Author name code: zacharias
ADS astronomy entries on 2022-09-14
author:"Zacharias, Pia" 
------------------------------------------------------------------------  

Title: High-resolution observations of the solar photosphere,
    chromosphere, and transition region. A database of coordinated IRIS
    and SST observations
Authors: Rouppe van der Voort, L. H. M.; De Pontieu, B.; Carlsson,
   M.; de la Cruz Rodríguez, J.; Bose, S.; Chintzoglou, G.; Drews, A.;
   Froment, C.; Gošić, M.; Graham, D. R.; Hansteen, V. H.; Henriques,
   V. M. J.; Jafarzadeh, S.; Joshi, J.; Kleint, L.; Kohutova, P.;
   Leifsen, T.; Martínez-Sykora, J.; Nóbrega-Siverio, D.; Ortiz, A.;
   Pereira, T. M. D.; Popovas, A.; Quintero Noda, C.; Sainz Dalda, A.;
   Scharmer, G. B.; Schmit, D.; Scullion, E.; Skogsrud, H.; Szydlarski,
   M.; Timmons, R.; Vissers, G. J. M.; Woods, M. M.; Zacharias, P.
Bibcode: 2020A&A...641A.146R
Altcode: 2020arXiv200514175R
  NASA's Interface Region Imaging Spectrograph (IRIS) provides
  high-resolution observations of the solar atmosphere through ultraviolet
  spectroscopy and imaging. Since the launch of IRIS in June 2013, we
  have conducted systematic observation campaigns in coordination with
  the Swedish 1 m Solar Telescope (SST) on La Palma. The SST provides
  complementary high-resolution observations of the photosphere and
  chromosphere. The SST observations include spectropolarimetric imaging
  in photospheric Fe I lines and spectrally resolved imaging in the
  chromospheric Ca II 8542 Å, Hα, and Ca II K lines. We present
  a database of co-aligned IRIS and SST datasets that is open for
  analysis to the scientific community. The database covers a variety
  of targets including active regions, sunspots, plages, the quiet Sun,
  and coronal holes.

Title: Disentangling flows in the solar transition region
Authors: Zacharias, P.; Hansteen, V. H.; Leenaarts, J.; Carlsson,
   M.; Gudiksen, B. V.
Bibcode: 2018A&A...614A.110Z
Altcode: 2018arXiv180407513Z
  Context. The measured average velocities in solar and stellar spectral
  lines formed at transition region temperatures have been difficult
  to interpret. The dominant redshifts observed in the lower transition
  region naturally leads to the question of how the upper layers of the
  solar (and stellar) atmosphere can be maintained. Likewise, no ready
  explanation has been made for the average blueshifts often found in
  upper transition region lines. However, realistic three-dimensional
  radiation magnetohydrodynamics (3D rMHD) models of the solar atmosphere
  are able to reproduce the observed dominant line shifts and may thus
  hold the key to resolve these issues. <BR /> Aims: These new 3D rMHD
  simulations aim to shed light on how mass flows between the chromosphere
  and corona and on how the coronal mass is maintained. These simulations
  give new insights into the coupling of various atmospheric layers
  and the origin of Doppler shifts in the solar transition region and
  corona. <BR /> Methods: The passive tracer particles, so-called corks,
  allow the tracking of parcels of plasma over time and thus the study of
  changes in plasma temperature and velocity not only locally, but also
  in a co-moving frame. By following the trajectories of the corks, we
  can investigate mass and energy flows and understand the composition
  of the observed velocities. <BR /> Results: Our findings show that
  most of the transition region mass is cooling. The preponderance of
  transition region redshifts in the model can be explained by the higher
  percentage of downflowing mass in the lower and middle transition
  region. The average upflows in the upper transition region can be
  explained by a combination of both stronger upflows than downflows
  and a higher percentage of upflowing mass. The most common combination
  at lower and middle transition region temperatures are corks that are
  cooling and traveling downward. For these corks, a strong correlation
  between the pressure gradient along the magnetic field line and the
  velocity along the magnetic field line has been observed, indicating a
  formation mechanism that is related to downward propagating pressure
  disturbances. Corks at upper transition region temperatures are
  subject to a rather slow and highly variable but continuous heating
  process. <BR /> Conclusions: Corks are shown to be an essential tool
  in 3D rMHD models in order to study mass and energy flows. We have
  shown that most transition region plasma is cooling after having been
  heated slowly to upper transition region temperatures several minutes
  before. Downward propagating pressure disturbances are identified as
  one of the main mechanisms responsible for the observed redshifts at
  transition region temperatures. <P />The movie associated to Fig. 3
  is available at <A href="https://www.aanda.org">http://www.aanda.org</A>

