux tubes in the convection zone and atmosphere of the Sun as well
  as to other astrophysical problems.

Title: Dynamic interaction of convection with magnetic flux sheets:
    first results of a new MHD code
Authors: Steiner, O.; Knölker, M.; Schüssler, M.
Bibcode: 1994ASIC..433..441S
Altcode:
  No abstract at ADS

Title: Distribution of Magnetic Flux on the Surface of Rapidly
    Rotating Stars
Authors: Caligari, P.; Schussler, M.; Stix, M.; Solanki, S. K.
Bibcode: 1994ASPC...64..387C
Altcode: 1994csss....8..387C
  No abstract at ADS

Title: Alpha-effect due to instability of magnetic flux tubes and
    the solar dynamo
Authors: Schmitt, D.; Ferriz-Mas, A.; Schüssler, M.
Bibcode: 1994smf..conf..101S
Altcode:
  No abstract at ADS

Title: Dynamics of erupting magnetic flux tubes
Authors: Moreno-Insertis, F.; Schüssler, M.; Caligari, P.
Bibcode: 1994ASIC..433..407M
Altcode: 1994ssm..work..407M
  The eruption of magnetic flux tubes from the overshoot layer due to
  instability and the dynamics of their subsequent rise through the solar
  convection zone are followed by numerical simulation. Special emphasis
  is put on the possibility of explaining observed regularities of the
  active regions at the surface (tilt angles, latitude of emergence,
  asymmetry between preceding and following parts, etc). Instability
  sets in with non-axisymmetric (undular) modes at azimuthal wavenumbers
  $m=1$ and $m=2$ if the field strength exceeds values of the order of
  $10^5$ G. At the same time, such strong initial fields are required
  to reproduce the observable properties of sunspots and active
  regions. Consequently, a consistent picture of storage, instability
  and eruption of solar magnetic fields emerges.

Title: Solar magnetic fields : proceedings of the international
    conference held in Freiburg, Germany, June 29-July 2, 1993
Authors: Schüssler, M.; Schmidt, Wolfgang
Bibcode: 1994smfp.book.....S
Altcode: 1994QB539.M23S74...
  No abstract at ADS

Title: Instability and eruption of magnetic flux tubes
Authors: Caligari, P.; Ferriz-Mas, A.; Moreno-Insertis, F.;
   Schüssler, M.
Bibcode: 1994smf..conf..139C
Altcode:
  No abstract at ADS

Title: Solar Magnetic Elements: Models Compared with Observations
Authors: Grossmann-Doerth, U.; Knolker, M.; Schussler, M.; Solanki,
   S. K.
Bibcode: 1994ASPC...68...96G
Altcode: 1994sare.conf...96G
  No abstract at ADS

Title: MHD simulations with adaptive mesh refinement
Authors: Steiner, O.; Grossmann-Doerth, U.; Knölker, M.; Schüssler,
   M.
Bibcode: 1994smf..conf..282S
Altcode:
  No abstract at ADS

Title: Flux tube dynamics in pre-main-sequence and giant stars.
Authors: Caligari, P.; Schaerer, D.; Schüssler, M.; Solanki, S.
Bibcode: 1994AGAb...10...92C
Altcode:
  No abstract at ADS

Title: Simulation of magneto-convection with radiative transfer
Authors: Steiner, O.; Grossmann-Doerth, U.; Knölker, M.; Schüssler,
   M.
Bibcode: 1994smf..conf..286S
Altcode:
  No abstract at ADS

Title: Instability and eruption of magnetic flux tubes in the solar
    convection zone.
Authors: Schussler, M.; Caligari, P.; Ferriz-Mas, A.; Moreno-Insertis,
   F.
Bibcode: 1994A&A...281L..69S
Altcode:
  We present a consistent model of storage, instability and dynamical
  eruption of magnetic flux tubes in the solar convection zone
  and underlying overshoot region. Using a convection zone model
  with self-consistent overshoot layer, we calculate equilibrium
  configurations of magnetic flux tubes and determine their linear
  stability properties, taking into consideration the effects of
  stratification and rotation. Instability of flux tubes stored in
  the overshoot layer with growth times below one year requires field
  strengths of the order of 10<SUP>5</SUP> G; in many cases, the dominant
  mode has an azimuthal wave number of m = 2. Numerical simulations are
  used to follow the nonlinear evolution of such unstable flux tubes and
  their rise through the convection zone, from which they emerge to form
  active regions. The results are in accordance with the following two
  requirements, based on observational facts: (a) the upward motion of
  the tubes is not significantly deflected by the Coriolis force so that
  they can emerge at low latitudes, (b) their inclination with respect
  to the East-West direction (tilt angle) as a function of the latitude
  of emergence is consistent with observations of acitve regions.

Title: Storage of Magnetic Flux in the Overshoot Region
Authors: Moreno-Insertis, F.; Schussler, M.; Ferriz-Mas, A.
Bibcode: 1993IAUS..157...41M
Altcode:
  No abstract at ADS

Title: Flux Tubes and Dynamos
Authors: Schussler, M.
Bibcode: 1993IAUS..157...27S
Altcode:
  No abstract at ADS

Title: Instabilities of magnetic flux tubes in a stellar convection
    zone I. Equatorial flux rings in differentially rotating stars
Authors: Ferriz-Mas, A.; Schüssler, M.
Bibcode: 1993GApFD..72..209F
Altcode:
  The stability properties of magnetic flux tubes in stellar
  convection zones including overshoot regions is of considerable
  interest in connection with the problems of magnetic flux storage
  and hydromagnetic dynamo action in the Sun and other cool stars. We
  have developed a general formalism based on the approximation of thin
  flux tubes which provides a basis for a linear stability analysis
  of arbitrary flux tube equilibria. As a first application, the
  stability of axisymmetric, toroidal flux tubes (flux rings) located
  in the equatorial plane of a star under the influence of differential
  rotation and stratification has been considered. Arbitrary angular
  velocity differences between the interior of the flux ring and its
  environment are permitted. It is found that the linear evolution of
  radial and azimuthal perturbations (i.e., within the equatorial plane)
  is decoupled from that of latitudinal perturbations (perpendicular to
  the plane). The latitudinal instability ('poleward slip') is found to
  be suppressed if the matter within the flux tube rotates faster than
  its environment by a sufficient amount. For perturbations within the
  equatorial plane, both stratification (sub-order superadiabatic) of the
  external gas and rotation are crucial. Angular momentum conservation
  tends to suppress axisymmetric modes. This effect is enhanced by a
  faster rotation of the gas within the flux tube. Non-axisymmetric modes
  are more unstable since the constraint of angular momentum conservation
  is broken. For these modes, a slower internal rotation rate has a
  stabilizing effect. Within a certain range of magnetic field strengths,
  a second region of stability exists within the region of unstable
  configurations, which can extend into the superadiabatically stratified
  (convectively unstable) region. The character of the different modes
  is discussed in conjunction with the topology of the stability diagram.

