Author name code: rempel
ADS astronomy entries on 2022-09-14
author:"Rempel, Matthias" 
------------------------------------------------------------------------  

Title: Chromospheric extension of the MURaM code
Authors: Przybylski, D.; Cameron, R.; Solanki, S. K.; Rempel, M.;
   Leenaarts, J.; Anusha, L. S.; Witzke, V.; Shapiro, A. I.
Bibcode: 2022A&A...664A..91P
Altcode: 2022arXiv220403126P
  Context. Detailed numerical models of the chromosphere and corona are
  required to understand the heating of the solar atmosphere. An accurate
  treatment of the solar chromosphere is complicated by the effects
  arising from non-local thermodynamic equilibrium (NLTE) radiative
  transfer. A small number of strong, highly scattering lines dominate the
  cooling and heating in the chromosphere. Additionally, the recombination
  times of ionised hydrogen are longer than the dynamical timescales,
  requiring a non-equilibrium (NE) treatment of hydrogen ionisation. <BR
  /> Aims: We describe a set of necessary additions to the MURaM code that
  allow it to handle some of the important NLTE effects. We investigate
  the impact on solar chromosphere models caused by NLTE and NE effects in
  radiation magnetohydrodynamic simulations of the solar atmosphere. <BR
  /> Methods: The MURaM code was extended to include the physical
  process required for an accurate simulation of the solar chromosphere,
  as implemented in the Bifrost code. This includes a time-dependent
  treatment of hydrogen ionisation, a scattering multi-group radiation
  transfer scheme, and approximations for NLTE radiative cooling. <BR />
  Results: The inclusion of NE and NLTE physics has a large impact on the
  structure of the chromosphere; the NE treatment of hydrogen ionisation
  leads to a higher ionisation fraction and enhanced populations in
  the first excited state throughout cold inter-shock regions of the
  chromosphere. Additionally, this prevents hydrogen ionisation from
  buffering energy fluctuations, leading to hotter shocks and cooler
  inter-shock regions. The hydrogen populations in the ground and first
  excited state are enhanced by 10<SUP>2</SUP>-10<SUP>3</SUP> in the
  upper chromosphere and by up to 10<SUP>9</SUP> near the transition
  region. <BR /> Conclusions: Including the necessary NLTE physics
  leads to significant differences in chromospheric structure and
  dynamics. The thermodynamics and hydrogen populations calculated using
  the extended version of the MURaM code are consistent with previous
  non-equilibrium simulations. The electron number and temperature
  calculated using the non-equilibrium treatment of the chromosphere
  are required to accurately synthesise chromospheric spectral
  lines. <P />Movies associated to Fig. 2 are only available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202141230/olm">https://www.aanda.org</A>

Title: Rapid Blue- and Red-shifted Excursions in H$\alpha$ line
    profiles synthesized from realistic 3D MHD simulations
Authors: Danilovic, S.; Bjørgen, J. P.; Leenaarts, J.; Rempel, M.
Bibcode: 2022arXiv220813749D
Altcode:
  Rapid blue- and red-shifted events (RBEs/RREs) may have an important
  role in mass-loading and heating the solar corona, but their nature
  and origin are still debatable. We aim to model these features to
  learn more about their properties, formation and origin. A realistic
  three-dimensional (3D) magneto-hydrodynamic (MHD) model of a solar
  plage region is created. Synthetic H$\alpha$ spectra are generated
  and the spectral signatures of these features are identified. The
  magnetic field lines associated with these events are traced and the
  underlying dynamic is studied. The model reproduces well many properties
  of RBEs and RREs, such as spatial distribution, lateral movement,
  length and lifetimes. Synthetic H$\alpha$ line profiles, similarly to
  observed ones, show strong blue- or red-shift and asymmetries. These
  line profiles are caused by the vertical component of velocity with
  magnitudes larger than $30-40$ km/s that appear mostly in the height
  range of $2-4$ Mm. By tracing magnetic field lines, we show that the
  vertical velocity that causes the appearance of RBE/RREs to appear is
  always associated with the component of velocity perpendicular to the
  magnetic field line. The study confirms the hypothesis that RBEs and
  RREs are signs of Alfv{é}nic waves with, in some cases, a significant
  contribution from slow magneto-acoustic mode.

Title: Effects of spectral resolution on simple magnetic field
    diagnostics of the Mg II h &amp; k lines
Authors: Centeno, Rebecca; Rempel, Matthias; Casini, Roberto; del
   Pino Aleman, Tanausu
Bibcode: 2022arXiv220807507C
Altcode:
  We study the effects of finite spectral resolution on the magnetic
  field values retrieved through the weak field approximation (WFA)
  from the cores of the Mg II h &amp; k lines. The retrieval of the
  line-of-sight (LOS) component of the magnetic field, $B_{\rm LOS}$,
  from synthetic spectra generated in a uniformly magnetized FAL-C
  atmosphere are accurate when restricted to the inner lobes of Stokes
  V. As we degrade the spectral resolution, partial redistribution (PRD)
  effects, that more prominently affect the outer lobes of Stokes V,
  are brought into the line core through spectral smearing, degrading the
  accuracy of the WFA and resulting in an inference bias, which is more
  pronounced the poorer the resolution. When applied to a diverse set of
  spectra emerging from a sunspot simulation, we find a good accuracy
  in the retrieved $B_{\rm LOS}$ when comparing it to the model value
  at the height where the optical depth in the line core is unity. The
  accuracy is preserved up to field strengths of B~1500 G. Limited
  spectral resolution results in a small bias toward weaker retrieved
  fields. The WFA for the transverse component of the magnetic field is
  also evaluated. Reduced spectral resolution degrades the accuracy of
  the inferences because spectral mixing results in the line effectively
  probing deeper layers of the atmosphere.

Title: Derivation of Boundary Conditions for Data-Driven Simulations
    of Active Regions and their Emission
Authors: Tremblay, Benoit; Malanushenko, Anna; Rempel, Matthias;
   Kazachenko, Maria
Bibcode: 2022cosp...44.2472T
Altcode:
  Coronal heating remains a major area of research in solar physics. In
  particular, the spatial dimensions and the structuring of heating
  processes have yet to be fully understood. Whereas observations suggest
  that plasma is heated in bundles of thin flux tubes, it's been theorized
  from simulations that emission in active regions can be structured
  in larger flux tubes with irregular boundaries. In the latter case,
  the emission can appear like the emission from loop bundles, with
  variations of the column depth at their boundaries causing an impression
  of individual loops. These scenarios have distinct implications for
  coronal heating and the study of coronal loops and thus need to be
  confirmed observationally. Our objective is to develop insight into
  the spatial properties of solar coronal heating using a statistical
  analysis of the emission from observed and simulated active regions. To
  this end, we perform data-driven MHD simulations of active regions. The
  MURaM simulation is being modified to work with photospheric inputs
  as boundary conditions, including observed vector magnetograms, and
  electric field maps and flow maps inferred from observations. We
  focus on electric field maps derived using the PDFI\_SS inversion
  technique and flow maps derived through supervised deep learning. More
  specifically, we train a convolutional neural network to emulate the
  MURaM simulation and infer MURaM-like flows from observational data,
  including large-scale flows in the granulation surrounding active
  regions. We present derivations of boundary conditions (i.e., electric
  field maps, flows maps) from SDO/HMI observations of selected active
  regions, and discuss the limitations and challenges associated with
  the methods. We detail ongoing efforts in driving the MURaM simulation
  from derived boundary conditions. Finally, we illustrate how these
  data-driven simulations will be used to study the structuring of the
  emission of active regions statistically and identify which scenario
  of coronal heating best matches observations.

Title: Predicted appearance of Magnetic Flux Rope and Sheared Magnetic
    Arcade Structures before a Coronal Mass Ejection via three-dimensional
    radiative Magnetohydrodynamic Modeling
Authors: Chintzoglou, Georgios; Cheung, Mark; Rempel, Matthias
Bibcode: 2022cosp...44.2406C
Altcode:
  Magnetic Flux Ropes (MFRs) are free-energy-carrying, three-dimensional
  magnetized plasma structures characterized by twisted magnetic field
  lines and are widely considered the core structure of Coronal Mass
  Ejections (CMEs) propagating in the interplanetary space. The way MFRs
  form remains unclear as different theories predict that either MFRs
  form during the initiation of the CME or pre-exist the onset of the
  CME. The term "pre-existing structure" is synonymous with "filament
  channels." On the one hand, the theories predicting on-the-fly MFR
  formation require Sheared Magnetic Arcades (SMAs; low twist but
  stressed magnetic structures) for the filament channel/pre-existing
  magnetic structure of CMEs. On the other hand, a growing number of
  works using SDO/AIA observations (combined with non-linear force-free
  extrapolations; NLFFF) suggest that MFRs may be the form of filament
  channels, therefore pre-existing the CME eruption. However, due to
  the inability to routinely measure the 3D magnetic field in the solar
  atmosphere, we cannot unambiguously interpret optical and EUV imaging
  observations as projected on the plane of the sky. Therefore, a raging
  debate on the nature of the pre-eruptive structure continues. It is
  also possible that the filament channel/pre-eruptive structure evolves
  from SMA to MFR slowly, further complicating the distinction between
  these two types of structures in the solar observations. This work
  presents realistic simulated optical and EUV observations synthesized
  on a time-evolving radiative MURaM MHD model at different times
  along the slow evolution of an SMA converting to an MFR. We discuss
  the implications of our results in the context of filament channel
  formation and CME initiation theory.

Title: Acoustic-gravity wave propagation characteristics in 3D
    radiation hydrodynamic simulations of the solar atmosphere
Authors: Fleck, Bernhard; Khomenko, Elena; Carlsson, Mats; Rempel,
   Matthias; Steiner, Oskar; Riva, Fabio; Vigeesh, Gangadharan
Bibcode: 2022cosp...44.2503F
Altcode:
  There has been tremendous progress in the degree of realism of
  three-dimensional radiation magneto-hydrodynamic simulations of the
  solar atmosphere in the past decades. Four of the most frequently
  used numerical codes are Bifrost, CO5BOLD, MANCHA3D, and MURaM. Here
  we test and compare the wave propagation characteristics in model
  runs from these four codes by measuring the dispersion relation
  of acoustic-gravity waves at various heights. We find considerable
  differences between the various models.

Title: A Statistical Approach to Study Fine Structure of EUV Emission
    in Active Regions
Authors: Malanushenko, Anna; Rempel, Matthias; Tremblay, Benoit;
   Kazachenko, Maria
Bibcode: 2022cosp...44.2526M
Altcode:
  Heating of the solar corona is one of the major problems in solar
  physics, and spatial dimension and structuring of the processes involved
  in heating are yet to be understood. Observations of the numerous
  thin coronal loops above active regions (ARs) suggest that coronal
  heating itself is highly variable on small scales, heating plasma in
  collections of thin flux tubes. It has recently been theorized, based
  on simulations, that emitting plasma in ARs can also be structured in
  larger flux tubes with irregular boundaries. The emission of these large
  flux tubes can appear like emission of loop bundles, with variations of
  the column depth at their boundaries causing an impression of individual
  loops. This "coronal veil" theory was argued to be a more general
  scenario, which better explains AR emission properties than previous
  models. If confirmed observationally, it will have a large impact
  on coronal heating studies, suggesting that existing measurements of
  temperature and density in coronal loops may need to be reevaluated. The
  observational validation of this hypothesis is as important as it is
  difficult. For a given coronal loop, it is difficult to tell whether it
  is a compact feature or a projection artifact. In this talk, we propose
  a new statistical approach to address this problem. Instead of trying
  to analyze each loop individually, we focus on scaling relationship
  between a number of loops in a given AR and the AR's total brightness in
  a given wavelength. We argue that these two quantities are related by a
  power law. We demonstrate in theoretical calculations how the power law
  coefficients will differ depending on whether the emission is structured
  into (a) compact features, (b) large features with irregular boundaries,
  or (c) extended and thin veil-like features. We demonstrate that these
  power laws exist in observations and discuss numerical experiments
  which may help us to determine which of these scenarios, if any,
  best describes observations. We further describe the observational
  statistics that can, in conjunction with numerical experiments, help
  us understand which of these scenarios take place in the Sun. We also
  present the first results from our project to collect these data.

Title: Heating of the solar chromosphere through current dissipation
Authors: da Silva Santos, J. M.; Danilovic, S.; Leenaarts, J.; de
   la Cruz Rodríguez, J.; Zhu, X.; White, S. M.; Vissers, G. J. M.;
   Rempel, M.
Bibcode: 2022A&A...661A..59D
Altcode: 2022arXiv220203955D
  Context. The solar chromosphere is heated to temperatures higher than
  predicted by radiative equilibrium. This excess heating is greater
  in active regions where the magnetic field is stronger. <BR />
  Aims: We aim to investigate the magnetic topology associated with an
  area of enhanced millimeter (mm) brightness temperatures in a solar
  active region mapped by the Atacama Large Millimeter/submillimeter
  Array (ALMA) using spectropolarimetric co-observations with the 1-m
  Swedish Solar Telescope (SST). <BR /> Methods: We used Milne-Eddington
  inversions, nonlocal thermodynamic equilibrium (non-LTE) inversions,
  and a magnetohydrostatic extrapolation to obtain constraints on the
  three-dimensional (3D) stratification of temperature, magnetic field,
  and radiative energy losses. We compared the observations to a snapshot
  of a magnetohydrodynamics simulation and investigate the formation
  of the thermal continuum at 3 mm using contribution functions. <BR />
  Results: We find enhanced heating rates in the upper chromosphere of up
  to ∼5 kW m<SUP>−2</SUP>, where small-scale emerging loops interact
  with the overlying magnetic canopy leading to current sheets as shown
  by the magnetic field extrapolation. Our estimates are about a factor
  of two higher than canonical values, but they are limited by the ALMA
  spatial resolution (∼1.2″). Band 3 brightness temperatures reach
  about ∼10<SUP>4</SUP> K in the region, and the transverse magnetic
  field strength inferred from the non-LTE inversions is on the order
  of ∼500 G in the chromosphere. <BR /> Conclusions: We are able to
  quantitatively reproduce many of the observed features including the
  integrated radiative losses in our numerical simulation. We conclude
  that the heating is caused by dissipation in current sheets. However,
  the simulation shows a complex stratification in the flux emergence
  region where distinct layers may contribute significantly to the
  emission in the mm continuum. <P />The movie is available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202243191/olm">https://www.aanda.org</A>

Title: The Coronal Veil
Authors: Malanushenko, A.; Cheung, M. C. M.; DeForest, C. E.; Klimchuk,
   J. A.; Rempel, M.
Bibcode: 2022ApJ...927....1M
Altcode: 2021arXiv210614877M
  Coronal loops, seen in solar coronal images, are believed to
  represent emission from magnetic flux tubes with compact cross
  sections. We examine the 3D structure of plasma above an active
  region in a radiative magnetohydrodynamic simulation to locate volume
  counterparts for coronal loops. In many cases, a loop cannot be linked
  to an individual thin strand in the volume. While many thin loops are
  present in the synthetic images, the bright structures in the volume
  are fewer and of complex shape. We demonstrate that this complexity
  can form impressions of thin bright loops, even in the absence of thin
  bright plasma strands. We demonstrate the difficulty of discerning
  from observations whether a particular loop corresponds to a strand in
  the volume, or a projection artifact. We demonstrate how apparently
  isolated loops could deceive observers, even when observations from
  multiple viewing angles are available. While we base our analysis
  on a simulation, the main findings are independent from a particular
  simulation setup and illustrate the intrinsic complexity involved in
  interpreting observations resulting from line-of-sight integration
  in an optically thin plasma. We propose alternative interpretation
  for strands seen in Extreme Ultraviolet images of the corona. The
  "coronal veil" hypothesis is mathematically more generic, and
  naturally explains properties of loops that are difficult to address
  otherwise-such as their constant cross section and anomalously high
  density scale height. We challenge the paradigm of coronal loops as
  thin magnetic flux tubes, offering new understanding of solar corona,
  and by extension, of other magnetically confined bright hot plasmas.

Title: The effect of small-scale magnetic fields on stellar convection
    and activity
Authors: Rempel, M.
Bibcode: 2022fysr.confE..39R
Altcode:
  The Sun is a unique star in the sense that we can observe it at high
  resolution and study phenomena at a detail that is hidden in stellar
  observations. This applies specifically to small-scale magnetic fields
  that are organized on the stellar surface on scale of granulation and
  smaller. Significant progress over the past 10-20 years in both solar
  observations and modeling through small-scale dynamo simulations point
  to a small-scale field of a large enough strength to have a dynamical
  impact on convection, differential rotation as well as large-scale
  magnetic activity. In this talk I will highlight lessons learned
  from the Sun that may have a broader impact on understanding stellar
  convection and magnetism.

Title: Probing the Physics of the Solar Atmosphere with the Multi-slit
    Solar Explorer (MUSE). II. Flares and Eruptions
Authors: Cheung, Mark C. M.; Martínez-Sykora, Juan; Testa, Paola;
   De Pontieu, Bart; Chintzoglou, Georgios; Rempel, Matthias; Polito,
   Vanessa; Kerr, Graham S.; Reeves, Katharine K.; Fletcher, Lyndsay; Jin,
   Meng; Nóbrega-Siverio, Daniel; Danilovic, Sanja; Antolin, Patrick;
   Allred, Joel; Hansteen, Viggo; Ugarte-Urra, Ignacio; DeLuca, Edward;
   Longcope, Dana; Takasao, Shinsuke; DeRosa, Marc L.; Boerner, Paul;
   Jaeggli, Sarah; Nitta, Nariaki V.; Daw, Adrian; Carlsson, Mats; Golub,
   Leon; The
Bibcode: 2022ApJ...926...53C
Altcode: 2021arXiv210615591C
  Current state-of-the-art spectrographs cannot resolve the fundamental
  spatial (subarcseconds) and temporal (less than a few tens of
  seconds) scales of the coronal dynamics of solar flares and eruptive
  phenomena. The highest-resolution coronal data to date are based on
  imaging, which is blind to many of the processes that drive coronal
  energetics and dynamics. As shown by the Interface Region Imaging
  Spectrograph for the low solar atmosphere, we need high-resolution
  spectroscopic measurements with simultaneous imaging to understand the
  dominant processes. In this paper: (1) we introduce the Multi-slit Solar
  Explorer (MUSE), a spaceborne observatory to fill this observational
  gap by providing high-cadence (&lt;20 s), subarcsecond-resolution
  spectroscopic rasters over an active region size of the solar transition
  region and corona; (2) using advanced numerical models, we demonstrate
  the unique diagnostic capabilities of MUSE for exploring solar coronal
  dynamics and for constraining and discriminating models of solar flares
  and eruptions; (3) we discuss the key contributions MUSE would make
  in addressing the science objectives of the Next Generation Solar
  Physics Mission (NGSPM), and how MUSE, the high-throughput Extreme
  Ultraviolet Solar Telescope, and the Daniel K Inouye Solar Telescope
  (and other ground-based observatories) can operate as a distributed
  implementation of the NGSPM. This is a companion paper to De Pontieu
  et al., which focuses on investigating coronal heating with MUSE.

Title: Convolutional Neural Networks and Stokes Response Functions
Authors: Centeno, Rebecca; Flyer, Natasha; Mukherjee, Lipi; Egeland,
   Ricky; Casini, Roberto; del Pino Alemán, Tanausú; Rempel, Matthias
Bibcode: 2022ApJ...925..176C
Altcode: 2021arXiv211203802C
  In this work, we study the information content learned by
  a convolutional neural network (CNN) when trained to carry out the
  inverse mapping between a database of synthetic Ca II intensity spectra
  and the vertical stratification of the temperature of the atmospheres
  used to generate such spectra. In particular, we evaluate the ability
  of the neural network to extract information about the sensitivity of
  the spectral line to temperature as a function of height. By training
  the CNN on sufficiently narrow wavelength intervals across the Ca
  II spectral profiles, we find that the error in the temperature
  prediction shows an inverse relationship to the response function
  of the spectral line to temperature, that is, different regions of
  the spectrum yield a better temperature prediction at their expected
  regions of formation. This work shows that the function that the CNN
  learns during the training process contains a physically meaningful
  mapping between wavelength and atmospheric height.

Title: A solar coronal loop in a box: Energy generation and heating
Authors: Breu, C.; Peter, H.; Cameron, R.; Solanki, S. K.; Przybylski,
   D.; Rempel, M.; Chitta, L. P.
Bibcode: 2022A&A...658A..45B
Altcode: 2021arXiv211211549B
  Context. Coronal loops are the basic building block of the upper solar
  atmosphere as seen in the extreme UV and X-rays. Comprehending how
  these are energized, structured, and evolve is key to understanding
  stellar coronae. <BR /> Aims: Here we investigate how the energy
  to heat the loop is generated by photospheric magneto-convection,
  transported into the upper atmosphere, and how the internal
  structure of a coronal magnetic loop forms. <BR /> Methods: In a 3D
  magnetohydrodynamics model, we study an isolated coronal loop rooted
  with both footpoints in a shallow layer within the convection zone
  using the MURaM code. To resolve its internal structure, we limited
  the computational domain to a rectangular box containing a single
  coronal loop as a straightened magnetic flux tube. Field-aligned heat
  conduction, gray radiative transfer in the photosphere and chromosphere,
  and optically thin radiative losses in the corona were taken into
  account. The footpoints were allowed to interact self-consistently
  with the granulation surrounding them. <BR /> Results: The loop is
  heated by a Poynting flux that is self-consistently generated through
  small-scale motions within individual magnetic concentrations in
  the photosphere. Turbulence develops in the upper layers of the
  atmosphere as a response to the footpoint motions. We see little
  sign of heating by large-scale braiding of magnetic flux tubes
  from different photospheric concentrations at a given footpoint. The
  synthesized emission, as it would be observed by the Atmospheric Imaging
  Assembly or the X-Ray Telescope, reveals transient bright strands that
  form in response to the heating events. Overall, our model roughly
  reproduces the properties and evolution of the plasma as observed
  within (the substructures of) coronal loops. <BR /> Conclusions:
  With this model we can build a coherent picture of how the energy
  flux to heat the upper atmosphere is generated near the solar surface
  and how this process drives and governs the heating and dynamics of
  a coronal loop. <P />Movie associated to Fig. 2 is available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202141451/olm">https://www.aanda.org</A>

Title: Probing the Physics of the Solar Atmosphere with the Multi-slit
    Solar Explorer (MUSE). I. Coronal Heating
Authors: De Pontieu, Bart; Testa, Paola; Martínez-Sykora, Juan;
   Antolin, Patrick; Karampelas, Konstantinos; Hansteen, Viggo; Rempel,
   Matthias; Cheung, Mark C. M.; Reale, Fabio; Danilovic, Sanja; Pagano,
   Paolo; Polito, Vanessa; De Moortel, Ineke; Nóbrega-Siverio, Daniel;
   Van Doorsselaere, Tom; Petralia, Antonino; Asgari-Targhi, Mahboubeh;
   Boerner, Paul; Carlsson, Mats; Chintzoglou, Georgios; Daw, Adrian;
   DeLuca, Edward; Golub, Leon; Matsumoto, Takuma; Ugarte-Urra, Ignacio;
   McIntosh, Scott W.; the MUSE Team
Bibcode: 2022ApJ...926...52D
Altcode: 2021arXiv210615584D
  The Multi-slit Solar Explorer (MUSE) is a proposed mission composed of
  a multislit extreme ultraviolet (EUV) spectrograph (in three spectral
  bands around 171 Å, 284 Å, and 108 Å) and an EUV context imager (in
  two passbands around 195 Å and 304 Å). MUSE will provide unprecedented
  spectral and imaging diagnostics of the solar corona at high spatial
  (≤0.″5) and temporal resolution (down to ~0.5 s for sit-and-stare
  observations), thanks to its innovative multislit design. By obtaining
  spectra in four bright EUV lines (Fe IX 171 Å, Fe XV 284 Å, Fe XIX-Fe
  XXI 108 Å) covering a wide range of transition regions and coronal
  temperatures along 37 slits simultaneously, MUSE will, for the first
  time, "freeze" (at a cadence as short as 10 s) with a spectroscopic
  raster the evolution of the dynamic coronal plasma over a wide range of
  scales: from the spatial scales on which energy is released (≤0.″5)
  to the large-scale (~170″ × 170″) atmospheric response. We use
  numerical modeling to showcase how MUSE will constrain the properties of
  the solar atmosphere on spatiotemporal scales (≤0.″5, ≤20 s) and
  the large field of view on which state-of-the-art models of the physical
  processes that drive coronal heating, flares, and coronal mass ejections
  (CMEs) make distinguishing and testable predictions. We describe the
  synergy between MUSE, the single-slit, high-resolution Solar-C EUVST
  spectrograph, and ground-based observatories (DKIST and others), and
  the critical role MUSE plays because of the multiscale nature of the
  physical processes involved. In this first paper, we focus on coronal
  heating mechanisms. An accompanying paper focuses on flares and CMEs.

Title: Probing the Physics of the Solar Atmosphere with the Multi-slit
Solar Explorer (MUSE): II. Flares and Eruptions
Authors: Cheung, Chun Ming Mark; Martinez-Sykora, Juan; Testa, Paola;
   De Pontieu, Bart; Chintzoglou, Georgios; Rempel, Matthias; Polito,
   Vanessa; Kerr, Graham; Reeves, Katharine; Fletcher, Lyndsay; Jin,
   Meng; Nobrega, Daniel; Danilovic, Sanja; Antolin, Patrick; Allred,
   Joel; Hansteen, Viggo; Ugarte-Urra, Ignacio; DeLuca, Edward; Longcope,
   Dana; Takasao, Shinsuke; DeRosa, Marc; Boerner, Paul; Jaeggli, Sarah;
   Nitta, Nariaki; Daw, Adrian; Carlsson, Mats; Golub, Leon
Bibcode: 2021AGUFMSH51A..08C
Altcode:
  Current state-of-the-art spectrographs cannot resolve the fundamental
  spatial (sub-arcseconds) and temporal scales (less than a few tens
  of seconds) of the coronal dynamics of solar flares and eruptive
  phenomena. The highest resolution coronal data to date are based on
  imaging, which is blind to many of the processes that drive coronal
  energetics and dynamics. As shown by IRIS for the low solar atmosphere,
  we need high-resolution spectroscopic measurements with simultaneous
  imaging to understand the dominant processes. In this paper: (1)
  we introduce the Multi-slit Solar Explorer (MUSE), a spaceborne
  observatory to fill this observational gap by providing high-cadence
  (&lt;20 s), sub-arcsecond resolution spectroscopic rasters over an
  active region size of the solar transition region and corona; (2)
  using advanced numerical models, we demonstrate the unique diagnostic
  capabilities of MUSE for exploring solar coronal dynamics, and for
  constraining and discriminating models of solar flares and eruptions;
  (3) we discuss the key contributions MUSE would make in addressing the
  science objectives of the Next Generation Solar Physics Mission (NGSPM),
  and how MUSE, the high-throughput EUV Solar Telescope (EUVST) and the
  Daniel K Inouye Solar Telescope (and other ground-based observatories)
  can operate as a distributed implementation of the NGSPM. This is a
  companion paper to De Pontieu et al. (2021, also submitted to SH-17),
  which focuses on investigating coronal heating with MUSE.

Title: Solar Atmosphere Radiative Transfer Model Comparison based
    on 3D MHD Simulations
Authors: Haberreiter, Margit; Criscuoli, Serena; Rempel, Matthias;
   Mendes Domingos Pereira, Tiago
Bibcode: 2021AGUFMSH43A..06H
Altcode:
  The reconstruction of the solar spectral irradiance (SSI) on various
  time scales is essential for the understanding of the Earths climate
  response to the SSI variability. The driver of the SSI variability
  is understood to be the intensity contrast of magnetic features
  present on the Sun with respect to the largely non-magnetic quiet
  Sun. However, different spectral synthesis codes lead to diverging
  projections of SSI variability. We present a study in which we compare
  three different radiative transfer codes and carry out a a detailed
  analysis of their performance. We perform the spectral synthesis at
  the continuum wavelength of 665 nm with the Code for Solar Irradiance
  (COSI), and the Rybicki-Hummer (RH), and Max Planck University of
  Chicago Radiative MHD (MURaM) codes for three 3D MHD simulations
  snapshots, a non-magnetic case, and MHD simulations with 100 G, and 200
  G magnetic field strength. We determine the intensity distributions, the
  intensity differences and ratios for the spectral synthesis codes. We
  identify that the largest discrepancies originate in the intergranular
  lanes where the most field concentration occurs. Overall, the applied
  radiative transfer codes give consistent intensity distributions. Also,
  the intensity variation as a function of magnetic field strength for
  the particular 100 G and 200 G snapshots agree within the 2-3% range.

Title: Analyzing the Structure of Coronal Loops in MURaM Radiation
    MHD Simulations
Authors: David, Mia; Rempel, Matthias; Malanushenko, Anna
Bibcode: 2021AGUFMSH45B2377D
Altcode:
  Coronal loops are emission features that trace out parts of the solar
  magnetic field in the corona, and as such they provide important
  information about the magnetic and plasma structure of the solar
  corona. Their thermal substructure is still an open question: their
  thickness is at the limit of resolution of the instruments observing
  them, and higher resolution instruments tend to find finer strands. This
  raises the question whether the finest strands are resolved with the
  currently available highest resolution instruments. In this project,
  we address this from a modeling perspective and look to answer the
  following questions. Does the number of strands identified in synthetic
  observations depend on the resolution of the numerical simulation? How
  many strands remain hidden in current observations that may otherwise
  be evident in future higher resolution observations? We look at
  simulations done with MURaM code of a bipolar active region that
  are available at three different numerical resolutions. We emulate
  observables at various resolutions, including one which exceeds that of
  current instruments. We synthesize data in resolution of Atmospheric
  Imaging Assembly onboard Solar Dynamics Observatory (SDO/AIA) and
  High-Resolution Coronal Imager (HiC). We find that the number of
  strands found in synthetic AIA does not depend on the resolution of
  the simulation, and that it is a small fraction of the strands found
  in the native resolution of the simulation. The number of strands
  seen in synthetic HiC data is a factor of 2-4 higher than that in
  synthetic AIA, and increases moderately with the resolution of the
  simulation. We compare the results with observations by studying an
  active region observed by AIA. We study the dependence of the number
  of loops counted on the viewing angle in both synthetic and observable
  data. We also report statistical properties of these strands.

Title: Modeling the Solar Atmosphere: From quiet Sun to Flares
Authors: Rempel, Matthias
Bibcode: 2021AGUFMSH43A..01R
Altcode:
  Comparison of models and observations requires simulations with a
  sufficient degree of realism, ideally simulations that allow for the
  computation of synthetic observables. This realism is in general
  a compromise between the sophistication of implemented physics,
  numerical resolution, extent of the spatial and temporal domain
  (including dimensionality and boundary conditions) as well as the
  initial state simulations are started from. I will review selected
  simulation results from the past decade that strike the balanced
  for realism in different ways, discuss their limitations and avenues
  for future improvement. These simulations will encompass the range
  from detailed studies of quiet Sun magnetism to active region
  scale simulations including the lower solar corona and flares. I
  will end this talk with more general remarks on challenges of
  simulation-observation comparison and challenges from the evolving
  compute and data infrastructure. Computing platforms continue to
  rely more heavily on GPUs and the availability of computing resources
  outpaces data storage capabilities. This requires in the future more
  scalable and integrated computation and data analysis pipelines that
  rely less on the storage of intermediate data products.

Title: A Statistical Approach to Study Spatial Characteristics of
    EUV Emission in Active Regions
Authors: Malanushenko, Anna; Egeland, Ricky; Kazachenko, Maria;
   Rempel, Matthias; Tremblay, Benoit
Bibcode: 2021AGUFMSH45B2360M
Altcode:
  Heating of the solar corona is one of the major problems in solar
  physics, and spatial dimension and structuring of the processes involved
  in heating are yet to be understood. Observations of the numerous
  thin coronal loops above active regions (ARs) suggest that coronal
  heating itself is highly variable on small scales, heating plasma in
  collections of thin flux tubes. It has recently been theorized, based
  on simulations, that emitting plasma in ARs can also be structured in
  larger flux tubes with irregular boundaries. The emission of these large
  flux tubes can appear like emission of loop bundles, with variations
  of the column depth at their boundaries causing an impression of
  individual loops. This "coronal veil" theory was argued to be a
  more general scenario, which better explains AR emission properties
  than previous models. If confirmed observationally, it will have a
  large impact on coronal heating studies, suggesting that existing
  measurements of temperature and density in coronal loops may need to
  be reevaluated. The observational validation of this hypothesis is as
  important as it is difficult. For a given coronal loop, it is difficult
  to tell whether it is a compact feature or a projection artifact. In
  this talk, we propose a new statistical approach to address this
  problem. Instead of trying to analyze each loop individually, we focus
  on scaling relationship between a number of loops in a given AR and the
  AR's total brightness in a given wavelength. We argue that these two
  quantities are related by a power law. We demonstrate in theoretical
  calculations how the power law coefficients will differ depending on
  whether the emission is structured into (a) compact features, (b)
  large features with irregular boundaries, or (c) extended and thin
  veil-like features. We demonstrate that these power laws exist in
  observations and discuss numerical experiments which may help us to
  determine which of these scenarios, if any, best describes observations.

Title: Visualizing the Solar Corona in Virtual Reality
Authors: Wolff, Milana; Dima, Gabriel; Rempel, Matthias; Lacatus,
   Daniela; Paraschiv, Alin; Lecinski, Alice; Malanushenko, Anna
Bibcode: 2021AGUFMSH45B2365W
Altcode:
  This work presents novel visualizations of the optically thin solar
  corona in a virtual reality environment created using the Unity
  development platform. Unity enables fast rendering and interaction
  with three dimensional datasets in an immersive setting. We depict
  data derived from coronal simulations generated by radiative
  magnetohydrodynamic MURaM. These visualizations represent synthetic
  emissivity values computed for a variety of coronal emission lines
  using high-resolution, time-dependent thermodynamic and magnetic
  datasets. Users can enter the virtual environment, accessible on desktop
  and mobile devices or with a virtual reality head-mounted display
  (such as Oculus or Vive headsets) and observe and interact with both
  static and dynamic structures in the solar corona from arbitrary
  vantage points. These types of direct interaction techniques with
  simulated large-scale structures enhance intuitive understanding of
  solar dynamics. We welcome ideas from the community on how to further
  leverage this technology.