Title: New insights on mass flows in and out of the solar transition
    region
Authors: Zacharias, Pia
Bibcode: 2017psio.confE..56Z
Altcode:
  No abstract at ADS

Title: Tracing Flows in the Solar Atmosphere Using the Bifrost
    Stellar Atmosphere Code
Authors: Zacharias, P.
Bibcode: 2015AGUFMSH31B2409Z
Altcode:
  All energy that heats the solar corona must pass through the
  transition region between the chromosphere and corona. Observations
  of transition region emission lines reveal the presence of redshifts
  up to temperatures of about 200,000 K and blueshifts for higher
  temperatures. The apparent large downward flows in the lower transition
  region would lead to an emptying of the corona, in contrast to what is
  being observed, thus some mechanism must be responsible for maintaining
  the mass balance between the photosphere and corona. We use the Bifrost
  stellar atmosphere code to perform 3D radiation MHD simulations of the
  solar atmosphere. The focus of this study is on the temporal evolution
  of mass flows into and out of the solar corona. By adding tracer
  particles to the simulations and analyzing their paths over time,
  we can provide new insights on the physical processes driving these
  mass flows and on their role in the chromosphere-corona mass cycle.

Title: Analysis of mass and energy flows in coronal loops using the
    Bifrost stellar atmosphere code
Authors: Zacharias, Pia; Leenaarts, Jorritt
Bibcode: 2014cosp...40E3750Z
Altcode:
  Identifying and quantifying processes that lead to the regulation of
  the mass and energy supply from the chromosphere to the outer layers
  of the solar atmosphere is key to advancing our understanding of the
  coupling between the solar chromosphere and corona. The combination
  of large-scale numerical simulations of the rapidly evolving
  detailed physical processes occurring in the solar atmosphere and
  high-resolution observations from instruments like IRIS, Hinode and
  SDO makes this undertaking more and more feasible. Using the Bifrost
  stellar atmosphere code, we have performed 3D radiation MHD simulations
  of the solar atmosphere extending from the convection zone to the
  corona to study the evolution of magnetic field structures and the
  underlying dynamics. By adding tracer particles to the simulation,
  we are able to follow the magnetic field lines properly. A detailed
  analysis of the plasma properties along those field lines and of the
  generation and propagation of waves travelling into the corona will
  be presented. By tracking the evolution of the observed perturbations,
  we provide new insights on the physical processes driving them and on
  their role in the chromosphere-corona mass cycle.

Title: Studies of the dynamics and energetics of cool plasma ejections
    into the corona
Authors: Zacharias, Pia; Bingert, Sven; Peter, Hardi
Bibcode: 2013enss.confE.142Z
Altcode:
  The corona is highly dynamic and shows transient events on various
  scales in space and time. Most of these features are related to
  changes in the magnetic field structure or impulsive heating caused
  by the conversion of magnetic to thermal energy. We are applying
  three-dimensional magnetohydrodynamic models in order to investigate
  the structure and dynamics in the upper solar atmosphere above a small
  active region. Emission line spectra are synthesized from the model
  and compared to spectra and images observed by current space-based
  instruments, such as the EUV Imaging Spectrometer (EIS) onboard the
  Hinode satellite and the Atmospheric Imaging Assembly (AIA) onboard the
  Solar Dynamics Observatory. We investigate mass and energy flows between
  the solar chromosphere and corona and discuss possible scenarios for a
  mass cycle between the lower and upper solar atmosphere. In particular,
  we have studied the processes that lead to the formation and ejection
  of a confined plasma ejection into the solar corona. A description of
  the nature of this particular feature will be presented which is found
  to be a hydrodynamic phenomenon triggered by a heating event above the
  chromosphere. A detailed analysis of 1D coronal loop models has been
  performed to understand how the plasma responds to a heating pulse. The
  results confirm the formation mechanism of the blob observed in the 3D
  model. This raises the question if other small-scale ejection features
  seen on the Sun could also be based on hydrodynamic processes instead
  of being plasmoid-type phenomena as it is usually assumed.