Title: On the Stability of Magnetic Flux Tubes in the Equator of
    a Star
Authors: Ferriz-Mas, A.; Schussler, M.
Bibcode: 1993IAUS..157...45F
Altcode:
  No abstract at ADS

Title: Modes of a flux ring lying in the equator of a star.
Authors: Ferriz-Mas, A.; Schüssler, M.
Bibcode: 1993spd..conf...69F
Altcode:
  No abstract at ADS

Title: Why rapid rotators have polar spots.
Authors: Schuessler, M.; Solanki, S. K.
Bibcode: 1992A&A...264L..13S
Altcode:
  Starspots on magnetically active, cool stars preferentially appear near
  the poles. We suggest that this preference of high latitudes is due
  to the rapid rotation to these stars which leads to a dominance of the
  Coriolis force over the buoyancy force in the dynamics of magnetic flux
  tubes. As a consequence, flux tubes erupting from the deep parts of the
  stellar convection zone follow a path nearly parallel to the axis of
  rotation and thus necessarily surface at high latitudes, unless their
  initial field strength exceeds a critical value for which buoyancy
  becomes dominant again. It is shown that for stars with rotation
  periods below about 10 days flux tubes with such large field strength
  (of the order of 10 exp 6 G) cannot be formed and stored since they are
  unstable with respect to non-axisymmetric disturbances. Consequently,
  magnetically active stars with rapid rotation exhibit magnetic flux
  eruption at high latitudes and polar starspots.

Title: Storage of magnetic flux tubes in a convective overshoot region
Authors: Moreno-Insertis, F.; Schuessler, M.; Ferriz-Mas, A.
Bibcode: 1992A&A...264..686M
Altcode:
  Consideration is given to the suppression of the radial and polar escape
  of magnetic flux in the form of toroidal flux tubes (flux rings) from
  low latitudes in the overshoot region below the solar convection zone
  through the combined action of the subadiabatic ambient stratification
  and the rotationally induced forces. It is shown that a flux ring which
  is initially in thermal equilibrium with its environment and rotates
  with the ambient angular velocity moves radially and latitudinally
  towards an equilibrium configuration of lower internal temperature
  and larger internal rotation rate with respect to the surrounding
  nonmagnetic gas. Flux rings perform superposed buoyancy and inertial
  oscillations around their equilibrium positions. From a study of the
  frequencies and amplitudes of these oscillations, it is concluded that
  flux rings with B of less than about 100,000 G can be kept within the
  overshoot region if the superadiabaticity is sufficiently negative,
  i.e., less than about -0.00004.

Title: MHD simulations with adaptive mesh refinement.
Authors: Steiner, O.; Grossmann-Doerth, U.; Knölker, M.; Schüssler,
   M.
Bibcode: 1992AGAb....7..213S
Altcode:
  No abstract at ADS

Title: Instability of magnetic flux tubes in the solar convection
    zone.
Authors: Caligari, P.; Moreno-Insertis, F.; Schüssler, M.
Bibcode: 1992AGAb....7..152C
Altcode:
  No abstract at ADS

Title: Convection
Authors: Schüssler, M.
Bibcode: 1992ASIC..373...81S
Altcode: 1992sla..conf...81S
  No abstract at ADS

Title: Small-Scale Photospheric Magnetic Fields
Authors: Schüssler, M.
Bibcode: 1992ASIC..373..191S
Altcode: 1992sla..conf..191S
  No abstract at ADS

Title: The effect of non-linear oscillations in magnetic flux tubes
    on Stokes V asymmetry
Authors: Grossmann-Doerth, U.; Schuessler, M.; Solanki, S. K.
Bibcode: 1991A&A...249..239G
Altcode:
  The present knowledge on, and interpretation of, the asymmetry of
  Stokes V of spectral lines thought to be formed in and around magnetic
  flux tubes is reviewed. Crude models of nonlinear oscillations in the
  flux tube were investigated in order to explain the observed values
  of Stokes V amplitude and area asymmetry. It was found that flux tube
  models with quasi-oscillatory motions, consisting of a slow upflow
  and a rapid downflow in the magnetic region, as well as a downflow
  in the nonmagnetic surroundings, may reproduce the observations,
  both asymmetries and zero-crossing shift of Stokes V.

Title: Structure and dynamics of magnetic fields in the solar
    convection zone.
Authors: Schüssler, M.
Bibcode: 1991NAWG.1991...25S
Altcode:
  No abstract at ADS

Title: Solar magnetic elements: results of MHD simulations.
Authors: Grossmann-Doerth, U.; Knölker, M.; Schüssler, M.;
   Weisshaar, E.
Bibcode: 1991AGAb....6...31G
Altcode:
  No abstract at ADS

Title: Filigree and flux tube physics.
Authors: Spruit, H. C.; Schuessler, M.; Solanki, S. K.
Bibcode: 1991sia..book..890S
Altcode:
  This review covers the properties of the small-scale (outside
  sunspots) magnetic field from observational and theoretical points
  of view. Special emphasis is put on the physics of small isolated
  magnetic concentrations (flux tubes). Topics discussed include the
  basic observational properties, the origin and disappearance of the
  small-scale field, the properties of individual magnetic elements,
  their influence on the solar irradiance and their interaction with
  solar oscillations.

