rempel

explanation blue bibcodes open ADS page with paths to full text

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

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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.
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. 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. 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. 
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 102-103 in the
upper chromosphere and by up to 109 near the transition
region. 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. 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>

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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.
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.

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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
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.

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Title: Derivation of Boundary Conditions for Data-Driven Simulations
of Active Regions and their Emission
Authors: Tremblay, Benoit; Malanushenko, Anna; Rempel, Matthias;
Kazachenko, Maria
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.

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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
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.

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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
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.

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Title: A Statistical Approach to Study Fine Structure of EUV Emission
in Active Regions
Authors: Malanushenko, Anna; Rempel, Matthias; Tremblay, Benoit;
Kazachenko, Maria
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.

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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.
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. 
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). 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. 
Results: We find enhanced heating rates in the upper chromosphere of up
to ∼5 kW m−2, 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 ∼104 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. 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. The movie is available at <A
href="https://www.aanda.org/10.1051/0004-6361/202243191/olm">https://www.aanda.org</A>

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Title: The Coronal Veil
Authors: Malanushenko, A.; Cheung, M. C. M.; DeForest, C. E.; Klimchuk,
J. A.; Rempel, M.
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.

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Title: The effect of small-scale magnetic fields on stellar convection
and activity
Authors: Rempel, M.
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.

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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
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.

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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
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.

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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.
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. 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. 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. 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. 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. 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>

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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
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.

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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
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.

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Title: Solar Atmosphere Radiative Transfer Model Comparison based
on 3D MHD Simulations
Authors: Haberreiter, Margit; Criscuoli, Serena; Rempel, Matthias;
Mendes Domingos Pereira, Tiago
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.

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Title: Analyzing the Structure of Coronal Loops in MURaM Radiation
MHD Simulations
Authors: David, Mia; Rempel, Matthias; Malanushenko, Anna
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
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
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
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.
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.
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. 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. 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. 
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−1, 1 km s−1, and 0.1 km
s−1, 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. 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. 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.
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. 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. 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. 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. 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.
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 × 104
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.
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. 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
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.
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. 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. 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. 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. 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. The animation is available at <A
href="https://www.aanda.org/10.1051/0004-6361/202040199/olm">https://www.aanda.org</A>
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
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.
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. 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
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
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.
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.
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-2 to a rise of 6.3 W m-2. 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-2 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.
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.

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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
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. 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. 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. 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.

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Title: On the Contribution of Quiet-Sun Magnetism to Solar Irradiance
Variations: Constraints on Quiet-Sun Variability and Grand-minimum
Scenarios
Authors: Rempel, M.
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 × 10-4 G-1. 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.

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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
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.

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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.
2020arXiv200400079J Altcode:
This white paper summarizes major scientific challenges and
opportunities in understanding magnetic reconnection and related
explosive phenomena as a fundamental plasma process.

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Title: Using the Butterfly Effect to Probe How the Sun Generates
Acoustic Noise
Authors: Lindsey, Charles; Rempel, Matthias
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.

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Title: Testing Data-driven Simulations of Solar Eruptive Flares
Using Synthetic Magnetograms from Flux Emergence Simulations
Authors: Fan, Y.; Rempel, M.
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.

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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.
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 vlos, and magnetic field B as
a function of the continuum optical-depth τc. However,
they do not directly provide the gas pressure Pg 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 Pg 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 τc to geometrical height z. Aims:
We aim at improving the determination of the gas pressure and density
via the application of magnetohydrostatic (MHS) equilibrium instead of
HE. 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 Pg
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 - τc conversion (i.e., Wilson depression). On the
other hand, the application of MHS equilibrium with similar boundary
conditions allows determination of Pg 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 - τc 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. 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 τc. 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.

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Title: Superstrong photospheric magnetic fields in sunspot penumbrae
Authors: Siu-Tapia, A.; Lagg, A.; van Noort, M.; Rempel, M.; Solanki,
S. K.
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. 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. 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. 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 vLOS (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
vLOS ∼ 16 - 18 km s-1 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
(vLOS ∼ 15 - 17 km s-1) 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. 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 (vLOS &gt; 10 km s-1
and up to ∼30 km s-1 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.

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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.
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. 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. 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. 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. 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.

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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.
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 systems1,2. On
the Sun, strong flares are usually found in newly emerging sunspot
regions3. 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 corona4
and on how three-dimensional magnetic reconnection allows for rapid
energy release5,6, 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 flares7,
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 plasmas8 up to and beyond 100 million K.

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Title: Combining magneto-hydrostatic constraints with Stokes profiles
inversions
Authors: Borrero, J. M.; Pastor Yabar, A.; Rempel, M.; Ruiz Cobo, B.
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.

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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
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.

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Title: Opposite Polarity Magnetic Fields and Convective Downflows
in a Simulated Sunspot Penumbra
Authors: Bharti, Lokesh; Rempel, Matthias
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.

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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
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.

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Title: Reversed Dynamo at Small Scales and Large Magnetic Prandtl
Number
Authors: Brandenburg, Axel; Rempel, Matthias
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.

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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.
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
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.
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. 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. 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. 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. 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.
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.
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
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.
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
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
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 1023
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 1031
(1032 ) 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
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
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
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
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.
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-1 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-1. 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.
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.
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
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 10Be and 14C, 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
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.
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 3x1011
erg/cm2/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.
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.

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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
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.

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Title: Characterizing the Motion of Photospheric Magnetic Bright
Points at High Resolution
Authors: Van Kooten, Samuel Jay; Cranmer, Steven R.; Rempel, Matthias
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.