Title: Efficient Numerical Treatment of Ambipolar and Hall Drift as
    Hyperbolic System
Authors: Rempel, M.; Przybylski, D.
Bibcode: 2021ApJ...923...79R
Altcode: 2021arXiv211113811R
  Partially ionized plasmas, such as the solar chromosphere, require
  a generalized Ohm's law including the effects of ambipolar and
  Hall drift. While both describe transport processes that arise
  from the multifluid equations and are therefore of hyperbolic
  nature, they are often incorporated in models as a diffusive, i.e.,
  parabolic process. While the formulation as such is easy to include
  in standard MHD models, the resulting diffusive time-step constraints
  do require often a computationally more expensive implicit treatment
  or super-time-stepping approaches. In this paper we discuss an
  implementation that retains the hyperbolic nature and allows for an
  explicit integration with small computational overhead. In the case of
  ambipolar drift, this formulation arises naturally by simply retaining
  a time derivative of the drift velocity that is typically omitted. This
  alone leads to time-step constraints that are comparable to the native
  MHD time-step constraint for a solar setup including the region from
  photosphere to lower solar corona. We discuss an accelerated treatment
  that can further reduce time-step constraints if necessary. In the case
  of Hall drift we propose a hyperbolic formulation that is numerically
  similar to that for the ambipolar drift and we show that the combination
  of both can be applied to simulations of the solar chromosphere at
  minimal computational expense.

Title: Characterization of magneto-convection in sunspots. The
    Gough-Tayler stability criterion in MURaM sunspot simulations
Authors: Schmassmann, M.; Rempel, M.; Bello González, N.;
   Schlichenmaier, R.; Jurčák, J.
Bibcode: 2021A&A...656A..92S
Altcode:
  Context. Observations have shown that in stable sunspots, the umbral
  boundary is outlined by a critical value of the vertical magnetic
  field component. However, the nature of the distinct magnetoconvection
  regimes in the umbra and penumbra is still unclear. <BR /> Aims: We
  analyse a sunspot simulation in an effort to understand the origin
  of the convective instabilities giving rise to the penumbral and
  umbral distinct regimes. <BR /> Methods: We applied the criterion
  from Gough &amp; Tayler (1966, MNRAS, 133, 85), accounting for the
  stabilising effect of the vertical magnetic field, to investigate
  the convective instabilities in a MURaM sunspot simulation. <BR />
  Results: We find: (1) a highly unstable shallow layer right beneath the
  surface extending all over the simulation box in which convection is
  triggered by radiative cooling in the photosphere; (2) a deep umbral
  core (beneath −5 Mm) stabilised against overturning convection
  that underlies a region with stable background values permeated
  by slender instabilities coupled to umbral dots; (3) filamentary
  instabilities below the penumbra nearly parallel to the surface and
  undulating instabilities coupled to the penumbra which originate
  in the deep layers. These deep-rooted instabilities result in the
  vigorous magneto-convection regime characteristic of the penumbra; (4)
  convective downdrafts in the granulation, penumbra, and umbra develop
  at about 2 km s<SUP>−1</SUP>, 1 km s<SUP>−1</SUP>, and 0.1 km
  s<SUP>−1</SUP>, respectively, indicating that the granular regime of
  convection is more vigorous than the penumbra convection regime, which,
  in turn, is more vigorous than the close-to-steady umbra; (5) the GT
  criterion outlines both the sunspot magnetopause and peripatopause,
  highlighting the tripartite nature of the sub-photospheric layers
  of magnetohydrodynamic (MHD) sunspot models; and, finally, (6)
  the Jurčák criterion is the photospheric counterpart of the GT
  criterion in deep layers. <BR /> Conclusions: The GT criterion as a
  diagnostic tool reveals the tripartite nature of sunspot structure
  with distinct regimes of magneto-convection in the umbra, penumbra,
  and granulation operating in realistic MHD simulations. <P
  />Movies associated with Figs. 2 and 3 are available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202141607/olm">https://www.aanda.org</A>

Title: Solar atmosphere radiative transfer model comparison based
    on 3D MHD simulations
Authors: Haberreiter, M.; Criscuoli, S.; Rempel, M.; Pereira, T. M. D.
Bibcode: 2021A&A...653A.161H
Altcode: 2021arXiv210902681H
  Context. The reconstruction of the solar spectral irradiance (SSI)
  on various time scales is essential for the understanding of the
  Earth's climate response to the SSI variability. <BR /> Aims: The
  driver of the SSI variability is understood to be the intensity
  contrast of magnetic features present on the Sun with respect to
  the largely non-magnetic quiet Sun. However, different spectral
  synthesis codes lead to diverging projections of SSI variability. In
  this study we compare three different radiative transfer codes and
  carry out a detailed analysis of their performance. <BR /> Methods:
  We perform the spectral synthesis at the continuum wavelength of
  665 nm with the Code for Solar Irradiance, and the Rybicki-Hummer,
  and Max Planck University of Chicago Radiative MHD codes for three 3D
  MHD simulations snapshots, a non-magnetic case, and MHD simulations
  with 100 G, and 200 G magnetic field strength. <BR /> Results: We
  determine the intensity distributions, the intensity differences and
  ratios for the spectral synthesis codes. We identify that the largest
  discrepancies originate in the intergranular lanes where the most
  field concentration occurs. <BR /> Conclusions: Overall, the applied
  radiative transfer codes give consistent intensity distributions. Also,
  the intensity variation as a function of magnetic field strength for
  the particular 100 G and 200 G snapshots agree within the 2-3% range.

Title: Measuring the Magnetic Origins of Solar Flares, Coronal Mass
    Ejections, and Space Weather
Authors: Judge, Philip; Rempel, Matthias; Ezzeddine, Rana; Kleint,
   Lucia; Egeland, Ricky; Berdyugina, Svetlana V.; Berger, Thomas; Bryans,
   Paul; Burkepile, Joan; Centeno, Rebecca; de Toma, Giuliana; Dikpati,
   Mausumi; Fan, Yuhong; Gilbert, Holly; Lacatus, Daniela A.
Bibcode: 2021ApJ...917...27J
Altcode: 2021arXiv210607786J
  We take a broad look at the problem of identifying the magnetic
  solar causes of space weather. With the lackluster performance
  of extrapolations based upon magnetic field measurements in the
  photosphere, we identify a region in the near-UV (NUV) part of the
  spectrum as optimal for studying the development of magnetic free energy
  over active regions. Using data from SORCE, the Hubble Space Telescope,
  and SKYLAB, along with 1D computations of the NUV spectrum and numerical
  experiments based on the MURaM radiation-magnetohydrodynamic and
  HanleRT radiative transfer codes, we address multiple challenges. These
  challenges are best met through a combination of NUV lines of bright Mg
  II, and lines of Fe II and Fe I (mostly within the 4s-4p transition
  array) which form in the chromosphere up to 2 × 10<SUP>4</SUP>
  K. Both Hanle and Zeeman effects can in principle be used to derive
  vector magnetic fields. However, for any given spectral line the τ
  = 1 surfaces are generally geometrically corrugated owing to fine
  structure such as fibrils and spicules. By using multiple spectral
  lines spanning different optical depths, magnetic fields across nearly
  horizontal surfaces can be inferred in regions of low plasma β, from
  which free energies, magnetic topology, and other quantities can be
  derived. Based upon the recently reported successful sub-orbital space
  measurements of magnetic fields with the CLASP2 instrument, we argue
  that a modest space-borne telescope will be able to make significant
  advances in the attempts to predict solar eruptions. Difficulties
  associated with blended lines are shown to be minor in an Appendix.

Title: First Results of the Chromospheric MURaM code
Authors: Przybylski, D. F.; Cameron, R.; Solanki, S.; Rempel, M.
Bibcode: 2021AAS...23810605P
Altcode:
  The solar chromosphere, spanning the region between the photosphere
  and the transition to the corona, remains one of the least understood
  parts of the Sun. This is partly because observing the chromosphere
  and interpreting these observations is full of pitfalls. Also, the
  simulation of the chromosphere is complex, as the particle densities
  and collisional rates are too low to maintain local thermodynamic
  equilibrium (LTE). Additionally, the recombination rates of hydrogen are
  larger than the dynamical timescales and the populations must be solved
  in non-equilibrium (NE). Realistic simulations of the chromosphere
  must treat the magneto-hydrodynamics, time-dependant atomic and
  molecular chemistry, and radiation transfer simultaneously. <P />The
  MURaM radiation-MHD code has previously been used for investigation
  of the connection between the solar photosphere and corona, ranging
  from small-scale dynamo generated 'quiet' sun fields to sunspots and
  complex active regions. Until now these simulations have been performed
  in LTE, greatly limiting their realism in the solar chromosphere. We
  have extended MURaM to include NLTE effects following the prescriptions
  used in the Bifrost code. The low viscocity and resistivity of the MURaM
  code leads to turbulent convection in the photosphere with kilo-Gauss
  mixed-polarity magnetic fields. This results in a dynamic chromosphere
  with strong shocks and a finely structured magnetic field. We discuss
  the implications of this new model towards observations of chromospheric
  spectral lines.

Title: A Comprehensive Radiative Magnetohydrodynamics Simulation
    of Active Region Scale Flux Emergence from the Convection Zone to
    the Corona
Authors: Chen, Feng; Rempel, Matthias; Fan, Yuhong
Bibcode: 2021arXiv210614055C
Altcode:
  We present a comprehensive radiative magnetohydrodynamic simulation of
  the quiet Sun and large solar active regions. The 197 Mm wide simulation
  domain spans from 18 (10) Mm beneath the photosphere to 113 Mm in the
  solar corona. Radiative transfer assuming local thermal equilibrium,
  optically-thin radiative losses, and anisotropic conduction transport
  provide the necessary realism for synthesizing observables to compare
  with remote sensing observations of the photosphere and corona. This
  model self-consistently reproduces observed features of the quiet
  Sun, emerging and developed active regions, and solar flares up to M
  class. Here, we report an overview of the first results. The surface
  magnetoconvection yields an upward Poynting flux that is dissipated in
  the corona and heats the plasma to over one million K. The quiescent
  corona also presents ubiquitous propagating waves, jets, and bright
  points with sizes down to 2 Mm. Magnetic flux bundles emerge into the
  photosphere and give rise to strong and complex active regions with
  over $10^{23}$ Mx magnetic flux. The coronal free magnetic energy,
  which is approximately 18\% of the total magnetic energy, accumulates
  to approximately $10^{33}$ erg. The coronal magnetic field is clearly
  non-force-free, as the Lorentz force needs to balance the pressure
  force and viscous stress as well as drive magnetic field evolution. The
  emission measure from $\log_{10}T{=}4.5$ to $\log_{10}T{&gt;}7$ provides
  a comprehensive view of the active region corona, such as coronal loops
  of various lengths and temperatures, mass circulation by evaporation
  and condensation, and eruptions from jets to large-scale mass ejections.

Title: On the (in)stability of sunspots
Authors: Strecker, H.; Schmidt, W.; Schlichenmaier, R.; Rempel, M.
Bibcode: 2021A&A...649A.123S
Altcode: 2021arXiv210311487S
  Context. The stability of sunspots is one of the long-standing unsolved
  puzzles in the field of solar magnetism and the solar cycle. The thermal
  and magnetic structure of the sunspot beneath the solar surface is
  not accessible through observations, thus processes in these regions
  that contribute to the decay of sunspots can only be studied through
  theoretical and numerical studies. <BR /> Aims: We study the effects
  that destabilise and stabilise the flux tube of a simulated sunspot
  in the upper convection zone. The depth-varying effects of fluting
  instability, buoyancy forces, and timescales on the flux tube are
  analysed. <BR /> Methods: We analysed a numerical simulation of a
  sunspot calculated with the MURaM code. The simulation domain has a
  lateral extension of more than 98 Mm × 98 Mm and extends almost 18
  Mm below the solar surface. The analysed data set of 30 hours shows a
  stable sunspot at the solar surface. We studied the evolution of the
  flux tube at defined horizontal layers (1) by means of the relative
  change in perimeter and area, that is, its compactness; and (2) with
  a linear stability analysis. <BR /> Results: The simulation shows a
  corrugation along the perimeter of the flux tube (sunspot) that proceeds
  fastest at a depth of about 8 Mm below the solar surface. Towards
  the surface and towards deeper layers, the decrease in compactness is
  damped. From the stability analysis, we find that above a depth of 2
  Mm, the sunspot is stabilised by buoyancy forces. The spot is least
  stable at a depth of about 3 Mm because of the fluting instability. In
  deeper layers, the flux tube is marginally unstable. The stability
  of the sunspot at the surface affects the behaviour of the field
  lines in deeper layers by magnetic tension. Therefore the fluting
  instability is damped at depths of about 3 Mm, and the decrease in
  compactness is strongest at a depth of about 8 Mm. The more vertical
  orientation of the magnetic field and the longer convective timescale
  lead to slower evolution of the corrugation process in layers deeper
  than 10 Mm. <BR /> Conclusions: The formation of large intrusions of
  field-free plasma below the surface destabilises the flux tube of
  the sunspot. This process is not visible at the surface, where the
  sunspot is stabilised by buoyancy forces. The onset of sunspot decay
  occurs in deeper layers, while the sunspot still appears stable in
  the photosphere. The intrusions eventually lead to the disruption
  and decay of the sunspot. <P />The animation is available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202040199/olm">https://www.aanda.org</A>
  <P />This paper is mainly based on Part I of
  the Ph.D. thesis "On the decay of sunspots", <A
  href="https://freidok.uni-freiburg.de/data/165760">https://freidok.uni-freiburg.de/data/165760</A>

Title: Critical Science Plan for the Daniel K. Inouye Solar Telescope
    (DKIST)
Authors: Rast, Mark P.; Bello González, Nazaret; Bellot Rubio,
   Luis; Cao, Wenda; Cauzzi, Gianna; Deluca, Edward; de Pontieu, Bart;
   Fletcher, Lyndsay; Gibson, Sarah E.; Judge, Philip G.; Katsukawa,
   Yukio; Kazachenko, Maria D.; Khomenko, Elena; Landi, Enrico; Martínez
   Pillet, Valentín; Petrie, Gordon J. D.; Qiu, Jiong; Rachmeler,
   Laurel A.; Rempel, Matthias; Schmidt, Wolfgang; Scullion, Eamon; Sun,
   Xudong; Welsch, Brian T.; Andretta, Vincenzo; Antolin, Patrick; Ayres,
   Thomas R.; Balasubramaniam, K. S.; Ballai, Istvan; Berger, Thomas E.;
   Bradshaw, Stephen J.; Campbell, Ryan J.; Carlsson, Mats; Casini,
   Roberto; Centeno, Rebecca; Cranmer, Steven R.; Criscuoli, Serena;
   Deforest, Craig; Deng, Yuanyong; Erdélyi, Robertus; Fedun, Viktor;
   Fischer, Catherine E.; González Manrique, Sergio J.; Hahn, Michael;
   Harra, Louise; Henriques, Vasco M. J.; Hurlburt, Neal E.; Jaeggli,
   Sarah; Jafarzadeh, Shahin; Jain, Rekha; Jefferies, Stuart M.; Keys,
   Peter H.; Kowalski, Adam F.; Kuckein, Christoph; Kuhn, Jeffrey R.;
   Kuridze, David; Liu, Jiajia; Liu, Wei; Longcope, Dana; Mathioudakis,
   Mihalis; McAteer, R. T. James; McIntosh, Scott W.; McKenzie, David
   E.; Miralles, Mari Paz; Morton, Richard J.; Muglach, Karin; Nelson,
   Chris J.; Panesar, Navdeep K.; Parenti, Susanna; Parnell, Clare E.;
   Poduval, Bala; Reardon, Kevin P.; Reep, Jeffrey W.; Schad, Thomas A.;
   Schmit, Donald; Sharma, Rahul; Socas-Navarro, Hector; Srivastava,
   Abhishek K.; Sterling, Alphonse C.; Suematsu, Yoshinori; Tarr, Lucas
   A.; Tiwari, Sanjiv; Tritschler, Alexandra; Verth, Gary; Vourlidas,
   Angelos; Wang, Haimin; Wang, Yi-Ming; NSO and DKIST Project; DKIST
   Instrument Scientists; DKIST Science Working Group; DKIST Critical
   Science Plan Community
Bibcode: 2021SoPh..296...70R
Altcode: 2020arXiv200808203R
  The National Science Foundation's Daniel K. Inouye Solar Telescope
  (DKIST) will revolutionize our ability to measure, understand,
  and model the basic physical processes that control the structure
  and dynamics of the Sun and its atmosphere. The first-light DKIST
  images, released publicly on 29 January 2020, only hint at the
  extraordinary capabilities that will accompany full commissioning of
  the five facility instruments. With this Critical Science Plan (CSP)
  we attempt to anticipate some of what those capabilities will enable,
  providing a snapshot of some of the scientific pursuits that the DKIST
  hopes to engage as start-of-operations nears. The work builds on the
  combined contributions of the DKIST Science Working Group (SWG) and
  CSP Community members, who generously shared their experiences, plans,
  knowledge, and dreams. Discussion is primarily focused on those issues
  to which DKIST will uniquely contribute.

Title: Acoustic-gravity wave propagation characteristics in
    three-dimensional radiation hydrodynamic simulations of the solar
    atmosphere
Authors: Fleck, B.; Carlsson, M.; Khomenko, E.; Rempel, M.; Steiner,
   O.; Vigeesh, G.
Bibcode: 2021RSPTA.37900170F
Altcode: 2020arXiv200705847F
  There has been tremendous progress in the degree of realism of
  three-dimensional radiation magneto-hydrodynamic simulations of the
  solar atmosphere in the past decades. Four of the most frequently
  used numerical codes are Bifrost, CO5BOLD, MANCHA3D and MURaM. Here
  we test and compare the wave propagation characteristics in model
  runs from these four codes by measuring the dispersion relation of
  acoustic-gravity waves at various heights. We find considerable
  differences between the various models. The height dependence of
  wave power, in particular of high-frequency waves, varies by up to
  two orders of magnitude between the models, and the phase difference
  spectra of several models show unexpected features, including ±180°
  phase jumps. <P />This article is part of the Theo Murphy meeting issue
  `High-resolution wave dynamics in the lower solar atmosphere'.

Title: Atmosphere and Ocean Responses to Extreme Low Solar Activity
    and Their Hemispheric Differences
Authors: Liu, Hanli; Solomon, Stanley; Rempel, Matthias; McInerney,
   Joseph; Danabasoglu, Gokhan
Bibcode: 2021cosp...43E.724L
Altcode:
  The total solar irradiance (TSI) changes by ~0.1% during solar
  cycles. The impact of the change on tropospheric climate is small in
  comparison with the climate variability and it is thus challenging to
  clearly quantify the solar signal. The rather weak signal also makes
  it difficult to investigate the processes involved in sun-climate
  connection. As a result the climate sensitivity to solar forcing
  is poorly quantified and understood. In this study, we seek to
  overcome this difficulty by driving a coupled whole atmosphere-ocean
  model--the NCAR CESM Whole Atmosphere Community Climate Model (WACCM)
  with the interactive ocean model (POP2)--with an extreme low solar
  forcing. The TSI and solar spectral irradiance (SSI) are obtained from
  MHD simulations using the MURaM code, and the TSI/SSI values obtained
  can be regarded as a lower theoretical limit as allowed by known
  solar physics principles. With this hypothetical low solar forcing,
  significant and complex changes are seen throughout the atmosphere
  and also in the ocean circulation. While the surface generally cools
  during the 200-year simulation, the evolution path of the cooling and
  the cooling rates are very different between the two hemispheres. Our
  analysis suggests that the interplay between the radiative forcing
  and dynamical feedback determines the response, and the dynamical
  feedback from atmosphere and ocean coupling, in particular in the form
  of atmospheric waves, differ between the two hemispheres. Additional
  simulations with extreme low SSI forcing in the ultraviolet (UV)
  only and in the visible/infrared (VIR) only show that they can
  cause troposphere/ocean responses similar to the full forcing case,
  albeit with different magnitudes. This unambiguously demonstrates
  the importance of middle atmosphere/lower atmosphere/ocean coupling
  in sun-climate connection and in studying the climate sensitivity to
  solar forcing.

Title: Flare simulations with the MURaM radiative MHD code
Authors: Rempel, Matthias; Cheung, Mark; Chintzoglou, Georgios
Bibcode: 2021cosp...43E1772R
Altcode:
  Over the past few years the MURaM radiative MHD code was expanded
  for its capability to simulate the coupled solar atmosphere from the
  upper convection zone into the lower solar corona. The code includes
  the essential physics to synthesize thermal emission ranging from
  the visible spectrum in the photosphere to EUV and soft X-ray from
  transition region and corona. A more sophisticated treatment of the
  chromosphere is currently under development. After a brief review of
  the code's capabilities and limitations we present a new setup that
  allows to create collisional polarity inversion lines (cPILs) and study
  the coronal response including flares. In the setup we start with a
  bipolar sunspot configuration and set the spots on collision course
  by imposing the appropriate velocity field at the footpoints in the
  subphotospheric boundary. We vary parameters such as the initial spot
  separation, collision speed and collision distance. While all setups
  lead to the formation of a sigmoid structure, only the cases with a
  close passing of the spots cause flares and mass eruptions. The energy
  release is in the $1-2\times 10^{31}$ erg range, putting the simulated
  flares into the upper C to lower M-class range. While the case with the
  more distant passing of the spots does not lead to a flare, the corona
  is nonetheless substantially heated, suggesting non-eruptive energy
  release mechanisms. We discuss the applicability/implications of our
  setups for investigating the way cPILs form and produce eruptions and
  present preliminary results.

Title: Flare Simulations with the MURaM Radiative MHD Code
Authors: Rempel, M.; Chintzoglou, G.; Cheung, C. M. M.
Bibcode: 2020AGUFMSH0500004R
Altcode:
  No abstract at ADS

Title: The Dimmest State of the Sun
Authors: Yeo, K. L.; Solanki, S. K.; Krivova, N. A.; Rempel, M.;
   Anusha, L. S.; Shapiro, A. I.; Tagirov, R. V.; Witzke, V.
Bibcode: 2020GeoRL..4790243Y
Altcode: 2021arXiv210209487Y
  How the solar electromagnetic energy entering the Earth's atmosphere
  varied since preindustrial times is an important consideration in
  the climate change debate. Detrimental to this debate, estimates
  of the change in total solar irradiance (TSI) since the Maunder
  minimum, an extended period of weak solar activity preceding the
  industrial revolution, differ markedly, ranging from a drop of 0.75
  W m<SUP>-2</SUP> to a rise of 6.3 W m<SUP>-2</SUP>. Consequently, the
  exact contribution by solar forcing to the rise in global temperatures
  over the past centuries remains inconclusive. Adopting a novel approach
  based on state-of-the-art solar imagery and numerical simulations, we
  establish the TSI level of the Sun when it is in its least-active state
  to be 2.0 ± 0.7 W m<SUP>-2</SUP> below the 2019 level. This means TSI
  could not have risen since the Maunder minimum by more than this amount,
  thus restricting the possible role of solar forcing in global warming.

Title: Major Scientific Challenges and Opportunities in Understanding
    Magnetic Reconnection and Related Explosive Phenomena in Solar and
    Heliospheric Plasmas
Authors: Ji, H.; Karpen, J.; Alt, A.; Antiochos, S.; Baalrud, S.;
   Bale, S.; Bellan, P. M.; Begelman, M.; Beresnyak, A.; Bhattacharjee,
   A.; Blackman, E. G.; Brennan, D.; Brown, M.; Buechner, J.; Burch, J.;
   Cassak, P.; Chen, B.; Chen, L. -J.; Chen, Y.; Chien, A.; Comisso,
   L.; Craig, D.; Dahlin, J.; Daughton, W.; DeLuca, E.; Dong, C. F.;
   Dorfman, S.; Drake, J.; Ebrahimi, F.; Egedal, J.; Ergun, R.; Eyink,
   G.; Fan, Y.; Fiksel, G.; Forest, C.; Fox, W.; Froula, D.; Fujimoto,
   K.; Gao, L.; Genestreti, K.; Gibson, S.; Goldstein, M.; Guo, F.; Hare,
   J.; Hesse, M.; Hoshino, M.; Hu, Q.; Huang, Y. -M.; Jara-Almonte, J.;
   Karimabadi, H.; Klimchuk, J.; Kunz, M.; Kusano, K.; Lazarian, A.; Le,
   A.; Lebedev, S.; Li, H.; Li, X.; Lin, Y.; Linton, M.; Liu, Y. -H.;
   Liu, W.; Longcope, D.; Loureiro, N.; Lu, Q. -M.; Ma, Z-W.; Matthaeus,
   W. H.; Meyerhofer, D.; Mozer, F.; Munsat, T.; Murphy, N. A.; Nilson,
   P.; Ono, Y.; Opher, M.; Park, H.; Parker, S.; Petropoulou, M.; Phan,
   T.; Prager, S.; Rempel, M.; Ren, C.; Ren, Y.; Rosner, R.; Roytershteyn,
   V.; Sarff, J.; Savcheva, A.; Schaffner, D.; Schoeffier, K.; Scime, E.;
   Shay, M.; Sironi, L.; Sitnov, M.; Stanier, A.; Swisdak, M.; TenBarge,
   J.; Tharp, T.; Uzdensky, D.; Vaivads, A.; Velli, M.; Vishniac, E.;
   Wang, H.; Werner, G.; Xiao, C.; Yamada, M.; Yokoyama, T.; Yoo, J.;
   Zenitani, S.; Zweibel, E.
Bibcode: 2020arXiv200908779J
Altcode:
  Magnetic reconnection underlies many explosive phenomena in the
  heliosphere and in laboratory plasmas. The new research capabilities in
  theory/simulations, observations, and laboratory experiments provide the
  opportunity to solve the grand scientific challenges summarized in this
  whitepaper. Success will require enhanced and sustained investments
  from relevant funding agencies, increased interagency/international
  partnerships, and close collaborations of the solar, heliospheric,
  and laboratory plasma communities. These investments will deliver
  transformative progress in understanding magnetic reconnection and
  related explosive phenomena including space weather events.

Title: A distinct magnetic property of the inner penumbral
    boundary. III. Analysis of simulated sunspots
Authors: Jurčák, Jan; Schmassmann, Markus; Rempel, Matthias; Bello
   González, Nazaret; Schlichenmaier, Rolf
Bibcode: 2020A&A...638A..28J
Altcode: 2020arXiv200403940J
  Context. Analyses of sunspot observations revealed a fundamental
  magnetic property of the umbral boundary: the invariance of the
  vertical component of the magnetic field. <BR /> Aims: We analyse
  the magnetic properties of the umbra-penumbra boundary in simulated
  sunspots and thus assess their similarity to observed sunspots. We
  also aim to investigate the role of the plasma β and the ratio of
  kinetic to magnetic energy in simulated sunspots in the convective
  motions because these quantities cannot be reliably determined from
  observations. <BR /> Methods: We used a set of non-gray simulation
  runs of sunspots with the MURaM code. The setups differed in terms
  of subsurface magnetic field structure and magnetic field boundary
  imposed at the top of the simulation domain. These data were used to
  synthesize the Stokes profiles, which were then degraded to the Hinode
  spectropolarimeter-like observations. Then, the data were treated
  like real Hinode observations of a sunspot, and magnetic properties
  at the umbral boundaries were determined. <BR /> Results: Simulations
  with potential field extrapolation produce a realistic magnetic field
  configuration on the umbral boundaries of the sunspots. Two simulations
  with a potential field upper boundary, but different subsurface
  magnetic field structures, differ significantly in the extent of their
  penumbrae. Increasing the penumbra width by forcing more horizontal
  magnetic fields at the upper boundary results in magnetic properties
  that are not consistent with observations. This implies that the size of
  the penumbra is given by the subsurface structure of the magnetic field,
  that is, by the depth and inclination of the magnetopause, which is
  shaped by the expansion of the sunspot flux rope with height. None of
  the sunspot simulations is consistent with the observed properties of
  the magnetic field and the direction of the Evershed flow at the same
  time. Strong outward-directed Evershed flows are only found in setups
  with an artificially enhanced horizontal component of the magnetic
  field at the top boundary that are not consistent with the observed
  magnetic field properties at the umbra-penumbra boundary. We stress
  that the photospheric boundary of simulated sunspots is defined by a
  magnetic field strength of equipartition field value.

Title: On the Contribution of Quiet-Sun Magnetism to Solar Irradiance
Variations: Constraints on Quiet-Sun Variability and Grand-minimum
    Scenarios
Authors: Rempel, M.
Bibcode: 2020ApJ...894..140R
Altcode: 2020arXiv200401795R
  While the quiet-Sun magnetic field shows only little variation with
  the solar cycle, long-term variations cannot be completely ruled
  out from first principles. We investigate the potential effect of
  quiet-Sun magnetism on spectral solar irradiance through a series of
  small-scale dynamo simulations with zero vertical flux imbalance (
  $\langle {B}_{z}\rangle =0$) and varying levels of small-scale magnetic
  field strength, and one weak network case with an additional flux
  imbalance corresponding to a flux density of $\langle {B}_{z}\rangle
  =100$ G. From these setups, we compute the dependence of the outgoing
  radiative energy flux on the mean vertical magnetic field strength
  in the photosphere at a continuum optical depth τ = 1 ( $\langle |
  {B}_{z}| {\rangle }_{\tau =1}$). We find that a quiet-Sun setup with
  a mean vertical field strength of $\langle | {B}_{z}| {\rangle }_{\tau
  =1}=69$ G is about 0.6% brighter than a non-magnetic reference case. We
  find a linear dependence of the outgoing radiative energy flux on the
  mean field strength $\langle | {B}_{z}| {\rangle }_{\tau =1}$ with
  a relative slope of 1.4 × 10<SUP>-4</SUP> G<SUP>-1</SUP>. With this
  sensitivity, only a moderate change of the quiet-Sun field strength
  by 10% would lead to a total solar irradiance variation comparable
  to the observed solar cycle variation. While this does provide strong
  indirect constraints on possible quiet-Sun variations during a regular
  solar cycle, it also emphasizes that potential variability over longer
  timescales could make a significant contribution to longer-term solar
  irradiance variations.

Title: Comparing Radiative Transfer Codes and Opacity Samplings for
    Solar Irradiance Reconstructions
Authors: Criscuoli, Serena; Rempel, Matthias; Haberreiter, Margit;
   Pereira, Tiago M. D.; Uitenbroek, Han; Fabbian, Damian
Bibcode: 2020SoPh..295...50C
Altcode:
  Some techniques developed to reproduce solar irradiance variations make
  use of synthetic radiative fluxes of quiet and magnetic features. The
  synthesis of radiative fluxes of astronomical objects is likely
  to be affected by uncertainties resulting from approximations and
  specific input employed for the synthesis. In this work we compare
  spectra obtained with three radiative transfer codes with the
  purpose of investigating differences in reproducing solar irradiance
  variations. Specifically, we compare spectral synthesis produced in
  non-local thermodynamic equilibrium obtained with COSI and RH using
  1-D atmosphere models. We also compare local thermodynamic equilibrium
  syntheses emerging from 3-D MURaM simulations of the solar atmosphere
  obtained with two sets of opacity tables generated with the ATLAS9
  package and with the RH code, and test the effects of opacity sampling
  on the emergent spectra. We find that, although the different codes
  and methodologies employed to synthesize the spectrum reproduce overall
  the observed solar spectrum with a similar degree of accuracy, subtle
  differences in quiet Sun spectra may translate into larger differences
  in the computation of the contrasts of magnetic features, which,
  in turn, critically affect the estimates of solar variability.

Title: Major Scientific Challenges and Opportunities in Understanding
    Magnetic Reconnection and Related Explosive Phenomena throughout
    the Universe
Authors: Ji, H.; Alt, A.; Antiochos, S.; Baalrud, S.; Bale, S.;
   Bellan, P. M.; Begelman, M.; Beresnyak, A.; Blackman, E. G.; Brennan,
   D.; Brown, M.; Buechner, J.; Burch, J.; Cassak, P.; Chen, L. -J.;
   Chen, Y.; Chien, A.; Craig, D.; Dahlin, J.; Daughton, W.; DeLuca, E.;
   Dong, C. F.; Dorfman, S.; Drake, J.; Ebrahimi, F.; Egedal, J.; Ergun,
   R.; Eyink, G.; Fan, Y.; Fiksel, G.; Forest, C.; Fox, W.; Froula, D.;
   Fujimoto, K.; Gao, L.; Genestreti, K.; Gibson, S.; Goldstein, M.; Guo,
   F.; Hesse, M.; Hoshino, M.; Hu, Q.; Huang, Y. -M.; Jara-Almonte, J.;
   Karimabadi, H.; Klimchuk, J.; Kunz, M.; Kusano, K.; Lazarian, A.;
   Le, A.; Li, H.; Li, X.; Lin, Y.; Linton, M.; Liu, Y. -H.; Liu, W.;
   Longcope, D.; Loureiro, N.; Lu, Q. -M.; Ma, Z-W.; Matthaeus, W. H.;
   Meyerhofer, D.; Mozer, F.; Munsat, T.; Murphy, N. A.; Nilson, P.;
   Ono, Y.; Opher, M.; Park, H.; Parker, S.; Petropoulou, M.; Phan, T.;
   Prager, S.; Rempel, M.; Ren, C.; Ren, Y.; Rosner, R.; Roytershteyn,
   V.; Sarff, J.; Savcheva, A.; Schaffner, D.; Schoeffier, K.; Scime, E.;
   Shay, M.; Sitnov, M.; Stanier, A.; TenBarge, J.; Tharp, T.; Uzdensky,
   D.; Vaivads, A.; Velli, M.; Vishniac, E.; Wang, H.; Werner, G.; Xiao,
   C.; Yamada, M.; Yokoyama, T.; Yoo, J.; Zenitani, S.; Zweibel, E.
Bibcode: 2020arXiv200400079J
Altcode:
  This white paper summarizes major scientific challenges and
  opportunities in understanding magnetic reconnection and related
  explosive phenomena as a fundamental plasma process.

Title: Using the Butterfly Effect to Probe How the Sun Generates
    Acoustic Noise
Authors: Lindsey, Charles; Rempel, Matthias
Bibcode: 2020SoPh..295...26L
Altcode:
  A major encumbrance to recognition of individual episodes of noise
  emission is the accumulation over hours of other noise emitted long
  before. This is true in simulations just as it is in the solar
  environment itself. The composite seismic signature of acoustic
  radiation accumulated over preceding hours drowns out the signature
  of newly emitted "acoustic pings." This problem could be alleviated in
  simulations by periodically damping the accumulated acoustic radiation
  - if this can be done benignly, i.e. in such a way that the onset
  transient of the damping (and its subsequent termination) does not emit
  its own acoustic noise. We introduce a way of doing this based upon a
  study of the butterfly effect in compressible radiative MHD simulations
  of convection that excites p-modes. This gives us an encouraging preview
  of what further development of this utility offers for an understanding
  of the character of p-mode generation in convective atmospheres.