Title: Solar magnetic elements
Authors: Schüssler, Manfred
Bibcode: 1991GApFD..62..271S
Altcode:
  The interaction of magnetic fields with convective flows and radiative
  energy transport leads to a remarkable spatial organization of the
  magnetic flux permeating the solar photosphere: Most of the flux
  outside sunspots is concentrated in small-scale structures of large
  fields strength. The properties of these flux concentrations are largely
  uniform suggesting the concept of a prototype structure, the magnetic
  element. This contribution gives a brief overview of the observational
  results and the theoretical concepts used to describe the physics of
  magnetic elements. Dynamical processes like flows, oscillations and
  waves are considered in somewhat more detail including some recent
  results of numerical simulations. In addition, some consequences of the
  intermittent nature of photospheric magnetic fields for the structure
  of the solar chromosphere are briefly discussed.

Title: Concentrated magnetic fields in the solar atmosphere.
Authors: Schüssler, M.
Bibcode: 1991ptpa.conf....1S
Altcode:
  The solar atmosphere provides the opportunity to study
  magnetohydrodynamic and plasma processes on scales and under
  circumstances inaccessible to experimental investigation on Earth. In
  this contribution the author gives a summary of the present knowledge
  about magnetic elements, the small-scale structures into which most
  of the magnetic flux in the solar photosphere is concentrated. They
  represent an example for the formation of dissipative structures in
  systems far from thermal equilibrium. He discusses in particular the
  observationally determined properties of magnetic elements as well as
  theoretical concepts and results concerning their formation, stationary
  state, instabilities and decay.

Title: Some developments in the theory of magnetic flux concentrations
    in the solar atmosphere
Authors: Knoelker, M.; Grossmann-Doerth, U.; Schuessler, M.;
   Weisshaar, E.
Bibcode: 1991AdSpR..11e.285K
Altcode: 1991AdSpR..11..285K
  Most of the magnetic flux in the solar photosphere is concentrated
  in small-scale structures of large field strength, called magnetic
  elements. We discuss briefly the observationally determined properties
  of magnetic elements and the theoretical concepts for the origin of
  magnetic flux filamentation and concentration. New results of model
  calculations for 2D magnetic flux sheets on the basis of numerical
  simulation of the compressible MHD equations including a full (grey)
  radiative transfer are presented. Synthetic Stokes profiles of spectral
  lines and continuum intensity distributions serve to compare the
  theoretical models with observational data. Among the key results are:
  (1) The upper layers of the magnetic structure become hotter than
  the environment due to radiative illumination effects; (2) a strong
  convective flow evolves with horizontal velocity of 2 km/s towards the
  flux sheet and a narrow ``downflow jet'' with velocity up to 6 km/s
  adjacent to the magnetic structure; (3) both flux sheet and non-magnetic
  environment oscillate with a period around 5 minutes. Comparison with
  observed properties of solar magnetic elements reveals: (4) Calculated
  and semi-empirical temperature profiles as function of height in the
  photosphere are in reasonable agreement; (5) the calculated velocity
  field around flux concentrations explains the area asymmetry of the
  observed Stokes V-profiles including their center-limb variation; (6)
  the calculated continuum intensity of a flux sheet model is compatible
  with the values inferred from high spatial resolution observations
  of bright points at solar disk center; (7) the observed center-limb
  variation of facular contrast at low or medium spatial resolution is
  reproduced by arranging calculated flux sheets in arrays. We stress the
  importance of MHD simulation models for the analysis and interpretation
  of data from future facilities for high spatial resolution observations
  like OSL and LEST.

Title: Model calculations of magnetic flux concentrations in the
    solar photosphere.
Authors: Grossmann-Doerth, U.; Knölker, M.; Schüssler, M.;
   Weisshaar, E.
Bibcode: 1990AGAb....5...44G
Altcode:
  No abstract at ADS

Title: Theoretical Aspects of Small-Scale Photospheric Magnetic Fields
Authors: Schüssler, M.
Bibcode: 1990IAUS..138..161S
Altcode:
  No abstract at ADS

Title: Non-linear dynamos. I - One-dimensional model of a thin
    layer dynamo
Authors: Schmitt, D.; Schuessler, M.
Bibcode: 1989A&A...223..343S
Altcode:
  A simple model of a boundary layer alpha-effect dynamo at the bottom
  of the solar convection zone is considered in order to study nonlinear
  solutions. After discussing the various modes that result from the
  solution of the linear eigenvalue problem the results of a numerical
  study are presented, including two different kinds of nonlinearities:
  (1) quenching of the alpha-effect for increasing amplitude of the
  magnetic field and (2) loss of magnetic flux from the dynamo region due
  to magnetic buoyancy or related instabilities. It turns out that the
  spectrum of nonlinear solutions, especially for alpha-effect quenching,
  is very complicated and that the results may sensitively depend on
  the initial conditions. Nonlinear modes appear which have no linear
  counterpart. The implications for the solar dynamo are discussed; it
  is argued that the flux loss nonlinearity leads to results which are in
  better accordance with the properties of the solar cycle. Consequences
  for the interpretation of observations of active stars are briefly
  mentioned.

Title: Stokes V asymmetry and shift of spectral lines
Authors: Grossmann-Doerth, U.; Schuessler, M.; Solanki, S. K.
Bibcode: 1989A&A...221..338G
Altcode:
  Further evidence is given for the interpretation of the observed
  unshifted and asymmetric Stokes V profiles in the solar atmosphere
  in terms of a magnetic flux concentration expanding with height and
  surrounded by a downflow. A general proof is given that a V profile
  originating in an atmosphere in which magnetic field and flow are
  spatially separated along the line of sight has a zero-crossing
  wavelength which is unshifted with respect to the rest wavelength of
  the line center. Heuristic considerations for the dependence of the
  asymmetry of Stokes V on Zeeman shift, Doppler shift, line width,
  and strength are described and confirmed by exploratory calculations.

Title: Observational aspects of magnetic flux sheet models
Authors: Grossmann-Doerth, U.; Knölker, M.; Schüssler, M.;
   Weisshaar, E.
Bibcode: 1989hsrs.conf..427G
Altcode:
  No abstract at ADS

Title: Dynamics of magnetic flux concentrations - The second-order
    thin flux tube approximation
Authors: Ferriz-Mas, A.; Schuessler, M.; Anton, V.
Bibcode: 1989A&A...210..425F
Altcode:
  The thin flux tube approximation for the dynamics of magnetic flux
  concentrations is extended up to second order in the radial expansion
  to consistently include azimuthal velocities and twisted magnetic
  fields. The linear wave modes of a flux tube as described by the
  new set of equations are calculated and compared with the results
  of the conventional zeroth-order thin flux tube approximation. By
  comparison with exact solutions which are available for a uniform
  flux tube in a non-stratified medium, it is shown that the thin flux
  tube approximation (based on an expansion approach) is best suited to
  describe surface modes of a magnetic structure while body waves are
  not so well represented.