Title: Mass flows between the chromosphere and corona - comparison
    of 1D and 3D coronal loop models
Authors: Zacharias, P.; Bingert, S.; Peter, H.
Bibcode: 2012AGUFMSH33B2239Z
Altcode:
  The ejection and return of cool transition region plasma into the
  corona will be discussed in the context of our three-dimensional
  magnetohydrodynamic (3D MHD) model of the solar corona. The dynamics
  of the ejection are investigated in more detail in a 1D loop model
  and are compared to observations from Hinode/EIS and SDO/AIA. Results
  from the 3D model analysis serve as input for the 1D loop model. In the
  3D case, a heating pulse is the main driver of the ejection. To mimic
  the situation in 1D, a heating pulse is injected at different heights
  along the loop with varying amplitude and width. As a consequence,
  the heating rate is strongly increased in a localized area and leads
  to enhanced evaporation that causes the material to rise. We present
  results that show the successful reproduction of the ejection in a 1D
  loop model following the injection of a heating pulse. In contrast to
  earlier studies, where similar heating events lead to both redshifts
  in transition region emission lines and blueshifts in coronal emission
  lines, preliminary results of our parameter study show exclusively
  upflows along the loop and almost no downflows during the heating
  phase. We will discuss these findings in terms of the mass cycle
  between the chromosphere and corona.

Title: Ejection of cool plasma into the corona - comparison of
    results from a 3D MHD model with results from AIA/SDO, EIS/Hinode
    and a 1D loop model
Authors: Zacharias, Pia; Bingert, Sven; Peter, Hardi
Bibcode: 2012decs.confE..48Z
Altcode:
  The formation and subsequent ejection of cool plasma into the
  corona will be discussed, as observed in our three-dimensional
  magnetohydrodynamic (3D MHD) model of the solar atmosphere extending
  from the photosphere into the corona. The model accounts properly
  for the energy balance, especially for heat conduction and radiative
  losses, allowing us to reliably synthesize the profiles of optically
  thin extreme ultraviolet emission lines and compare them to existing
  observations. A detailed description of the nature of this particular
  phenomenon will be provided. The analysis of the various forces
  acting upon the plasma in the 3D model shows that the pressure
  gradient which is driving the ejection is due to Ohmic dissipation of
  currents resulting from the braiding of the magnetic field lines by
  photospheric plasma motions. Preliminary results of a parameter study
  on the reproduction of the phenomenon in a one-dimensional loop model
  support the scenario of a heating event that leads to the ejection
  of cool plasma into the corona in both, the 1D loop model and the 3D
  model. In addition, results of the numerical model will be compared to
  observations from the Extreme Ultraviolet Imaging Spectrometer (EIS)
  onboard Hinode and the Atmospheric Imaging Assembly (AIA) onboard SDO,
  where we have also found evidence of cool plasma ejecta that are moving
  along magnetic field lines.