Title: Testing Data-driven Simulations of Solar Eruptive Flares
    Using Synthetic Magnetograms from Flux Emergence Simulations
Authors: Fan, Y.; Rempel, M.
Bibcode: 2019AGUFMSH33B3393F
Altcode:
  To understand the feasibility of data-driven simulations of
  solar eruptive events using the electric field inferred from the
  observed time sequences of vector magnetograms, we have performed
  synthetic data driven simulations using synthetic magnetograms and
  electric fields extracted from interior-to-corona flux emergence
  simulations. We have carried out coronal simulations of eruptive
  flares with the MFE MHD code driven by the boundary data at the
  base of the corona extracted from the flux emergence simulations
  with the MURaM MHD code. We performed simulations driven with only
  the horizontal v×B electric field and the vector B field extracted
  from the MURaM simulation at the transition region height, but with
  the thermodynamics self-determined from the MFE coronal simulation,
  which includes the coronal heating due to numerical dissipation,
  radiative cooling, and field aligned thermal conduction. The coronal
  heating is due to dissipation of the Poynting flux from the lower
  boundary electric field due to magneto-convection. We find that the
  driven coronal simulations produce coronal emissions in AIA channels
  that are qualitatively similar to those produced by MURaM, and most
  importantly re-produce the main eruptive flares with sigmoid brightening
  during the evolution. These experiments suggest that with only the VxB
  electric field and the B field at the lower boundary (which would be
  the situation using the observed vector magnetograms), it is possible
  for coronal MHD simulations to reproduce the coronal magnetic field
  evolution and onset of eruptions.

Title: Combining magnetohydrostatic constraints with Stokes profiles
    inversions. I. Role of boundary conditions
Authors: Borrero, J. M.; Pastor Yabar, A.; Rempel, M.; Ruiz Cobo, B.
Bibcode: 2019A&A...632A.111B
Altcode:
  Context. Inversion codes for the polarized radiative transfer
  equation, when applied to spectropolarimetric observations (i.e.,
  Stokes vector) in spectral lines, can be used to infer the temperature
  T, line-of-sight velocity v<SUB>los</SUB>, and magnetic field B as
  a function of the continuum optical-depth τ<SUB>c</SUB>. However,
  they do not directly provide the gas pressure P<SUB>g</SUB> or density
  ρ. In order to obtain these latter parameters, inversion codes rely
  instead on the assumption of hydrostatic equilibrium (HE) in addition
  to the equation of state (EOS). Unfortunately, the assumption of HE is
  rather unrealistic across magnetic field lines, causing estimations
  of P<SUB>g</SUB> and ρ to be unreliable. This is because the role
  of the Lorentz force, among other factors, is neglected. Unreliable
  gas pressure and density also translate into an inaccurate conversion
  from optical depth τ<SUB>c</SUB> to geometrical height z. <BR /> Aims:
  We aim at improving the determination of the gas pressure and density
  via the application of magnetohydrostatic (MHS) equilibrium instead of
  HE. <BR /> Methods: We develop a method to solve the momentum equation
  under MHS equilibrium (i.e., taking the Lorentz force into account)
  in three dimensions. The method is based on the iterative solution of
  a Poisson-like equation. Considering the gas pressure P<SUB>g</SUB>
  and density ρ from three-dimensional magnetohydrodynamic (MHD)
  simulations of sunspots as a benchmark, we compare the results from the
  application of HE and MHS equilibrium using boundary conditions with
  different degrees of realism. Employing boundary conditions that can
  be applied to actual observations, we find that HE retrieves the gas
  pressure and density with an error smaller than one order of magnitude
  (compared to the MHD values) in only about 47% of the grid points in
  the three-dimensional domain. Moreover, the inferred values are within
  a factor of two of the MHD values in only about 23% of the domain. This
  translates into an error of about 160 - 200 km in the determination of
  the z - τ<SUB>c</SUB> conversion (i.e., Wilson depression). On the
  other hand, the application of MHS equilibrium with similar boundary
  conditions allows determination of P<SUB>g</SUB> and ρ with an error
  smaller than an order of magnitude in 84% of the domain. The inferred
  values are within a factor of two in more than 55% of the domain. In
  this latter case, the z - τ<SUB>c</SUB> conversion is obtained with an
  accuracy of 30 - 70 km. Inaccuracies are due in equal part to deviations
  from MHS equilibrium and to inaccuracies in the boundary conditions. <BR
  /> Results: Compared to HE, our new method, based on MHS equilibrium,
  significantly improves the reliability in the determination of the
  density, gas pressure, and conversion between geometrical height z and
  continuum optical depth τ<SUB>c</SUB>. This method could be used in
  conjunction with the inversion of the radiative transfer equation for
  polarized light in order to determine the thermodynamic, kinematic,
  and magnetic parameters of the solar atmosphere.

Title: Superstrong photospheric magnetic fields in sunspot penumbrae
Authors: Siu-Tapia, A.; Lagg, A.; van Noort, M.; Rempel, M.; Solanki,
   S. K.
Bibcode: 2019A&A...631A..99S
Altcode: 2019arXiv190913619S
  Context. Recently, there have been some reports of unusually strong
  photospheric magnetic fields (which can reach values of over 7 kG)
  inferred from Hinode SOT/SP sunspot observations within penumbral
  regions. These superstrong penumbral fields are even larger than the
  strongest umbral fields on record and appear to be associated with
  supersonic downflows. The finding of such fields has been controversial
  since they seem to show up only when spatially coupled inversions
  are performed. <BR /> Aims: Here, we investigate and discuss the
  reliability of those findings by studying in detail observed spectra
  associated with particularly strong magnetic fields at the inner edge
  of the penumbra of active region 10930. <BR /> Methods: We applied
  classical diagnostic methods and various inversions with different
  model atmospheres to the observed Stokes profiles in two selected
  pixels with superstrong magnetic fields, and compared the results
  with a magnetohydrodynamic simulation of a sunspot whose penumbra
  contains localized regions with strong fields (nearly 5 kG at τ = 1)
  associated with supersonic downflows. <BR /> Results: The different
  inversions provide different results: while the SPINOR 2D inversions
  consider a height-dependent single-component model and return B &gt;
  7 kG and supersonic positive v<SUB>LOS</SUB> (corresponding to a
  counter-Evershed flow), height-dependent two-component inversions
  suggest the presence of an umbral component (almost at rest)
  with field strengths ∼4 - 4.2 kG and a penumbral component with
  v<SUB>LOS</SUB> ∼ 16 - 18 km s<SUP>-1</SUP> and field strengths up
  to ∼5.8 kG. Likewise, height-independent two-component inversions
  find a solution for an umbral component and a strongly redshifted
  (v<SUB>LOS</SUB> ∼ 15 - 17 km s<SUP>-1</SUP>) penumbral component
  with B ∼ 4 kG. According to a Bayesian information criterion,
  the inversions providing a better balance between the quality of
  the fits and the number of free parameters considered by the models
  are the height-independent two-component inversions, but they lie
  only slightly above the SPINOR 2D inversions. Since it is expected
  that the physical parameters all display considerable gradients with
  height, as supported by magnetohydrodynamic (MHD) sunspot simulations,
  the SPINOR 2D inversions are the preferred ones. <BR /> Conclusions:
  According to the MHD sunspot simulation analyzed here, the presence
  of counter-Evershed flows in the photospheric penumbra can lead to
  the necessary conditions for the observation of ∼5 kG fields at the
  inner penumbra. Although a definite conclusion about the potential
  existence of fields in excess of 7 kG cannot be given, their nature
  could be explained (based on the simulation results) as the consequence
  of the extreme dynamical effects introduced by highly supersonic
  counter-Evershed flows (v<SUB>LOS</SUB> &gt; 10 km s<SUP>-1</SUP>
  and up to ∼30 km s<SUP>-1</SUP> according to SPINOR 2D). The latter
  are much faster and more compressive downflows than those found in
  the MHD simulations and therefore could lead to field intensification
  up to considerably stronger fields. Also, a lower gas density would
  lead to a deeper depression of the τ = 1 surface, making possible
  the observation of deeper-lying stronger fields. The superstrong
  magnetic fields are expected to be nearly force-free, meaning that
  they can attain much larger strengths than expected when considering
  only balance between magnetic pressure and the local gas pressure.

Title: Three-dimensional modeling of chromospheric spectral lines
    in a simulated active region
Authors: Bjørgen, Johan P.; Leenaarts, Jorrit; Rempel, Matthias;
   Cheung, Mark C. M.; Danilovic, Sanja; de la Cruz Rodríguez, Jaime;
   Sukhorukov, Andrii V.
Bibcode: 2019A&A...631A..33B
Altcode: 2019arXiv190601098B
  Context. Because of the complex physics that governs the formation of
  chromospheric lines, interpretation of solar chromospheric observations
  is difficult. The origin and characteristics of many chromospheric
  features are, because of this, unresolved. <BR /> Aims: We focus on
  studying two prominent features: long fibrils and flare ribbons. To
  model these features, we use a 3D magnetohydrodynamic simulation of
  an active region, which self-consistently reproduces both of these
  features. <BR /> Methods: We modeled the Hα, Mg II k, Ca II K,
  and Ca II 8542 Å lines using the 3D non-LTE radiative transfer
  code Multi3D. To obtain non-LTE electron densities, we solved the
  statistical equilibrium equations for hydrogen simultaneously with the
  charge conservation equation. We treated the Ca II K and Mg II k lines
  with partially coherent scattering. <BR /> Results: This simulation
  reproduces long fibrils that span between the opposite-polarity
  sunspots and go up to 4 Mm in height. They can be traced in all lines
  owing to density corrugation. In contrast to previous studies, Hα,
  Mg II h&amp;k, and Ca II H&amp;K are formed at similar height in this
  model. Although some of the high fibrils are also visible in the Ca II
  8542 Å line, this line tends to sample loops and shocks lower in the
  chromosphere. Magnetic field lines are aligned with the Hα fibrils,
  but the latter holds to a lesser extent for the Ca II 8542 Å line. The
  simulation shows structures in the Hα line core that look like flare
  ribbons. The emission in the ribbons is caused by a dense chromosphere
  and a transition region at high column mass. The ribbons are visible in
  all chromospheric lines, but least prominent in Ca II 8542 Å line. In
  some pixels, broad asymmetric profiles with a single emission peak
  are produced similar to the profiles observed in flare ribbons. They
  are caused by a deep onset of the chromospheric temperature rise
  and large velocity gradients. <BR /> Conclusions: The simulation
  produces long fibrils similar to what is seen in observations. It
  also produces structures similar to flare ribbons despite the lack
  of nonthermal electrons in the simulation. The latter suggests that
  thermal conduction might be a significant agent in transporting flare
  energy to the chromosphere in addition to nonthermal electrons.

Title: A comprehensive three-dimensional radiative magnetohydrodynamic
    simulation of a solar flare
Authors: Cheung, M. C. M.; Rempel, M.; Chintzoglou, G.; Chen, F.;
   Testa, P.; Martínez-Sykora, J.; Sainz Dalda, A.; DeRosa, M. L.;
   Malanushenko, A.; Hansteen, V.; De Pontieu, B.; Carlsson, M.; Gudiksen,
   B.; McIntosh, S. W.
Bibcode: 2019NatAs...3..160C
Altcode: 2018NatAs...3..160C
  Solar and stellar flares are the most intense emitters of X-rays and
  extreme ultraviolet radiation in planetary systems<SUP>1,2</SUP>. On
  the Sun, strong flares are usually found in newly emerging sunspot
  regions<SUP>3</SUP>. The emergence of these magnetic sunspot groups
  leads to the accumulation of magnetic energy in the corona. When
  the magnetic field undergoes abrupt relaxation, the energy released
  powers coronal mass ejections as well as heating plasma to temperatures
  beyond tens of millions of kelvins. While recent work has shed light
  on how magnetic energy and twist accumulate in the corona<SUP>4</SUP>
  and on how three-dimensional magnetic reconnection allows for rapid
  energy release<SUP>5,6</SUP>, a self-consistent model capturing how
  such magnetic changes translate into observable diagnostics has remained
  elusive. Here, we present a comprehensive radiative magnetohydrodynamics
  simulation of a solar flare capturing the process from emergence to
  eruption. The simulation has sufficient realism for the synthesis of
  remote sensing measurements to compare with observations at visible,
  ultraviolet and X-ray wavelengths. This unifying model allows us to
  explain a number of well-known features of solar flares<SUP>7</SUP>,
  including the time profile of the X-ray flux during flares, origin
  and temporal evolution of chromospheric evaporation and condensation,
  and sweeping of flare ribbons in the lower atmosphere. Furthermore,
  the model reproduces the apparent non-thermal shape of coronal X-ray
  spectra, which is the result of the superposition of multi-component
  super-hot plasmas<SUP>8</SUP> up to and beyond 100 million K.

Title: Combining magneto-hydrostatic constraints with Stokes profiles
    inversions
Authors: Borrero, J. M.; Pastor Yabar, A.; Rempel, M.; Ruiz Cobo, B.
Bibcode: 2019arXiv191014131B
Altcode:
  Inversion codes for the polarized radiative transfer equation can
  be used to infer the temperature $T$, line-of-sight velocity $v_{\rm
  los}$, and magnetic field $\rm{\bf B}$ as a function of the continuum
  optical-depth $\tau_{\rm c}$. However, they do not directly provide
  the gas pressure $P_{\rm g}$ or density $\rho$. In order to obtain
  these latter parameters, inversion codes rely instead on the assumption
  of hydrostatic equilibrium (HE) in addition to the equation of state
  (EOS). Unfortunately, the assumption of HE is rather unrealistic across
  magnetic field lines. This is because the role of the Lorentz force,
  among other factors, is neglected. This translates into an inaccurate
  conversion from optical depth $\tau_{\rm c}$ to geometrical height
  $z$. We aim at improving this conversion via the application of
  magneto-hydrostatic (MHS) equilibrium instead of HE. We develop a
  method to solve the momentum equation under MHS equilibrium (i.e.,
  taking the Lorentz force into account) in three dimensions. The method
  is based on the solution of a Poisson-like equation. Considering the
  gas pressure $P_{\rm g}$ and density $\rho$ from three-dimensional
  magneto-hydrodynamic (MHD) simulations of sunspots as a benchmark, we
  compare the results from the application of HE and MHS equilibrium. We
  find that HE retrieves the gas pressure and density within an order
  of magnitude of the MHD values in only about 47 \% of the domain. This
  translates into an error of about $160-200$ km in the determination of
  the $z-\tau_{\rm c}$ conversion. On the other hand, the application of
  MHS equilibrium allows determination of $P_{\rm g}$ and $\rho$ within
  an order of magnitude in 84 \% of the domain. In this latter case, the
  $z-\tau_{\rm c}$ conversion is obtained with an accuracy of $30-70$ km.

Title: Principles Of Heliophysics: a textbook on the universal
    processes behind planetary habitability
Authors: Schrijver, Karel; Bagenal, Fran; Bastian, Tim; Beer,
   Juerg; Bisi, Mario; Bogdan, Tom; Bougher, Steve; Boteler, David;
   Brain, Dave; Brasseur, Guy; Brownlee, Don; Charbonneau, Paul; Cohen,
   Ofer; Christensen, Uli; Crowley, Tom; Fischer, Debrah; Forbes, Terry;
   Fuller-Rowell, Tim; Galand, Marina; Giacalone, Joe; Gloeckler, George;
   Gosling, Jack; Green, Janet; Guetersloh, Steve; Hansteen, Viggo;
   Hartmann, Lee; Horanyi, Mihaly; Hudson, Hugh; Jakowski, Norbert;
   Jokipii, Randy; Kivelson, Margaret; Krauss-Varban, Dietmar; Krupp,
   Norbert; Lean, Judith; Linsky, Jeff; Longcope, Dana; Marsh, Daniel;
   Miesch, Mark; Moldwin, Mark; Moore, Luke; Odenwald, Sten; Opher, Merav;
   Osten, Rachel; Rempel, Matthias; Schmidt, Hauke; Siscoe, George;
   Siskind, Dave; Smith, Chuck; Solomon, Stan; Stallard, Tom; Stanley,
   Sabine; Sojka, Jan; Tobiska, Kent; Toffoletto, Frank; Tribble, Alan;
   Vasyliunas, Vytenis; Walterscheid, Richard; Wang, Ji; Wood, Brian;
   Woods, Tom; Zapp, Neal
Bibcode: 2019arXiv191014022S
Altcode:
  This textbook gives a perspective of heliophysics in a way that
  emphasizes universal processes from a perspective that draws attention
  to what provides Earth (and similar (exo-)planets) with a relatively
  stable setting in which life as we know it can thrive. The book is
  intended for students in physical sciences in later years of their
  university training and for beginning graduate students in fields of
  solar, stellar, (exo-)planetary, and planetary-system sciences.

Title: Opposite Polarity Magnetic Fields and Convective Downflows
    in a Simulated Sunspot Penumbra
Authors: Bharti, Lokesh; Rempel, Matthias
Bibcode: 2019ApJ...884...94B
Altcode: 2019arXiv190806439B
  Recent numerical simulations and observations of sunspots show a
  significant amount of opposite polarity magnetic fields within the
  sunspot penumbra. Most of the opposite polarity fields are associated
  with convective downflows. We present an analysis of 3D MHD simulations
  through forward modeling of synthetic Stokes profiles of the Fe I
  6301.5 Å and Fe I 6302.5 Å lines. The synthetic Stokes profiles
  are spatially and spectrally degraded considering typical instrument
  properties. Line bisector shifts of the Fe I 6301.5 Å line are
  used to determine line-of-sight velocities. Far wing magnetograms are
  constructed from the Stokes V profiles of the Fe I 6302.5 Å line. While
  we find an overall good agreement between observations and simulations,
  the fraction of opposite polarity magnetic fields, the downflow filling
  factor, and the opposite polarity-downflow association are strongly
  affected by spatial smearing and presence of strong gradients in the
  line-of-sight magnetic fields and velocity. A significant fraction
  of opposite polarity magnetic fields and downflows is hidden in the
  observations due to typical instrumental noise. Comparing simulations
  that differ by more than a factor of two in grid spacing, we find that
  these quantities are robust within the simulations.

Title: What the Sudden Death of Solar Cycles Can Tell Us About the
    Nature of the Solar Interior
Authors: McIntosh, Scott W.; Leamon, Robert J.; Egeland, Ricky;
   Dikpati, Mausumi; Fan, Yuhong; Rempel, Matthias
Bibcode: 2019SoPh..294...88M
Altcode: 2019arXiv190109083M
  We observe the abrupt end of solar-activity cycles at the Sun's
  Equator by combining almost 140 years of observations from ground and
  space. These "terminator" events appear to be very closely related to
  the onset of magnetic activity belonging to the next solar cycle at
  mid-latitudes and the polar-reversal process at high latitudes. Using
  multi-scale tracers of solar activity we examine the timing of these
  events in relation to the excitation of new activity and find that the
  time taken for the solar plasma to communicate this transition is of
  the order of one solar rotation - but it could be shorter. Utilizing
  uniquely comprehensive solar observations from the Solar Terrestrial
  Relations Observatory (STEREO) and Solar Dynamics Observatory (SDO)
  we see that this transitional event is strongly longitudinal in
  nature. Combined, these characteristics suggest that information
  is communicated through the solar interior rapidly. A range of
  possibilities exist to explain such behavior: for example gravity
  waves on the solar tachocline, or that the magnetic fields present
  in the Sun's convection zone could be very large, with a poloidal
  field strengths reaching 50 kG - considerably larger than conventional
  explorations of solar and stellar dynamos estimate. Regardless of the
  mechanism responsible, the rapid timescales demonstrated by the Sun's
  global magnetic-field reconfiguration present strong constraints on
  first-principles numerical simulations of the solar interior and,
  by extension, other stars.

Title: Reversed Dynamo at Small Scales and Large Magnetic Prandtl
    Number
Authors: Brandenburg, Axel; Rempel, Matthias
Bibcode: 2019ApJ...879...57B
Altcode: 2019arXiv190311869B
  We show that at large magnetic Prandtl numbers, the Lorentz force does
  work on the flow at small scales and drives fluid motions, whose energy
  is dissipated viscously. This situation is the opposite of that in a
  normal dynamo, where the flow does work against the Lorentz force. We
  compute the spectral conversion rates between kinetic and magnetic
  energies for several magnetic Prandtl numbers and show that normal
  (forward) dynamo action occurs on large scales over a progressively
  narrower range of wavenumbers as the magnetic Prandtl number is
  increased. At higher wavenumbers, reversed dynamo action occurs,
  i.e., magnetic energy is converted back into kinetic energy at small
  scales. We demonstrate this in both direct numerical simulations
  forced by volume stirring and in large eddy simulations (LESs) of solar
  convectively driven small-scale dynamos. Low-density plasmas such as
  stellar coronae tend to have large magnetic Prandtl numbers, i.e., the
  viscosity is large compared with the magnetic diffusivity. The regime
  in which viscous dissipation dominates over resistive dissipation for
  large magnetic Prandtl numbers was also previously found in LESs of the
  solar corona, i.e., our findings are a more fundamental property of MHD
  that is not just restricted to dynamos. Viscous energy dissipation is a
  consequence of positive Lorentz force work, which may partly correspond
  to particle acceleration in close-to-collisionless plasmas. This is,
  however, not modeled in the MHD approximation employed. By contrast,
  resistive energy dissipation on current sheets is expected to be
  unimportant in stellar coronae.

Title: Radiative MHD Simulation of a Solar Flare
Authors: Cheung, Mark; Rempel, Matthias D.; Chintzoglou, Georgios;
   Chen, Feng; Testa, Paola; Martinez-Sykora, Juan; Sainz Dalda, Alberto;
   DeRosa, Marc L.; Malanushenko, Anna; Hansteen, Viggo; Carlsson, Mats;
   De Pontieu, Bart; Gudiksen, Boris; McIntosh, Scott W.
Bibcode: 2019AAS...23431005C
Altcode:
  We present a radiative MHD simulation of a solar flare. The
  computational domain captures the near-surface layers of the convection
  zone and overlying atmosphere. Inspired by the observed evolution of
  NOAA Active Region (AR) 12017, a parasitic bipolar region is imposed
  to emerge in the vicinity of a pre-existing sunspot. The emergence of
  twisted magnetic flux generates shear flows that create a pre-existing
  flux rope underneath the canopy field of the sunspot. Following erosion
  of the overlying bootstrapping field, the flux rope erupts. Rapid
  release of magnetic energy results in multi-wavelength synthetic
  observables (including X-ray spectra, narrowband EUV images, Doppler
  shifts of EUV lines) that are consistent with flare observations. This
  works suggests the super-position of multi-thermal, superhot (up
  to 100 MK) plasma may be partially responsible for the apparent
  non-thermal shape of coronal X-ray sources in flares. Implications
  for remote sensing observations of other astrophysical objects is also
  discussed. This work is an important stepping stone toward high-fidelity
  data-driven MHD models.

Title: On the Challenges of synthetizing solar and stellar spectra
    for Irradiance reconstructions
Authors: Criscuoli, Serena; Rempel, Matthias D.; Haberreiter, Margit;
   Pereira, Tiago; Uitenbroek, Han; Fabbian, Damian
Bibcode: 2019AAS...23421702C
Altcode:
  Syntheses of solar and stellar spectra strongly depend on the adopted
  approximations and atomic and molecular databases. We compare LTE and
  NLTE syntheses of solar spectra obtained with widely used radiative
  transfer codes, utilizing both 3D-MHD simulations and 1D-static
  atmosphere models. We show that although different codes reproduce
  reasonably well the observed spectrum, subtle differences may translate
  into discrepancies of several tens of percents in the estimate of
  solar and stellar spectral irradiance variability.

Title: Constraining non-linear dynamo models using quasi-biennial
    oscillations from sunspot area data
Authors: Inceoglu, F.; Simoniello, R.; Arlt, R.; Rempel, M.
Bibcode: 2019A&A...625A.117I
Altcode: 2019arXiv190403724I
  Context. Solar magnetic activity exhibits variations with periods
  between 1.5 and 4 years, the so-called quasi-biennial oscillations
  (QBOs), in addition to the well-known 11-year Schwabe cycles. Solar
  dynamo is thought to be the mechanism responsible for the generation
  of QBOs. <BR /> Aims: In this work, we analyse sunspot areas to
  investigate the spatial and temporal behaviour of the QBO signal
  and study the physical mechanisms responsible using simulations from
  fully non-linear mean-field flux-transport dynamos. <BR /> Methods:
  We investigated the behaviour of the QBOs in the sunspot area data for
  the full disk, and the northern and southern hemispheres, using wavelet
  and Fourier analyses. We also ran solar dynamos with two different
  approaches to generating a poloidal field from an existing toroidal
  field, namely Babcock-Leighton and turbulent α mechanisms. We then
  studied the simulated magnetic field strengths as well as meridional
  circulation and differential rotation rates using the same methods. <BR
  /> Results: The results from the sunspot areas show that the QBOs are
  present in the full disk and hemispheric sunspot areas. These QBOs show
  slightly different spatial and temporal behaviours, indicating slightly
  decoupled solar hemispheres. The QBO signal is generally intermittent
  and in-phase with the sunspot area data, surfacing when the solar
  activity is at its maximum. The results from the BL-dynamos show that
  they are neither capable of generating the slightly decoupled behaviour
  of solar hemispheres nor can they generate QBO-like signals. The
  turbulent α-dynamos on the other hand generated decoupled hemispheres
  and some QBO-like shorter cycles. <BR /> Conclusions: In conclusion,
  our simulations show that the turbulent α-dynamos with the Lorentz
  force seem more efficient in generating the observed temporal and
  spatial behaviour of the QBO signal compared with the BL-dynamos.

Title: Major Scientific Challenges and Opportunities in Understanding
    Magnetic Reconnection and Related Explosive Phenomena throughout
    the Universe
Authors: Ji, Hantao; Alt, A.; Antiochos, S.; Baalrud, S.; Bale, S.;
   Bellan, P. M.; Begelman, M.; Beresnyak, A.; Blackman, E. G.; Brennan,
   D.; Brown, M.; Buechner, J.; Burch, J.; Cassak, P.; Chen, L. -J.;
   Chen, Y.; Chien, A.; Craig, D.; Dahlin, J.; Daughton, W.; DeLuca, E.;
   Dong, C. F.; Dorfman, S.; Drake, J.; Ebrahimi, F.; Egedal, J.; Ergun,
   R.; Eyink, G.; Fan, Y.; Fiksel, G.; Forest, C.; Fox, W.; Froula, D.;
   Fujimoto, K.; Gao, L.; Genestreti, K.; Gibson, S.; Goldstein, M.;
   Guo, F.; Hesse, M.; Hoshino, M.; Hu, Q.; Huang, Y. -M.; Jara-Almonte,
   J.; Karimabadi, H.; Klimchuk, J.; Kunz, M.; Kusano, K.; Lazarian,
   A.; Le, A.; Li, H.; Li, X.; Lin, Y.; Linton, M.; Liu, Y. -H.; Liu,
   W.; Longcope, D.; Louriero, N.; Lu, Q. -M.; Ma, Z. -W.; Matthaeus,
   W. H.; Meyerhofer, D.; Mozer, F.; Munsat, T.; Murphy, N. A.; Nilson,
   P.; Ono, Y.; Opher, M.; Park, H.; Parker, S.; Petropoulou, M.; Phan,
   T.; Prager, S.; Rempel, M.; Ren, C.; Ren, Y.; Rosner, R.; Roytershteyn,
   V.; Sarff, J.; Savcheva, A.; Schaffner, D.; Schoeffier, K.; Scime, E.;
   Shay, M.; Sitnov, M.; Stanier, A.; TenBarge, J.; Tharp, T.; Uzdensky,
   D.; Vaivads, A.; Velli, M.; Vishniac, E.; Wang, H.; Werner, G.; Xiao,
   C.; Yamada, M.; Yokoyama, T.; Yoo, J.; Zenitani, S.; Zweibel, E.
Bibcode: 2019BAAS...51c...5J
Altcode: 2019astro2020T...5J
  This is a group white paper of 100 authors (each with explicit
  permission via email) from 51 institutions on the topic of magnetic
  reconnection which is relevant to 6 thematic areas. Grand challenges
  and research opportunities are described in observations, numerical
  modeling and laboratory experiments in the upcoming decade.

Title: The Solar Photospheric Continuum Brightness as a Function of
    Mean Magnetic Flux Density. I. The Role of the Magnetic Structure
    Size Distribution
Authors: Peck, C. L.; Rast, M. P.; Criscuoli, S.; Rempel, M.
Bibcode: 2019ApJ...870...89P
Altcode:
  Solar irradiance models indicate that irradiance variations are
  dominated by changes in the disk-coverage of magnetic structures,
  whose brightness is thought to be determined by their size and average
  magnetic flux density. Recent results suggest that the brightness of
  small-scale magnetic structures also depends on the mean magnetic flux
  of the extended region surrounding them due to reduced convective
  vigor. Low spatial resolution, however, may limit the ability to
  distinguish the role of magnetic structure size distributions from that
  of the mean magnetic flux. Using high-resolution 3D MHD simulations,
  we investigate the brightness of magnetic structures embedded in
  regions characterized by different mean magnetic flux. In agreement
  with previous results, we find reduced brightness with increasing
  mean magnetic flux when comparing the pixel-by-pixel continuum
  brightness versus magnetic field strength. Evaluating equivalently
  sized magnetic structures, however, we find no significant dependence
  of the magnetic structure brightness on the mean magnetic flux of the
  region in which they are embedded. Rather, we find that simulations
  with larger mean magnetic flux generate larger, and therefore darker,
  magnetic structures whose contributions result in an overall darkening
  of the region. The differences in magnetic structure size distributions
  alone can explain the reduced brightness of regions with larger mean
  magnetic flux. This implies that, for the range of mean magnetic flux
  of the simulations, convective suppression plays at most a secondary
  role in determining radiative output of magnetized regions. Quantifying
  the role of convective transport over a wider range of mean magnetic
  flux is the subject of the second paper in this series.

Title: Solar Eruptions during Magnetic Flux Emergence from the
    Convection Zone to the Corona
Authors: Chen, Feng; Fan, Yuhong; Rempel, Matthias; Nimmo, Kenzie
Bibcode: 2018cosp...42E.599C
Altcode:
  We present a realistic numerical model of magnetic flux emergence from
  the convection zone to the corona. The magnetic and velocity fields from
  a solar convective dynamo simulation are used as a time-dependent bottom
  boundary to drive the radiation magnetohydrodynamic simulations. The
  sophisticated treatments on the radiation and thermal conduction in
  the simulation allow a direct comparison between model synthesized
  observables and real observations. The main results are: (1) The quiet
  Sun corona is heated to over 1 MK by the energy flux provided by the
  small-scale magnetic field that is maintained by a local dynamo. (2)
  Emerging flux bundles create several active regions in a 200 Mm
  wide domain. The coronal temperature is significantly increased as
  active regions are forming at the photosphere. (3) Synthetic EUV
  images show coronal loops with various lengths and temperature. (4)
  More than 100 flares, with 1/3 reaching C class and above, occur in
  the simulation. The magnetic energy is mostly release through the
  work done by the Lorentz force, which is quickly thermalized by the
  viscosity. Moreover, the energy released during the flares and soft
  X-ray flux, i.e., the flare class nicely reproduce the relationship
  derived from observations. (5) The biggest flare reaches M2.5 and
  releases about 5e31 ergs magnetic energy. Plasma in cusp-shaped
  post-flare loops is heated to several tens MK. The flare is accompanied
  by the ejection of a giant flux rope that originates from highly
  sheared magnetic field at the polarity inversion line of a sunspot pair.

Title: Small-scale Dynamo Simulations: Magnetic Field Amplification
    in Exploding Granules and the Role of Deep and Shallow Recirculation
Authors: Rempel, M.
Bibcode: 2018ApJ...859..161R
Altcode: 2018arXiv180508390R
  We analyze recent high-resolution photospheric small-scale dynamo
  simulations that were computed with the MURaM radiative MHD code. We
  focus our analysis on newly forming downflow lanes in exploding
  granules, as they show how weakly magnetized regions in the photosphere
  (the center of granules) evolve into strongly magnetized regions
  (downflow lanes). We find that newly formed downflow lanes initially
  exhibit mostly a laminar converging flow that amplifies the vertical
  magnetic field embedded in the granule from a few 10 G to field
  strengths exceeding 800 G. This results in extended magnetic sheets that
  have a length comparable to granular scales. Field amplification by
  turbulent shear first happens a few 100 km beneath the visible layers
  of the photosphere. Shallow recirculation transports the resulting
  turbulent field into the photosphere within minutes, after which
  the newly formed downflow lane shows a mix of strong magnetic sheets
  and turbulent field components. We stress in particular the role of
  shallow and deep recirculation for the organization and strength of
  magnetic field in the photosphere and discuss the photospheric and
  sub-photospheric energy conversion associated with the small-scale
  dynamo process. While the energy conversion through the Lorentz
  force depends only weakly on the saturation field strength (and
  therefore deep or shallow recirculation), it is strongly dependent
  on the magnetic Prandtl number. We discuss the potential of these
  findings for further constraining small-scale dynamo models through
  high-resolution observations.

Title: Vector Magnetograms - From Photosphere to the Base of the
    Solar Corona
Authors: Malanushenko, Anna V.; Rempel, Matthias; Cheung, Chun
   Ming Mark
Bibcode: 2018tess.conf20234M
Altcode:
  The magnetic field in solar active regions is currently a major
  topic of research in solar physics. While hard to measure directly,
  it is commonly modeled with the use of photospheric magnetograms. An
  assumption that is often made in such modeling is that the plasma
  beta is small in the rarefied corona and therefore an equilibrium
  configuration requires that the Lorentz force vanishes through
  the volume. While this assumption greatly simplifies the modeling,
  it also complicates the use of the photospheric magnetic field as a
  boundary condition, as the photosphere is not in general a low-beta
  environment. While vector magnetograms at the base of the low-beta
  corona are not routinely available, the photospheric magnetograms
  continue to be widely used for coronal modeling. Additional steps,
  such as pre-processing, can be taken during the modeling to make these
  data as consistent with the low-beta equilibria as possible. In this
  work, we attempt to analyze how much do magnetograms of the coronal
  base differ from those of the photosphere, analyze their morphology,
  magnitude and how they change with height. For this, we analyze some
  of the most realistic full-MHD simulations of active regions made
  with MURaM code. They simulation volume includes upper convection
  zone, photosphere, transition region, and the corona. While they are
  not simulations of a specific active region, they appear extremely
  realistic in wide range of diagnostics, from the magnetic field in the
  photosphere, to the coronal morphology, to evolution typically observed
  in active regions. We study these simulations and the synthetic data
  they produce, focusing on the applicability of vector magnetograms to
  low-beta coronal magnetic modeling. We also describe some alternative
  methods of gathering vector magnetograms of the chromosphere from
  the coronal morphology, and compare them to the actual structures of
  the simulations.