Title: Models of Magnetic Flux Sheets
Authors: Grossmann-Doerth, U.; Knölker, M.; Schüssler, M.;
   Weisshaar, E.
Bibcode: 1989ASIC..263..481G
Altcode: 1989ssg..conf..481G
  No abstract at ADS

Title: Theoretical aspects and modelling of photospheric flux tubes.
Authors: Knölker, M.; Schüssler, M.
Bibcode: 1989ftsa.conf...17K
Altcode:
  This paper discusses some aspects of the theoretical description of
  concentrated magnetic fields in the solar photosphere. The authors
  focus on processes leading to the formation and destruction of magnetic
  elements and on the properties of their quasi-equilibrium state. Results
  of 2D model calculations of flux slabs are discussed in some detail with
  emphasis on the continuum intensity and its center-to-limb variation.

Title: Book-Review - Solar and Stellar Physics - 5TH European Solar
    Meeting - Titisee / Schwarzwald - Germany - 1987APR27-30
Authors: Schroter, E. H.; Schussler, M.; Staude, J.
Bibcode: 1989AN....310...66S
Altcode:
  No abstract at ADS

Title: Book-Review - Solar and Stellar Physics / 5TH European Solar
    Meeting / Titisee / Schwarzwald Germany - 1987APR
Authors: Schroter, E. H.; Schussler, M.; Venkatakrishnan, P.
Bibcode: 1988BASI...16..248S
Altcode:
  No abstract at ADS

Title: Unshifted, asymmetric Stokes V-profiles - Possible solution
    of a riddle
Authors: Grossmann-Doerth, U.; Schuessler, M.; Solanki, S. K.
Bibcode: 1988A&A...206L..37G
Altcode:
  We show that Stokes V-profiles originating in an atmosphere
  in which a magnetic field and a systematic velocity field are
  spatially separated along the line of sight are asymmetric with
  unshifted zero-crossings. Such a configuration is probably realized
  in the peripheral parts of magnetic flux concentrations in the solar
  photosphere: Since the magnetic field flares out with height, the line
  of sight traverses both a static magnetic region and a downdraft in
  the non-magnetic surroundings. V-profiles formed in these parts of the
  flux concentation will be strongly asymmetric but unshifted while the
  V-profiles from the central parts are symmetric and unshifted. The
  resulting mean V-profiles show the characteristics of the observed
  profiles, i.e. they are unshifted and asymmetric.

Title: Model calculations of magnetic flux tubes. IV - Convective
    energy transport and the nature of intermediate size flux
    concentrations
Authors: Knoelker, M.; Schuessler, M.
Bibcode: 1988A&A...202..275K
Altcode:
  Results of 2D-MHD simulations of solar (sub-) photospheric magnetic flux
  concentrations with sizes between 500 km and 1000 km are presented. A
  residual level of convective energy transport within the magnetic
  structure is assumed to be maintained by oscillatory convection or by
  motions driven by the Rayleigh-Taylor instability since pure radiative
  transport leads to density inversions. The calculated flux sheet models
  demonstrate the decreasing efficiency of heating by lateral influx of
  radiation with increasing size of the structure. Flux concentrations
  with sizes greater than about 500 km appear darker than the mean
  pahotosphere if observed at the center of the solar disk but display a
  significant brightness enhancement near the limb. The results are used
  to resolve the apparent contradiction between the data obtained with
  the Fourier Transform Spectrometer in network and plage regions which
  imply small, hot and bright structures and spectra with high spatial
  resolution (less than about 1 arcsec) which reveal comparatively
  large (1-2 arcsecs), dark magnetic structures in active regions. All
  observations can be reconciled if the larger structures are clusters
  of small magnetic elements and partly suppress the convective energy
  transport.

Title: Book-Review - Solar and Stellar Physics
Authors: Schroter, E. H.; Schussler, M.
Bibcode: 1988JBAA...98..262S
Altcode:
  No abstract at ADS

Title: Book-Review - Solar and Stellar Physics
Authors: Schroter, E. H.; Schussler, M.
Bibcode: 1988S&T....75S.498S
Altcode:
  No abstract at ADS

Title: Model calculations of magnetic flux tubes. III - Properties
    of solar magnetic elements
Authors: Knoelker, M.; Schuessler, M.; Weisshaar, E.
Bibcode: 1988A&A...194..257K
Altcode:
  The paper presents the results of 2D-MHD simulations of small magnetic
  flux concentrations in the solar photosphere. Consideration is given
  to the effects of extended boundary layers (i.e., a smooth transition
  to the nonmagnetic environment and inhibition of convective energy
  transport in the flux concentration surroundings). A comparison is made
  with observational data, and it is found that the boundary layer is
  likely to be thin compared to the diameter of a magnetic element while
  convective transport does not seem to be strongly inhibited outside the
  flux concentration. It is found that all models of magnetic elements
  are slightly unstable against fluting in the absence of a surrounding
  whirl flow.

Title: Continuum intensity of magnetic flux concentrations - Are
    magnetic elements bright points?
Authors: Schuessler, M.; Solanki, S. K.
Bibcode: 1988A&A...192..338S
Altcode:
  The authors use the weakening of temperature sensitive spectral
  lines (Fe I λ5250.2 and λ5247.1) in a strong plage region to
  derive a lower limit (≈1.4 of the quiet photospheric value) for the
  continuum intensity in magnetic flux concentrations. This suggests an
  identification of magnetic elements with the observed photospheric
  bright points. The authors discuss the implications of this result
  for the quantitative determination of magnetic flux and propose a
  procedure for obtaining information on the physical structure of the
  non-magnetic surroundings of flux concentrations.