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Title: Are Internetwork Magnetic Fields in the Solar Photosphere
Horizontal or Vertical?
Authors: Lites, B. W.; Rempel, M.; Borrero, J. M.; Danilovic, S.
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.
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.

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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
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.

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Title: 3D MHD simulation of a Solar Flare
Authors: Rempel, M.; Cheung, M. C. M.; HGCR Team
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.
2016A&A...594A.103D Altcode: 2016arXiv160706242D
Context. Information on the origin of internetwork magnetic field is
hidden at the smallest spatial scales. Aims: We try to retrieve
the power spectra with certainty to the highest spatial frequencies
allowed by current instrumentation. Methods: To accomplish this,
we use a 2D inversion code that is able to recover information up to
the instrumental diffraction limit. 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. 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.
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. 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. 
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. 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. 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.
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
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.
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
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
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
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
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.
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.
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
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.

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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.
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. 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.
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-1, the naked spot is surrounded by
an inflow of 1-2 km s-1 in close proximity. However, both
spots are surrounded by an outflow on larger scales with a few 100 m
s-1 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
1021 Mx day-1. 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.
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.
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.
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}⊙ } (solar radius) of
horizontal extent and a radial extent from 0.715 to 0.96 {{R}⊙
}. 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.
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 ≈ 102 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.
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.
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 ⊙ 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 ⊙. 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\prime r v\prime _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\prime _rv\prime _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\prime _rv\prime
_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.
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+). 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 σB
≤ 35 (Gauss), σγ ≤ 1.2°, and σv ≤
10 m s-1, respectively. Additionally, M-E inversion codes
agree with the numerical simulations, when compared at a fixed optical
depth, within σB ≤ 130 (Gauss), σγ ≤ 5°,
and σv ≤ 320 m s-1. 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 σB ≤ 90 (Gauss), σγ ≤ 3°,
and σv ≤ 90 m s-1.

---------------------------------------------------------
Title: Time-distance Helioseismology of Two Realistic Sunspot
Simulations
Authors: DeGrave, K.; Jackiewicz, J.; Rempel, M.
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
~1002 Mm2 × 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.
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.
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
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.
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.
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.
2014ApJ...788..127D Altcode: 2014arXiv1404.4645D
Linear time-distance helioseismic inversions are carried out
for vector flow velocities using travel times measured from two
~1002 Mm2 × 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.
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 ⊙, 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 ⊙,
where R ⊙ is the solar radius. (2) In the deeper convection
zone (&lt;0.9 R ⊙), 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 eq-0.25B eq throughout the
convection zone, where B eq 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.
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 × 1022 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.
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.
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.
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
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.
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.

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Title: Magnetic Field Intensification by the Three-dimensional
"Explosion" Process
Authors: Hotta, H.; Rempel, M.; Yokoyama, T.
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.

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Title: On the Amplitude of Convective Velocities in the Deep Solar
Interior
Authors: Miesch, Mark S.; Featherstone, Nicholas A.; Rempel, Matthias;
Trampedach, Regner
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-1 in the upper CZ (r ~ 0.95R) and at least 8 m
s-1 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.

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

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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.
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.

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Title: High-latitude Solar Torsional Oscillations during Phases of
Changing Magnetic Cycle Amplitude
Authors: Rempel, M.
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.

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Title: Numerical Sunspot Models: Robustness of Photospheric Velocity
and Magnetic Field Structure
Authors: Rempel, M.
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.

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Title: Comparison of Multi-Height Observations with a 3D MHD
Sunspot Model
Authors: Jaeggli, S. A.; Lin, H.; Uitenbroek, H.; Rempel, M.
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.

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Title: Numerical calculation of convection with reduced speed of
sound technique
Authors: Hotta, H.; Rempel, M.; Yokoyama, T.; Iida, Y.; Fan, Y.
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. 
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. 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. 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.

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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.
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.

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Title: Helioseismology of a Realistic Magnetoconvective Sunspot
Simulation
Authors: Braun, D. C.; Birch, A. C.; Rempel, M.; Duvall, T. L.
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
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.
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.
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.

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Title: The role of magnetic field in supergranular scale selection
Authors: Lord, J. W.; Rast, M. P.; Rempel, M.
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.
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
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.

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Title: Mechanisms of sunspot formation
Authors: Cheung, M. C. M.; Rempel, M.
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
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
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.
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.
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
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.

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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.
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.
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 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. The National
Center for Atmospheric Research is sponsored by the National 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.
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. 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.
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.
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-1
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.
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
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.
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.
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.
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.
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.
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.
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 rhov1/2. 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.
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.
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.
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.
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. 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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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
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.
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.
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.
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.
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. 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
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.
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.
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.
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
2009astro2010S..74R Altcode: 2009astro2010S..74F
No abstract at ADS

---------------------------------------------------------
Title: Large Scale Flows in the Solar Convection Zone
Authors: Brun, Allan Sacha; Rempel, Matthias
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
2009astro2010S.262S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Solar and stellar activity cycles
Authors: Rempel, Matthias
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.
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
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.
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. 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. 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. 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. 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.
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.
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
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
3×108 m2 s-1. 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.
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.
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.
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.
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.
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.
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
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.
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
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-1 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.
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.
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
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.
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
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.
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.
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.
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.
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.
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.
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. 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
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.
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). 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
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.
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 ad &lt; -10-4, 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;
-10-4 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
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
105G) 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 105G 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.
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.
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.
2001ApJ...552L.171R Altcode:
A strong superequipartition magnetic field strength on the order of
10 T (105 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
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.
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.
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.
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.
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.
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.