Title: Statistical study of the release of magnetic energy during
    flares in a large-scale MHD simulation
Authors: Chen, Feng; Nimmo, Kenzie; Rempel, Matthias; Fan, Yuhong
Bibcode: 2018tess.conf10421C
Altcode:
  We analyze how the magnetic energy is release and converted into
  other forms of energy in (the impulsive phase of) flares that
  occur in a large scale realistic MHD simulation of magnetic flux
  emergence from the convection zone to the corona. The magnetic
  and velocity fields from a solar convective dynamo simulation are
  used as a time-dependent bottom boundary to drive the radiation
  magnetohydrodynamic simulations. "Realistic" referred to that the
  sophisticated treatments on the radiation and thermal conduction in the
  simulation allow a direct and quantitative comparison between model
  synthesized observables and real observations. The main results are:
  (1) The quiet Sun corona is heated to over 1 MK by the energy flux
  provided by the small-scale magnetic field that is maintained by a
  local dynamo. Emerging flux bundles bring more than 10<SUP>23</SUP>
  Mx flux to the photosphere in a period of about 50 hours and give rise
  to several active regions. The coronal temperature is significantly
  increased as active regions are forming at the photosphere. (2) More
  than 100 flares, which are identified by peaks in the magnetic energy
  releasing rate, occur in the simulation. Synthesized GOES soft X-ray
  flux shows that about 1/3 of them reaching C class and above. The
  largest one reaches M2.5 and releases about 5e31 ergs of magnetic
  energy, and is associated with a flux rope ejection. (3) The magnetic
  energy is mostly release through the work done by the Lorentz force,
  which is quickly thermalized by the viscosity, i.e. converted to the
  internal energy of the plasma. Then above half of the energy released
  is taken away by the radiative loss during the impulsive phase. (4)
  The synthesized GOES soft X-ray flux, i.e., the flare class is well
  correlated with the magnetic energy released during the flares. The
  relation shows that an M (X) class flare corresponds to 10<SUP>31</SUP>
  (10<SUP>32</SUP> ) ergs of magnetic energy released.

Title: Transport of Internetwork Magnetic Flux Elements in the Solar
Photosphere : Signatures of Large-Scale Flows and their Effect on
    Transport Statistics
Authors: Agrawal, Piyush; Rast, Mark; Gosic, Milan; Rempel, Matthias;
   Bellot Rubio, Luis
Bibcode: 2018tess.conf21704A
Altcode:
  The motions of small-scale magnetic &lt;span class="s1" flux elements
  in the solar photosphere can provide some measure of the Lagrangian
  properties of the convective &lt;span class="s1" flow. Measurements of
  these motions have been critical in estimating the turbulent diffusion
  coef&lt;span class="s1" ficient in &lt;span class="s1" flux-transport
  dynamo models and in determining the Alfvén wave excitation spectrum
  for coronal heating models. We examine the motions of internetwork
  &lt;span class="s1" flux elements in Hinode&lt;span class="s1"
  /Narrowband Filter Imager magnetograms and study the scaling of
  their mean squared displacement and the shape of their displacement
  probability distribution as a function of time. We &lt;span class="s1"
  find that the mean squared displacement scales super-diffusively with
  a slope of about 1.48. Super-diffusive scaling has been observed in
  other studies for temporal increments as small as 5 s, increments over
  which ballistic scaling would be expected. Using high-cadence MURaM
  simulations, we show that the observed super-diffusive scaling at short
  increments is a consequence of random changes in barycenter positions
  due to &lt;span class="s1" flux evolution. We also &lt;span class="s1"
  find that for long temporal increments, beyond granular lifetimes,
  the observed displacement distribution deviates from that expected
  for a diffusive process, evolving from Rayleigh to Gaussian. This
  change in distribution can be modeled analytically by accounting for
  supergranular advection along with granular motions. These results
  complicate the interpretation of magnetic element motions as strictly
  advective or diffusive on short and long timescales and suggest that
  measurements of magnetic element motions must be used with caution
  in turbulent diffusion or wave excitation models. We propose that
  passive tracer motions in measured photospheric &lt;span class="s1"
  flows may yield more robust transport statistics.

Title: Measuring the Spatio-temporal Statistics of Magnetic Flux
    Emergence
Authors: Lamb, Derek A.; Glueck, Deborah; Rempel, Matthias
Bibcode: 2018tess.conf21163L
Altcode:
  The large-scale solar magnetic field, in the form of sunspots and the
  associated active regions, exhibits more-or-less predictable patterns
  of flux emergence associated with the solar cycle: cycle periods fall
  in a small range of 9-13 years, and sunspot emergence in each cycle
  starts at latitudes of approximately 30 degrees and progresses towards
  the equators. The small-scale magnetic field is observed at all phases
  of the solar cycle and at all latitudes. Do the properties of flux
  emergence change at different scales, or are there smooth transitions
  between small- and large-scale flux emergence? We describe our first
  steps toward addressing this question, by algorithmically and manually
  identifying flux emergence in sequences of SDO/HMI magnetograms. We
  measure several properties of the individual flux emergence events, such
  as the flux emergence rate, the bipole orientation and separation speed,
  and compare the statistical distributions of these properties as a
  function of the total emerged flux. We make some preliminary comparisons
  to flux emergence events identified in small-scale dynamo simulations.

Title: Simulations of quiet Sun magnetism: On the role of deep and
    shallow recirculation in small-scale dynamo simulations
Authors: Rempel, Matthias
Bibcode: 2018tess.conf11505R
Altcode:
  Observations suggest that small-scale magnetic field in the solar
  photosphere is mostly independent from the strength of nearby network
  field as well as independent of the solar cycle. This supports the
  view that the origin of small-scale magnetism is due to a small-scale
  dynamo that operates independently from the large-scale dynamo
  responsible for the solar cycle. The saturation field strength and
  structure of the resulting magnetic field in the photosphere depends
  critically on the contributions from deep and shallow recirculation
  within the strongly stratified convection zone. We analyze recent
  high resolution photospheric small-scale dynamo simulations that were
  computed with the MURaM radiative MHD code. We focus the analysis on
  newly forming downflow lanes in exploding granules since they show
  how weakly magnetized regions in the photosphere (center of granules)
  evolve into the most strongly magnetized regions (downflow lanes). We
  find that newly formed downflow lanes exhibit initially mostly a
  laminar converging flow that amplifies the vertical magnetic field
  embedded in the granule from initially a few 10 G to field strengths
  of up to 1 kG on a time scale of about 2 minutes. This results in
  extended magnetic sheets that have a length comparable to granular
  scales. These sheets are a consequence of deep recirculation. Field
  amplification by turbulent shear happens first a few 100 km beneath the
  visible layers of the photosphere. Shallow recirculation transports
  the resulting turbulent field into the photosphere within minutes,
  after which the newly formed downflow lane shows a mix of strong
  magnetic sheets and turbulent field components. Furthermore, deep
  recirculation leads to a magnetic flux imbalance on larger scales
  that can maintain a quiet Sun (mixed polarity) magnetic network solely
  through small-scale dynamo action. We discuss the potential of these
  findings for further constraining small-scale dynamo models through
  high resolution observations.

Title: Transport of Internetwork Magnetic Flux Elements in the
    Solar Photosphere
Authors: Agrawal, Piyush; Rast, Mark P.; Gošić, Milan; Bellot Rubio,
   Luis R.; Rempel, Matthias
Bibcode: 2018ApJ...854..118A
Altcode: 2017arXiv171101290A
  The motions of small-scale magnetic flux elements in the solar
  photosphere can provide some measure of the Lagrangian properties of
  the convective flow. Measurements of these motions have been critical
  in estimating the turbulent diffusion coefficient in flux-transport
  dynamo models and in determining the Alfvén wave excitation spectrum
  for coronal heating models. We examine the motions of internetwork
  flux elements in Hinode/Narrowband Filter Imager magnetograms and
  study the scaling of their mean squared displacement and the shape of
  their displacement probability distribution as a function of time. We
  find that the mean squared displacement scales super-diffusively with
  a slope of about 1.48. Super-diffusive scaling has been observed in
  other studies for temporal increments as small as 5 s, increments
  over which ballistic scaling would be expected. Using high-cadence
  MURaM simulations, we show that the observed super-diffusive scaling
  at short increments is a consequence of random changes in barycenter
  positions due to flux evolution. We also find that for long temporal
  increments, beyond granular lifetimes, the observed displacement
  distribution deviates from that expected for a diffusive process,
  evolving from Rayleigh to Gaussian. This change in distribution can be
  modeled analytically by accounting for supergranular advection along
  with granular motions. These results complicate the interpretation
  of magnetic element motions as strictly advective or diffusive on
  short and long timescales and suggest that measurements of magnetic
  element motions must be used with caution in turbulent diffusion or
  wave excitation models. We propose that passive tracer motions in
  measured photospheric flows may yield more robust transport statistics.

Title: Evershed and Counter-Evershed Flows in Sunspot MHD Simulations
Authors: Siu-Tapia, A. L.; Rempel, M.; Lagg, A.; Solanki, S. K.
Bibcode: 2018ApJ...852...66S
Altcode: 2017arXiv171201202S
  There have been a few reports in the literature of counter-Evershed
  flows observed in well-developed sunspot penumbrae, i.e., flows
  directed toward the umbra along penumbral filaments. Here, we
  investigate the driving forces of such counter-Evershed flows in a
  radiative magnetohydrodynamic simulation of a sunspot, and compare
  them to the forces acting on the normal Evershed flow. The simulation
  covers a timespan of 100 solar hours and generates an Evershed outflow
  exceeding 8 km s<SUP>-1</SUP> in the penumbra along radially aligned
  filaments where the magnetic field is almost horizontal. Additionally,
  the simulation produces a fast counter-Evershed flow (i.e., an inflow
  near τ =1) in some regions within the penumbra, reaching peak flow
  speeds of ∼12 km s<SUP>-1</SUP>. The counter-Evershed flows are
  transient and typically last a few hours before they turn into outflows
  again. By using the kinetic energy equation and evaluating its various
  terms in the simulation box, we found that the Evershed flow occurs
  due to overturning convection in a strongly inclined magnetic field,
  while the counter-Evershed flows can be well-described as siphon flows.

Title: Terminator 2020: Get Ready for the "Event" of The Next Decade
Authors: McIntosh, S. W.; Leamon, R. J.; Fan, Y.; Rempel, M.;
   Dikpati, M.
Bibcode: 2017AGUFMSH22B..06M
Altcode:
  The abrupt end of solar activity cycles 22 and 23 at the Sun's
  equator are observed with instruments from the Solar and Heliospheric
  Observatory (SOHO), Solar Terrestrial Relations Observatory (STEREO),
  and Solar Dynamics Observatory (SDO). These events are remarkable in
  that they rapidly trigger the onset of magnetic activity belonging
  to the next solar cycle at mid-latitudes. The triggered onset of new
  cycle flux emergence leads to blossoming of the new cycle shortly
  thereafter. Using small-scale tracers of magnetic solar activity we
  examine the timing of the cycle ``termination points'' in relation
  to the excitation of new activity and find that the time taken
  for the solar plasma to communicate this transition is less than
  one solar rotation, and possibly as little as a eight days. This
  very short transition time implies that the mean magnetic field
  present in the Sun's convection zone is approximately 80 kG. This
  value may be considerably larger than conventional explorations
  estimate and therefore, have a significant dynamical impact on the
  physical appearance of solar activity, and considerably impacting
  our ability to perform first-principles numerical simulations of the
  same. Should solar cycle 24 [and 25] continue in their progression
  we anticipate that a termination event of this type should occur in
  the 2020 timeframe. PSP will have a front row seat to observe this
  systemic flip in solar magnetism and the induced changes in our star's
  radiative and partiuculate output. Such observations may prove to be
  critical in assessing the Sun's ability to force short term evolution
  in the Earth's atmosphere.

Title: Numerical MHD Coronal Simulations: Energy Statistics and
    FORWARD Analysis.
Authors: Nimmo, K.; Rempel, M.; Chen, F.; Gibson, S. E.; Fan, Y.
Bibcode: 2017AGUFMSH43A2800N
Altcode:
  We analyse a recent realistic radiative MHD simulation of the solar
  corona that was computed with the extended version of the MURaM
  code. The simulation covers the uppermost 8Mm of the solar convection
  zone and reaches 115Mm into the solar corona. The simulation covers 48
  hours of solar time and simulates the evolution of a complex active
  region. The energy release in the corona is highly intermittent and
  we identify a total of 118 individual events including flares and a
  coronal mass ejection, which we analyse in further detail. From the
  simulation we compute an X-ray flux mimicking observations by the GOES
  (Geostationary Operational Environmental Satellite) satellite in the
  wavelength range 1-8 Å. The power law index for the GOES X-ray flux
  for flares of class C and above in this simulation is found to be
  1.33452. We analyze the correlation between synthetic coronal emission
  during flares and the magnetic energy release in the corona. The latter
  is a quantity that cannot be directly determined in observations.The
  FORWARD code is a tool used for the purpose of coronal magnetometry. It
  can be used to compute synthetic observables from coronal models. We
  focus on the interpretation of the High Altitude Observatory's CoMP
  observations. The CoMP (COronal Multi-channel Polarimeter) instrument
  measures the intensity and the linear and circular polarisation of
  FeXIII at 1074.7nm.We discuss some important limitations of coronal
  emission line polarimetry when simulating an extremely active solar
  region, with emphasis on the influence of high velocities, temperatures
  and densities on the FORWARD output.

Title: The Nature of Grand Minima and Maxima from Fully Nonlinear
    Flux Transport Dynamos
Authors: Inceoglu, Fadil; Arlt, Rainer; Rempel, Matthias
Bibcode: 2017ApJ...848...93I
Altcode: 2017arXiv171008644I
  We aim to investigate the nature and occurrence characteristics of
  grand solar minimum and maximum periods, which are observed in the
  solar proxy records such as <SUP>10</SUP>Be and <SUP>14</SUP>C, using
  a fully nonlinear Babcock-Leighton type flux transport dynamo including
  momentum and entropy equations. The differential rotation and meridional
  circulation are generated from the effect of turbulent Reynolds stress
  and are subjected to back-reaction from the magnetic field. To generate
  grand minimum- and maximum-like periods in our simulations, we used
  random fluctuations in the angular momentum transport process, namely
  the Λ-mechanism, and in the Babcock-Leighton mechanism. To characterize
  the nature and occurrences of the identified grand minima and maxima
  in our simulations, we used the waiting time distribution analyses,
  which reflect whether the underlying distribution arises from a random
  or a memory-bearing process. The results show that, in the majority of
  the cases, the distributions of grand minima and maxima reveal that the
  nature of these events originates from memoryless processes. We also
  found that in our simulations the meridional circulation speed tends
  to be smaller during grand maximum, while it is faster during grand
  minimum periods. The radial differential rotation tends to be larger
  during grand maxima, while it is smaller during grand minima. The
  latitudinal differential rotation, on the other hand, is found to be
  larger during grand minima.

Title: Emergence of Magnetic Flux Generated in a Solar Convective
    Dynamo. I. The Formation of Sunspots and Active Regions, and The
    Origin of Their Asymmetries
Authors: Chen, Feng; Rempel, Matthias; Fan, Yuhong
Bibcode: 2017ApJ...846..149C
Altcode: 2017arXiv170405999C
  We present a realistic numerical model of sunspot and active region
  formation based on the emergence of flux bundles generated in a solar
  convective dynamo. To this end, we use the magnetic and velocity fields
  in a horizontal layer near the top boundary of the solar convective
  dynamo simulation to drive realistic radiative-magnetohydrodynamic
  simulations of the uppermost layers of the convection zone. The
  main results are as follows. (1) The emerging flux bundles rise with
  the mean speed of convective upflows and fragment into small-scale
  magnetic elements that further rise to the photosphere, where bipolar
  sunspot pairs are formed through the coalescence of the small-scale
  magnetic elements. (2) Filamentary penumbral structures form when the
  sunspot is still growing through ongoing flux emergence. In contrast
  to the classical Evershed effect, the inflow seems to prevail over the
  outflow in a large part of the penumbra. (3) A well-formed sunspot is a
  mostly monolithic magnetic structure that is anchored in a persistent
  deep-seated downdraft lane. The flow field outside the spot shows a
  giant vortex ring that comprises an inflow below 15 Mm depth and an
  outflow above 15 Mm depth. (4) The sunspots successfully reproduce the
  fundamental properties of the observed solar active regions, including
  the more coherent leading spots with a stronger field strength, and
  the correct tilts of bipolar sunspot pairs. These asymmetries can be
  linked to the intrinsic asymmetries in the magnetic and flow fields
  adapted from the convective dynamo simulation.

Title: Realistic radiative MHD simulation of a solar flare
Authors: Rempel, Matthias D.; Cheung, Mark; Chintzoglou, Georgios;
   Chen, Feng; Testa, Paola; Martinez-Sykora, Juan; Sainz Dalda, Alberto;
   DeRosa, Marc L.; Viktorovna Malanushenko, Anna; Hansteen, Viggo H.;
   De Pontieu, Bart; Carlsson, Mats; Gudiksen, Boris; McIntosh, Scott W.
Bibcode: 2017SPD....4840001R
Altcode:
  We present a recently developed version of the MURaM radiative
  MHD code that includes coronal physics in terms of optically thin
  radiative loss and field aligned heat conduction. The code employs
  the "Boris correction" (semi-relativistic MHD with a reduced speed
  of light) and a hyperbolic treatment of heat conduction, which allow
  for efficient simulations of the photosphere/corona system by avoiding
  the severe time-step constraints arising from Alfven wave propagation
  and heat conduction. We demonstrate that this approach can be used
  even in dynamic phases such as a flare. We consider a setup in which
  a flare is triggered by flux emergence into a pre-existing bipolar
  active region. After the coronal energy release, efficient transport
  of energy along field lines leads to the formation of flare ribbons
  within seconds. In the flare ribbons we find downflows for temperatures
  lower than ~5 MK and upflows at higher temperatures. The resulting
  soft X-ray emission shows a fast rise and slow decay, reaching a peak
  corresponding to a mid C-class flare. The post reconnection energy
  release in the corona leads to average particle energies reaching 50
  keV (500 MK under the assumption of a thermal plasma). We show that
  hard X-ray emission from the corona computed under the assumption of
  thermal bremsstrahlung can produce a power-law spectrum due to the
  multi-thermal nature of the plasma. The electron energy flux into the
  flare ribbons (classic heat conduction with free streaming limit) is
  highly inhomogeneous and reaches peak values of about 3x10<SUP>11</SUP>
  erg/cm<SUP>2</SUP>/s in a small fraction of the ribbons, indicating
  regions that could potentially produce hard X-ray footpoint sources. We
  demonstrate that these findings are robust by comparing simulations
  computed with different values of the saturation heat flux as well as
  the "reduced speed of light".

Title: 3D Collision of Active Region-Sized Emerging Flux Tubes in
    the Solar Convection Zone and its Manifestation in the Photospheric
    Surface
Authors: Chintzoglou, Georgios; Cheung, Mark; Rempel, Matthias D.
Bibcode: 2017SPD....4830004C
Altcode:
  We present observations obtained with the Solar Dynamics Observatory’s
  Helioseismic Magnetic Imager (SDO/HMI) of target NOAA Active Regions
  (AR) 12017 and 12644, which initially were comprised of a simple bipole
  and later on became quadrupolar via parasitic bipole emergence right
  next to their leading polarities. Once these ARs became quadrupolar,
  they spewed multiple Coronal Mass Ejections (CMEs) and a multitude
  of highly energetic flares (a large number of M class flares). The
  proximity of the parasitic bipole to one of the two pre-existing
  sunspots forms a compact polarity inversion line (PIL). This type of
  quadrupolar ARs are known to be very flare- and CME-productive due
  to the continuous interaction of newly emerging non-potential flux
  with pre-existing flux in the photosphere. We show that well before
  the emergence of the parasitic bipole, the pre-existing polarity
  (typically a well-developed sunspot) undergoes interesting precursor
  dynamic evolution, namely (a) displacement of pre-existing sunspot’s
  position, (b) progressive and significant oblateness of its initially
  nearly-circular shape, and (c) opposite polarity enhancement in the
  divergent moat flow around the sunspot. We employ high-resolution
  radiative-convective 3D MHD simulations of an emerging parasitic bipole
  to show that all these activity aspects seen in the photosphere are
  associated with the collision of a parasitic bipole with the nearby
  pre-existing polarity below the photospheric surface. Given the rich
  flare and CME productivity of this class of ARs and the precursor-like
  dynamic evolution of the pre-existing polarity, this work presents
  the potential for predicting inclement space weather.

Title: Realistic simulation of the emergence of magnetic field
    generated in a solar convective dynamo from the convection zone into
    the corona
Authors: Chen, Feng; Rempel, Matthias D.; Fan, Yuhong
Bibcode: 2017SPD....4840501C
Altcode:
  We present a comprehensive realistic numerical model of emergence of
  magnetic flux generated in a solar convective dynamo from the convection
  zone to the corona. The magnetic and velocity fields in a horizontal
  layer near the top boundary of the solar convective dynamo simulation
  are used as a time-dependent bottom boundary to drive the radiation
  magnetohydrodynamic simulations of the emergence of the flux bundles
  through the upper most convection zone to more than 100 Mm above the
  surface of the Sun. The simualtion allows a direct comparison bewtween
  model synthesized observable and real obervations of flux emergence
  processes through different layers of the solar atmopshere.Emerging
  flux bundles bring more than 1e23 Mx flux to the photosphere in a
  period of about 50 hours and give rise to several active regions in a
  horizontal domain of 200 Mm. The mean corona temperature is about 1 MK
  for the quiet Sun and is significantly increased after active regions
  form at the photosphere. The flux emergence process produces a lot of
  dynamical features, such as coronal bright points, jets, waves and
  propagating disturbances, as well as flares and mass ejections. The
  biggest flare reaches M2.5 as indicated by synthetic GOES-15 soft
  X-ray flux. The total magnetic energy released during the eruption is
  about 5e31 ergs. The flare leads to a significant corona heating. The
  mean temperature in the coronal reaches more than 5 MK. And plasma
  in cusp-shaped post-flare loops is heated to several tens MK. The
  flare is accompanied by the ejection of a giant flux rope that carries
  cool and dense plasma. The flux rope is formed during the eruption by
  the reconnection between a sheared arcade that rises up from the low
  atmosphere above a bipolar sunspot pair and overlying fieldlines that
  are mostly perpendicular to the axis of the sheared arcade.

Title: Characterizing the Motion of Photospheric Magnetic Bright
    Points at High Resolution
Authors: Van Kooten, Samuel Jay; Cranmer, Steven R.; Rempel, Matthias
Bibcode: 2017shin.confE..68V
Altcode:
  Magnetic bright points on the solar photosphere, visible in both
  continuum and G-band images, indicate footpoints of kilogauss magnetic
  flux tubes extending to the corona. The power spectrum of transverse
  bright point motion is thus also the power spectrum of Alfven wave
  excitation, with these waves transporting energy up flux tubes into
  the corona. This spectrum is a key input in coronal and heliospheric
  models. After briefly reviewing observations of bright point motion, we
  present a power spectrum of bright point motion derived from radiative
  MHD simulations, exploiting spatial resolution higher than can be
  obtained in observations while using automated tracking to produce
  large data quantities. We find slightly higher amounts of power at
  all frequencies compared to observational spectra while confirming
  the spectrum shape of recent observations. This provides a prediction
  for DKIST observations of bright points, which will achieve similar
  resolution. We also present results from tracing test particles
  in the horizontal plasma flow, finding similar power spectra but
  differing motion paths. Finally, we introduce a simplified, laminar
  model of granulation, with which we explore the roles of turbulence
  and of the properties of the granulation pattern in determining bright
  point motion.

Title: Are Internetwork Magnetic Fields in the Solar Photosphere
    Horizontal or Vertical?
Authors: Lites, B. W.; Rempel, M.; Borrero, J. M.; Danilovic, S.
Bibcode: 2017ApJ...835...14L
Altcode:
  Using many observations obtained during 2007 with the
  Spectro-Polarimeter of the Hinode Solar Optical Telescope, we explore
  the angular distribution of magnetic fields in the quiet internetwork
  regions of the solar photosphere. Our work follows from the insight of
  Stenflo, who examined only linear polarization signals in photospheric
  lines, thereby avoiding complications of the analysis arising from the
  differing responses to linear and circular polarization. We identify
  and isolate regions of a strong polarization signal that occupy only
  a few percent of the observed quiet Sun area yet contribute most to
  the net linear polarization signal. The center-to-limb variation of
  the orientation of linear polarization in these strong signal regions
  indicates that the associated magnetic fields have a dominant vertical
  orientation. In contrast, the great majority of the solar disk is
  occupied by much weaker linear polarization signals. The orientation of
  the linear polarization in these regions demonstrates that the field
  orientation is dominantly horizontal throughout the photosphere. We
  also apply our analysis to Stokes profiles synthesized from the
  numerical MHD simulations of Rempel as viewed at various oblique
  angles. The analysis of the synthetic data closely follows that of
  the observations, lending confidence to using the simulations as a
  guide for understanding the physical origins of the center-to-limb
  variation of linear polarization in the quiet Sun area.

Title: Extension of the MURaM Radiative MHD Code for Coronal
    Simulations
Authors: Rempel, M.
Bibcode: 2017ApJ...834...10R
Altcode: 2016arXiv160909818R
  We present a new version of the MURaM radiative magnetohydrodynamics
  (MHD) code that allows for simulations spanning from the upper
  convection zone into the solar corona. We implement the relevant
  coronal physics in terms of optically thin radiative loss, field
  aligned heat conduction, and an equilibrium ionization equation of
  state. We artificially limit the coronal Alfvén and heat conduction
  speeds to computationally manageable values using an approximation
  to semi-relativistic MHD with an artificially reduced speed of light
  (Boris correction). We present example solutions ranging from quiet
  to active Sun in order to verify the validity of our approach. We
  quantify the role of numerical diffusivity for the effective coronal
  heating. We find that the (numerical) magnetic Prandtl number determines
  the ratio of resistive to viscous heating and that owing to the very
  large magnetic Prandtl number of the solar corona, heating is expected
  to happen predominantly through viscous dissipation. We find that
  reasonable solutions can be obtained with values of the reduced speed
  of light just marginally larger than the maximum sound speed. Overall
  this leads to a fully explicit code that can compute the time evolution
  of the solar corona in response to photospheric driving using numerical
  time steps not much smaller than 0.1 s. Numerical simulations of the
  coronal response to flux emergence covering a time span of a few days
  are well within reach using this approach.

Title: Lower solar atmosphere and magnetism at ultra-high spatial
    resolution
Authors: Collet, Remo; Criscuoli, Serena; Ermolli, Ilaria; Fabbian,
   Damian; Guerreiro, Nuno; Haberreiter, Margit; Peck, Courtney; Pereira,
   Tiago M. D.; Rempel, Matthias; Solanki, Sami K.; Wedemeyer-Boehm, Sven
Bibcode: 2016arXiv161202348C
Altcode:
  We present the scientific case for a future space-based telescope
  aimed at very high spatial and temporal resolution imaging of the
  solar photosphere and chromosphere. Previous missions (e.g., HINODE,
  SUNRISE) have demonstrated the power of observing the solar photosphere
  and chromosphere at high spatial resolution without contamination from
  Earth's atmosphere. We argue here that increased spatial resolution
  (from currently 70 km to 25 km in the future) and high temporal cadence
  of the observations will vastly improve our understanding of the
  physical processes controlling solar magnetism and its characteristic
  scales. This is particularly important as the Sun's magnetic field
  drives solar activity and can significantly influence the Sun-Earth
  system. At the same time a better knowledge of solar magnetism can
  greatly improve our understanding of other astrophysical objects.

Title: 3D MHD simulation of a Solar Flare
Authors: Rempel, M.; Cheung, M. C. M.; HGCR Team
Bibcode: 2016usc..confE...4R
Altcode:
  We present results from a numerical 3D simulation of a solar flare
  triggered by flux emergence into a pre-existing bipolar active
  region. The simulation is performed with a recently developed version
  of the MURaM radiative MHD code and includes coronal physics in terms of
  optically thin radiative loss and field-aligned heat conduction. Severe
  time-step constraints arising from Alfven wave propagation and heat
  conduction are avoided through the use of the Boris correction and a
  hyperbolic treatment of heat conduction. In the simulation we find a
  flare releasing about 5x10^30 erg over a time of about 1-2 minutes. The
  efficient transport of energy along field lines leads to the formation
  of flare ribbons within seconds and at later times to chromospheric
  evaporation filling coronal flare loops. Since the efficiency of
  energy transport by electrons (classic heat conduction vs. non-thermal
  electrons) is one of the main uncertainties, we compare simulations
  with different values for the saturation of the heat flux. We present
  synthetic observables in the form of UV, EUV and soft and hard Xray
  emission.

Title: Observed and simulated power spectra of kinetic and magnetic
    energy retrieved with 2D inversions
Authors: Danilovic, S.; Rempel, M.; van Noort, M.; Cameron, R.
Bibcode: 2016A&A...594A.103D
Altcode: 2016arXiv160706242D
  Context. Information on the origin of internetwork magnetic field is
  hidden at the smallest spatial scales. <BR /> Aims: We try to retrieve
  the power spectra with certainty to the highest spatial frequencies
  allowed by current instrumentation. <BR /> Methods: To accomplish this,
  we use a 2D inversion code that is able to recover information up to
  the instrumental diffraction limit. <BR /> Results: The retrieved power
  spectra have shallow slopes that extend further down to much smaller
  scales than has been found before. They do not seem to show any power
  law. The observed slopes at subgranular scales agree with those obtained
  from recent local dynamo simulations. Small differences are found for
  the vertical component of kinetic energy that suggest that observations
  suffer from an instrumental effect that is not taken into account. <BR
  /> Conclusions: Local dynamo simulations quantitatively reproduce the
  observed magnetic energy power spectra on the scales of granulation
  down to the resolution limit of Hinode/SP, within the error bars
  inflicted by the method used and the instrumental effects replicated.

Title: Internetwork magnetic field as revealed by two-dimensional
    inversions
Authors: Danilovic, S.; van Noort, M.; Rempel, M.
Bibcode: 2016A&A...593A..93D
Altcode: 2016arXiv160700772D
  Context. Properties of magnetic field in the internetwork regions
  are still fairly unknown because of rather weak spectropolarimetric
  signals. <BR /> Aims: We address the matter by using the two-dimensional
  (2D) inversion code, which is able to retrieve the information on
  smallest spatial scales up to the diffraction limit, while being less
  susceptible to noise than most of the previous methods used. <BR />
  Methods: Performance of the code and the impact of various effects
  on the retrieved field distribution is tested first on the realistic
  magneto-hydrodynamic (MHD) simulations. The best inversion scenario
  is then applied to the real data obtained by Spectropolarimeter (SP)
  on board Hinode. <BR /> Results: Tests on simulations show that: (1)
  the best choice of node position ensures a decent retrieval of all
  parameters; (2) the code performs well for different configurations
  of magnetic field; (3) slightly different noise levels or slightly
  different defocus included in the spatial point spread function
  (PSF) produces no significant effect on the results; and (4)
  temporal integration shifts the field distribution to a stronger,
  more horizontally inclined field. <BR /> Conclusions: Although the
  contribution of the weak field is slightly overestimated owing to noise,
  2D inversions are able to recover well the overall distribution of the
  magnetic field strength. Application of the 2D inversion code on the
  Hinode SP internetwork observations reveals a monotonic field strength
  distribution. The mean field strength at optical depth unity is ~
  130 G. At higher layers, field strength drops as the field becomes
  more horizontal. Regarding the distribution of the field inclination,
  tests show that we cannot directly retrieve it with the observations
  and tools at hand, however, the obtained distributions are consistent
  with those expected from simulations with a quasi-isotropic field
  inclination after accounting for observational effects.

Title: A low upper limit on the subsurface rise speed of solar
    active regions
Authors: Birch, A. C.; Schunker, H.; Braun, D. C.; Cameron, R.; Gizon,
   L.; Lo ptien, B.; Rempel, M.
Bibcode: 2016SciA....2E0557B
Altcode: 2016arXiv160705250B
  Magnetic field emerges at the surface of the Sun as sunspots and active
  regions. This process generates a poloidal magnetic field from a rising
  toroidal flux tube, it is a crucial but poorly understood aspect of
  the solar dynamo. The emergence of magnetic field is also important
  because it is a key driver of solar activity. We show that measurements
  of horizontal flows at the solar surface around emerging active regions,
  in combination with numerical simulations of solar magnetoconvection,
  can constrain the subsurface rise speed of emerging magnetic flux. The
  observed flows imply that the rise speed of the magnetic field is
  no larger than 150 m/s at a depth of 20 Mm, that is, well below the
  prediction of the (standard) thin flux tube model but in the range
  expected for convective velocities at this depth. We conclude that
  convective flows control the dynamics of rising flux tubes in the upper
  layers of the Sun and cannot be neglected in models of flux emergence.

Title: Turbulent transport of Small-scale magnetic flux elements on
    Solar Photosphere
Authors: Agrawal, Piyush; Rempel, Matthias; Bellot Rubio, Luis;
   Rast, Mark
Bibcode: 2016SPD....47.1201A
Altcode:
  We study the transport of small-scale magnetic elements on the solar
  photosphere using both observations and simulations. Observational
  data was obtained from Hinode - Solar Optical Telescope (SOT/SP)
  instrument and simulations from MURaM code. The magnetic flux elements
  were tracked in both data sets and statistics were obtained. We compute
  the probability density of the Eulerian distances traveled by the flux
  elements along Lagrangian trajectories. For a two-dimensional random
  walk process this distribution should be Rayleigh. Preliminary results
  show that the measured probability distribution in both the observed
  and simulated data approximates a random walk, on time scale close to
  the lifetime of granules, but deviates from it for longer times. This
  implies that diffusion may not be an appropriate framework for transport
  in the solar photosphere. We explore the roles of flux cancelation
  and element trapping in producing this result. Work is ongoing.