Title: Models of small magnetic flux concentrations in the solar
    photosphere.
Authors: Grossmann-Doerth, U.; Knölker, M.; Schüssler, M.
Bibcode: 1988AGAb....1...11G
Altcode:
  No abstract at ADS

Title: Spurious variation of photospheric magnetic flux
Authors: Grossmann-Doerth, U.; Pahlke, K. -D.; Schuessler, M.
Bibcode: 1987A&A...176..139G
Altcode:
  The response of a Babcock-type magnetograph to emission from different
  stages in the evolution of a solar active region is simulated by
  computing the Stokes I and V profiles of the 525.02-nm and 630.25-nm
  lines of Fe I in model atmospheres representing sunspot umbrae,
  magnetic knots, and magnetic elements. The results are presented in
  tables and graphs and characterized. Breakup of a single sunspot into
  small hot elements with the same total magnetic flux is shown to produce
  a significant (as much as factor of 10) spurious loss of signal in the
  magnetograph. It is suggested that recent observations of flux-loss
  events (in unipolar regions of the solar photosphere without magnetic
  neutral lines) be carefully reexamined and analyzed.

Title: Solar and Stellar Physics
Authors: Schröter, Egon-Horst; Schüssler, Manfred
Bibcode: 1987LNP...292.....S
Altcode: 1987ssp..conf.....S
  These proceedings bring together ideas from solar and stellar
  physics. The sun is near enough for rather detailed observations and
  one chapter is devoted to the more recent experimental data from
  observations from space. On the other hand the multitude of stars
  provides a wide range of physical parameters to test hypotheses in
  solar and stellar astrophysics. The reader will find an illuminating
  overview of these fields ranging from the dynamo in the convection zone
  to the stellar envelopes and winds in the outer regions. In particular
  the importance of small-scale magnetohydrodynamic processes for the
  activity phenomena plays an important role in the contributions to this
  volume. For both students and researchers the general introduction
  by N. O. Weiss makes an excellent guide to this very active field of
  research. (See also Lecture Notes in Physics Vol. 291.)

Title: Structure and Dynamics of Small Magnetic Flux Concentrations:
    Observation versus Theory
Authors: Schüssler, M.
Bibcode: 1987rfsm.conf..223S
Altcode:
  Observational results and theoretical model calculations for
  small magnetic flux concentrations in the solar photosphere are
  compared. The formation of flux concentrations and their magnetic,
  thermal and velocity structure are considered. Flux tube geometry,
  interaction with the environment and, finally, their destruction are
  discussed. In the conclusion, the necessity of further development and
  support of MHD model calculations in connection with new observational
  projects is emphasized.

Title: Magnetic Fields and the Rotation of the Solar Convection Zone
Authors: Schuessler, M.
Bibcode: 1987ASSL..137..303S
Altcode: 1987isav.symp..303S
  Arguments are presented in favour of the conjecture that magnetic fields
  in the convection zone are concentrated, highly fragmented and passive
  to velocity fields (convection, rotation) with the only exception of
  the uppermost layers, where buoyancy becomes dominant. A firmer basis is
  given to the concept of an "anchor" or "coupling depth". It is concluded
  that large active regions have their origin in a region of dominating
  differential rotation at the lower boundary layer of the convetion zone
  from where they are injected into the convection zone proper. It is
  shown that this view is consistent with observations of the evolution
  of active regions and favours the cluster model of sunspots.

Title: Solar Astrophysics: A Selection of Problems, Developments
    and Projects
Authors: Schüssler, M.
Bibcode: 1987MitAG..68...41S
Altcode:
  This contribution discusses the scientific basis of two large
  future projects in solar physics with German participation: LEST
  (Large Earthbound Solar Telescope) and HRSO (High Resolution Solar
  Observatory). The author starts with an overview of the current
  problem areas in solar physics and the relation to astrophysics in
  general. Emphasis is laid on the key role of small-scale structures for
  processes of broad astrophysical relevance and the crucial part played
  by magnetic fields and their intermittent structure. The requirements
  following from this state of the scientific discourse do not only
  concern observation and measurement: data evaluation and interpetation,
  numerical simulation and theory must be supported adequately in order to
  make sensible use of the data gathered with expensive new observational
  facilities. The author discusses the interaction and cooperation of
  these sub-disciplines and lays down the requirements for each of them.

Title: WORKSHOP I: Solar/Stellar Activity and Winds
Authors: Schüssler, M.
Bibcode: 1986MitAG..65..150S
Altcode:
  No abstract at ADS

Title: Berichte von den Workshops.
Authors: Schüssler, M.; Trümper, J.; Offermann, D.; Porcas, R.
Bibcode: 1986MitAG..65..149S
Altcode:
  Workshop I: Solar/stellar activity and winds (M. Schüssler),
  Workshop II: "ROSAT", Stand des Projekts (J. Trümper), Workshop
  III: Variabilität der mittleren Atmosphäre - Vorbereitung eines
  Shuttle-Experiments (D. Offermann), Workshop IV: VLBI of galactic
  objects (R. Porcas).

Title: MHD Models of Solar Photospheric Magnetic Flux Concentrations
Authors: Schüssler, M.
Bibcode: 1986ssmf.conf..103S
Altcode:
  This contribution has two parts: the first part attempts to review
  the different approaches and approximations used for the theoretical
  description of small magnetic flux concentrations. Emphasis is laid on
  calculations for individual structures (flux tubes) while large-scale
  numerical simulations are not discussed. In the second part, a few
  specific topics are considered in somewhat more detail. It is pointed
  out that detailed model calculations are needed to provide predictions
  which can be tested by observations (forthcoming high-resolution
  measurements from space). Such models must describe very small
  structures like thermal and resistive boundary layers of only a few
  km width.

Title: Heating of solar magnetic elements by downflows
Authors: Hasan, S. S.; Schuessler, M.
Bibcode: 1985A&A...151...69H
Altcode:
  The idea that magnetic elements in the photosphere and lower
  chromosphere of the sun are heated by downflowing gas is quantitatively
  examined. The time-dependent hydromagnetic equations are solved
  numerically in the slender flux tube approximation. Viscous terms are
  retained, and the radiative exchange of heat between the flux tube
  and the ambient medium are included. Hydrogen ionization and its
  thermodynamic consequences are treated self-consistently. Starting
  from a state of hydrostatic and thermal equilibrium, the temporal
  response due to the onset of a downflow in the tube is studied. After a
  transient phase lasting a few minutes, a stationary state results that
  is substantially hotter than the ambient medium over a fairly large
  height range. Chapman's facular model can be reproduced remarkably well
  by adjusting the mass flux entering the tube at the upper boundary. The
  results are comparatively insensitive to viscosity (nu less than or
  equal to 10 to the 12th sq cm/s), while radiative heat exchange is
  significant. Some observational implications are discussed, and it is
  suggested that the necessary mass flux could be provided by overstable
  oscillations during their downflow phase.