Title: Ejection of cool plasma into the hot corona
Authors: Zacharias, P.; Peter, H.; Bingert, S.
Bibcode: 2011A&A...532A.112Z
Altcode: 2011arXiv1106.5972Z
  Context. The corona is highly dynamic and shows transient events
  on various scales in space and time. Most of these features are
  related to changes in the magnetic field structure or impulsive
  heating caused by the conversion of magnetic to thermal energy. <BR
  /> Aims: We investigate the processes that lead to the formation,
  ejection and fall of a confined plasma ejection that was observed in
  a numerical experiment of the solar corona. By quantifying physical
  parameters such as mass, velocity, and orientation of the plasma
  ejection relative to the magnetic field, we provide a description of
  the nature of this particular plasma ejection. <BR /> Methods: The
  time-dependent three-dimensional magnetohydrodynamic (3D MHD) equations
  are solved in a box extending from the chromosphere, which serves as
  a reservoir for mass and energy, to the lower corona. The plasma is
  heated by currents that are induced through field line braiding as
  a consequence of photospheric motions included in the model. Spectra
  of optically thin emission lines in the extreme ultraviolet range are
  synthesized, and magnetic field lines are traced over time. We determine
  the trajectory of the plasma ejection and identify anomalies in the
  profiles of the plasma parameters. <BR /> Results: Following strong
  heating just above the chromosphere, the pressure rapidly increases,
  leading to a hydrodynamic explosion above the upper chromosphere in the
  low transition region. The explosion drives the plasma, which needs
  to follow the magnetic field lines. The ejection is then moving more
  or less ballistically along the loop-like field lines and eventually
  drops down onto the surface of the Sun. The speed of the ejection is in
  the range of the sound speed, well below the Alfvén velocity. <BR />
  Conclusions: The plasma ejection observed in a numerical experiment
  of the solar corona is basically a hydrodynamic phenomenon, whereas
  the rise of the heating rate is of magnetic nature. The granular
  motions in the photosphere lead (by chance) to a strong braiding of
  the magnetic field lines at the location of the explosion that in
  turn is causing strong currents which are dissipated. Future studies
  need to determine if this process is a ubiquitous phenomenon on the
  Sun on small scales. Data from the Atmospheric Imaging Assembly on
  the Solar Dynamics Observatory (AIA/SDO) might provide the relevant
  information. <P />Appendix and movie are available in electronic form
  at <A href="http://www.aanda.org">http://www.aanda.org</A>

Title: Investigation of mass flows in the transition region and
    corona in a three-dimensional numerical model approach
Authors: Zacharias, P.; Peter, H.; Bingert, S.
Bibcode: 2011A&A...531A..97Z
Altcode: 2011arXiv1105.5491Z
  Context. The origin of solar transition region redshifts is not
  completely understood. Current research is addressing this issue
  by investigating three-dimensional magneto-hydrodynamic models
  that extend from the photosphere to the corona. <BR /> Aims: By
  studying the average properties of emission line profiles synthesized
  from the simulation runs and comparing them to observations with
  present-day instrumentation, we investigate the origin of mass
  flows in the solar transition region and corona. <BR /> Methods:
  Doppler shifts were determined from the emission line profiles of
  various extreme-ultraviolet emission lines formed in the range of T =
  10<SUP>4</SUP>-10<SUP>6</SUP> K. Plasma velocities and mass flows
  were investigated for their contribution to the observed Doppler
  shifts in the model. In particular, the temporal evolution of plasma
  flows along the magnetic field lines was analyzed. <BR /> Results:
  Comparing observed vs. modeled Doppler shifts shows a good correlation
  in the temperature range log (T/[K]) = 4.5-5.7, which is the basis of
  our search for the origin of the line shifts. The vertical velocity
  obtained when weighting the velocity by the density squared is shown
  to be almost identical to the corresponding Doppler shift. Therefore,
  a direct comparison between Doppler shifts and the model parameters
  is allowed. A simple interpretation of Doppler shifts in terms of
  mass flux leads to overestimating the mass flux. Upflows in the model
  appear in the form of cool pockets of gas that heat up slowly as they
  rise. Their low temperature means that these pockets are not observed
  as blueshifts in the transition region and coronal lines. For a set of
  magnetic field lines, two different flow phases could be identified. The
  coronal part of the field line is intermittently connected to subjacent
  layers of either strong or weak heating, leading either to mass flows
  into the loop (observed as a blueshift) or to the draining of the loop
  (observed as a redshift).

Title: On the nature of coronal loops above the quiet sun network
Authors: Bingert, S.; Zacharias, P.; Peter, H.; Gudiksen, B. V.
Bibcode: 2010AdSpR..45..310B
Altcode:
  The structure and dynamics of a box in a stellar corona can be
  modeled employing a 3D MHD model for different levels of magnetic
  activity. Depending on the magnetic flux through the surface the
  nature of the resulting coronal structures can be quite different. We
  investigate a model of an active region for two sunspots surrounded by
  magnetic field patches comparable in magnetic flux to the sunspots. The
  model results in emission from the model corona being concentrated in
  loop structures. In Gudiksen and Nordlund (2005) the loops seen in EUV
  and X-ray emission outline the magnetic field, following the general
  paradigm. However, in our model, where the magnetic field is far from
  a force-free state, the loops seen in X-ray emission do not follow
  the magnetic field lines. This result is of interest especially for
  loops as found in areas where the magnetic field emerging from active
  regions interacts with the surrounding network.