Title: Forward and Inverse Modeling of Helioseismic Holography
    Measurements of MHD Simulations of Convection and Sunspot Flows
Authors: DeGrave, Kyle; Braun, Douglas; Birch, Aaron; Crouch, Ashley
   D.; Javornik, Brenda; Rempel, Matthias D.
Bibcode: 2016SPD....4720303D
Altcode:
  We test and validate newly-developed, empirically-derived sensitivity
  kernels for use in helioseismic analysis. These kernels are based on
  the Born approximation and derived from applying direct measurements to
  artificial realizations of incoming and scattered wavefields. These
  kernels are employed in a series of forward and inverse modeling
  of flows from the near-surface layers of two publicly available
  magnetohydrodynamic (MURaM-based) solar simulations - a quiet-Sun
  simulation, and one containing a sunspot. Forward travel times
  computed using the kernels generally compare favorably in non-magnetic
  regions. One finding of note is the presence of flow-like artifacts in
  the sunspot measurements which appear when the spot umbra or penumbra
  falls within the measurement pupils. Inversions for the horizontal flow
  components are able to reproduce the large-scale supergranule-sized
  flows in the upper 3Mm of both domains, but are compromised by noise
  at greater depths. In spite of the magnetic artifact, the moat flow
  surrounding the spot is at least qualitatively recovered. This work
  is supported by the NASA Heliophysics Division through NNH12CF68C,
  NNH12CF23C, and NNX16AG88G, and by the NSF Solar-Terrestrial Program
  through grant AGS-1127327.

Title: Formation of sunspots and active regions through the emergence
    of magnetic flux generated in a solar convective dynamo
Authors: Chen, Feng; Rempel, Matthias D.; Fan, Yuhong
Bibcode: 2016SPD....4730306C
Altcode:
  We present a realistic numerical model of sunspot and active region
  formation through the emergence of flux tubes generated in a solar
  convective dynamo. The magnetic and velocity fields in a horizontal
  layer near the top boundary of the solar convective dynamo simulation
  are used as a time-dependent bottom boundary to drive the near surface
  layer radiation MHD simulations of magneto-convection and flux emergence
  with the MURaM code. The latter code simulates the emergence of the
  flux tubes through the upper most layer of the convection zone to
  the photosphere.The emerging flux tubes interact with the convection
  and break into small scale magnetic elements that further rise to the
  photosphere. At the photosphere, several bipolar pairs of sunspots are
  formed through the coalescence of the small scale magnetic elements. The
  sunspot pairs in the simulation successfully reproduce the fundamental
  observed properties of solar active regions, including the more coherent
  leading spots with a stronger field strength, and the correct tilts
  of the bipolar pairs. These asymmetries come most probably from the
  intrinsic asymmetries in the emerging fields imposed at the bottom
  boundary, where the horizontal fields are already tilted and the leading
  sides of the emerging flux tubes are usually up against the downdraft
  lanes of the giant cells. It is also found that penumbrae with numerous
  filamentary structures form in regions of strong horizontal magnetic
  fields that naturally comes from the ongoing flux emergence. In contrast
  to previous models, the penumbrae and umbrae are divided by very sharp
  boarders, which is highly consistent with observations.

Title: Coronal extension of the MURaM radiative MHD code: From quiet
    sun to flare simulations
Authors: Rempel, Matthias D.; Cheung, Mark
Bibcode: 2016SPD....4720803R
Altcode:
  We present a new version of the MURaM radiative MHD code, which
  includes a treatment of the solar corona in terms of MHD, optically thin
  radiative loss and field-aligned heat conduction. In order to relax the
  severe time-step constraints imposed by large Alfven velocities and heat
  conduction we use a combination of semi-relativistic MHD with reduced
  speed of light ("Boris correction") and a hyperbolic formulation of
  heat conduction. We apply the numerical setup to 4 different setups
  including a mixed polarity quiet sun, an open flux region, an arcade
  solution and an active region setup and find all cases an amount of
  coronal heating sufficient to maintain a corona with temperatures from
  1 MK (quiet sun) to 2 MK (active region, arcade). In all our setups
  the Poynting flux is self-consistently created by photospheric and
  sub-photospheric magneto-convection in the lower part of our simulation
  domain. Varying the maximum allowed Alfven velocity ("reduced speed of
  light") leads to only minor changes in the coronal structure as long
  as the limited Alfven velocity remains larger than the speed of sound
  and about 1.5-3 times larger than the peak advection velocity. We also
  found that varying details of the numerical diffusivities that govern
  the resistive and viscous energy dissipation do not strongly affect
  the overall coronal heating, but the ratio of resistive and viscous
  energy dependence is strongly dependent on the effective numerical
  magnetic Prandtl number. We use our active region setup in order to
  simulate a flare triggered by the emergence of a twisted flux rope
  into a pre-existing bipolar active region. Our simulation yields a
  series of flares, with the strongest one reaching GOES M1 class. The
  simulation reproduces many observed properties of eruptions such as
  flare ribbons, post flare loops and a sunquake.

Title: Distortions of Magnetic Flux Tubes in the Presence of Electric
    Currents
Authors: Malanushenko, Anna; Rempel, Matthias; Cheung, Mark
Bibcode: 2016SPD....47.0322M
Altcode:
  Solar coronal loops possess several peculiar properties, which
  have been a subject of intensive research for a long time. These in
  particular include the lack of apparent expansion of coronal loops
  and the increased pressure scale height in loops compared to the
  diffuse background. Previously, Malanushenko &amp; Schrijver (2013)
  proposed that these could be explained by the fact that magnetic
  flux tubes expand with height in a highly anisotropic manner. They
  used potential field models to demonstrate that flux tubes that have
  circular cross section at the photosphere, in the corona turn into
  a highly elongates structures, more resembling thick ribbons. Such
  ribbons, viewed along the expanding edge, would appear as thin, crisp
  structures of a constant cross-section with an increased pressure scale
  height, and when viewed along the non-expanding side, would appear
  as faint, wide and underdense features. This may also introduce a
  selection bias,when a set of loops is collected for a further study,
  towards those viewed along the expanding edge.However, some of the
  past studies have indicated that strong electric currents flowing in a
  given flux tube may result in the tube maintaining a relatively constant
  cross-sectional shape along its length. Given that Malanushenko &amp;
  Schrijver (2013) focused on a potential, or current-free, field model of
  an active region, the extend to which their analysis could be applied
  to the real solar fields, was unclear.In the present study, we use a
  magnetic field created by MURaM, a highly realistic state-of-the-art
  radiative MHD code (Vogler et al, 2005; Rempel et al, 2009b). MURaM was
  shown to reproduce a wide variety of observed features of the solar
  corona (e.g., Hansteen et al, 2010; Cheung et al. 2007, 2008; Rempel
  2009a,b). We analyze the distortions of magnetic flux tubes in a MURaM
  simulation of an active region corona. We quantify such distortions and
  correlate them with a number of relevant parameters of flux tubes, with
  a particular emphasis on the electric currents in the simulated corona.

Title: Physics &amp; Diagnostics of the Drivers of Solar Eruptions
Authors: Cheung, Mark; Rempel, Matthias D.; Martinez-Sykora, Juan;
   Testa, Paola; Hansteen, Viggo H.; Viktorovna Malanushenko, Anna;
   Sainz Dalda, Alberto; DeRosa, Marc L.; De Pontieu, Bart; Carlsson,
   Mats; Chen, Feng; McIntosh, Scott W.; Gudiksen, Boris
Bibcode: 2016SPD....47.0607C
Altcode:
  We provide an update on our NASA Heliophysics Grand Challenges Research
  (HGCR) project on the ‘Physics &amp; Diagnostics of the Drivers of
  Solar Eruptions’. This presentation will focus on results from a
  data-inspired, 3D radiative MHD model of a solar flare. The model
  flare results from the interaction of newly emerging flux with a
  pre-existing active region. Synthetic observables from the model
  reproduce observational features compatible with actual flares. These
  include signatures of coronal magnetic reconnection, chromospheric
  evaporation, EUV flare arcades, sweeping motion of flare ribbons
  and sunquakes.

Title: Interpreting Irradiance Distributions Using High-Resolution
    3D MHD Simulations
Authors: Peck, Courtney; Rast, Mark; Criscuoli, Serena; Uitenbroek,
   Han; Rempel, Matthias D.
Bibcode: 2016SPD....4730302P
Altcode:
  We present initial results of studies aimed at understanding the
  impact of the unresolved magnetic field distribution on solar spectral
  irradiance. Using high-resolution 3D MHD simulations (from MURaM code)
  and spectral synthesis (with the RH code), we examine the emergent
  spectra of two atmospheres with similar mean field strengths but
  differing imposed-field conditions at wavelengths spanning from
  visible to infrared. Comparing the contrast against the magnetic
  field strength for the two magnetic simulations, we find differences
  in the distributions of contrasts versus field strength. We repeat
  the analysis after convolving the images with the PSF of a typical
  solar telescope (1-meter) and discuss the potential implications for
  irradiance modeling and future steps.

Title: Large-scale magnetic fields at high Reynolds numbers in
    magnetohydrodynamic simulations
Authors: Hotta, H.; Rempel, M.; Yokoyama, T.
Bibcode: 2016Sci...351.1427H
Altcode:
  The 11-year solar magnetic cycle shows a high degree of coherence in
  spite of the turbulent nature of the solar convection zone. It has
  been found in recent high-resolution magnetohydrodynamics simulations
  that the maintenance of a large-scale coherent magnetic field is
  difficult with small viscosity and magnetic diffusivity (≲1012square
  centimenters per second). We reproduced previous findings that indicate
  a reduction of the energy in the large-scale magnetic field for lower
  diffusivities and demonstrate the recovery of the global-scale magnetic
  field using unprecedentedly high resolution. We found an efficient
  small-scale dynamo that suppresses small-scale flows, which mimics the
  properties of large diffusivity. As a result, the global-scale magnetic
  field is maintained even in the regime of small diffusivities—that
  is, large Reynolds numbers.

Title: The Effects of Magnetic Field Morphology on the Determination
    of Oxygen and Iron Abundances in the Solar Photosphere
Authors: Moore, Christopher S.; Uitenbroek, Han; Rempel, Matthias;
   Criscuoli, Serena; Rast, Mark
Bibcode: 2016AAS...22712501M
Altcode:
  The solar chemical abundance (or a scaled version of it) is
  implemented in numerous astrophysical analyses. Thus, an accurate
  and precise estimation of the solar elemental abundance is crucial
  in astrophysics.We have explored the impact of magnetic fields
  on the determination of the solar photospheric oxygen andiron
  abundances using 3D radiation-magnetohydrodynamic (MHD) simulations
  of convection. Specifically, weexamined differences in abundance
  deduced from three classes of atmospheres simulated with the MURaM
  code: apure hydrodynamic (HD) simulation, an MHD simulation with
  a local dynamo magnetic field that has saturated withan unsigned
  vertical field strength of 80 G at the optical depth unity surface,
  and an MHD simulation with an initially imposed vertical mean field
  of 80 G. We use differential equivalent width analysis for diagnosing
  abundances derived from five oxygen and four iron spectral lines of
  differing wavelength, oscillator strength, excitation potential, and
  Lande g-factor, and find that the morphology of the magnetic field
  is important to the outcome of abundance determinations. The largest
  deduced abundance differences are found in the vertical mean field
  simulations and small scale unresolved field resulting from the local
  dynamo has a smaller impact on abundance determinations.

Title: Evolution of Fine-scale Penumbral Magnetic Structure and
    Formation of Penumbral Jets
Authors: Tiwari, S. K.; Moore, R. L.; Rempel, M.; Winebarger, A. R.
Bibcode: 2015AGUFMSH13D2461T
Altcode:
  Sunspot penumbra consists of spines (more vertical field) and
  penumbral filaments (interspines). Spines are outward extension of
  umbra. Penumbral filaments are recently found, both in observations
  and magnetohydrodynamic (MHD) simulations, to be magnetized stretched
  granule-like convective cells, with strong upflows near the head
  that continues along the central axis with weakening strength of the
  flow. Strong downflows are found at the tails of filaments and weak
  downflows along the sides of it. These lateral downflows often contain
  opposite polarity magnetic field to that of spines; most strongly near
  the heads of filaments. In spite of this advancement in understanding
  of small-scale structure of sunspot penumbra, how the filaments and
  spines evolve and interact remains uncertain. <P />Penumbral jets,
  bright, transient features, seen in the chromosphere, are one of
  several dynamic events in sunspot penumbra. It has been proposed
  that these penumbral microjets result from component (acute angle)
  reconnection of the magnetic field in spines with that in interspines
  and could contribute to transition-region and coronal heating above
  sunspots. In a recent investigation, it was proposed that the jets
  form as a result of reconnection between the opposite polarity field
  at edges of filaments with spine field, and it was found that these
  jets do not significantly directly heat the corona above sunspots. We
  discuss how the proposed formation of penumbral jets is integral to the
  formation mechanism of penumbral filaments and spines, and may explain
  why penumbral jets are few and far between. We also point out that
  the generation of the penumbral jets could indirectly drive coronal
  heating via generation of MHD waves or braiding of the magnetic field.

Title: Numerical Simulations of Sunspot Decay: On the
    Penumbra-Evershed Flow-Moat Flow Connection
Authors: Rempel, M.
Bibcode: 2015ApJ...814..125R
Altcode: 2015arXiv151101410R
  We present a series of high-resolution sunspot simulations that cover
  a timespan of up to 100 hr. The simulation domain extends about 18 Mm
  in depth beneath the photosphere and 98 Mm horizontally. We use open
  boundary conditions that do not maintain the initial field structure
  against decay driven by convective motions. We consider two setups:
  a sunspot simulation with penumbra, and a “naked-spot” simulation
  in which we removed the penumbra after 20 hr through a change in the
  magnetic top boundary condition. While the sunspot has an Evershed
  outflow of 3-4 km s<SUP>-1</SUP>, the naked spot is surrounded by
  an inflow of 1-2 km s<SUP>-1</SUP> in close proximity. However, both
  spots are surrounded by an outflow on larger scales with a few 100 m
  s<SUP>-1</SUP> flow speed in the photosphere. While the sunspot has
  an almost constant magnetic flux content for the simulated timespan
  of three to four days, the naked spot decays steadily at a rate of
  10<SUP>21</SUP> Mx day<SUP>-1</SUP>. A region with reduced downflow
  filling factor, which is more extended for the sunspot, surrounds both
  spots. The absence of downflows perturbs the upflow/downflow mass flux
  balance and leads to a large-scale radially overturning flow system;
  the photospheric component of this flow is the observable moat flow. The
  reduction of the downflow filling factor also inhibits the submergence
  of magnetic field in the proximity of the spots, which stabilizes them
  against decay. While this effect is present for both spots, it is more
  pronounced for the sunspot and explains the almost stationary magnetic
  flux content.

Title: Towards a Data-Optimized Coronal Magnetic Field Model (DOC-FM):
    Synthetic Test Beds and Multiwavelength Forward Modeling
Authors: Gibson, S. E.; Dalmasse, K.; Fan, Y.; Fineschi, S.; MacKay,
   D.; Rempel, M.; White, S. M.
Bibcode: 2015AGUFMSH54B..04G
Altcode:
  Understanding the physical state of the solar corona is key to
  deciphering the origins of space weather as well as to realistically
  representing the environment to be navigated by missions such as
  Solar Orbiter and Solar Probe Plus. However, inverting solar coronal
  observations to reconstruct this physical state -- and in particular
  the three-dimensional coronal magnetic field - is complicated by
  limited lines of sight and by projection effects. On the other hand,
  the sensitivity of multiwavelength observations to different physical
  mechanisms implies a potential for simultaneous probing of different
  parts of the coronal plasma. In order to study this complementarity, and
  to ultimately establish an optimal set of observations for constraining
  the three-dimensional coronal magnetic field, we are developing a suite
  of representative simulations to act as diagnostic test beds. We will
  present three such test beds: a coronal active region, a quiescent
  prominence, and a global corona. Each fully define the physical state
  of density, temperature, and vector magnetic field in three dimensions
  throughout the simulation domain. From these test beds, and using the
  FORWARD SolarSoft IDL codes, we will create a broad range of synthetic
  data. Radio observables will include intensity and circular polarization
  (including gyroresonance effects) and Faraday rotation for a range of
  frequencies. Infrared and visible forbidden line diagnostics of Zeeman
  and saturated Hanle effects will yield full Stokes vector (I, Q, U,
  V) synthetic data, and UV permitted line Hanle diagnostics will yield
  intensity and linear polarization. In addition, we will synthesize
  UV and SXR imager data, UV/EUV spectrometric data, and white light
  brightness and polarized brightness. All of these synthetic data,
  along with the "ground truth" physical state of the simulations from
  which they are derived, will be made available to the community for
  the purpose of testing coronal inversion techniques.

Title: Solar Differential rotation Maintained by Small- and
    Large-scale Convection
Authors: Hotta, H.; Rempel, M.; Yokoyama, T.
Bibcode: 2015ASPC..498..154H
Altcode:
  We investigate the solar differential rotation with special interest
  for the near surface shear layer (NSSL) in a high-resolution
  hydrodynamic numerical calculation. The sun is rotating
  differentially. Helioseismology has revealed the detailed structure of
  the solar differential rotation. One of the most important features
  is the NSSL. It is thought that the solar differential rotation is
  maintained by the turbulent thermal convection. In the NSSL convection
  time scales are short, leading to a regime with weak influence of
  rotation on convection. In order to reproduce the NSSL by the numerical
  calculations, we must use a large number of grids and integrate a
  large number of time steps for covering the broad spatial and temporal
  scales. This requirements for the NSSL is achieved using our recent
  efficient numerical method. In the calculation, the global scale and the
  10 Mm-scale convection is established simultaneously. Then the solar
  like NSSL is partially reproduced. Around the NSSL, the convection
  transports the angular momentum radially inward and generates the
  poleward meridional flow. The small scale convection acts as the
  turbulent viscosity on the meridional flow. The turbulent viscous
  stress balances with the Coriolis force in the NSSL.

Title: Efficient Small-scale Dynamo in the Solar Convection Zone
Authors: Hotta, H.; Rempel, M.; Yokoyama, T.
Bibcode: 2015ApJ...803...42H
Altcode: 2015arXiv150203846H
  We investigate small-scale dynamo action in the solar convection
  zone through a series of high-resolution MHD simulations in a
  local Cartesian domain with 1 {{R}<SUB>⊙ </SUB>} (solar radius) of
  horizontal extent and a radial extent from 0.715 to 0.96 {{R}<SUB>⊙
  </SUB>}. The dependence of the solution on resolution and diffusivity
  is studied. For a grid spacing of less than 350 km, the rms magnetic
  field strength near the base of the convection zone reaches 95% of
  the equipartition field strength (i.e., magnetic and kinetic energy
  are comparable). For these solutions the Lorentz force feedback on
  the convection velocity is found to be significant. The velocity near
  the base of the convection zone is reduced to 50% of the hydrodynamic
  one. In spite of the significant decrease of the convection velocity,
  the reduction in the enthalpy flux is relatively small, since the
  magnetic field also suppresses the horizontal mixing of the entropy
  between up- and downflow regions. This effect increases the amplitude
  of the entropy perturbation and makes convective energy transport
  more efficient. We discuss potential implications of these results
  for solar global convection and dynamo simulations.

Title: Photon Mean Free Paths, Scattering, and Ever-Increasing
    Telescope Resolution
Authors: Judge, P. G.; Kleint, L.; Uitenbroek, H.; Rempel, M.;
   Suematsu, Y.; Tsuneta, S.
Bibcode: 2015SoPh..290..979J
Altcode: 2014arXiv1409.7866J; 2015SoPh..tmp....3J
  We revisit an old question: what are the effects of observing stratified
  atmospheres on scales below a photon mean free path λ? The mean free
  path of photons emerging from the solar photosphere and chromosphere
  is ≈ 10<SUP>2</SUP> km. Using current 1 m-class telescopes, λ is
  on the order of the angular resolution. But the Daniel K. Inoue Solar
  Telescope will have a diffraction limit of 0.020″ near the atmospheric
  cutoff at 310 nm, corresponding to 14 km at the solar surface. Even
  a small amount of scattering in the source function leads to physical
  smearing due to this solar "fog", with effects similar to a degradation
  of the telescope point spread function. We discuss a unified picture
  that depends simply on the nature and amount of scattering in the
  source function. Scalings are derived from which the scattering in the
  solar atmosphere can be transcribed into an effective Strehl ratio,
  a quantity useful to observers. Observations in both permitted (e.g.,
  Fe I 630.2 nm) and forbidden (Fe I 525.0 nm) lines will shed light on
  both instrumental performance as well as on small-scale structures in
  the solar atmosphere.

Title: The Effects of Magnetic Field Morphology on the Determination
    of Oxygen and Iron Abundances in the Solar Photosphere
Authors: Moore, Christopher S.; Uitenbroek, Han; Rempel, Matthias;
   Criscuoli, Serena; Rast, Mark P.
Bibcode: 2015ApJ...799..150M
Altcode:
  We have explored the impact of magnetic fields on the determination
  of the solar photospheric oxygen and iron abundances using
  three-dimensional radiation-magnetohydrodynamic (MHD) simulations
  of convection. Specifically, we examined differences in abundance
  deduced from three classes of atmospheres simulated with the MURaM
  code: a pure hydrodynamic (HD) simulation, an MHD simulation with
  a local dynamo magnetic field that has saturated with an unsigned
  vertical field strength of 80 G at τ = 1, and an MHD simulation with
  an initially imposed vertical mean field of 80 G. We use differential
  equivalent width analysis for diagnosing abundances derived from
  five oxygen and four iron lines of differing wavelength, oscillator
  strength, excitation potential, and Landé g-factor, and find that
  the morphology of the magnetic field is important to the outcome of
  abundance determinations. The largest deduced abundance differences are
  found in the vertical mean field simulations, where the O I and Fe I
  abundance corrections compared to the pure HD case are ~+0.011 dex and
  +0.065 dex respectively. Small scale unresolved field resulting from
  the local dynamo has a smaller impact on abundance determinations,
  with corrections of -0.0001 dex and +0.0044 dex in the magnetized
  compared to the pure HD simulations. While the overall influence of
  magnetic field on abundance estimates is found to be small, we stress
  that such estimates are sensitive not only to the magnitude of magnetic
  field but also to its morphology.

Title: High-resolution Calculation of the Solar Global Convection
    with the Reduced Speed of Sound Technique. II. Near Surface Shear
    Layer with the Rotation
Authors: Hotta, H.; Rempel, M.; Yokoyama, T.
Bibcode: 2015ApJ...798...51H
Altcode: 2014arXiv1410.7093H
  We present a high-resolution, highly stratified numerical simulation of
  rotating thermal convection in a spherical shell. Our aim is to study
  in detail the processes that can maintain a near surface shear layer
  (NSSL) as inferred from helioseismology. Using the reduced speed of
  sound technique, we can extend our global convection simulation to 0.99
  R <SUB>⊙</SUB> and include, near the top of our domain, small-scale
  convection with short timescales that is only weakly influenced by
  rotation. We find the formation of an NSSL preferentially in high
  latitudes in the depth range of r = 0.95-0.975 R <SUB>⊙</SUB>. The
  maintenance mechanisms are summarized as follows. Convection under the
  weak influence of rotation leads to Reynolds stresses that transport
  angular momentum radially inward in all latitudes. This leads to
  the formation of a strong poleward-directed meridional flow and an
  NSSL, which is balanced in the meridional plane by forces resulting
  from the &lt; v<SUP>\prime </SUP><SUB>r</SUB> v<SUP>\prime </SUP>_θ
  &gt; correlation of turbulent velocities. The origin of the required
  correlations depends to some degree on latitude. In high latitudes, a
  positive correlation &lt; v<SUP>\prime </SUP>_rv<SUP>\prime </SUP>_θ
  &gt; is induced in the NSSL by the poleward meridional flow whose
  amplitude increases with the radius, while a negative correlation is
  generated by the Coriolis force in bulk of the convection zone. In low
  latitudes, a positive correlation &lt; v<SUP>\prime </SUP>_rv<SUP>\prime
  </SUP>_θ &gt; results from rotationally aligned convection cells
  ("banana cells"). The force caused by these Reynolds stresses is in
  balance with the Coriolis force in the NSSL.

Title: Comparison of inversion codes for polarized line formation
    in MHD simulations. I. Milne-Eddington codes
Authors: Borrero, J. M.; Lites, B. W.; Lagg, A.; Rezaei, R.; Rempel, M.
Bibcode: 2014A&A...572A..54B
Altcode: 2014arXiv1409.3376B
  Milne-Eddington (M-E) inversion codes for the radiative transfer
  equation are the most widely used tools to infer the magnetic field
  from observations of the polarization signals in photospheric and
  chromospheric spectral lines. Unfortunately, a comprehensive comparison
  between the different M-E codes available to the solar physics
  community is still missing, and so is a physical interpretation of their
  inferences. In this contribution we offer a comparison between three
  of those codes (VFISV, ASP/HAO, and HeLIx<SUP>+</SUP>). These codes are
  used to invert synthetic Stokes profiles that were previously obtained
  from realistic non-grey three-dimensional magnetohydrodynamical (3D MHD)
  simulations. The results of the inversion are compared with each other
  and with those from the MHD simulations. In the first case, the M-E
  codes retrieve values for the magnetic field strength, inclination and
  line-of-sight velocity that agree with each other within σ<SUB>B</SUB>
  ≤ 35 (Gauss), σ<SUB>γ</SUB> ≤ 1.2°, and σ<SUB>v</SUB> ≤
  10 m s<SUP>-1</SUP>, respectively. Additionally, M-E inversion codes
  agree with the numerical simulations, when compared at a fixed optical
  depth, within σ<SUB>B</SUB> ≤ 130 (Gauss), σ<SUB>γ</SUB> ≤ 5°,
  and σ<SUB>v</SUB> ≤ 320 m s<SUP>-1</SUP>. Finally, we show that
  employing generalized response functions to determine the height at
  which M-E codes measure physical parameters is more meaningful than
  comparing at a fixed geometrical height or optical depth. In this case
  the differences between M-E inferences and the 3D MHD simulations
  decrease to σ<SUB>B</SUB> ≤ 90 (Gauss), σ<SUB>γ</SUB> ≤ 3°,
  and σ<SUB>v</SUB> ≤ 90 m s<SUP>-1</SUP>.

Title: Time-distance Helioseismology of Two Realistic Sunspot
    Simulations
Authors: DeGrave, K.; Jackiewicz, J.; Rempel, M.
Bibcode: 2014ApJ...794...18D
Altcode: 2014arXiv1408.2262D
  Linear time-distance helioseismic inversions are carried out using
  several filtering schemes to determine vector flow velocities within two
  ~100<SUP>2</SUP> Mm<SUP>2</SUP> × 20 Mm realistic magnetohydrodynamic
  sunspot simulations of 25 hr. One simulation domain contains a model
  of a full sunspot (i.e., one with both an umbra and penumbra), while
  the other contains a pore (i.e., a spot without a penumbra). The goal
  is to test current helioseismic methods using these state-of-the-art
  simulations of magnetic structures. We find that horizontal
  flow correlations between inversion and simulation flow maps are
  reasonably high (~0.5-0.8) in the upper 3 Mm at distances exceeding
  25-30 Mm from spot center, but are substantially lower at smaller
  distances and larger depths. Inversions of forward-modeled travel
  times consistently outperform those of our measured travel times in
  terms of horizontal flow correlations, suggesting that our inability
  to recover flow structure near these active regions is largely due to
  the fact that we are unable to accurately measure travel times near
  strong magnetic features. In many cases the velocity amplitudes from
  the inversions underestimate those of the simulations by up to 50%,
  possibly indicating nonlinearity of the forward problem. In every case,
  we find that our inversions are unable to recover the vertical flow
  structure of the simulations at any depth.

Title: The Role of Subsurface Flows in Solar Surface Convection:
    Modeling the Spectrum of Supergranular and Larger Scale Flows
Authors: Lord, J. W.; Cameron, R. H.; Rast, M. P.; Rempel, M.;
   Roudier, T.
Bibcode: 2014ApJ...793...24L
Altcode: 2014arXiv1407.2209L
  We model the solar horizontal velocity power spectrum at scales
  larger than granulation using a two-component approximation to the
  mass continuity equation. The model takes four times the density
  scale height as the integral (driving) scale of the vertical motions
  at each depth. Scales larger than this decay with height from the
  deeper layers. Those smaller are assumed to follow a Kolmogorov
  turbulent cascade, with the total power in the vertical convective
  motions matching that required to transport the solar luminosity in a
  mixing length formulation. These model components are validated using
  large-scale radiative hydrodynamic simulations. We reach two primary
  conclusions. (1) The model predicts significantly more power at low
  wavenumbers than is observed in the solar photospheric horizontal
  velocity spectrum. (2) Ionization plays a minor role in shaping the
  observed solar velocity spectrum by reducing convective amplitudes in
  the regions of partial helium ionization. The excess low wavenumber
  power is also seen in the fully nonlinear three-dimensional radiative
  hydrodynamic simulations employing a realistic equation of state. This
  adds to other recent evidence suggesting that the amplitudes of
  large-scale convective motions in the Sun are significantly lower
  than expected. Employing the same feature tracking algorithm used
  with observational data on the simulation output, we show that the
  observed low wavenumber power can be reproduced in hydrodynamic
  models if the amplitudes of large-scale modes in the deep layers
  are artificially reduced. Since the large-scale modes have reduced
  amplitudes, modes on the scale of supergranulation and smaller remain
  important to convective heat flux even in the deep layers, suggesting
  that small-scale convective correlations are maintained through the
  bulk of the solar convection zone.

Title: Numerical Simulations of Quiet Sun Magnetism: On the
    Contribution from a Small-scale Dynamo
Authors: Rempel, M.
Bibcode: 2014ApJ...789..132R
Altcode: 2014arXiv1405.6814R
  We present a series of radiative MHD simulations addressing the origin
  and distribution of the mixed polarity magnetic field in the solar
  photosphere. To this end, we consider numerical simulations that cover
  the uppermost 2-6 Mm of the solar convection zone and we explore scales
  ranging from 2 km to 25 Mm. We study how the strength and distribution
  of the magnetic field in the photosphere and subsurface layers depend
  on resolution, domain size, and boundary conditions. We find that 50% of
  the magnetic energy at the τ = 1 level comes from fields with the less
  than 500 G strength and that 50% of the energy resides on scales smaller
  than about 100 km. While the probability distribution functions are
  essentially independent of resolution, properly describing the spectral
  energy distribution requires grid spacings of 8 km or smaller. The
  formation of flux concentrations in the photosphere exceeding 1
  kG requires a mean vertical field strength greater than 30-40 G at
  τ = 1. The filling factor of kG flux concentrations increases with
  overall domain size as the magnetic field becomes organized by larger,
  longer-lived flow structures. A solution with a mean vertical field
  strength of around 85 G at τ = 1 requires a subsurface rms field
  strength increasing with depth at the same rate as the equipartition
  field strength. We consider this an upper limit for the quiet Sun field
  strength, which implies that most of the convection zone is magnetized
  close to the equipartition. We discuss these findings in view of recent
  high-resolution spectropolarimetric observations of quiet Sun magnetism.

Title: Validating Time-Distance Helioseismology With Realistic Quiet
    Sun Simulations
Authors: DeGrave, Kyle; Jackiewicz, Jason; Rempel, Matthias
Bibcode: 2014AAS...22421803D
Altcode:
  Linear time-distance helioseismic inversions are carried out for vector
  flow velocities using travel times measured from two ~100^2 Mm^2 x 20 Mm
  realistic magnetohydrodynamic quiet-Sun simulations of about 20 hr. The
  goal is to test current seismic methods on these state-of-the-art
  simulations. We find that horizontal flow maps correlate well with the
  simulations in the upper ~3 Mm of the domains for several filtering
  schemes, including phase-speed, ridge, and combined phase-speed and
  ridge measurements. In several cases, however, the velocity amplitudes
  from the inversions severely underestimate those of the simulations,
  possibly indicating nonlinearity of the forward problem. We also find
  that results of the inversions for the vertical velocity component
  depend significantly on the type of data filtering. In particular,
  phase-speed filters show better results than the other methods. In
  many cases, the vertical flows are irretrievable due to high levels
  of noise, suggesting a need for statistical averaging.

Title: Numerical simulations of sunspot decay: On the role of a
    penumbra and subsurface field structure
Authors: Rempel, Matthias D.
Bibcode: 2014AAS...22420204R
Altcode:
  We present high-resolution simulations of decaying sunspots that cover
  a time span of up to 100 hours. The simulations reach 18Mm deep into
  the convection zone and use open boundaries that do not maintain the
  initial field structure against decay driven by convective motions. We
  discuss three experiments: A sunspot simulation with penumbra, a
  "naked-spot" simulation in which we removed the penumbra after 20
  hours, and a sunspot simulation with penumbra, but a less coherent
  subsurface field structure. In all three simulations we study the
  decay process and large-scale flows in proximity of the spots. Over the
  time span covered by the simulation the spot with penumbra is almost
  stationary with regard to the total flux content, but shows a steady
  decay of the flux present in the umbra area at a rate comparable to
  the "naked-spot" experiment. A less coherent sub-surface magnetic
  field structure leads within 12-24 hours to a less coherent surface
  appearance, i.e. details of the subsurface structure do not remain
  hidden from the photosphere. In all three experiments the dominant
  subsurface flow patterns are outflows.

Title: Using Synthetic Data From Convection Simulations To Test
    Helioseismic Holography Inversions For Three-Dimensional Vector Flows
Authors: Crouch, Ashley D.; Birch, Aaron; Braun, Douglas; Javornik,
   Brenda; Rempel, Matthias D.
Bibcode: 2014AAS...22421807C
Altcode:
  We investigate the efficacy of helioseismic holography for inferring
  the three-dimensional vector flows in the near-surface layers of the
  solar interior. Synthetic helioseismic data are taken from compressible
  convection simulations. Travel times are measured from the synthetic
  data using helioseismic holography. Kernels for the sensitivity
  of travel times to subsurface flows are calculated using the Born
  approximation. Inversions for subsurface flows are then performed
  using subtractive optimally localized averaging. This provides an
  opportunity to evaluate the accuracy of the inversion technique. We
  compare the actual flows present in the convection simulations to the
  flows retrieved by the inversion. We discuss the influence of the
  regularization used by the inversion, and the effects of noise and
  spatial resolution. This work is supported by the NASA SDO Science
  Center program (NNH09CE41C), the NASA Heliophysics Guest Investigator
  program (NNH12CF68C), and the NASA LWS TR&amp;T tools and methods
  program (NNH09CF68C). The National Center for Atmospheric Research is
  sponsored by the National Science Foundation.