Title: Model Calculations of Solar Photospheric Flux Concentrations
Authors: Knoelker, M.; Schussler, M.; Weisshaar, E.
Bibcode: 1985tphr.conf..195K
Altcode:
  No abstract at ADS

Title: Model calculations of solar photospheric flux concentrations.
Authors: Knölker, M.; Schüssler, M.; Weisshaar, E.
Bibcode: 1985MPARp.212..195K
Altcode:
  No abstract at ADS

Title: On the structure of magnetic fields in the solar convection
    zone.
Authors: Schuessler, M.
Bibcode: 1984ESASP.220...67S
Altcode: 1984ESPM....4...67S
  The following concepts are discussed in turn and tentatively put
  together in a picture of a boundary layer dynamo near the bottom
  of the convection zone: expulsion of magnetic flux and vorticity,
  fragmentation and accumulation of fields, the dominating forces that
  govern the evolution of a magnetic structure.

Title: The interchange instability of small flux tubes
Authors: Schuessler, M.
Bibcode: 1984A&A...140..453S
Altcode:
  The effect of stationary velocity fields on the interchange (fluting)
  instability of magnetic flux tubes is investigated theoretically. It is
  shown through a series of calculations that converging convective cells
  and internal flows along the fieldlines cannot stabilize but instead
  can lead to a slightly destabilizing contribution. A small flux tube
  (magnetic element) is found to be stable if it is surrounded by a
  whirl which forms in a narrow intergranular down draft. Estimates for
  the case of the sun (with an assumed whirl velocity of 2 km/s) yield
  stable tubes in between the unstable tubes with magnetic fluxes of more
  than 5-10 x 10 to the 17th mx when stabilized by the centrifugal force
  of the whirl, and more than 2-6 x 10 to the 19th mx when stabilized
  for buoyancy. Some of the implications of the estimates for current
  models of the solar surface magnetic fields are discussed.

Title: Model calculations of magnetic flux tubes. I - Equations and
    method. II - Stationary results for solar magnetic elements
Authors: Deinzer, W.; Hensler, G.; Schuessler, M.; Weisshaar, E.
Bibcode: 1984A&A...139..426D
Altcode:
  The equations, boundary conditions, geometry, approximations, and
  numerical methods used in model studies of the physics of magnetic flux
  tubes in stellar convection zones and atmospheres are presented. The
  methods described are then used to study the smallest flux tubes in
  the solar atmosphere, i.e., the magnetic elements that are thought to
  be responsible for facular points and filigree. The models proposed
  here are dynamical and include flows; the material properties (e.g.,
  opacity and heat capacity) are calculated self-consistently. Although
  many limitations remain, it is shown that a considerable contribution
  to the understanding of the physics of photospheric flux concentrations
  can be made even at this level of sophistication. In addition, models of
  stationary magnetic flux elements are presented which are considered to
  constitute the basic structure of the solar photospheric magnetic field.

Title: Model Calculations of Magnetic Flux Tubes - Part Two -
    Stationary Results for Solar Magnetic Elements
Authors: Deinzer, W.; Hensler, G.; Schussler, M.; Weisshaar, E.
Bibcode: 1984A&A...139..435D
Altcode:
  Based on the methods of a preceding paper (Deinzer et al., 1984) we
  present models of stationary magnetic flux elements which are thought
  to build the basic strncture of the solar photospheric magnetic
  field. The main results are: a) A model with a density reduction to
  half of the ambient value and a reduction of the vertical convective
  transport coefficient by a factor 0.2 is well comparable with the semi-
  empirical model of Chapman (1979). b) The tube is surrounded by a
  dark region of gas which is significantly cooler than the undisturbed
  photo sphere at the same height. This reduces the "hot wall effect"
  and points to the necessity of an additional heating process for the
  upper layers of the flux tube atmosphere. c) The inclination of the
  isotherms with respect to the lines of constant gravitational potential
  in the nonmagnetic surroundings of the tube leads to a downflow around
  the magnetic strncture with a velocity of ∼ 1 km s<SUP>-1</SUP>. d)
  The energetics of the observable layers (τ ≍ 1) of the tube is
  dominated by horizontal radiative influx. Convective transport along
  the tube plays a secondary role.

Title: Evidence for the 22-YEAR-CYCLE in the Longitudinal Distribution
    of Sunspots
Authors: Balthasar, Horst; Schüssler, Manfred
Bibcode: 1984SoPh...93..177B
Altcode:
  It was shown in an earlier paper that preferred hemispheres of solar
  activity alternate with the 22-year magnetic cycle, when analyzed in
  the 27.0 day Bartels rotation. Using data which cover the time between
  1818 and 1983 we trace back this result to 1880 (cycle 12). Before
  1880 no significant correlations are found.

Title: Equilibrium Models for Thin Flux Tubes in the Solar Convection
    Zone
Authors: Anton, V.; Schüssler, M.
Bibcode: 1984MitAG..62..219A
Altcode:
  No abstract at ADS

Title: Preferred longitudes of sunspot groups and high-speed solar
wind streams: evidence for a "solar memory".
Authors: Balthasar, H.; Schuessler, M.
Bibcode: 1983SoPh...87...23B
Altcode:
  Correlation analysis of the mean longitude distribution of sunspot
  groups (taken from the Greenwich Photoheliographic Results)
  and high-speed solar wind streams (inferred from the C9 index for
  geomagnetic disturbances) with the Bartels rotation period P = 27.0
  days shows anti-correlation for individual cycles.

Title: Self-consistent models for small photospheric flux tubes
Authors: Deinzer, W.; Hensler, G.; Schmitt, D.; Schuessler, M.;
   Weisshaar, E.
Bibcode: 1983IAUS..102...67D
Altcode:
  The present investigation is concerned with results of a numerical study
  of magnetic field concentrations in the solar photosphere and upper
  convection zone. A two-dimensional time dependent code was developed
  for the full MHD equations in slab geometry for a compressible medium,
  taking into account the momentum equation, the equation of continuity,
  the induction equation for infinite conductivity, and the energy
  equation. A finite-element technique is used in the development
  procedure. Attention is given to the description of convective energy
  transport by the mixing-length formalism, a nearly static flux tube
  model, and the use of the two-dimensional radiative transfer code of
  Mihalas et al. (1978) for LTE diagnostics of the radiation field.