Title: Spectral analysis of 3D MHD models of coronal structures
Authors: Zacharias, Pia; Bingert, Sven; Peter, Hardi
Bibcode: 2009AdSpR..43.1451Z
Altcode: 2009arXiv0904.2312Z
  We study extreme-ultraviolet emission line spectra derived from
  three-dimensional magnetohydrodynamic models of structures in the
  corona. In order to investigate the effects of increased magnetic
  activity at photospheric levels in a numerical experiment, a much
  higher magnetic flux density is applied at the photosphere as compared
  to the Sun. Thus, we can expect our results to highlight the differences
  between the Sun and more active, but still solar-like stars. We discuss
  signatures seen in extreme-ultraviolet emission lines synthesized from
  these models and compare them to observed signatures in the spatial
  distribution and temporal evolution of Doppler shifts in lines formed
  in the transition region and corona. This is of major interest to test
  the quality of the underlying magnetohydrodynamic model to heat the
  corona, i.e. currents in the corona driven by photospheric motions
  (flux braiding).

Title: Doppler shifts in the transition region and corona. Mass
    cycle between the chromosphere and the corona
Authors: Zacharias, P.; Bingert, S.; Peter, H.
Bibcode: 2009MmSAI..80..654Z
Altcode:
  Emission lines in the transition region and corona show persistent
  line shifts. It is a major challenge to understand the dynamics in
  the upper atmosphere and thus these line shifts, which are a signature
  of the mass cycle between the chromosphere and the corona. We examine
  EUV emission line profiles synthesized from a 3D MHD coronal model of
  a solar-like corona, in particular of an active region surrounded by
  strong chromospheric network. This allows us to investigate the physical
  processes leading to the line Doppler shifts, since we have access to
  both, the synthetic spectra and the physical parameters, i.e. magnetic
  field, temperature and density in the simulation box. By analyzing the
  evolution of the flows along field lines together with the changing
  magnetic structure we can investigate the mass cycle. We find evidence
  that loops are loaded with mass during a reconnection process, leading
  to upflows. After the loops disconnect from the reconnection site,
  they cool and drain which leads to the observed redshifts. Previous 1D
  loop models (neglecting the 3D nature) assumed that heating leads to
  evaporation and upflows followed by a cooling phase after the heating
  stops. The scenario modeled here is quite different, as it shows
  that the continuously changing three-dimensional magnetic structure
  is of pivotal importance to understand the mass balance between the
  chromosphere and the corona.

Title: Spectral Analysis of 3D MHD Models of Quiet Sun and Active
    Region Structures
Authors: Zacharias, P.; Bingert, S.; Peter, H.
Bibcode: 2008ESPM...12.3.39Z
Altcode:
  We study EUV emission line spectra derived from 3D MHD models
  of structures in the corona, in particular of an active region
  surrounded by a strong chromospheric network. The 3D MHD models account
  properly for the energy balance, especially for heat conduction and
  radiative losses. This allows us to reliably synthesize the profiles
  of EUV emission lines observable with current EUV spectrometers,
  e.g. SUMER/SOHO and EIS/Hinode. We investigate the temporal evolution
  and spatial distribution of the Doppler shifts of the EUV emission
  lines synthesized from these models. This is of major interest
  for the underlying mechanism of the heating of the solar corona,
  i.e. dissipation of currents in the corona driven by photospheric
  motions (flux braiding). Based on the 3D MHD models we can also derive
  intensity maps as they will be observed through the coronal channels
  of the AIA-instrument onboard SDO and we can analyze how to process the
  AIA maps in order to derive physical quantities such as temperatures and
  densities. Since we have access not only to the synthetic spectra, but
  also to the magnetic field in the box we can explore how the magnetic
  field relates to the emission in various coronal lines. In the present
  paper we will show results on the evolution of spectral properties
  such as line shifts or widths in different modeled coronal structures
  and compare these to results from SUMER/SOHO as well as EIS/Hinode data.