Title: Validating Time-Distance Helioseismology with Realistic
    Quiet-Sun Simulations
Authors: DeGrave, K.; Jackiewicz, J.; Rempel, M.
Bibcode: 2014ApJ...788..127D
Altcode: 2014arXiv1404.4645D
  Linear time-distance helioseismic inversions are carried out
  for vector flow velocities using travel times measured from two
  ~100<SUP>2</SUP> Mm<SUP>2</SUP> × 20 Mm realistic magnetohydrodynamic
  quiet-Sun simulations of about 20 hr. The goal is to test current
  seismic methods on these state-of-the-art simulations. Using recent
  three-dimensional inversion schemes, we find that inverted horizontal
  flow maps correlate well with the simulations in the upper ~3 Mm of the
  domains for several filtering schemes, including phase-speed, ridge, and
  combined phase-speed and ridge measurements. In several cases, however,
  the velocity amplitudes from the inversions severely underestimate
  those of the simulations, possibly indicating nonlinearity of the
  forward problem. We also find that, while near-surface inversions of
  the vertical velocities are best using phase-speed filters, in almost
  all other example cases these flows are irretrievable due to noise,
  suggesting a need for statistical averaging to obtain better inferences.

Title: High-resolution Calculations of the Solar Global Convection
    with the Reduced Speed of Sound Technique. I. The Structure of the
    Convection and the Magnetic Field without the Rotation
Authors: Hotta, H.; Rempel, M.; Yokoyama, T.
Bibcode: 2014ApJ...786...24H
Altcode: 2014arXiv1402.5008H
  We carry out non-rotating high-resolution calculations of the solar
  global convection, which resolve convective scales of less than
  10 Mm. To cope with the low Mach number conditions in the lower
  convection zone, we use the reduced speed of sound technique (RSST),
  which is simple to implement and requires only local communication in
  the parallel computation. In addition, the RSST allows us to expand
  the computational domain upward to about 0.99 R <SUB>⊙</SUB>, as
  it can also handle compressible flows. Using this approach, we study
  the solar convection zone on the global scale, including small-scale
  near-surface convection. In particular, we investigate the influence
  of the top boundary condition on the convective structure throughout
  the convection zone as well as on small-scale dynamo action. Our main
  conclusions are as follows. (1) The small-scale downflows generated in
  the near-surface layer penetrate into deeper layers to some extent and
  excite small-scale turbulence in the region &gt;0.9 R <SUB>⊙</SUB>,
  where R <SUB>⊙</SUB> is the solar radius. (2) In the deeper convection
  zone (&lt;0.9 R <SUB>⊙</SUB>), the convection is not influenced by
  the location of the upper boundary. (3) Using a large eddy simulation
  approach, we can achieve small-scale dynamo action and maintain a
  field of about 0.15B <SUB>eq</SUB>-0.25B <SUB>eq</SUB> throughout the
  convection zone, where B <SUB>eq</SUB> is the equipartition magnetic
  field to the kinetic energy. (4) The overall dynamo efficiency varies
  significantly in the convection zone as a consequence of the downward
  directed Poynting flux and the depth variation of the intrinsic
  convective scales.

Title: Numerical Simulations of Active Region Scale Flux Emergence:
    From Spot Formation to Decay
Authors: Rempel, M.; Cheung, M. C. M.
Bibcode: 2014ApJ...785...90R
Altcode: 2014arXiv1402.4703R
  We present numerical simulations of active region scale flux emergence
  covering a time span of up to 6 days. Flux emergence is driven by
  a bottom boundary condition that advects a semi-torus of magnetic
  field with 1.7 × 10<SUP>22</SUP> Mx flux into the computational
  domain. The simulations show that, even in the absence of twist,
  the magnetic flux is able the rise through the upper 15.5 Mm of the
  convection zone and emerge into the photosphere to form spots. We find
  that spot formation is sensitive to the persistence of upflows at the
  bottom boundary footpoints, i.e., a continuing upflow would prevent
  spot formation. In addition, the presence of a torus-aligned flow
  (such flow into the retrograde direction is expected from angular
  momentum conservation during the rise of flux ropes through the
  convection zone) leads to a significant asymmetry between the pair
  of spots, with the spot corresponding to the leading spot on the Sun
  being more axisymmetric and coherent, but also forming with a delay
  relative to the following spot. The spot formation phase transitions
  directly into a decay phase. Subsurface flows fragment the magnetic
  field and lead to intrusions of almost field free plasma underneath
  the photosphere. When such intrusions reach photospheric layers, the
  spot fragments. The timescale for spot decay is comparable to the
  longest convective timescales present in the simulation domain. We
  find that the dispersal of flux from a simulated spot in the first
  two days of the decay phase is consistent with self-similar decay by
  turbulent diffusion.

Title: Magnetoconvection models and what we need the ATST to tell us
Authors: Rempel, Matthias D.
Bibcode: 2013SPD....4440103R
Altcode:
  So called "realistic" magnetoconvection simulations of the solar
  photosphere include all relevant physical ingredients in terms of
  equation of state and 3 dimensional radiative transfer in addition
  to MHD. In that sense they do not have any explicit free parameters,
  however, implicit degrees of freedom are present since simulations
  are limited to a finite volume, have a finite resolution and can be
  only run for a finite time. This results in dependencies on boundary
  conditions, on the numerical treatment of unresolved scales and
  on the chosen initial state. A successful numerical model of the
  solar photosphere and underlying convection zone is only possible
  if these implicit degrees of freedom are sufficiently constrained
  through observations. In this talk I will discuss two examples of
  recent high resolution simulations: the quiet sun photosphere and
  sunspot fine structure. Numerical simulations of quiet sun magnetism
  can explain most of the observed unsigned magnetic flux density as a
  consequence of a small scale dynamo process. These simulations make
  however unrealistic assumptions about the small scale dissipation
  and depend to some degree on the assumed bottom boundary condition
  several Mm beneath the photosphere. Observations are needed to verify
  the validity of this modeling approach. Sunspot simulations have
  successfully linked convective energy transport in the penumbra with
  penumbral fine structure. Current observations provide several indirect
  hints on convective flows, but higher resolution is needed to settle
  this aspect. So far sunspot simulations cannot explain from first
  principles the existence and extent of a penumbra, since this aspect
  depends strongly on the magnetic top boundary condition. Observations
  of the detailed magnetic field structure in the upper photosphere,
  chromosphere and lower corona above sunspots are needed to guide
  modeling. The National Center for Atmospheric Research is sponsored
  by the National Science Foundation.

Title: Formation of Magnetic Structures during Emergence of Untwisted
    Flux Rope
Authors: Fang, Fang; Fan, Y.; Rempel, M.
Bibcode: 2013SPD....44..102F
Altcode:
  Ideal MHD simulations have shown that the twist of the magnetic flux
  rope before emergence plays an important role in the coherency of the
  emerged magnetic structures. Recently, with more realistic simulations
  with turbulent convection, it is found that magnetic structures can
  form at the photosphere from emergence of uniform magnetic fields. The
  discrepancy therefore leads to a controversial question that whether the
  twist exists before the emergence or is formed afterwards by surface
  flows. In light of this, we carry out simulations on the emergence of
  untwisted flux rope from the convection zone into the corona, using
  more realistic treatment of the thermodynamic processes in the solar
  interior and the outer atmosphere. In our coupled simulations, we
  study the interaction between the convective motion and the magnetic
  fields and also the formation of coronal structures in comparison
  with observations.

Title: The High-latitude Branch of the Solar Torsional Oscillation
    in the Rising Phase of Cycle 24
Authors: Howe, R.; Christensen-Dalsgaard, J.; Hill, F.; Komm, R.;
   Larson, T. P.; Rempel, M.; Schou, J.; Thompson, M. J.
Bibcode: 2013ApJ...767L..20H
Altcode:
  We use global heliseismic data from the Global Oscillation Network
  Group, the Michelson Doppler Imager on board the Solar and Heliospheric
  Observatory, and the Helioseismic and Magnetic Imager on board the Solar
  Dynamics Observatory, to examine the behavior, during the rising phase
  of Solar Cycle 24, of the migrating zonal flow pattern known as the
  torsional oscillation. Although the high-latitude part of the pattern
  appears to be absent in the new cycle when the flows are derived by
  subtracting a mean across a full solar cycle, it can be seen if we
  subtract the mean over a shorter period in the rising phase of each
  cycle, and these two mean rotation profiles differ significantly
  at high latitudes. This indicates that the underlying high-latitude
  rotation has changed; we speculate that this is in response to weaker
  polar fields, as suggested by a recent model.

Title: The solar dynamo - where do we stand, where do we go?
Authors: Rempel, Matthias
Bibcode: 2013enss.confE.117R
Altcode:
  Understanding the origin of the large scale solar magnetic field and its
  temporal evolution is one of the still unsolved key questions in solar
  physics. While large scale dynamos are understood on a fundamental
  level for more than 5 decades, the details of how the solar dynamo
  operates are still heavily debated. In this talk I will review the
  various approaches taken in the past (mean field models vs. 3D numerical
  simulations) and discuss their intrinsic strengths and weaknesses. I
  will present a collection of recent modeling results including cyclic
  behavior in 3D numerical simulations, the connection between dynamo
  action and torsional oscillations, the role of flux transport and near
  surface field evolution (Babcock-Leighton alpha-effects) as well as
  flux emergence and sunspot formation. I will close the talk with an
  outlook on future developments.

Title: The Sunspot Penumbra in the Photosphere: Results from Forward
    Synthesized Spectroscopy
Authors: Tritschler, A.; Uitenbroek, H.; Rempel, M.
Bibcode: 2012ASPC..463...89T
Altcode:
  We present first results from a spectral synthesis of the
  Zeeman-insensitive Fe 1 557.6 nm line for two different viewing angles
  (0° and 30°) using numerical simulations of a sunspot as an input
  model. We performed a bisector analysis to calculate two-dimensional
  maps of line-of-sight Doppler velocities and the line width. We analyze
  azimuthal cuts of the LOS velocity at different penumbral radii and
  calculate the radial behavior of azimuthal averages of line width and
  intensity. Both are compared with observational results. The properties
  of dark cores in penumbral filaments are discussed briefly. Within the
  limitations of this study, we find that the results from the forward
  synthesized spectroscopy are in good agreement with the observations,
  corroborating that the photospheric structure and dynamics of the
  penumbra is a signature of overturning anisotropic magneto-convection.

Title: Magnetic Field Intensification by the Three-dimensional
    "Explosion" Process
Authors: Hotta, H.; Rempel, M.; Yokoyama, T.
Bibcode: 2012ApJ...759L..24H
Altcode: 2012arXiv1210.0949H
  We investigate an intensification mechanism for the magnetic field
  near the base of the solar convection zone that does not rely on
  differential rotation. Such mechanism in addition to differential
  rotation has been suggested by studies of flux emergence, which
  typically require field strength in excess of those provided by
  differential rotation alone. We study here a process in which potential
  energy of the superadiabatically stratified convection zone is converted
  into magnetic energy. This mechanism, known as the "explosion of
  magnetic flux tubes," has been previously studied in thin flux tube
  approximation as well as two-dimensional magnetohydrodynamic (MHD)
  simulations; here we expand the investigation to three-dimensional
  MHD simulations. Our main result is that enough intensification can
  be achieved in a three-dimensional magnetic flux sheet as long as the
  spatial scale of the imposed perturbation normal to the magnetic field
  is sufficiently large. When this spatial scale is small, the flux sheet
  tends to rise toward the surface, resulting in a significant decrease
  of the magnetic field amplification.

Title: On the Amplitude of Convective Velocities in the Deep Solar
    Interior
Authors: Miesch, Mark S.; Featherstone, Nicholas A.; Rempel, Matthias;
   Trampedach, Regner
Bibcode: 2012ApJ...757..128M
Altcode: 2012arXiv1205.1530M
  We obtain lower limits on the amplitude of convective velocities
  in the deep solar convection zone (CZ) based only on the observed
  properties of the differential rotation and meridional circulation
  together with simple and robust dynamical balances obtained from
  the fundamental magnetohydrodynamics equations. The linchpin of the
  approach is the concept of gyroscopic pumping whereby the meridional
  circulation across isosurfaces of specific angular momentum is linked
  to the angular momentum transport by the convective Reynolds stress. We
  find that the amplitude of the convective velocity must be at least
  30 m s<SUP>-1</SUP> in the upper CZ (r ~ 0.95R) and at least 8 m
  s<SUP>-1</SUP> in the lower CZ (r ~ 0.75R) in order to be consistent
  with the observed mean flows. Using the base of the near-surface shear
  layer as a probe of the rotational influence, we are further able to
  show that the characteristic length scale of deep convective motions
  must be no smaller than 5.5-30 Mm. These results are compatible with
  convection models but suggest that the efficiency of the turbulent
  transport assumed in advection-dominated flux-transport dynamo models
  is generally not consistent with the mean flows they employ.

Title: Numerical models of sunspot formation and fine structure
Authors: Rempel, M.
Bibcode: 2012RSPTA.370.3114R
Altcode:
  No abstract at ADS

Title: Waves as the Source of Apparent Twisting Motions in Sunspot
    Penumbrae
Authors: Bharti, L.; Cameron, R. H.; Rempel, M.; Hirzberger, J.;
   Solanki, S. K.
Bibcode: 2012ApJ...752..128B
Altcode: 2012arXiv1204.2221B
  The motion of dark striations across bright filaments in a sunspot
  penumbra has become an important new diagnostic of convective gas
  flows in penumbral filaments. The nature of these striations has,
  however, remained unclear. Here, we present an analysis of small-scale
  motions in penumbral filaments in both simulations and observations. The
  simulations, when viewed from above, show fine structure with dark lanes
  running outward from the dark core of the penumbral filaments. The
  dark lanes either occur preferentially on one side or alternate
  between both sides of the filament. We identify this fine structure
  with transverse (kink) oscillations of the filament, corresponding to
  a sideways swaying of the filament. These oscillations have periods in
  the range of 5-7 minutes and propagate outward and downward along the
  filament. Similar features are found in observed G-band intensity time
  series of penumbral filaments in a sunspot located near disk center
  obtained by the Broadband Filter Imager on board the Hinode. We also
  find that some filaments show dark striations moving to both sides
  of the filaments. Based on the agreement between simulations and
  observations we conclude that the motions of these striations are
  caused by transverse oscillations of the underlying bright filaments.

Title: High-latitude Solar Torsional Oscillations during Phases of
    Changing Magnetic Cycle Amplitude
Authors: Rempel, M.
Bibcode: 2012ApJ...750L...8R
Altcode:
  Torsional oscillations are variations of the solar differential rotation
  that are strongly linked to the magnetic cycle of the Sun. Helioseismic
  inversions have revealed significant differences in the high-latitude
  branch of torsional oscillations between cycle 23 and cycle 24. Here we
  employ a non-kinematic flux-transport dynamo model that has been used
  previously to study torsional oscillations and simulate the response of
  the high-latitude branch to a change in the amplitude of the magnetic
  cycle. It is found that a reduction of the cycle amplitude leads to
  an increase in the amplitude of differential rotation that is mostly
  visible as a drop in the high-latitude rotation rate. Depending on the
  amplitude of this adjustment the high-latitude torsional oscillation
  signal can become temporarily hidden due to the unknown changing
  mean rotation rate that is required to properly define the torsional
  oscillation signal.

Title: Numerical Sunspot Models: Robustness of Photospheric Velocity
    and Magnetic Field Structure
Authors: Rempel, M.
Bibcode: 2012ApJ...750...62R
Altcode: 2012arXiv1203.0534R
  MHD simulations of sunspots have successfully reproduced many aspects
  of sunspot fine structure as a consequence of magneto-convection
  in inclined magnetic field. We study how global sunspot properties
  and penumbral fine structure depend on the magnetic top boundary
  condition as well as on grid spacing. The overall radial extent of
  the penumbra is subject to the magnetic top boundary condition. All
  other aspects of sunspot structure and penumbral fine structure are
  resolved at an acceptable level starting from a grid resolution of 48
  [24] km (horizontal [vertical]). We find that the amount of inverse
  polarity flux and the overall amount of overturning convective motions
  in the penumbra are robust with regard to both resolution and boundary
  conditions. At photospheric levels Evershed flow channels are strongly
  magnetized. We discuss in detail the relation between velocity and
  magnetic field structure in the photosphere and point out observational
  consequences.

Title: Comparison of Multi-Height Observations with a 3D MHD
    Sunspot Model
Authors: Jaeggli, S. A.; Lin, H.; Uitenbroek, H.; Rempel, M.
Bibcode: 2012ASPC..456...67J
Altcode:
  In sunspots the contribution to the horizontal pressure support from the
  curvature force and the geometrical height of formation which magnetic
  field measurements sample are poorly constrained observationally due to
  the effect of radiative transfer. In cool atmospheres, observations of
  the sunspot photosphere probe geometrically deeper layers, information
  on the magnetic field gradients cannot be easily derived even using
  multi-wavelength, multi-height observations. Recent MHD atmosphere
  models of sunspots analyzed with the Rybiki-Hummer radiative transfer
  code allow for direct comparison with simultaneous multi-height
  observations of the Fe I magnetic field diagnostics at 1565 and 630.2
  nm in sunspots observed using the Facility Infrared Spectropolarimeter
  at the Dunn Solar Telescope.

Title: Numerical calculation of convection with reduced speed of
    sound technique
Authors: Hotta, H.; Rempel, M.; Yokoyama, T.; Iida, Y.; Fan, Y.
Bibcode: 2012A&A...539A..30H
Altcode: 2012arXiv1201.1061H
  Context. The anelastic approximation is often adopted in numerical
  calculations with low Mach numbers, such as those including stellar
  internal convection. This approximation requires so-called frequent
  global communication, because of an elliptic partial differential
  equation. Frequent global communication is, however, negative factor for
  the parallel computing performed with a large number of CPUs. <BR />
  Aims: We test the validity of a method that artificially reduces the
  speed of sound for the compressible fluid equations in the context of
  stellar internal convection. This reduction in the speed of sound leads
  to longer time steps despite the low Mach number, while the numerical
  scheme remains fully explicit and the mathematical system is hyperbolic,
  thus does not require frequent global communication. <BR /> Methods:
  Two- and three-dimensional compressible hydrodynamic equations are
  solved numerically. Some statistical quantities of solutions computed
  with different effective Mach numbers (owing to the reduction in the
  speed of sound) are compared to test the validity of our approach. <BR
  /> Results: Numerical simulations with artificially reduced speed
  of sound are a valid approach as long as the effective Mach number
  (based on the lower speed of sound) remains less than 0.7.

Title: Properties of Umbral Dots as Measured from the New Solar
    Telescope Data and MHD Simulations
Authors: Kilcik, A.; Yurchyshyn, V. B.; Rempel, M.; Abramenko, V.;
   Kitai, R.; Goode, P. R.; Cao, W.; Watanabe, H.
Bibcode: 2012ApJ...745..163K
Altcode: 2011arXiv1111.3997K
  We studied bright umbral dots (UDs) detected in a moderate size sunspot
  and compared their statistical properties to recent MHD models. The
  study is based on high-resolution data recorded by the New Solar
  Telescope at the Big Bear Solar Observatory and three-dimensional (3D)
  MHD simulations of sunspots. Observed UDs, living longer than 150 s,
  were detected and tracked in a 46 minute long data set, using an
  automatic detection code. A total of 1553 (620) UDs were detected
  in the photospheric (low chromospheric) data. Our main findings
  are (1) none of the analyzed UDs is precisely circular, (2) the
  diameter-intensity relationship only holds in bright umbral areas, and
  (3) UD velocities are inversely related to their lifetime. While nearly
  all photospheric UDs can be identified in the low chromospheric images,
  some small closely spaced UDs appear in the low chromosphere as a single
  cluster. Slow-moving and long-living UDs seem to exist in both the low
  chromosphere and photosphere, while fast-moving and short-living UDs
  are mainly detected in the photospheric images. Comparison to the 3D
  MHD simulations showed that both types of UDs display, on average, very
  similar statistical characteristics. However, (1) the average number
  of observed UDs per unit area is smaller than that of the model UDs,
  and (2) on average, the diameter of model UDs is slightly larger than
  that of observed ones.

Title: Helioseismology of a Realistic Magnetoconvective Sunspot
    Simulation
Authors: Braun, D. C.; Birch, A. C.; Rempel, M.; Duvall, T. L.
Bibcode: 2012ApJ...744...77B
Altcode:
  We compare helioseismic travel-time shifts measured from a realistic
  magnetoconvective sunspot simulation using both helioseismic holography
  and time-distance helioseismology, and measured from real sunspots
  observed with the Helioseismic and Magnetic Imager instrument on
  board the Solar Dynamics Observatory and the Michelson Doppler Imager
  instrument on board the Solar and Heliospheric Observatory. We
  find remarkable similarities in the travel-time shifts measured
  between the methodologies applied and between the simulated and real
  sunspots. Forward modeling of the travel-time shifts using either Born
  or ray approximation kernels and the sound-speed perturbations present
  in the simulation indicates major disagreements with the measured
  travel-time shifts. These findings do not substantially change with
  the application of a correction for the reduction of wave amplitudes
  in the simulated and real sunspots. Overall, our findings demonstrate
  the need for new methods for inferring the subsurface structure of
  sunspots through helioseismic inversions.

Title: Sunspot Modeling: From Simplified Models to Radiative MHD
    Simulations
Authors: Rempel, Matthias; Schlichenmaier, Rolf
Bibcode: 2011LRSP....8....3R
Altcode:
  We review our current understanding of sunspots from the scales of their
  fine structure to their large scale (global) structure including the
  processes of their formation and decay. Recently, sunspot models have
  undergone a dramatic change. In the past, several aspects of sunspot
  structure have been addressed by static MHD models with parametrized
  energy transport. Models of sunspot fine structure have been relying
  heavily on strong assumptions about flow and field geometry (e.g.,
  flux-tubes, "gaps", convective rolls), which were motivated in
  part by the observed filamentary structure of penumbrae or the
  necessity of explaining the substantial energy transport required
  to maintain the penumbral brightness. However, none of these models
  could self-consistently explain all aspects of penumbral structure
  (energy transport, filamentation, Evershed flow). In recent years,
  3D radiative MHD simulations have been advanced dramatically to the
  point at which models of complete sunspots with sufficient resolution
  to capture sunspot fine structure are feasible. Here, overturning
  convection is the central element responsible for energy transport,
  filamentation leading to fine structure, and the driving of strong
  outflows. On the larger scale these models are also in the progress
  of addressing the subsurface structure of sunspots as well as sunspot
  formation. With this shift in modeling capabilities and the recent
  advances in high resolution observations, the future research will be
  guided by comparing observation and theory.

Title: Testing Helioseismic Measurements of the Solar Meridional
    Flow with Numerical Simulations
Authors: Hartlep, T.; Zhao, J.; Kosovichev, A. G.; Mansour, N. N.;
   Rempel, M.; Pipin, V.
Bibcode: 2011AGUFMSH52B..03H
Altcode:
  The meridional flow is of fundamental importance for understanding
  magnetic flux transport in the solar interior. Reliable measurements of
  the flow could provide important constraints for dynamo theories. The
  actual shape and strength of the meridional flow, particularly in the
  deep interior, remains unknown. Detecting such weak flows with a speed
  of 10-20 m/s in the deep solar interior is a challenging problem for
  helioseismology. Numerical simulations of helioseismic wave propagation
  provide means for testing and calibrating measurement techniques,
  and can help increase our confidence in the inferences obtained from
  helioseismic inversions. We have developed a 3D numerical spectral code
  to simulate the propagation of acoustic waves in the whole-Sun. With
  this code, we simulate the propagation of stochastic wave fields given
  mean meridional flows of different strength and circulation patterns
  (including flow models with deep and shallow stagnation points). Our
  helioseismic measurement techniques are based on estimating acoustic
  travel times from wave-field cross-correlations (time-distance
  helioseismology method). We investigate various cross-correlation
  schemes, and study the sensitivity of acoustic travel times to the
  depth and speed of the meridional flow. Using the numerical simulation
  results we discuss the prospects of measuring the Sun's meridional
  flow from Solar Dynamics Observatory (SDO/HMI) data.

Title: Numerical simulations of the subsurface structure of sunspots
Authors: Rempel, M.; Cheung, M.; Birch, A. C.; Braun, D. C.
Bibcode: 2011AGUFMSH52B..02R
Altcode:
  Knowledge of the subsurface magnetic field and flow structure of
  sunspots is essential for understanding the processes involved in their
  formation, dynamic evolution and decay. Information on the subsurface
  structure can be obtained by either direct numerical modeling or
  helioseismic inversions. Numerical simulations have reached only
  in recent years the point at which entire sunspots or even active
  regions can be modeled including all relevant physical processes
  such as 3D radiative transfer and a realistic equation of state. We
  present in this talk results from a series of different models: from
  simulations of individual sunspots (with and without penumbrae) in
  differently sized computational domains to simulations of the active
  region formation process (flux emergence). It is found in all models
  that the subsurface magnetic field fragments on an intermediate scale
  (larger than the scale of sunspot fine structure such as umbral dots);
  most of these fragmentations become visible as light bridges or flux
  separation events in the photosphere. The subsurface field strength is
  found to be in the 5-10 kG range. The simulated sunspots are surrounded
  by large scale flows, the most dominant and robust flow component is
  a deep reaching outflow with an amplitude reaching about 50% of the
  convective RMS velocity at the respective depth. The simulated sunspots
  show helioseismic signatures (frequency dependent travel time shifts)
  similar to those in observed sunspots. On the other hand it is clear
  from the simulations that these signatures originate in the upper
  most 2-3 Mm of the convection zone, since only there substantial
  perturbations of the wave speed are present. The contributions from
  deeper layers are insignificant, in particular a direct comparison
  between an 8 Mm and 16 Mm deep simulation leads to indiscernible
  helioseismic differences. The National Center for Atmospheric Research
  is sponsored by the National Science Foundation. This work is in part
  supported through the NASA SDO Science Center.

Title: The role of magnetic field in supergranular scale selection
Authors: Lord, J. W.; Rast, M. P.; Rempel, M.
Bibcode: 2011AGUFMSH53C..03L
Altcode:
  We examine the role of the magnetic field in solar surface convection
  using the MURaM radiative MHD code. Using two 74x74x16 Mm simulations,
  one without magnetic field and one with an initially uniform and
  vertical 10 Gauss field, we investigate the role of magnetic field in
  supergranular scale selection. We find that the simulation with magnetic
  field has two peaks in the photospheric kinetic energy spectrum, one
  corresponding to granular size scales and a second peak near 24 Mm,
  while the purely hydrodynamic simulation has a single peak near the
  size scale of granulation (Figure 1). We examine two possible physical
  mechanisms which may underlie this increased power at low wavenumbers:
  the decreased opacity in magnetic elements near the photosphere which
  increases the radiative cooling there and the coupling, by regions
  of high magnetic flux density in convective downflows, of deeper
  larger scale motions to the photosphere. These mechanisms imply two
  very different processes. The first suggests that supergranulation
  is organized in the photosphere where radiation escapes the system
  (top down) and the second suggests that the large scale convection
  deep in the sun influences the scales observed in the photosphere
  (bottom up). Temporal cross correlation is used to examine which
  direction information is moving during pattern formation across
  scales. Additionally, a series of experiments were conducted to
  isolate individual physical effects, artificially increasing and
  decreasing the radiative losses in regions of strong magnetic flux,
  reducing the importance of magnetic tension, and constraining the box
  depth to understand the sensitivity of the size scales observed to
  the boundary conditions imposed.

Title: Properties of Umbral Dots as Measured from the New Solar
    Telescope Data and MHD Simulations
Authors: Yurchyshyn, V.; Kilcik, A.; Rempel, M.; Abramenko, V.; Kitai,
   R.; Goode, P. R.; Cao, W.; Watanabe, H.
Bibcode: 2011sdmi.confE..86Y
Altcode:
  We studied bright umbral dots (UDs) detected in the main sunspot
  of AR NOAA 11108 and compare their statistical properties to a
  state-of-the-art MHD model of a sunspot. The study is based on
  high resolution data recorded on September 20, 2010 by the New Solar
  Telescope (NST) at Big Bear Solar Observatory and 3D MHD simulations of
  sunspots. The 46 min data set included photospheric (0.3nm TiO filter
  centered at 705.7 nm) and chromospheric (0.025nm Hα Lyot filter)
  adaptive optics corrected and speckle reconstructed images. Bright
  UDs, living longer than 150 s, were detected and tracked using an
  automatic UD detection code. Total 1553 (620) UDs were detected
  in the photospheric (chromospheric) data. Our main findings are:
  i) none of the analyzed UDs is of an exact circular shape, ii) the
  diameter-intensity relationship only works for bright umbral areas, and
  iii) UD velocities inversely related to their life time. Comparison of
  photospheric and chromospheric data showed that nearly all photospheric
  UDs can be identified in the chromospheric images. However, it appears
  that some small closely spaced UDs appear in the chromospheric images
  as a single cluster, which may lead to the underestimation of the total
  number of detected chromospheric UDs. Also, while slow moving and long
  living UDs seem to exist in both chromosphere and photosphere, fast
  moving and short living ones are detected mainly in the photospheric
  images. Comparison of model and observed data shows that both types
  of UDs display very similar statistical characteristics. The main
  difference between parameters of model and observed UDs is that i)
  the average number of observed UDs per unit area is smaller than that
  of the model UDs, and ii) on average, the diameter of model UDs is
  slightly larger than that of observed ones.

Title: Subsurface Magnetic Field and Flow Structure of Simulated
    Sunspots
Authors: Rempel, Matthias
Bibcode: 2011ApJ...740...15R
Altcode: 2011arXiv1106.6287R
  We present a series of numerical sunspot models addressing the
  subsurface field and flow structure in up to 16 Mm deep domains covering
  up to two days of temporal evolution. Changes in the photospheric
  appearance of the sunspots are driven by subsurface flows in several Mm
  depth. Most of magnetic field is pushed into a downflow vertex of the
  subsurface convection pattern, while some fraction of the flux separates
  from the main trunk of the spot. Flux separation in deeper layers
  is accompanied in the photosphere with light bridge formation in the
  early stages and formation of pores separating from the spot at later
  stages. Over a timescale of less than a day we see the development of a
  large-scale flow pattern surrounding the sunspots, which is dominated
  by a radial outflow reaching about 50% of the convective rms velocity
  in amplitude. Several components of the large scale flow are found
  to be independent from the presence of a penumbra and the associated
  Evershed flow. While the simulated sunspots lead to blockage of heat
  flux in the near surface layers, we do not see compelling evidence for
  a brightness enhancement in their periphery. We further demonstrate
  that the influence of the bottom boundary condition on the stability
  and long-term evolution of the sunspot is significantly reduced in a 16
  Mm deep domain compared to the shallower domains considered previously.

Title: Mechanisms of sunspot formation
Authors: Cheung, M. C. M.; Rempel, M.
Bibcode: 2011sdmi.confE..34C
Altcode:
  We present numerical MHD simulations that model the rise of magnetic
  flux tubes through the upper 16 Mm of the solar convection zone and
  into the photosphere. Due to the strong stratification (a density
  contrast of 10^4), the emerging field is initially dispersed over
  a wide area. Nevertheless, the dispersed flux is eventually able to
  reorganize into coherent spots with photospheric field strengths of 3
  kG. In the models, sunspot formation is weakly sensitive to the initial
  subsurface field strength and to the presence of magnetic twist. As a
  consequence sunspots can form from untwisted flux tubes with as little
  as 5 kG average field strength at 16 Mm depth. The physical mechanisms
  which enables this robust formation process to occur will be discussed.

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: Comparison of numerical simulations and observations of
    helioseismic MHD waves in sunspots
Authors: Parchevsky, K. V.; Zhao, J.; Kosovichev, A. G.; Rempel, M.
Bibcode: 2011IAUS..273..422P
Altcode:
  Numerical 3D simulations of MHD waves in magnetized regions
  with background flows are very important for the understanding of
  propagation and transformation of waves in sunspots. Such simulations
  provide artificial data for testing and calibration of helioseismic
  techniques used for analysis of data from space missions SOHO/MDI,
  SDO/HMI, and HINODE. We compare with helioseismic observations
  results of numerical simulations of MHD waves in different models
  of sunspots. The simulations of waves excited by a localized source
  provide a detailed picture of the interaction of the MHD waves with
  the magnetic field and background flows (deformation of the waveform,
  wave transformation, amplitude variations and anisotropy). The observed
  cross-covariance function represents an effective Green's function of
  helioseismic waves. As an initial step, we compare it with simulations
  of waves generated by a localized source. More thorough analysis
  implies using multiple sources and comparison of the observed and
  simulated cross-covariance functions. We plan to do such calculations
  in the nearest future. Both, the simulations and observations show
  that the wavefront inside the sunspot travels ahead of a reference
  ``quiet Sun'' wavefront, when the wave enters the sunspot. However,
  when the wave passes the sunspot, the time lag between the wavefronts
  becomes unnoticeable.

Title: Towards physics-based helioseismic inversions of subsurface
    sunspot structure
Authors: Braun, D. C.; Birch, A. C.; Crouch, A. D.; Rempel, M.
Bibcode: 2011IAUS..273..379B
Altcode:
  Numerical computations of wave propagation through sunspot-like magnetic
  field structures are critical to developing and testing methods to
  deduce the subsurface structure of sunspots and active regions. We
  show that helioseismic analysis applied to the MHD sunspot simulations
  of Rempel and collaborators, as well as to translation-invariant
  models of umbral-like fields, yield wave travel-time measurements in
  qualitative agreement with those obtained in real sunspots. However,
  standard inversion methods applied to these data fail to reproduce the
  true wave-speed structure beneath the surface of the model. Inversion
  methods which incorporate direct effects of the magnetic field,
  including mode conversion, may be required.

Title: 3D numerical MHD modeling of sunspots with radiation transport
Authors: Rempel, Matthias
Bibcode: 2011IAUS..273....8R
Altcode: 2010arXiv1011.0981R
  Sunspot fine structure has been modeled in the past by a combination
  of idealized magneto-convection simulations and simplified models
  that prescribe the magnetic field and flow structure to a large
  degree. Advancement in numerical methods and computing power has
  enabled recently 3D radiative MHD simulations of entire sunspots with
  sufficient resolution to address details of umbral dots and penumbral
  filaments. After a brief review of recent developments we focus on the
  magneto-convective processes responsible for the complicated magnetic
  structure of the penumbra and the mechanisms leading to the driving of
  strong horizontal outflows in the penumbra (Evershed effect). The bulk
  of energy and mass is transported on scales smaller than the radial
  extent of the penumbra. Strong horizontal outflows in the sunspot
  penumbra result from a redistribution of kinetic energy preferring
  flows along the filaments. This redistribution is facilitated primarily
  through the Lorentz force, while horizontal pressure gradients play
  only a minor role. The Evershed flow is strongly magnetized: While
  we see a strong reduction of the vertical field, the horizontal field
  component is enhanced within filaments.