Title: Simulation interplanetarer Alféenwellen
Authors: Richter, A. K.; Schüssler, M.; Weisshaar, E.
Bibcode: 1983MitAG..60..289R
Altcode:
  No abstract at ADS

Title: Stellar dynamo theory
Authors: Schuessler, M.
Bibcode: 1983IAUS..102..213S
Altcode:
  Three topics of current interest are treated in the review: (1)
  The success and shortcomings of dynamo models for the solar cycle
  are explained, and oscillator models discussed briefly. (2) The
  intermittent (flux tube) nature of magnetic fields in convection zones
  leads to new conjectures about stellar dynamos. Arguments are given
  that the dynamo may operate in the overshoot region below a convective
  envelope. Mean-field theory for intermittent fields is illustrated. (3)
  The author reviews nonlinear dynamo models and some attempts to
  interpret observational results concerning late-type active stars.

Title: On changes of the rotation velocities of stable, recurrent
    sunspots and their interpretation with a flux tube model
Authors: Balthasar, H.; Schuessler, M.; Woehl, H.
Bibcode: 1982SoPh...76...21B
Altcode:
  The angular rotation velocities of stable, recurrent sunspots
  were investigated using data from the Greenwich Photoheliographic
  Results 1940 until 1968. We found constant rotation velocities
  during the passages on the solar disk with errors of about ±4 m
  s<SUP>−1</SUP>. During their lifetime these spots show a decreasing
  braking of their rotation velocities from 0.8 to 0.3 m s<SUP>−1</SUP>
  per day. A plausible interpretation is found by assuming the spots
  to be coupled to a slowly rising subsurface flux tube and a rotation
  velocity which increases with depth.

Title: Zweidimensionale Dynamomodelle auf der Basis magnetischer
    Flußröhren
Authors: Schüssler, M.
Bibcode: 1982MitAG..55...69S
Altcode:
  No abstract at ADS

Title: Modelle photosphärischer Magnetfeld-Konzentrationen
Authors: Deinzer, W.; Hensler, G.; Schmitt, D.; Schüssler, M.;
   Weisshaar, E.
Bibcode: 1982MitAG..55...65D
Altcode:
  No abstract at ADS

Title: The solar torsional oscillation and dynamo models of the
    solar cycle
Authors: Schuessler, M.
Bibcode: 1981A&A....94L..17S
Altcode:
  It is shown that the solar torsional oscillation and overlapping
  of cycles recently discovered by Howard and LaBonte (1980) can
  easily be produced by the Lorentz force due to a migrating dynamo
  wave. Comparison with a special flux tube dynamo model (Schuessler,
  1980) shows reasonable agreement with the observations.

Title: Ein solarer Dynamo auf der Basis magnetischer Flußröhren.
Authors: Schüssler, M.
Bibcode: 1981MitAG..52..136S
Altcode:
  No abstract at ADS

Title: Ein solarer Dynamo auf der Basis Magnetischer Flußröhren
Authors: Schüssler, M.
Bibcode: 1981MitAG..52Q.136S
Altcode:
  No abstract at ADS

Title: Flux tube dynamo approach to the solar cycle
Authors: Schuessler, M.
Bibcode: 1980Natur.288..150S
Altcode:
  In the past two decades dynamo models of solar activity and the physical
  foundations of the solar cycle have become acceptable. However, recent
  observations revealing the concentrated form of photospheric magnetic
  flux<SUP>1</SUP>, the distribution and cyclic appearance of X-ray
  bright points<SUP>2,3</SUP> and ephemeral active regions<SUP>4</SUP>,
  as well as the discovery of sunspot brightness variations during
  the solar cycle<SUP>5,6</SUP>, have raised some questions about the
  dynamo theory of solar activity<SUP>7</SUP>. Both observation and
  theory<SUP>8</SUP> suggest that most of the magnetic flux in the solar
  convection zone is in the form of concentrated isolated magnetic flux
  tubes. The turbulent dynamo theory<SUP>9,10</SUP> has, therefore,
  to be modified and attempts have been made to include concentrated
  fields<SUP>11</SUP>. Nevertheless, the dynamo problem for a convective
  medium pervaded by concentrated flux tubes, the `flux tube dynamo',
  has not yet been solved. The problem is to regenerate the poloidal
  magnetic field out of which toroidal field is produced by differential
  rotation. Here the consequences and the structure of the resulting
  cycle are considered if a field regeneration process operating on flux
  tubes is assumed. A calculation similar to Leighton's magneto-kinematic
  model<SUP>12</SUP> shows that a flux tube dynamo model can operate and
  reproduce the essential features of the solar cycle. It can also explain
  the cyclic variation of sunspot brightness and ephemeral active regions.

Title: Neues zur Theorie der Sonnenaktivität.
Authors: Schuessler, M.
Bibcode: 1980S&W....19..331S
Altcode:
  The current status of the theory of photospheric magnetic fields and
  the solar cycle theory is reviewed. Some new observations concerning
  the photospheric magnetic fields, the bright X-ray spots, and the ratio
  of the umbra radius to the penumbra radius are discussed, and their
  importance for these theories and their further development is examined.

Title: Flows along magnetic flux tubes. I - Equilibrium and buoyancy
    of a slender magnetic loop in the interior of a star
Authors: Schuessler, M.
Bibcode: 1980A&A....89...26S
Altcode:
  It has been speculated that loop formation may enhance the buoyant
  loss of magnetic flux from stellar interiors. In this paper, the
  effect on magnetic buoyancy of a downflow along the arms of a slender,
  isolated magnetic loop formed from an initially horizontal flux tube
  is investigated excluding the uppermost subsurface layers of the
  star. Loop formation, in these circumstances, is shown to be incapable
  of accelerating the buoyant loss of magnetic flux from regions in
  radiative equilibrium. For convective regions as the envelope of the
  sun, only very broad loops with a radius of curvature of the order
  of their distance to the stellar center may grow and do not enhance
  the buoyant loss significantly. Thus horizontal (or azimuthal) flux
  tubes built by differential rotation stay horizontal for most of their
  buoyant rise through the star.