Title: On the Nature of Coronal Loops
Authors: Bingert, S.; Zacharias, P.; Peter, H.; Gudiksen, B.
Bibcode: 2008ESPM...12.3.29B
Altcode:
  The structure and dynamics of a box in a stellar corona can be
  modeled employing a 3D MHD model for different levels of magnetic
  activity. <P />Depending on the magnetic flux through the surface the
  nature of the resulting coronal structures can be quite different. <P
  />We will compare two different models of an active region, one for
  two basically isolated sunspots, and another one for two sunspots
  surrounded by magnetic field patches similar to the chromospheric
  network. <P />The current paradigm is that these loops follow magnetic
  field lines as pearls on a string, and thus the majority of present
  corona models describe structures following the field lines. <P />Our
  study challenges this paradigm by showing through a three-dimensional
  model that coronal structures in complex magnetic field geometries might
  appear loop-like while they are not aligned with the magnetic field. <P
  />Using a forward model approach, both models result in emission from
  the corona being concentrated in loop structures. <P />In the first case
  the loops seen in EUV and X-ray emission are following the magnetic
  field. <P />However, in the second case, where the magnetic field is
  far from a force-free state, the loops seen in X-ray emission do not
  follow the magnetic field, but are more related to the current sheets
  formed in response to the footpoint motions of the magnetic field. <P
  />This result is of interest especially for loops as found in areas
  where the magnetic field emerging from active regions interacts with
  the surrounding network or in the complex magnetic structures within
  chromospheric network patches.

Title: On the nature of coronal loops
Authors: Bingert, Sven; Zacharias, Pia; Peter, Hardi
Bibcode: 2008cosp...37..302B
Altcode: 2008cosp.meet..302B
  The structure and dynamics of a box in a stellar corona can be
  modeled employing a 3D MHD model for different levels of magnetic
  activity. In these models we account for the mass, momentum and energy
  balance including heat conduction and radiative losses. The heating
  is through current dissipation in the corona driven by photospheric
  motions (flux braiding). Depending on the magnetic flux through
  the surface the nature of the resulting coronal structures can be
  quite different. We will compare two different models of an active
  region, one for two basically isolated sunspots, and another one for
  two sunspots surrounded by strong magnetic field patches mimicking a
  strong chromospheric network. Both models result in emission from the
  model corona being concentrated in loop structures. In the first case
  the loops seen in EUV and X-ray emission are aligned with the magnetic
  field, following the general paradigm. However, in the second case,
  where the magnetic field is far from a force-free state, the loops seen
  in X-ray emission do not follow the magnetic field, but are related to
  the current sheets formed in response to the footpoint motions of the
  magnetic field. For the Sun this result is of interest especially for
  loops as found in areas where the magnetic field emerging from active
  regions interacts with the surrounding network. These models are now
  ready to face detailed comparisons with EUV spectroscopic observations
  from Hinode and imaging from SDO/AIA.

Title: 3D MHD models compared to EUV observations of quiet Sun and
    active region structures
Authors: Zacharias, Pia; Bingert, Sven; Peter, Hardi
Bibcode: 2008cosp...37.3579Z
Altcode: 2008cosp.meet.3579Z
  We study EUV emission line spectra derived from 3D MHD models of
  structures in the corona, in particular of an active region surrounded
  by a strong chromospheric network. The 3D MHD models account properly
  for the energy balance, especially for heat conduction and radiative
  losses, which allows us to reliably synthesize the profiles of
  EUV emission lines observable with current EUV spectrometers,
  i.e. SUMER/SOHO and EIS/Hinode. Thus we can directly compare these
  synthesized spectra to real observations with these instruments. We
  will discuss differences of models with different levels of magnetic
  activity in terms of signatures seen in EUV lines synthesized from
  these models and compare them to signatures found, e.g., in the spatial
  distribution and temporal evolution of Doppler shifts of lines formed
  in the transition region and corona. This is of major interest to test
  the quality of the underlying model to heat the corona, i.e. currents
  in the corona driven by photospheric motions (flux braiding). Based
  on the 3D MHD model we can also derive intensity maps as they will
  be observed through the coronal channels of the AIA-instrument on
  SDO. Through this we can provide some guidance on how to process the
  AIA maps in order to derive physical quantities such as temperature
  and density through a combination of filters.