Title: A more realistic representation of overshoot at the base of
    the solar convective envelope as seen by helioseismology
Authors: Christensen-Dalsgaard, J.; Monteiro, M. J. P. F. G.; Rempel,
   M.; Thompson, M. J.
Bibcode: 2011MNRAS.414.1158C
Altcode: 2011MNRAS.tmp..440C; 2011arXiv1102.0235C
  The stratification near the base of the Sun's convective envelope
  is governed by processes of convective overshooting and element
  diffusion, and the region is widely believed to play a key role in
  the solar dynamo. The stratification in that region gives rise to
  a characteristic signal in the frequencies of solar p modes, which
  has been used to determine the depth of the solar convection zone
  and to investigate the extent of convective overshoot. Previous
  helioseismic investigations have shown that the Sun's spherically
  symmetric stratification in this region is smoother than that in a
  standard solar model without overshooting, and have ruled out simple
  models incorporating overshooting, which extend the region of adiabatic
  stratification and have a more-or-less abrupt transition to subadiabatic
  stratification at the edge of the overshoot region. In this paper we
  consider physically motivated models which have a smooth transition
  in stratification bridging the region from the lower convection zone
  to the radiative interior beneath. We find that such a model is in
  better agreement with the helioseismic data than a standard solar model.

Title: Numerical Simulations of Sunspots: From the Scale of Sine
    Structure to the Scale of Active Regions
Authors: Rempel, Matthias D.
Bibcode: 2011SPD....42.1001R
Altcode: 2011BAAS..43S.1001R
  Over that past five years magneto-convective sunspot models have seen
  a dramatic improvement to the point at which simulations of entire
  sunspots with sufficient detail for resolving sunspot fine structure
  are possible. After a brief review of recent developments I will focus
  on three different classes of numerical sunspot models. 1.) Sunspot
  simulations at the highest currently affordable resolution that
  focus on details of sunspot fine structure: I will highlight the
  magneto-convective processes that are responsible for the energy
  transport, filamentation and driving of the Evershed flow in sunspot
  penumbrae. 2.) Sunspot models at lower resolution that can be evolved
  for time scales of several days in computational domains with horizontal
  extents beyond 50 Mm: These models start to address the subsurface field
  and flow structure <P />of sunspots and their surroundings as well as
  processes related to sunspot decay. In addition these simulations are
  used as a testbed for helioseismic inversion methods. 3.) Sunspot models
  on the scale of active regions: These models capture the last stages of
  the flux emergence and sunspot formation process in the upper most 10
  to 20 Mm of the convection zone. After the initial flux dispersal due
  to the strong expansion of emerging flux a re-amplification of flux
  into 3 kG sunspots is found as a robust result. <P />The National
  Center for Atmospheric Research is sponsored by the National <P
  />Science Foundation.

Title: Towards Reliable Physics-based Helioseismic Inversions of
    Sunspot Structure
Authors: Braun, Douglas; Birch, A.; Crouch, A.; Clack, C.; Dombroski,
   D.; Rempel, M.; Duvall, T., Jr.
Bibcode: 2011SPD....42.1603B
Altcode: 2011BAAS..43S.1603B
  Inversion methods capable of reliably probing the subsurface structure
  beneath regions of strong magnetic fields, such as sunspots,
  remain elusive. We will review progress of a SDO Science Center
  project, funded to (among other goals) develop and evaluate new
  methods for this problem. Progress to date has included extensive
  production of magneto-convective sunspot models for the testing and
  validation of existing methods, for which a 27 hour run of artificial
  photospheric Dopplergrams is available online to the community. We
  will also summarize progress on the use of magnetostatic models for
  the development and testing of novel inversion methods designed to
  distinguish between magnetic field and thermal perturbations. <P />This
  work is supported by NASA contracts NNH09CE41C and NNG07EI51C.

Title: Local Helioseismology of Magnetoconvective Sunspot Simulations
    and the Reliability of Standard Inversion Methods
Authors: Braun, Douglas; Birch, A.; Rempel, M.; Duvall, T.; J.
Bibcode: 2011SPD....42.1607B
Altcode: 2011BAAS..43S.1607B
  Controversy exists in the interpretation and modeling of helioseismic
  signals in and around magnetic regions like sunspots. We show the
  results of applying local helioseismic inversions to travel-time shift
  measurements from realistic magnetoconvective sunspot simulations. We
  compare travel-time maps made from several simulations, using
  different measurements (helioseismic holography and center-annulus
  time distance helioseismology), and made on real sunspots observed
  with the HMI instrument onboard the Solar Dynamics Observatory. We
  find remarkable similarities in the travel-time perturbations
  measured between: 1) simulations extending both 8 and 16 Mm deep,
  2) the methodology (holography or time-distance) applied, and 3)
  the simulated and real sunspots. The application of RLS inversions,
  using Born approximation kernels, to narrow frequency-band travel-time
  shifts from the simulations demonstrates that standard methods fail
  to reliably reproduce the true wave speed structure. These findings
  emphasize the need for new methods for inferring the subsurface
  structure of active regions. Artificial Dopplergrams from our
  simulations are available to the community at www.hao.ucar.edu under
  "Data" and "Sunspot Models." This work is supported by NASA under the
  SDO Science Center project (contract NNH09CE41C).

Title: Penumbral Fine Structure and Driving Mechanisms of Large-scale
    Flows in Simulated Sunspots
Authors: Rempel, M.
Bibcode: 2011ApJ...729....5R
Altcode: 2011arXiv1101.2200R
  We analyze in detail the penumbral structure found in a recent radiative
  magnetohydrodynamic simulation. Near τ = 1, the simulation produces
  penumbral fine structure consistent with the observationally inferred
  interlocking comb structure. Fast outflows exceeding 8 km s<SUP>-1</SUP>
  are present along almost horizontal stretches of the magnetic field;
  in the outer half of the penumbra, we see opposite polarity flux
  indicating flux returning beneath the surface. The bulk of the penumbral
  brightness is maintained by small-scale motions turning over on scales
  shorter than the length of a typical penumbral filament. The resulting
  vertical rms velocity at τ = 1 is about half of that found in the quiet
  Sun. Radial outflows in the sunspot penumbra have two components. In
  the uppermost few 100 km, fast outflows are driven primarily through
  the horizontal component of the Lorentz force, which is confined to
  narrow boundary layers beneath τ = 1, while the contribution from
  horizontal pressure gradients is reduced in comparison to granulation
  as a consequence of anisotropy. The resulting Evershed flow reaches its
  peak velocity near τ = 1 and falls off rapidly with height. Outflows
  present in deeper layers result primarily from a preferred ring-like
  alignment of convection cells surrounding the sunspot. These flows
  reach amplitudes of about 50% of the convective rms velocity rather
  independent of depth. A preference for the outflow results from a
  combination of Lorentz force and pressure driving. While the Evershed
  flow dominates by velocity amplitude, most of the mass flux is present
  in deeper layers and likely related to a large-scale moat flow.

Title: The Need for Physics-based Inversions of Sunspot Structure
    and Flows
Authors: Braun, D. C.; Birch, A. C.; Crouch, A. D.; Rempel, M.
Bibcode: 2011JPhCS.271a2010B
Altcode:
  Current controversy exists in the interpretation and modeling
  of helioseismic signals in and around magnetic regions like
  sunspots. Unresolved issues include the dependence of the sign of
  both the inferred flows and wave speed on the type of filtering used,
  and the discrepancy between the relatively deep two-layer wave-speed
  models derived from standard time-distance methods and shallow, positive
  wave-speed models derived using forward models which include effects of
  mode conversion To make full use of the year-round, almost limb-to-limb,
  coverage provided by the Solar Dynamics Observatory, an efficient and
  reliable inversion method incorporating possible magnetic effects and
  the currently unexplained sensitivity to methodology is critical.

Title: Solar Convection Zone Dynamics
Authors: Rempel, Matthias
Bibcode: 2011sswh.book...23R
Altcode: 2010arXiv1010.5858R
  A comprehensive understanding of the solar magnetic cycle requires
  detailed modeling of the solar interior including the maintenance and
  variation of large scale flows (differential rotation and meridional
  flow), the solar dynamo and the flux emergence process connecting the
  magnetic field in the solar convection zone with magnetic field in the
  photosphere and above. Due to the vast range of time and length scales
  encountered, a single model of the entire convection zone is still
  out of reach. However, a variety of aspects can be modeled through a
  combined approach of 3D MHD models and simplified descriptions. We
  will briefly review our current theoretical understanding of these
  processes based on numerical models of the solar interior.

Title: Sunspot Seismology with the Solar Dynamics Observatory
    Helioseismic and Magnetic Imager
Authors: Braun, D. C.; Birch, A. C.; Crouch, A. D.; Clack, C.;
   Dombroski, D.; Rempel, M.
Bibcode: 2010AGUFMSH14A..05B
Altcode:
  The Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics
  Observatory (SDO) promises to yield detailed information about the
  subsurface dynamics and structure of solar active regions. A SDO Science
  Center was recently funded and initiated by NASA to (among other goals)
  enable the reliable measurements of subsurface flow, magnetic field,
  and sound speed in regions of strong magnetic fields. Using analyses
  of sunspots observed with HMI/SDO, we illustrate the challenges of
  this goal and suggest a plan for the development and implementation of
  new physics-based modeling of the subsurface structure of sunspots. Key
  components of this effort will be discussed, including numerical forward
  modeling of the wave propagation through model sunspots. These efforts
  incorporate both magnetostatic and magneto-convective models. This
  work is supported by the NASA SDO Science Center and Heliophysics GI
  programs through contracts NNH09CE41C and NNG07EI51C.

Title: Interaction of MHD Waves with Sunspots
Authors: Parchevsky, K.; Zhao, J.; Kosovichev, A. G.; Rempel, M.
Bibcode: 2010AGUFM.S32A..07P
Altcode:
  Understanding of MHD wave propagation, transformation and scattering
  by sunspots and their interaction with the non-uniform background
  magnetic field and flows is very important for improving helioseismic
  inversion procedures. Such simulations also provide artificial data
  for testitng and calibration techniques used for analysis of data from
  space missions SOHO/MDI, SDO/HMI, and HINODE. We developed 3D linear MHD
  code for numerical simulation of excitation and propagation of MHD waves
  in non-uniform medium in presence of the background magnetic field and
  flows. We present simulations of MHD wave propagation in magnetostatic
  and dynamic models of sunspots. We consider separately two cases when
  the waves are excited by point sources, located at different distances
  from the spot, and by stochastic noise source. The results are compared
  with the waveforms of the cross-correlation function extracted from
  the observational data. We discuss the differences between the models
  and observations in terms of the amplitude variations and travel-time
  shifts. Comparison of the simulations with helioseismic observations
  allows us to test the sunspot and helioseismic models, and suggest
  improvements. The numerically simulated helioseismic data are publicly
  accessible for the helioseismic community for testing and verification
  of various ambient noise imaging techniques of helioseismology
  (time-distance, holography, and ring diagrams).

Title: Formation of Solar Active Regions (Invited)
Authors: Rempel, M.
Bibcode: 2010AGUFMSH42A..02R
Altcode:
  The flux emergence process transporting magnetic field from the solar
  interior into the photosphere and beyond is central to our understanding
  of solar magnetism. However, due to the wast range of length and time
  scales as well as different physical regimes encountered from the base
  of the solar convection zone to the solar corona, a fully coherent
  picture of this process does not yet exist. In this talk I review
  models addressing the flux emergence within the bulk of the convection
  zone as well as models of the last stages of flux emergence and active
  region formation in the upper most 10-20 Mm of the convection zone and
  photopshere. I will discuss the prospects of coupling these models in
  the near future. The latter had been hampered in the past primarily by
  the fact that realistic MHD simulations of the upper convection zone
  and photosphere were restricted to rather small domains compared to the
  typical scale of an active region. Over that past 5 years a combination
  of advancement in numerical methods and high performance computing
  has enabled numerical simulations on the scale of entire sunspots
  with sufficient resolution to capture the essence of sunspot fine
  structure. Currently these simulations are expanded even further to the
  scale of active regions to address the flux emergence process and active
  region formation in the photosphere. The National Center for Atmospheric
  Research is sponsored by the National Science Foundation. This work
  is in part supported by the NASA SDO Science Center.

Title: Discovery of a 1.6 Year Magnetic Activity Cycle in the
    Exoplanet Host Star ι Horologii
Authors: Metcalfe, T. S.; Basu, S.; Henry, T. J.; Soderblom, D. R.;
   Judge, P. G.; Knölker, M.; Mathur, S.; Rempel, M.
Bibcode: 2010ApJ...723L.213M
Altcode: 2010arXiv1009.5399M
  The Mount Wilson Ca HK survey revealed magnetic activity variations in
  a large sample of solar-type stars with timescales ranging from 2.5
  to 25 years. This broad range of cycle periods is thought to reflect
  differences in the rotational properties and the depths of the surface
  convection zones for stars with various masses and ages. In 2007, we
  initiated a long-term monitoring campaign of Ca II H and K emission
  for a sample of 57 southern solar-type stars to measure their magnetic
  activity cycles and their rotational properties when possible. We report
  the discovery of a 1.6 year magnetic activity cycle in the exoplanet
  host star ι Horologii and obtain an estimate of the rotation period
  that is consistent with Hyades membership. This is the shortest activity
  cycle so far measured for a solar-type star and may be related to the
  short-timescale magnetic variations recently identified in the Sun
  and HD 49933 from helioseismic and asteroseismic measurements. Future
  asteroseismic observations of ι Hor can be compared to those obtained
  near the magnetic minimum in 2006 to search for cycle-induced shifts in
  the oscillation frequencies. If such short activity cycles are common
  in F stars, then NASA's Kepler mission should observe their effects
  in many of its long-term asteroseismic targets.

Title: Modeling the Subsurface Structure of Sunspots
Authors: Moradi, H.; Baldner, C.; Birch, A. C.; Braun, D. C.; Cameron,
   R. H.; Duvall, T. L.; Gizon, L.; Haber, D.; Hanasoge, S. M.; Hindman,
   B. W.; Jackiewicz, J.; Khomenko, E.; Komm, R.; Rajaguru, P.; Rempel,
   M.; Roth, M.; Schlichenmaier, R.; Schunker, H.; Spruit, H. C.;
   Strassmeier, K. G.; Thompson, M. J.; Zharkov, S.
Bibcode: 2010SoPh..267....1M
Altcode: 2009arXiv0912.4982M; 2010SoPh..tmp..171M
  While sunspots are easily observed at the solar surface, determining
  their subsurface structure is not trivial. There are two main
  hypotheses for the subsurface structure of sunspots: the monolithic
  model and the cluster model. Local helioseismology is the only means
  by which we can investigate subphotospheric structure. However, as
  current linear inversion techniques do not yet allow helioseismology to
  probe the internal structure with sufficient confidence to distinguish
  between the monolith and cluster models, the development of physically
  realistic sunspot models are a priority for helioseismologists. This
  is because they are not only important indicators of the variety of
  physical effects that may influence helioseismic inferences in active
  regions, but they also enable detailed assessments of the validity of
  helioseismic interpretations through numerical forward modeling. In
  this article, we provide a critical review of the existing sunspot
  models and an overview of numerical methods employed to model wave
  propagation through model sunspots. We then carry out a helioseismic
  analysis of the sunspot in Active Region 9787 and address the serious
  inconsistencies uncovered by Gizon et al. (2009a, 2009b). We find that
  this sunspot is most probably associated with a shallow, positive
  wave-speed perturbation (unlike the traditional two-layer model)
  and that travel-time measurements are consistent with a horizontal
  outflow in the surrounding moat.

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: Seismic Discrimination of Thermal and Magnetic Anomalies in
    Sunspot Umbrae
Authors: Lindsey, C.; Cally, P. S.; Rempel, M.
Bibcode: 2010ApJ...719.1144L
Altcode:
  Efforts to model sunspots based on helioseismic signatures need to
  discriminate between the effects of (1) a strong magnetic field that
  introduces time-irreversible, vantage-dependent phase shifts, apparently
  connected to fast- and slow-mode coupling and wave absorption and (2)
  a thermal anomaly that includes cool gas extending an indefinite depth
  beneath the photosphere. Helioseismic observations of sunspots show
  travel times considerably reduced with respect to equivalent quiet-Sun
  signatures. Simulations by Moradi &amp; Cally of waves skipping
  across sunspots with photospheric magnetic fields of order 3 kG show
  travel times that respond strongly to the magnetic field and relatively
  weakly to the thermal anomaly by itself. We note that waves propagating
  vertically in a vertical magnetic field are relatively insensitive to
  the magnetic field, while remaining highly responsive to the attendant
  thermal anomaly. Travel-time measurements for waves with large skip
  distances into the centers of axially symmetric sunspots are therefore
  a crucial resource for discrimination of the thermal anomaly beneath
  sunspot umbrae from the magnetic anomaly. One-dimensional models of
  sunspot umbrae based on compressible-radiative-magnetic-convective
  simulations such as by Rempel et al. can be fashioned to fit
  observed helioseismic travel-time spectra in the centers of sunspot
  umbrae. These models are based on cooling of the upper 2-4 Mm of the
  umbral subphotosphere with no significant anomaly beneath 4.5 Mm. The
  travel-time reductions characteristic of these models are primarily
  a consequence of a Wilson depression resulting from a strong downward
  buoyancy of the cooled umbral medium.

Title: Spectropolarimetric analysis of 3D MHD sunspot simulations
Authors: Borrero, J. M.; Rempel, M.; Solanki, S. K.
Bibcode: 2010AN....331..567B
Altcode:
  We have employed 3D non-grey MHD simulations of sunspots to compute
  theoretical Stokes profiles and compare the levels of circular and
  linear polarization in the simulations with those observed in a real
  sunspot. We find that the spatial distribution and average values
  of these quantities agree very well with the observations, although
  the polarization levels in the simulations are slightly larger. This
  can be explained by a slightly larger magnetic field strength or a
  larger temperature gradient in the simulated penumbra as compared to
  the observations.

Title: Developing Physics-Based Procedures for Local Helioseismic
    Probing of Sunspots and Magnetic Regions
Authors: Birch, Aaron; Braun, D. C.; Crouch, A.; Rempel, M.; Fan,
   Y.; Centeno, R.; Toomre, J.; Haber, D.; Hindman, B.; Featherstone,
   N.; Duvall, T., Jr.; Jackiewicz, J.; Thompson, M.; Stein, R.; Gizon,
   L.; Cameron, R.; Saidi, Y.; Hanasoge, S.; Burston, R.; Schunker, H.;
   Moradi, H.
Bibcode: 2010AAS...21630805B
Altcode:
  We have initiated a project to test and improve the local helioseismic
  techniques of time-distance and ring-diagram analysis. Our goals are
  to develop and implement physics-based methods that will (1) enable the
  reliable determinations of subsurface flow, magnetic field, and thermal
  structure in regions of strong magnetic fields and (2) be quantitatively
  tested with realistic solar magnetoconvection simulations in the
  presence of sunspot-like magnetic fields. We are proceeding through a
  combination of improvements in local helioseismic measurements, forward
  modeling of the helioseismic wavefield, kernel computations, inversions,
  and validation through numerical simulations. As improvements over
  existing techniques are made they will be applied to the SDO/HMI
  observations. This work is funded through the the NASA Heliophysics
  Science Division through the Solar Dynamics Observatory (SDO) Science
  Center program.

Title: Numerical Simulations of Sunspot Fine Structure
Authors: Rempel, Matthias D.
Bibcode: 2010AAS...21621105R
Altcode:
  Sunspot fine structure has been modeled in the past by a combination
  of idealized magneto-convection simulations and simplified models
  that prescribe the magnetic field and flow structure to a large
  degree. Advancement in numerical methods and computing power has
  enabled recently 3D radiative MHD simulations of entire sunspots with
  sufficient resolution to address details of umbral dots and penumbral
  filaments. After a brief review of recent developments I will focus
  on the magneto-convective processes responsible for the complicated
  magnetic structure of the penumbra and the mechanisms leading to the
  driving of strong horizontal outflows (Evershed effect). Overturning
  convective motions are the central element for understanding sunspot
  fine structure. The expansion of upflowing plasma leads to a strong
  reduction of the magnetic field strength allowing for overturning
  convection, which weakens the magnetic field further due to flux
  expulsion. The latter has a stronger effect on the vertical magnetic
  field component, leading to the formation of elongated filaments with
  increased inclination angle. Strong horizontal outflows can be explained
  through a redistribution of kinetic energy preferring flows along
  the filaments. This redistribution is facilitated primarily through
  the Lorentz force, horizontal pressure gradients play only a minor
  role. In the near surface layers energy is primarily transported by
  convective motions turning over laterally, the contribution from large
  scale flows is negligible. <P />The National Center for Atmospheric
  Research is sponsored by the National Science Foundation.

Title: Activity Cycles of Southern Asteroseismic Targets
Authors: Metcalfe, Travis S.; Judge, P. G.; Basu, S.; Henry, T. J.;
   Soderblom, D. R.; Knoelker, M.; Rempel, M.
Bibcode: 2010AAS...21542416M
Altcode: 2010BAAS...42..333M
  The Mount Wilson Ca HK survey revealed magnetic activity variations in
  a large sample of solar-type stars with timescales ranging from 2.5
  to 25 years. This broad range of cycle periods is thought to reflect
  differences in the rotational properties and the depths of the surface
  convection zones for stars with various masses and ages. Asteroseismic
  data will soon provide direct measurements of these quantities for
  individual stars, but many of the most promising targets are in the
  southern sky (e.g., alpha Cen A &amp; B, beta Hyi, mu Ara, tau Cet,
  nu Ind), while long-term magnetic activity cycle surveys are largely
  confined to the north. In 2007 we began using the SMARTS 1.5-m telescope
  to conduct a long-term monitoring campaign of Ca II H &amp; K emission
  for a sample of 57 southern solar-type stars to measure their magnetic
  activity cycles and their rotational properties when possible. This
  sample includes the most likely southern asteroseismic targets to be
  observed by the Stellar Oscillations Network Group (SONG), currently
  scheduled to begin operations in 2012. We present selected results from
  the first two years of the survey, and from the longer time baseline
  sampled by a single-epoch survey conducted in 1992.

Title: Radiative MHD Modeling of Sunspot Fine Structure
Authors: Rempel, M.
Bibcode: 2009ASPC..415..351R
Altcode:
  For a long time 3D radiative MHD simulations of sunspots were out
  of reach. With increasing computing power there has been recently
  substantial progress in modeling magneto-convection in strong magnetic
  field regions and complete sunspots including the transition from umbra
  toward plage-like solar granulation. 3D simulations point toward a
  unified understanding of sunspot fine structure in terms of a magneto
  convection process in a background field with varying inclination
  angle. We summarize here briefly the most recent developments.

Title: Group Discussion: Solar Activity: The Role of Convection,
    the Tachocline and the Dynamo, and Applications of Data Assimilation
Authors: Rempel, M.; Dikpati, M.
Bibcode: 2009ASPC..416..551R
Altcode:
  We summarize opinions expressed and outstanding issues established
  during the group discussion on current understanding of the physics of
  the solar and stellar convection zones, including dynamos and the solar
  supergranulation. This includes discussion of prospective developments
  in data assimilation that are hoped will lead to deeper insight into
  some of the outstanding issues.

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: Numerical sunspot models - subsurface structure and
    helioseismic forward modeling (Invited)
Authors: Rempel, M.; Birch, A. C.; Braun, D. C.
Bibcode: 2009AGUFMSH11B..02R
Altcode:
  The magnetic and thermal subsurface structure of sunspots has been
  debated for decades. While local helioseismic inversions allow in
  principle to constrain the subsurface structure of sunspots, a full
  inversion is still not possible due to the complicated interaction
  between waves and magnetic field. As an alternative it is possible to
  address this problem through forward modeling. Over the past few years
  numerical MHD models of entire sunspots including radiative transfer and
  a realistic equation of state have become possible. These simulations
  include p-modes excited by convection and the full interaction of these
  modes with the magnetic and thermal structure of the sunspot. In this
  talk I will present recent progress in MHD modeling of sunspots with
  special emphasis on the thermal and magnetic structure of numerical
  sunspot models. It turns out that modeled sunspots so far impose
  rather shallow perturbations to sound and fast mode speeds in the
  upper most 2 Mm. Nevertheless the seismic signatures are very similar
  to observed sunspots.

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: Radiative-MHD Simulations of Sunspot Structure
Authors: Rempel, M.
Bibcode: 2009ASPC..416..461R
Altcode:
  For a long time 3D numerical simulations of sunspots were out
  of reach. With increasing computing power there has been recently
  substantial progress in modeling magneto-convection in strong magnetic
  field regions and complete sunspots including the transition from
  umbra toward plage-like solar granulation. We summarize here briefly
  the most recent developments and discuss future directions.

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: Radiative MHD Simulations of Sunspot Structure-Challenges
    and recent developments
Authors: Rempel, Matthias
Bibcode: 2009AIPC.1171..315R
Altcode:
  For a long time 3D radiative MHD simulations of sunspots were out
  of reach. With increasing computing power there has been recently
  substantial progress in modeling magneto-convection in strong magnetic
  field regions and complete sunspots including the transition from umbra
  toward plage like solar granulation. 3D simulations point toward a
  unified understanding of sunspot fine structure in terms of a magneto
  convection process in a background field with varying inclination
  angle. We summarize here briefly the most recent developments.

Title: Activity Cycles of Southern Asteroseismic Targets
Authors: Metcalfe, T. S.; Judge, P. G.; Basu, S.; Henry, T. J.;
   Soderblom, D. R.; Knoelker, M.; Rempel, M.
Bibcode: 2009arXiv0909.5464M
Altcode:
  The Mount Wilson Ca HK survey revealed magnetic activity variations in
  a large sample of solar-type stars with timescales ranging from 2.5
  to 25 years. This broad range of cycle periods is thought to reflect
  differences in the rotational properties and the depths of the surface
  convection zones for stars with various masses and ages. Asteroseismic
  data will soon provide direct measurements of these quantities for
  individual stars, but many of the most promising targets are in the
  southern sky (e.g., alpha Cen A &amp; B, beta Hyi, mu Ara, tau Cet,
  nu Ind), while long-term magnetic activity cycle surveys are largely
  confined to the north. In 2007 we began using the SMARTS 1.5-m telescope
  to conduct a long-term monitoring campaign of Ca II H &amp; K emission
  for a sample of 57 southern solar-type stars to measure their magnetic
  activity cycles and their rotational properties when possible. This
  sample includes the most likely southern asteroseismic targets to be
  observed by the Stellar Oscillations Network Group (SONG), currently
  scheduled to begin operations in 2012. We present selected results from
  the first two years of the survey, and from the longer time baseline
  sampled by a single-epoch survey conducted in 1992.

Title: Creation and destruction of magnetic field
Authors: Rempel, Matthias
Bibcode: 2009hppl.book...42R
Altcode:
  No abstract at ADS

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: Helioseismology of a Realistic MHD Sunspot Simulation
Authors: Braun, Douglas; Birch, A. C.; Rempel, M.
Bibcode: 2009SPD....40.0303B
Altcode:
  We have recently measured travel times and absorption of p modes
  propagating through a realistic numerical model of solar convection in
  the presence of a sunspot-like structure. Both the mean travel-time
  perturbations and the absorption in the simulation are remarkably
  similar to those observed in typical sunspots. Therefore, simulations
  of this type provide both the means to understand the physics behind
  the helioseismic observations and the opportunity to validate existing
  and future models of the subsurface structure of sunspots. We will
  compare helioseismic measurements made with the simulation with
  those of a typical sunspot observed with MDI. We will discuss the
  implications of these comparisons for structural inversions of sunspots
  and understanding the role of MHD mode conversion in interpreting
  helioseismic observations. This work is supported by NASA contracts
  NNH09CE41C and NNG07EI51C.

Title: Helioseismic Inversions applied to a Realistic MHD Sunspot
    Simulation
Authors: Birch, Aaron; Braun, D. C.; Rempel, M.
Bibcode: 2009SPD....40.0713B
Altcode:
  Local helioseismology applied to the realistic magneto-convection
  sunspot simulations of Rempel et al. produces solar-like wave travel
  times. We apply standard ray-theory based inversions to infer subsurface
  wave speed from these travel times. We find that the inferred wave-speed
  perturbations are similar to the wave-speed perturbations found from
  the analysis of typical sunspots observed with MDI. We show, however,
  that the ray theory inversions fail to retrieve the true time-averaged
  sound speed or fast-mode speed from the simulations. We propose some
  alternative strategies for inferring the subsurface structure of
  sunspots. <P />This work is supported by NASA contracts NNH09CE41C
  and NNG07EI51C.

Title: Large Scale Flows in the Solar Convection Zone
Authors: Brun, Allan Sacha; Rempel, Matthias
Bibcode: 2009SSRv..144..151B
Altcode: 2008SSRv..tmp..173B
  We discuss the current theoretical understanding of the large scale
  flows observed in the solar convection zone, namely the differential
  rotation and meridional circulation. Based on multi-D numerical
  simulations we describe which physical processes are at the origin of
  these large scale flows, how they are maintained and what sets their
  unique profiles. We also discuss how dynamo generated magnetic field
  may influence such a delicate dynamical balance and lead to a temporal
  modulation of the amplitude and profiles of the solar large scale flows.

Title: Planetary Dynamos from a Solar Perspective
Authors: Christensen, U. R.; Schmitt, D.; Rempel, M.
Bibcode: 2009SSRv..144..105C
Altcode: 2008SSRv..tmp..164C
  Direct numerical simulations of the geodynamo and other planetary
  dynamos have been successful in reproducing the observed magnetic
  fields. We first give an overview on the fundamental properties
  of planetary magnetism. We review the concepts and main results of
  planetary dynamo modeling, contrasting them with the solar dynamo. In
  planetary dynamos the density stratification plays no major role
  and the magnetic Reynolds number is low enough to allow a direct
  simulation of the magnetic induction process using microscopic values
  of the magnetic diffusivity. The small-scale turbulence of the flow
  cannot be resolved and is suppressed by assuming a viscosity far in
  excess of the microscopic value. Systematic parameter studies lead
  to scaling laws for the magnetic field strength or the flow velocity
  that are independent of viscosity, indicating that the models are in
  the same dynamical regime as the flow in planetary cores. Helical flow
  in convection columns that are aligned with the rotation axis play an
  important role for magnetic field generation and forms the basis for a
  macroscopic α-effect. Depending on the importance of inertial forces
  relative to rotational forces, either dynamos with a dominant axial
  dipole or with a small-scale multipolar magnetic field are found. Earth
  is predicted to lie close to the transition point between both classes,
  which may explain why the dipole undergoes reversals. Some models
  fit the properties of the geomagnetic field in terms of spatial power
  spectra, magnetic field morphology and details of the reversal behavior
  remarkably well. Magnetic field strength in the dipolar dynamo regime
  is controlled by the available power and found to be independent of
  rotation rate. Predictions for the dipole moment agree well with the
  observed field strength of Earth and Jupiter and moderately well for
  other planets. Dedicated dynamo models for Mercury, Saturn, Uranus
  and Neptune, which assume stably stratified layers above or below the
  dynamo region, can explain some of the unusual field properties of
  these planets.

Title: Planetary Dynamos from a Solar Perspective
Authors: Christensen, U. R.; Schmitt, D.; Rempel, M.
Bibcode: 2009odsm.book..105C
Altcode:
  Direct numerical simulations of the geodynamo and other planetary
  dynamos have been successful in reproducing the observed magnetic
  fields. We first give an overview on the fundamental properties
  of planetary magnetism. We review the concepts and main results of
  planetary dynamo modeling, contrasting them with the solar dynamo. In
  planetary dynamos the density stratification plays no major role
  and the magnetic Reynolds number is low enough to allow a direct
  simulation of the magnetic induction process using microscopic values
  of the magnetic diffusivity. The small-scale turbulence of the flow
  cannot be resolved and is suppressed by assuming a viscosity far in
  excess of the microscopic value. Systematic parameter studies lead
  to scaling laws for the magnetic field strength or the flow velocity
  that are independent of viscosity, indicating that the models are in
  the same dynamical regime as the flow in planetary cores. Helical flow
  in convection columns that are aligned with the rotation axis play an
  important role for magnetic field generation and forms the basis for a
  macroscopic α-effect. Depending on the importance of inertial forces
  relative to rotational forces, either dynamos with a dominant axial
  dipole or with a small-scale multipolar magnetic field are found. Earth
  is predicted to lie close to the transition point between both classes,
  which may explain why the dipole undergoes reversals. Some models
  fit the properties of the geomagnetic field in terms of spatial power
  spectra, magnetic field morphology and details of the reversal behavior
  remarkably well. Magnetic field strength in the dipolar dynamo regime
  is controlled by the available power and found to be independent of
  rotation rate. Predictions for the dipole moment agree well with the
  observed field strength of Earth and Jupiter and moderately well for
  other planets. Dedicated dynamo models for Mercury, Saturn, Uranus
  and Neptune, which assume stably stratified layers above or below the
  dynamo region, can explain some of the unusual field properties of
  these planets.

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: Magnetic flux emergence on the Sun and Sun-like stars
Authors: Rempel, Matthias; Fan, Yuhong; Birch, Aaron; Braun, Douglas
Bibcode: 2009astro2010S..74R
Altcode: 2009astro2010S..74F
  No abstract at ADS

Title: Large Scale Flows in the Solar Convection Zone
Authors: Brun, Allan Sacha; Rempel, Matthias
Bibcode: 2009odsm.book..151B
Altcode:
  We discuss the current theoretical understanding of the large scale
  flows observed in the solar convection zone, namely the differential
  rotation and meridional circulation. Based on multi-D numerical
  simulations we describe which physical processes are at the origin of
  these large scale flows, how they are maintained and what sets their
  unique profiles. We also discuss how dynamo generated magnetic field
  may influence such a delicate dynamical balance and lead to a temporal
  modulation of the amplitude and profiles of the solar large scale flows.

Title: Dynamos and magnetic fields of the Sun and other cool stars,
    and their role in the formation and evolution of stars and in the
    habitability of planets
Authors: Schrijver, Karel; Carpenter, Ken; Karovska, Margarita; Ayres,
   Tom; Basri, Gibor; Brown, Benjamin; Christensen-Dalsgaard, Joergen;
   Dupree, Andrea; Guinan, Ed; Jardine, Moira; Miesch, Mark; Pevtsov,
   Alexei; Rempel, Matthias; Scherrer, Phil; Solanki, Sami; Strassmeier,
   Klaus; Walter, Fred
Bibcode: 2009astro2010S.262S
Altcode:
  No abstract at ADS

Title: Solar and stellar activity cycles
Authors: Rempel, Matthias
Bibcode: 2008JPhCS.118a2032R
Altcode:
  A variety of different dynamo models have been proposed for the
  Sun. While the basic ingredients of the solar dynamo are known, there is
  no general agreement about the combination and the relative importance
  of these basic processes. Unfortunately observational constraints
  are not strong enough to clearly distinguish between different dynamo
  models. Studying stellar magnetism of the lower main-sequence allows
  us to impose additional constraints on the fundamental dynamo process
  and allows to investigate how the properties of dynamos change when
  rotation and convection zone depth are different from the solar
  values. We briefly summarize the current state of this field and give
  an outlook for future improvement including theoretical considerations,
  new observations and the contributions expected from asteroseismology.