Title: Theoretical aspects of early type magnetic stars.
Authors: Schüssler, M.
Bibcode: 1980Nukl...25.1425S
Altcode:
  No abstract at ADS

Title: Non-linear dynamo theory: finite amplitude magnetic fields
    with large scale circulation in a compressible stratified medium.
Authors: Schuessler, M.
Bibcode: 1979A&A....72..348S
Altcode:
  In the framework of the dynamo theory of the solar cycle self-consistent
  numerical solutions of the nonlinear mean-field MHD equations (including
  Lorentz force) within a compressible stratified medium are given for
  a Cartesian geometry. For both steady (alpha-squared) and oscillatory
  (alpha-omega) turbulent dynamos the growth of the magnetic field
  is limited by a mean flow driven by the Lorentz force. Magnetic
  buoyancy supports this mechanism but is not able to suppress
  dynamo action totally or to set narrow limits to the dynamo models
  investigated. Flow velocities of the order of 1 m/s are sufficient
  to limit the magnetic-field amplitude to about 10 mT (mean toroidal
  field of the sun). For an oscillatory dynamo of the solar type the
  flow pattern has a one-cell geometry with fluid rising to the surface
  near the spot zone (zone of maximum toroidal field in the vicinity of
  the equator), flowing towards the pole, and sinking down there. This
  may account for the observed poleward motion of the prominence zone.

Title: Magnetic buoyancy revisited: analytical and numerical results
    for rising flux tubes.
Authors: Schuessler, M.
Bibcode: 1979A&A....71...79S
Altcode:
  The buoyancy of magnetic flux tubes in the sun is reconsidered by
  investigating the effect of a shear flow (differential rotation) on the
  balance of magnetic-flux gain and loss, by examining the hydrostatic
  pressure stratification inside and outside a horizontal flux tube,
  and by numerically integrating the full set of MHD equations as
  an initial-value problem for the two-dimensional case. The results
  obtained show that: (1) flux tubes with a strong magnetic field can
  be produced in the solar convection zone for any realistic choice of
  parameters by consistent consideration of flux gain via differential
  rotation and flux loss due to magnetic buoyancy; (2) the buoyancy
  force can be determined by integration of the pressure deviation over
  the surface of a flux tube; and (3) rising flux tubes with a strong
  magnetic field experience distortion and eventually fragment.

Title: Neue Ergebnisse zum Auftrieb magnetischer Flußröhren in
    der Konvektionszone der Sonne
Authors: Schüssler, M.
Bibcode: 1979MitAG..45..199S
Altcode:
  No abstract at ADS

Title: Nichtlineare Dynamomodelle im kompressiblen geschichteten
    Medium
Authors: Schüssler, M.
Bibcode: 1979MitAG..45..198S
Altcode:
  No abstract at ADS

Title: Diffusion of a strong internal magnetic field through the
    radiative envelope of a 2.25 M sun-star.
Authors: Schuessler, M.; Paehler, A.
Bibcode: 1978A&A....68...57S
Altcode:
  Summary. Strong internal magnetic fields of stars are probably
  relevant for triggering supernova explosions, pulsar and white dwarf
  magnetic fields and the fields of some magnetic stars. We present
  numerical calculations for the evolution of a magnetic field produced
  by turbulent dynamo action in the convective core of a 2.25 M0-
  star. The diffusion of the field through the radiative envelope is
  investigated. The field reaches detectable surface values in a time
  less than 1 % of the familiar magnetic timescale T = R2. -1. For stars
  with M Mc (Mc = 2.25.. .5 M0 depending on the core field strength)
  the diffusion time is less than or at most of the order of the main
  sequence lifetime. Consequently, a core dynamo can serve as a model
  for magnetic stars in the mass range 1.5 M0 M Mc. Stars with 1.2 M0 M
  1.5 M0 probably possess a field which is a remnant from dynamo action
  during the Hayashi phase. Stars with M Mc have main sequence lifetimes
  too short to allow for growing of a detectable surface field. Their
  hidden fields are required, however, for the production of the observed
  pulsar magnetic fields. Key words: stars - magnetic field - stellar
  structure and evolution - a-effect - dynamo-theory

Title: On buoyant magnetic flux tube in the solar convection zone.
Authors: Schuessler, M.
Bibcode: 1977A&A....56..439S
Altcode:
  Analytical calculations and numerical results are presented
  for slender buoyant magnetic flux tubes moving through the solar
  convective envelope. Conservation of mass and magnetic flux is assumed
  together with thermal equilibrium with the surrounding gas. Two braking
  mechanisms are investigated: aerodynamic drag and viscous forces. The
  resulting vertical velocities are always of the order of a few meters
  per second, causing timescales of the order of the solar cycle for
  ascension through the whole convection zone. Thus there is enough time
  for the fields to be amplified by differential rotation and the alpha
  effect in order to build up an alpha-omega dynamo.

Title: Zur Wechselwirkung von Magnetfeldern und Strömungen im
kompressiblen Medium unter dem Aspekt des Dynamo-Problems Title:
    Zur Wechselwirkung von Magnetfeldern und Strömungen im kompressiblen
    Medium unter dem Aspekt des Dynamo-Problems
Authors: Schüssler, Manfred
Bibcode: 1977PhDT.......317S
Altcode:
  No abstract at ADS

Title: A Dynamo Model for Magnetic Stars with Long Periods
Authors: Schüssler, M.
Bibcode: 1976paps.coll...39S
Altcode: 1976IAUCo..32...39S
  No abstract at ADS

Title: Axisymmetric alpha <SUP>2</SUP>-dynamos in the Hayashi-phase.
Authors: Schuessler, M.
Bibcode: 1975A&A....38..263S
Altcode:
  The evolution of axisymmetric magnetic fields during the Hayashi-phase
  is investigated on the basis of the alpha-effect dynamo mechanism. In
  particular, the influence of the transition from the fully convective
  to the fully radiative main sequence state (for A stars) is considered
  for rigid rotation. The fields are calculated by solving the induction
  equation as an initial value problem. The numerical results show
  that a dynamo-built magnetic field can 'survive' the transition while
  conserving the main part of its total flux. The topology of the field
  can change significantly during the evolution. For a certain phase
  of the transition to the main sequence there are oscillating fields,
  possibly giving an explanation for magnetic stars with long periods.