Title: Asteroseismological Studies of Long-Period Variable Subdwarf
    B Stars. II. Two-Color Photometry of PG 1338+481
Authors: Randall, S. K.; Green, E. M.; Fontaine, G.; Brassard, P.;
   Terndrup, D. M.; Brown, N.; Fontaine, M.; Zacharias, P.; Chayer, P.
Bibcode: 2006ApJ...645.1464R
Altcode:
  We present the results of an observational campaign for the long-period
  variable subdwarf B star PG 1338+481. Seven continuous weeks of
  observing time at the Steward Observatory 1.55 m Kuiper telescope
  on Mount Bigelow, Arizona, and the 1.3 m MDM telescope at Kitt Peak
  rendered ~250 hr of simultaneous U/R time series photometry, as well as
  an extra ~70 hr of R-band-only data. The analysis of the combined light
  curves resulted in the extraction of 13 convincing periodicities in the
  2100-7200 s range, with amplitudes up to ~0.3% and ~0.2% in the U and
  R, respectively. Comparing the ratios of amplitudes in the two wave
  bands to those predicted from theory suggests the presence of dipole
  modes, a notion that is further supported by the period spacing between
  the highest amplitude peaks. If confirmed, this poses a challenge to
  current nonadiabatic theory. At the quantitative level, we find that the
  distribution of the observed period spectrum is highly nonuniform and
  much sparser than that predicted from a representative model. We provide
  a possible interpretation in the text. The asteroseismological analysis
  attempted for PG 1338+481 on the basis of six observed periodicities
  believed to constitute consecutive dipole modes renders encouraging
  results. Fixing the effective temperature and surface gravity to
  the spectroscopic estimates, we successfully isolate just one family
  of optimal models that can reproduce the measured periods to better
  than 1%. While the stellar parameters thus inferred must be regarded
  as preliminary, the achieved fit bodes well for future asteroseismic
  analyses of long-period variable subdwarf B stars. <P />Some of the
  observations reported here were obtained at the MMT Observatory, a joint
  facility of the University of Arizona and the Smithsonian Institution.

Title: Fluctuations in Babcock-Leighton Dynamos. I. Period Doubling
    and Transition to Chaos
Authors: Charbonneau, Paul; St-Jean, Cédric; Zacharias, Pia
Bibcode: 2005ApJ...619..613C
Altcode:
  We present a large series of numerical simulations of the solar
  magnetic activity cycle based on the Babcock-Leighton mechanism for the
  regeneration of the solar poloidal magnetic field. While the primary
  cycle period changes very little as the dynamo number is increased,
  the model shows a well-defined transition to chaos through a sequence
  of period-doubling bifurcations, i.e., the sequential appearance of
  modulations of the primary cycle's amplitude, with associated periods
  equal to twice the periods characterizing the amplitude variations prior
  to a given bifurcation. This behavior arises through the unavoidable
  time delay built into this type of solar dynamo model, rather than
  through the effects of complex, nonlinear magnetic back-reaction on
  the fluid motions driving the dynamo process. It is noteworthy that
  a chaotic regime exists in this numerical model, given that the only
  nonlinearity present is a simple algebraic amplitude-quenching factor in
  one of the governing partial differential equations. The results also
  represent a rare instance in which the complex dynamical behavior of a
  spatially extended, diffusive solar dynamo model can be reproduced in
  detail on the basis of the simplest of low-order dynamical systems,
  namely a one-dimensional iterative map. The numerical results also
  demonstrate the central role of meridional circulation in setting the
  primary cycle period in this class of dynamo models; despite variations
  by many orders of magnitude in the dynamo number and concomitant large
  and sometimes even chaotic variations in amplitude, the cycle period
  remains tightly locked to the meridional circulation turnover time.