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: Observation and Modeling of the Solar-Cycle Variation of the
    Meridional Flow
Authors: Gizon, Laurent; Rempel, Matthias
Bibcode: 2008SoPh..251..241G
Altcode: 2008arXiv0803.0950G; 2008SoPh..tmp...58G
  We present independent observations of the solar-cycle variation
  of flows near the solar surface and at a depth of about 60 Mm,
  in the latitude range ± 45°. We show that the time-varying
  components of the meridional flow at these two depths have opposite
  sign, whereas the time-varying components of the zonal flow are in
  phase. This is in agreement with previous results. We then investigate
  whether the observations are consistent with a theoretical model of
  solar-cycle-dependent meridional circulation based on a flux-transport
  dynamo combined with a geostrophic flow caused by increased radiative
  loss in the active region belt (the only existing quantitative
  model). We find that the model and the data are in qualitative
  agreement, although the amplitude of the solar-cycle variation of the
  meridional flow at 60 Mm is underestimated by the model.

Title: A solar mean field dynamo benchmark
Authors: Jouve, L.; Brun, A. S.; Arlt, R.; Brandenburg, A.; Dikpati,
   M.; Bonanno, A.; Käpylä, P. J.; Moss, D.; Rempel, M.; Gilman, P.;
   Korpi, M. J.; Kosovichev, A. G.
Bibcode: 2008A&A...483..949J
Altcode:
  Context: The solar magnetic activity and cycle are linked to an
  internal dynamo. Numerical simulations are an efficient and accurate
  tool to investigate such intricate dynamical processes. <BR />Aims:
  We present the results of an international numerical benchmark
  study based on two-dimensional axisymmetric mean field solar dynamo
  models in spherical geometry. The purpose of this work is to provide
  reference cases that can be analyzed in detail and that can help in
  further development and validation of numerical codes that solve such
  kinematic problems. <BR />Methods: The results of eight numerical
  codes solving the induction equation in the framework of mean field
  theory are compared for three increasingly computationally intensive
  models of the solar dynamo: an αΩ dynamo with constant magnetic
  diffusivity, an αΩ dynamo with magnetic diffusivity sharply varying
  with depth and an example of a flux-transport Babcock-Leighton dynamo
  which includes a non-local source term and one large single cell of
  meridional circulation per hemisphere. All cases include a realistic
  profile of differential rotation and thus a sharp tachocline. <BR
  />Results: The most important finding of this study is that all codes
  agree quantitatively to within less than a percent for the αΩ dynamo
  cases and within a few percent for the flux-transport case. Both
  the critical dynamo numbers for the onset of dynamo action and the
  corresponding cycle periods are reasonably well recovered by all
  codes. Detailed comparisons of butterfly diagrams and specific cuts of
  both toroidal and poloidal fields at given latitude and radius confirm
  the good quantitative agreement. <BR />Conclusions: We believe that
  such a benchmark study will be a very useful tool since it provides
  detailed standard cases for comparison and reference.

Title: Non-kinematic flux-transport dynamos with variable meridional
    flow
Authors: Rempel, M.
Bibcode: 2007AN....328.1096R
Altcode:
  A single counter clockwise flow cell is the assumption underlying
  most flux-transport dynamo models to date. On the other hand, global
  3D simulations of the solar convection zone by Miesch et al. indicate
  that the meridional flow is strongly variable and shows at a given
  time a multi-cellular flow structure, with only the long term average
  reflecting a more regular flow field. We investigate the influence
  of such a highly time variable meridional flow on a flux-transport
  dynamo model. In our model the differential rotation and meridional
  flow are driven self-consistently through a parameterization of
  the Reynolds-stress (Λ-effect) and also macroscopic Lorentz-force
  feedback is considered. We achieve the time variable flow by adding
  random fluctuations with a given correlation time and length scale to
  both components of the turbulent angular momentum flux. We find that
  a significant amount of random fluctuations can be tolerated before
  the dynamo loses its coherence, provided that the correlation time
  scale of the random component is significantly shorter than the cycle
  length. Stronger constraints on the amplitude of random fluctuations
  come from helioseismic constraints on the variability of differential
  rotation.

Title: Joint Discussion 17 Highlights of recent progress in the
    seismology of the Sun and Sun-like stars
Authors: Bedding, Timothy R.; Brun, Allan S.; Christensen-Dalsgaard,
   Jørgen; Crouch, Ashley; De Cat, Peter; García, Raphael A.; Gizon,
   Laurent; Hill, Frank; Kjeldsen, Hans; Leibacher, John W.; Maillard,
   Jean-Pierre; Mathis, S.; Rabello-Soares, M. Cristina; Rozelot,
   Jean-Pierre; Rempel, Matthias; Roxburgh, Ian W.; Samadi, Réza; Talon,
   Suzanne; Thompson, Michael J.
Bibcode: 2007HiA....14..491B
Altcode:
  The seismology and physics of localized structures beneath the surface
  of the Sun takes on a special significance with the completion in
  2006 of a solar cycle of observations by the ground-based Global
  Oscillation Network Group (GONG) and by the instruments on board the
  Solar and Heliospheric Observatory (SOHO). Of course, the spatially
  unresolved Birmingham Solar Oscillation Network (BiSON) has been
  observing for even longer. At the same time, the testing of models of
  stellar structure moves into high gear with the extension of deep probes
  from the Sun to other solar-like stars and other multi-mode pulsators,
  with ever-improving observations made from the ground, the success of
  the MOST satellite, and the recently launched CoRoT satellite. Here
  we report the current state of the two closely related and rapidly
  developing fields of helio- and asteroseimology.

Title: Origin of Solar Torsional Oscillations
Authors: Rempel, Matthias
Bibcode: 2007ApJ...655..651R
Altcode: 2006astro.ph.10221R
  Helioseismology has revealed many details of solar differential
  rotation and its time variation, known as torsional oscillations. So
  far there is no generally accepted theoretical explanation for
  torsional oscillations, even though a close relation to the solar
  activity cycle is evident. On the theoretical side, nonkinematic
  dynamo models (including the Lorentz force feedback on differential
  rotation) have been used to explain torsional oscillations. In this
  paper we use a slightly different approach by forcing torsional
  oscillations in a mean field differential rotation model. Our aim
  is not a fully self-consistent model, but rather to point out a few
  general properties of torsional oscillations, and their possible
  origins, that are independent from a particular dynamo model. We find
  that the poleward-propagating high-latitude branch of the torsional
  oscillations can be explained as a response of the coupled differential
  rotation/meridional flow system to periodic forcing in midlatitudes
  of either mechanical (Lorentz force) or thermal nature. The speed
  of the poleward propagation sets constraints on the value of the
  turbulent viscosity in the solar convection zone to be less than
  3×10<SUP>8</SUP> m<SUP>2</SUP> s<SUP>-1</SUP>. We also show that
  the equatorward-propagating low-latitude branch is most likely not a
  consequence of mechanical forcing (Lorentz force) alone, but rather
  of thermal origin due to the Taylor-Proudman theorem.

Title: The Uncombed Penumbra
Authors: Borrero, J. M.; Rempel, M.; Solanki, S. K.
Bibcode: 2006ASPC..358...19B
Altcode: 2006astro.ph..2130B
  The uncombed penumbral model explains the structure of the sunspot
  penumbra in terms of thick magnetic fibrils embedded in a surrounding,
  magnetic atmosphere. This model has been successfully applied to explain
  the polarization signals emerging from the sunspot penumbra. Thick
  penumbral fibrils face some physical problems, however. In this
  contribution we will offer possible solutions to these shortcomings.

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: Non-kinematic flux-transport dynamos and torsional oscillations
Authors: Rempel, M.
Bibcode: 2006ESASP.624E..18R
Altcode: 2006soho...18E..18R
  No abstract at ADS

Title: Time-varying component of the solar meridional flow
Authors: Gizon, L.; Rempel, M.
Bibcode: 2006ESASP.624E.129G
Altcode: 2006soho...18E.129G
  No abstract at ADS

Title: Solar Convection Zone Dynamics: How Sensitive Are Inversions
    to Subtle Dynamo Features?
Authors: Howe, R.; Rempel, M.; Christensen-Dalsgaard, J.; Hill, F.;
   Komm, R.; Larsen, R. M.; Schou, J.; Thompson, M. J.
Bibcode: 2006ApJ...649.1155H
Altcode:
  The nearly 10 year span of medium-degree helioseismic data from the
  Global Oscillation Network Group and the Michelson Doppler Imager has
  allowed us to study the evolving flows in the solar convection zone
  over most of solar cycle 23. Using two independent two-dimensional
  rotation inversion techniques and extensive studies of the resolution
  using artificial data from different assumed flow profiles, including
  those generated from sample mean field dynamo models, we attempt to
  assess the reality of certain features seen in the inferred rotation
  profiles. Our results suggest that the findings from observations of
  a substantial depth dependence of the phase of the zonal flow pattern
  in the low latitudes, and the penetration of the flows deep into the
  convection zone, are likely to be real rather than artifacts of the
  inversion process.

Title: Non-kinematic flux-transport dynamos and torsional oscillations
Authors: Rempel, M.
Bibcode: 2006IAUJD..17E...6R
Altcode:
  We present a non-kinematic, flux-transport dynamo model for the S
  un that combines a mean - field model for differential rotation and
  meridional flow with the mean - field induction equation. The induced
  magnetic field is allowed to feed back on differential rotation and
  meridional flow through the macroscopic Lorentz force, leading to
  solar cycle variations of zonal and meridional flows. We show that
  the dynamo saturates through this feedback at a field strength of
  around 10 - 2 0 kG and that the equator ward transport of field by
  the meridional flow at the base of the convection zone (essential for
  flux-transport dynamos) is not significantly reduced. The non-linear
  dynamo is capable of explaining the high- latitude branch of torsional
  oscillations (having correct amplitude and phase relation with respect
  to the magnetic butterfly diagram), but cannot explain the low-
  latitude branch through macroscopic Lorentz-force feedback. We present
  a compound model that includes a parameterisation of enhanced radiative
  losses in the active region belt (following the idea of Spruit 2003,
  Solar Physics 213, 1) and show that this can provide the correct
  oscillation pattern in low latitudes close to the surface. Thermally-
  driven inflows into the active region belt produced by this model are
  also consistent with observations.

Title: Flux-Transport Dynamos with Lorentz Force Feedback on
Differential Rotation and Meridional Flow: Saturation Mechanism and
    Torsional Oscillations
Authors: Rempel, Matthias
Bibcode: 2006ApJ...647..662R
Altcode: 2006astro.ph..4446R
  In this paper we discuss a dynamic flux-transport dynamo model that
  includes the feedback of the induced magnetic field on differential
  rotation and meridional flow. We consider two different approaches
  for the feedback: mean field Lorentz force and quenching of transport
  coefficients such as turbulent viscosity and heat conductivity. We find
  that even strong feedback on the meridional flow does not change the
  character of the flux-transport dynamo significantly; however, it leads
  to a significant reduction of differential rotation. To a large degree
  independent of the dynamo parameters, the saturation takes place when
  the toroidal field at the base of the convection zone reaches between
  1.2 and 1.5 T, and the energy converted into magnetic energy corresponds
  to about 0.1%-0.2% of the solar luminosity. The torsional oscillations
  produced through Lorentz force feedback on differential rotation show
  a dominant poleward propagating branch with the correct phase relation
  to the magnetic cycle. We show that incorporating enhanced surface
  cooling of the active region belt (as proposed by Spruit) leads to an
  equatorward propagating branch in good agreement with observations.

Title: The uncombed penumbra
Authors: Borrero, J. M.; Rempel, M.; Solanki, S. K.
Bibcode: 2006astro.ph..2129B
Altcode:
  The uncombed penumbral model explains the structure of the sunspot
  penumbra in terms of thick magnetic fibrils embedded in a magnetic
  surrounding atmosphere. This model has been successfully applied
  to explain the polarization signals emerging from the sunspot
  penumbra. Thick penumbral fibrils face some physical problems,
  however. In this contribution we will offer possible solutions to
  these shortcomings.

Title: Transport of Toroidal Magnetic Field by the Meridional Flow
    at the Base of the Solar Convection Zone
Authors: Rempel, Matthias
Bibcode: 2006ApJ...637.1135R
Altcode: 2006astro.ph.10133R
  In this paper we discuss the transport of toroidal magnetic field
  by a weak meridional flow at the base of the convection zone. We
  use the differential rotation and meridional flow model developed by
  Rempel and incorporate feedback of a purely toroidal magnetic field
  in two ways: directly through the Lorentz force (magnetic tension)
  and indirectly through quenching of the turbulent viscosity, which
  affects the parameterized turbulent angular momentum transport in the
  model. In the case of direct Lorentz force feedback, we find that a
  meridional flow with an amplitude of around 2 m s<SUP>-1</SUP> can
  transport a magnetic field with a strength of 20-30 kG. Quenching of
  turbulent viscosity leads to deflection of the meridional flow from
  the magnetized region and a significant reduction of the transport
  velocity if the magnetic field is above equipartition strength.

Title: How Sensitive are Rotation Inversions to Subtle Features of
    the Dynamo?
Authors: Howe, R.; Rempel, M.; Christensen-Dalsgaard, J.; Schou, J.;
   Thompson, M. J.; Komm, R.; Hill, F.
Bibcode: 2005ASPC..346...99H
Altcode:
  Global rotation inversions can probe the pattern of zonal flows
  well into the convection zone. In this paper, we test the ability
  of the inversions to constrain the predictions of dynamo models. A
  flux-transport dynamo model, including a mean-field theory of
  differential rotation and allowing for feedback of the Lorentz force
  on differential rotation and meridional flow, was used to produce a
  22-year cycle of simulated rotation profiles. These were then subjected
  to simulated inversions with realistic mode sets and errors, in order
  to test how well the subtle subsurface features of the input profile
  could be recovered. The preliminary results are quite encouraging.

Title: Fighting the Taylor-Proudman constraint -- How to get
    differential rotation solar-like?
Authors: Rempel, M.
Bibcode: 2005ASPC..346...75R
Altcode:
  We present a model for the solar differential rotation and
  meridional circulation based on a mean-field parametrization of the
  Reynolds-stresses that drive the differential rotation. We include
  the subadiabatic part of the tachocline and show that this, in
  conjunction with turbulent heat conductivity within the convection
  zone and upper overshoot region, provides the key physics to break
  the Taylor-Proudman constraint, which dictates normally differential
  rotation with contour lines parallel to the axis of rotation. We show
  that solar-like differential rotation with contour lines almost aligned
  with the radial direction is a very robust result of the model, which
  does not depend on the details of the Reynolds-stress and the assumed
  viscosity, as long as the Reynolds-stress transports angular momentum
  towards the equator. The meridional flow is more sensitive to the
  details of the assumed Reynolds-stress, but a one-cell flow, equatorward
  at the base of the convection zone and poleward in the upper half of
  the convection zone, is the preferred flow pattern for a variety of
  different assumptions concerning the Reynolds-stress. Incorporating the
  feedback of a toroidal magnetic field through Lorentz force into this
  models allows us to estimate up to which field strength meridional flow
  can transport toroidal magnetic field at the base of the convection
  zone equatorward. We find an upper limit of 2 to 3 T (20 to 30 kG)
  in our investigation.

Title: Influence of Random Fluctuations in the Λ-Effect on Meridional
    Flow and Differential Rotation
Authors: Rempel, Matthias
Bibcode: 2005ApJ...631.1286R
Altcode: 2006astro.ph.10132R
  We present a mean field model based on the approach taken by Rempel in
  order to investigate the influence of stochastic fluctuations in the
  Reynolds stresses on meridional flow and differential rotation. The
  stochastic fluctuations found in the meridional flow pattern directly
  resemble the stochastic fluctuations of the Reynolds stresses, while
  the stochastic fluctuations in the differential rotation are smaller by
  almost 2 orders of magnitude. It is further found that the correlation
  length and timescale of the stochastic fluctuations have only a weak
  influence on meridional flow, but a significant influence on the
  magnitude of variations in the differential rotation. We analyze
  the energy fluxes within the model to estimate timescales for the
  replenishment of differential rotation and meridional flow. We find
  that the timescale for the replenishment of differential rotation (~10
  years) is nearly 4 orders of magnitude longer than the timescale for
  the replenishment of meridional flow, which explains the differences
  in the response to stochastic fluctuations of the Reynolds stress
  found for both flow fields.

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: Concentration of Toroidal Magnetic Field in the Solar
    Tachocline by η-Quenching
Authors: Gilman, Peter A.; Rempel, Matthias
Bibcode: 2005ApJ...630..615G
Altcode: 2005astro.ph..4003G
  We show that if the turbulent magnetic diffusivity used in solar dynamos
  is assumed to be ``quenched'' by increasing toroidal fields, much
  larger amplitude and more concentrated toroidal fields can be induced
  by differential rotation from an assumed poloidal field than if there is
  no quenching. This amplification and concentration mechanism is weakened
  and bounded by jXB feedbacks on the differential rotation. Nevertheless,
  it is strong enough to contribute to the creation of ~100 kG toroidal
  fields near the base of the convection zone, perhaps in conjunction
  with the ``exploding flux tube'' process. Such high fields are necessary
  for sunspots to occur in low solar latitudes.

Title: Comments on "Full-sphere simulations of circulation-dominated
solar dynamo: Exploring the parity issue"
Authors: Dikpati, M.; Rempel, M.; Gilman, P. A.; MacGregor, K. B.
Bibcode: 2005A&A...437..699D
Altcode:
  Using two distinct simulation codes that respectively apply
  semi-implicit and fully explicit schemes, we perform calculations
  of a 2D kinematic Babcock-Leighton type flux-transport dynamo with
  Chatterjee et al.'s parameter settings. We show that their solutions are
  diffusion-dominated, rather than circulation-dominated as their title
  implies. We also have been unable to reproduce several properties of
  their dynamo solutions, namely we obtain a much faster cycle with ~
  4 times shorter period than theirs, with highly overlapping cycles;
  a polar field value of ∼ 2 kG if one has to produce a ~ 100 kG
  toroidal field at convection zone base; and quadrupolar parity as
  opposed to Chatterjee et al.'s dipolar parity solutions.

Title: Solar Differential Rotation and Meridional Flow: The Role of
    a Subadiabatic Tachocline for the Taylor-Proudman Balance
Authors: Rempel, M.
Bibcode: 2005ApJ...622.1320R
Altcode: 2006astro.ph..4451R
  We present a simple model for the solar differential rotation
  and meridional circulation based on a mean field parameterization
  of the Reynolds stresses that drive the differential rotation. We
  include the subadiabatic part of the tachocline and show that this,
  in conjunction with turbulent heat conductivity within the convection
  zone and overshoot region, provides the key physics to break the
  Taylor-Proudman constraint, which dictates differential rotation
  with contour lines parallel to the axis of rotation in case of
  an isentropic stratification. We show that differential rotation
  with contour lines inclined by 10°-30° with respect to the axis
  of rotation is a robust result of the model, which does not depend
  on the details of the Reynolds stress and the assumed viscosity, as
  long as the Reynolds stress transports angular momentum toward the
  equator. The meridional flow is more sensitive with respect to the
  details of the assumed Reynolds stress, but a flow cell, equatorward
  at the base of the convection zone and poleward in the upper half of
  the convection zone, is the preferred flow pattern.

Title: Dynamos with feedback of of j x B force on meridional flow
    and differential rotation
Authors: Rempel, M.; Dikpati, M.; MacGregor, K.
Bibcode: 2005ESASP.560..913R
Altcode: 2005csss...13..913R
  No abstract at ADS

Title: How Sensitive are Rotation Inversions to Subtle Features of
    the Dynamo?
Authors: Howe, R.; Rempel, M.; Christensen-Dalsgaard, J.; Hill, F.;
   Komm, R. W.; Schou, J.; Thompson, M. J.
Bibcode: 2004ESASP.559..468H
Altcode: 2004soho...14..468H
  No abstract at ADS

Title: Overshoot at the Base of the Solar Convection Zone: A
    Semianalytical Approach
Authors: Rempel, M.
Bibcode: 2004ApJ...607.1046R
Altcode:
  Despite the importance of overshoot at the base of the solar convection
  zone for the storage of strong toroidal magnetic field produced there
  by the solar dynamo, uncertainties concerning the depth and mean
  subadiabatic stratification remain large. Overshoot models based
  on the nonlocal mixing-length theory generally produce a shallow,
  weakly subadiabatic region with a sharp transition to the radiative
  interior, whereas several numerical simulations lead to significantly
  subadiabatic overshoot with penetration depth of more than a pressure
  scale height. We present a semianalytical convection zone/overshoot
  region model based on the assumption that the convective energy flux
  is governed by coherent downflow structures starting at the top of
  the domain and continuing all the way down into the overshoot region,
  which allows for modeling both the parameter regime addressed by
  nonlocal mixing-length approach and the regime addressed by numerical
  simulations. It turns out that the main differences between the nonlocal
  mixing-length approach and numerical simulations (nearly adiabatic
  vs. strongly subadiabatic overshoot) are caused by the much larger
  energy flux used in numerical simulations as a consequence of larger
  thermal diffusivities required by numerical constraints. The depth of
  the overshoot region is determined predominantly by the mixing between
  downflows and upflows in the convection zone. Furthermore, our model
  shows that the sharp transition between the nearly adiabatic overshoot
  and radiative interior, a typical result of the nonlocal mixing-length
  approach, can be avoided by assuming an ensemble of downflows with
  different strength.

Title: Dynamos with feedback of jxB Force on Meridional Flow and
    Differential Rotation
Authors: Rempel, M.; Dikpati, M.; MacGregor, K.
Bibcode: 2004AAS...204.8802R
Altcode: 2004BAAS...36..819R
  Recently, flux-transport dynamos have been successful in
  reproducing various observed features of the large scale solar
  magnetic fields. However, these studies addressed the transport of
  magnetic fields by the meridional circulation in a purely kinematic
  regime. The toroidal field strength at the base of the solar convection
  zone inferred from studies of rising magnetic flux tubes is around
  100 KG and thus orders of magnitude larger than the equipartition
  field strength estimated from a meridional flow velocity of a few
  m/s. Therefore it is crucial for flux-transport dynamos to address
  the feedback of the jxB on the meridional flow. We present a "dynamic"
  dynamo model, in which we couple a mean-field Reynolds-stress approach
  for the differential rotation and meridional circulation with the
  axisymmetric dynamo equations. This provides a self-consistent model
  that allows to study the back-reaction of the mean-field Lorentz force
  of the dynamo generated field on differential rotation and meridional
  circulation. This model gives an estimate of the magnetic field strength
  up to which a transport of magnetic field by the weak meridional
  flow and amplification by the shear in the differential rotation
  is possible. Additional to this the model also provides solar cycle
  variations in differential rotation and meridional circulation, which
  can be compared to helioseismic data. We also show that the feedback
  of the Lorentz-force on the meridional flow can be included into a
  kinematic dynamo model in terms of a "quenching" of the stream function,
  which deflects the flow from regions of strong toroidal magnetic
  field. From both studies we conclude that flux-transport dynamos work
  even with strong feedback of the jxB force, primarily because of two
  reasons: 1) The transport of the weak poloidal magnetic field, which
  is the sources of strong toroidal field, is not affected strongly. 2)
  The meridional flow results from a small difference between large
  forces, so that the transport capability is much larger than a simple
  estimate based on equipartition argument. <P />This work is partially
  supported by NASA grants W-10107 and W-10175. The National Center for
  Atmospheric Research is sponsored by the National Science Foundation.

Title: Stability Analysis of Tachocline Latitudinal Differential
    Rotation and Coexisting Toroidal Band Using a Shallow-Water Model
Authors: Dikpati, Mausumi; Gilman, Peter A.; Rempel, Matthias
Bibcode: 2003ApJ...596..680D
Altcode:
  Recently global, quasi-two-dimensional instabilities of tachocline
  latitudinal differential rotation have been studied using a so-called
  shallow-water model. While purely hydrodynamic shallow-water type
  disturbances were found to destabilize only the overshoot tachocline,
  the MHD analysis showed that in the presence of a broad toroidal
  field, both the radiative and overshoot parts of the tachocline can
  be unstable. We explore here instability in the shallow-water solar
  tachocline with concentrated toroidal bands placed at a wide range
  of latitudes, emulating different phases of the solar cycle. In
  equilibrium, the poleward magnetic curvature stress of the band is
  balanced either by an equatorward hydrostatic pressure gradient or
  by the Coriolis force from a prograde jet inside the band. We find
  that toroidal bands placed almost at all latitudes make the system
  unstable to shallow-water disturbances. For bands without prograde
  jets, the instability persists well above 100 kG peak field, while
  a jet stabilizes the band at a field of ~40 kG. The jet imparts
  gyroscopic inertia to the toroidal band inhibiting it from unstably
  ``tipping'' its axis away from rotation axis. Like previously
  studied HD and MHD shallow-water instabilities in the tachocline,
  unstable shallow-water modes found here produce kinetic helicity and
  hence a tachocline α-effect these narrow kinetic helicity profiles
  should generate narrowly confined poloidal fields, which will help
  formation of the narrow toroidal field. Toroidal bands poleward of
  15° latitude suppress midlatitude hydrodynamic α-effects. However,
  even strong toroidal bands equatorward of 15° allow this hydrodynamic
  α-effect. Such bands should occur during the late declining phase
  of a solar cycle and, thus, could help the onset of a new cycle by
  switching on the mid latitude α-effect.

Title: Convective Overshoot at the Base of the Solar Convection Zone -
    a Semi-Analytical Approach
Authors: Rempel, M.
Bibcode: 2003SPD....34.2607R
Altcode: 2003BAAS...35..855R
  Despite the importance of overshoot at the base of the solar convection
  zone for the storage of strong toroidal magnetic field produced there
  by the solar dynamo, the uncertainties concerning the depth and mean
  subadiabatic stratification are large. Overshoot models of the past,
  based on the non local mixing-length theory, generally produce a
  shallow weakly subadiabatic region with a sharp transition to the
  radiative interior, whereas several numerical simulations lead to
  significantly subadiabatic overshoot with penetration depth of more
  than a pressure scale height. I present a semi-analytical convection
  zone/overshoot model based on the assumption that the convective energy
  flux is governed by downflow structures with a low filling factor, which
  allows for modeling both, the parameter regime addressed by non-local
  mixing-length approach as well as the regime addressed by numerical
  simulations. It turns out that the main discrepancies between the
  non-local mixing-length approach and numerical simulations are due to
  the much larger energy flux used in numerical simulations. Furthermore
  this model shows that the sharp transition between the nearly adiabatic
  overshoot and radiative interior, a typical result of the non-local
  mixing-length approach which is in contradiction with helioseismology,
  can be avoided by assuming an ensemble of downflows with different
  strength (Mach number). <P />NCAR is sponsored by the National Science
  Foundation.

Title: Storage and Equilibrium of Toroidal Magnetic Fields in the
Solar Tachocline: A Comparison between MHD Shallow-Water and Full
    MHD Approaches
Authors: Rempel, Matthias; Dikpati, Mausumi
Bibcode: 2003ApJ...584..524R
Altcode:
  Recently Dikpati &amp; Gilman have shown, using a shallow-water
  model of the solar tachocline that allows the top surface to deform,
  that a tachocline with the observed broad differential rotation and a
  strong toroidal field is prolate. A strong toroidal field ring requires
  extra mass on its poleward side to provide a hydrostatic latitudinal
  pressure gradient to balance the poleward curvature stress. In
  a parallel study using a different approach, Rempel, Schüssler,
  &amp; Tóth have shown that such a latitudinal pressure gradient is
  found in a strongly subadiabatic stratification, whereas a weakly
  subadiabatic stratification leads to a complementary equilibrium state
  of the overshoot tachocline in which the magnetic curvature stress is
  balanced by a prograde rotational jet inside the toroidal ring. We show
  that the shallow-water model with height deformation is a first-order
  approach to the equilibrium state found by Rempel, Schüssler, &amp;
  Tóth for a strongly subadiabatic stratification. We also show that the
  shallow-water model can be generalized to allow for the equilibrium
  state found for a weakly subadiabatic stratification by suppressing
  the shell deformation associated with the toroidal field and allowing
  the differential rotation to be modified.

Title: Thermal properties of magnetic flux tubes. II. Storage of
    flux in the solar overshoot region
Authors: Rempel, M.
Bibcode: 2003A&A...397.1097R
Altcode:
  We consider the consequences of radiative heating for the storage
  of magnetic flux in the overshoot region at the bottom of the
  solar convection zone. In the first part of the paper, we study the
  evolution of axisymmetric flux tubes (flux rings), which are initially
  in neutrally buoyant mechanical equilibrium. Radiative heating leads to
  a slow upward drift of the flux ring with a velocity depending on the
  degree of subadiabaticity of the stratification. Maintaining the flux
  tubes within the overshoot region for time intervals comparable with
  the solar cycle period requires a strongly subadiabatic stratification
  with delta =nabla -nabla <SUB>ad</SUB> &lt; -10<SUP>-4</SUP>, which
  is not predicted by most current overshoot models (e.g., Skaley &amp;
  Stix \cite{skaley91}; van Ballegooijen \cite{Ballegooijen:1982b};
  Schmitt et al. \cite{Schmitt:etal:1984}). The drag force exerted
  by equatorward flow due to meridional circulation permits states of
  mechanical and thermal equilibrium in the overshoot region, but these
  apply only to very thin magnetic flux tubes containing less than 1%
  of the flux of a large sunspot. In the second part, we consider the
  influence of radiative heating (and cooling) on magnetic flux stored
  in the form of a magnetic layer. In contrast to the case of isolated
  flux tubes, the suppression of the convective energy transport within
  the magnetic layer affects the overall stratification of the overshoot
  region. In the case of a quenching of the convective heat conductivity
  by a factor of the order 100, the overshoot layer receives a net cooling
  leading to a stronger subadiabaticity, so that values of delta &lt;
  -10<SUP>-4</SUP> are reached. The stabilization of the stratification
  relaxes the conditions for flux storage. Stronger quenching of the heat
  conductivity leads to larger temperature perturbations (of both signs)
  and to the destabilization of the upper part of the overshoot layer,
  with the likely consequence of rapid magnetic flux loss.

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: Equilibrium And Instability Of Toroidal Field Bands And
    Rotational Jets In The Solar Tachocline
Authors: Gilman, P. A.; Rempel, M.; Dikpati, M.
Bibcode: 2002AAS...200.0416G
Altcode: 2002BAAS...34..645G
  Recently Dikpati &amp; Gilman (2001, ApJ, 552, 348) have shown,
  using a shallow-water model of the solar tachocline that allows
  the top surface to deform, that a tachocline with the observed broad
  differential rotation and a strong toroidal field is prolate. A strong
  toroidal field ring requires extra mass on its poleward side to provide
  a hydrostatic latitudinal pressure gradient to balance the poleward
  curvature stress. In a parallel study using a different approach,
  Rempel et al (2000, A&amp;A, 363, 789) have shown that a weakly
  subadiabatic stratification leads to a complementary equilibrium state
  of the overshoot tachocline in which the magnetic curvature stress is
  balanced by a prograde rotational jet inside the toroidal ring. We show
  that the shallow water model yields a similar equilibrium state if we
  suppress the shell deformation and allow the differential rotation to
  be modified. We are analyzing the stability of such an equilibrium
  tachocline by using the MHD shallow-water model of Gilman &amp;
  Dikpati (2002, ApJ, submitted). We expect to show that the combination
  of toroidal band and rotational jet is virtually always unstable to
  disturbances with longitudinal wave number m&gt;0, except perhaps when
  the band is extremely narrow. This instability could wipe out the jet,
  and lead to some poleward migration of the toroidal field, as well as
  the excitation of longitudinally periodic magnetic patterns that might
  provide sites for magnetic bouyancy to produce spots as well as other
  photospheric magnetic features. This work is supported by NASA grants
  W-19752 and S-10145-X. The National Center for Atmospheric Research
  is sponsored by the National Science Foundation.

Title: Numerical Simulations of Convective Overshoot
Authors: Rempel, M.; Rast, M. P.
Bibcode: 2002AAS...200.0417R
Altcode: 2002BAAS...34..646R
  The structure of the overshoot region at the base of solar convection
  zone is crucial to the storage of strong toroidal magnetic field
  produced there by the solar dynamo. Both the mean thermodynamic
  stratification and the statistical properties of the convective
  fluctuations affect the storage capabilities of the region. Overshoot
  models of the past, based on the non local mixing-length theory,
  generally produce a shallow weakly subadiabatic region with a steep
  transition to the radiative interior. A more recent estimation by
  Xiong &amp; Deng (Mon. Not. R. Astron. Soc. 327, 1137) suggests a
  larger subadiabaticity and a smoother transition to the radiative
  gradient. Numerical studies have to date contributed little to
  constraining these simpler models, largely because they are unable
  to match the very low values of radiative conductivity found in the
  solar interior. The abnormally high values of conductivity generally
  employed lead to much more vigorous convection and much deeper
  convective penetration than anticipated. To address this deficiency
  directly we adopt a formulation which explicitly separates of the
  thermal conductivity into a turbulent and a radiative component,
  and employ a novel thermal relaxation scheme which accelerates the
  approach to equilibrium in the deep radiative layers even at very low
  values of the latter. This separation also enables adjustment of the
  convective properties apart from the radiative ones in the lower half
  of the convection zone. Preliminary results suggest that the structure
  of the overshoot region is highly sensitive to the properties of the
  convection in the lower half of the convection zone. NCAR is sponsored
  by the National Since Foundation.

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: Struktur und Ursprung starker Magnetfelder am Boden der solaren
Konvektionszone Title: Struktur und Ursprung starker Magnetfelder
am Boden der solaren Konvektionszone Title: Structure and origin of
    strong magnetic field at the base of the solar convection zone;
Authors: Rempel, Matthias Dieter
Bibcode: 2001PhDT.......204R
Altcode:
  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: 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: 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: Stability of a flux tube model for prominences
Authors: Rempel, M.; Schmitt, D.; Glatzel, W.
Bibcode: 1999A&A...343..615R
Altcode:
  We discuss the stability of a flux tube model for quiescent solar
  prominences. The main result is that the configurations are stable
  only up to a critical width (defined as the extension of the central
  part of the flux tube with prominence matter at low temperatures)
  of about 1 000 km to 3 000 km. The dependence of the critical width
  on the prominence parameters height, temperature, density contrast,
  external magnetic field, external gas pressure and external temperature
  is analysed. The normal modes and eigenfrequencies obtained numerically
  cover the range of observational data for prominence oscillations.

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.

