Author name code: steiner
ADS astronomy entries on 2022-09-14
author:"Steiner, Oskar"
------------------------------------------------------------------------
Title: Observation of a small-scale magnetic vortex associated with
a chromospheric swirl: signatures of a small-scale magnetic tornado
Authors: Milena Diaz Castillo, Saida; Steiner, Oskar; Fischer,
Catherine; Berdyugina, Svetlana; Rezaei, Reza
Bibcode: 2022cosp...44.2521M
Altcode:
\newcommand{\ion}[2]{#1\,{\textsc{#2}}} High-resolution solar
observations revealed the existence of small-scale swirling vortices
in chromospheric intensity maps and velocity diagnostics. These events
are commonly localized in the quiet sun intergranular space and are
often related to small-scale magnetic flux concentrations at the solar
surface. Frequently, vortices have been observed in the vicinity of
magnetic flux concentrations, indicating a link between swirls and the
evolution of the small-scale magnetic fields. Vortices were also studied
with MHD numerical simulations of the solar atmosphere, revealing
their complexity, dynamics, and magnetic nature. In particular, it
has been suggested that the chromospheric swirling plasma motion is
due to a coherently rotating magnetic field structure, which again
is driven by a photospheric vortex flow at its footpoint. In this
contribution, we present a comprehensive description of the evolution
of an isolated small-scale magnetic element interacting with a vortex
flow, which in turn is related to a chromospheric swirl. We study
observations taken with the CRisp Imaging SpectroPolarimeter (CRISP)
instrument and the CHROMospheric Imaging Spectrometer (CHROMIS) at the
1m Swedish Solar Telescope (SST) in April 2019 as part of a SOLARNET
access program. The data were taken at quiet-Sun disk-center, recording
full Stokes photospheric maps in the \ion{Fe}{i} line at 617\,nm, full
Stokes data in the \ion{Ca}{ii} infrared triplet line at 854\,nm, and
spectroscopic maps in the H$\alpha$ 656\,nm, \ion{Ca}{ii} K 393\,nm,
and \ion{Ca}{ii} H 396\,nm lines. Utilizing the multi-wavelength
data and applying height-dependent Stokes inversion and local
correlation tracking methods, we are able to analyse the magnetic
field dynamics in the presence of vortex structures at photospheric
and chromospheric layers. The temporal evolution of the magnetic
element shows an appreciable increase in the magnetic field strength
during the interaction with the vortex flow, reaching kG values for
a few minutes. We also find a clear evidence of a Rapid Blue-shift
Excursion (RBE) associated with the magnetic field intensification
event propagating along the chromospheric vortex. In addition, we
explore the polarization signatures in the photosphere to reveal the
intrinsic structure of the magnetic element. Marginal but consistent
detection of linear polarization signals in the surroundings of the
magnetic element before intensification suggests a magnetic field
torsion. Our analysis indicates that we have observed a rotating
magnetic object reaching from the photosphere to the chromosphere,
resembling a small-scale magnetic tornado.
Title: Acoustic-gravity wave propagation characteristics in 3D
radiation hydrodynamic simulations of the solar atmosphere
Authors: Fleck, Bernhard; Khomenko, Elena; Carlsson, Mats; Rempel,
Matthias; Steiner, Oskar; Riva, Fabio; Vigeesh, Gangadharan
Bibcode: 2022cosp...44.2503F
Altcode:
There has been tremendous progress in the degree of realism of
three-dimensional radiation magneto-hydrodynamic simulations of the
solar atmosphere in the past decades. Four of the most frequently
used numerical codes are Bifrost, CO5BOLD, MANCHA3D, and MURaM. Here
we test and compare the wave propagation characteristics in model
runs from these four codes by measuring the dispersion relation
of acoustic-gravity waves at various heights. We find considerable
differences between the various models.
Title: Small scale Alfvénic vortices in the solar atmosphere
Authors: Canivete Cuissa, José Roberto; Steiner, Oskar; Battaglia,
Andrea
Bibcode: 2022cosp...44.2551C
Altcode:
Vortices are one of the fundamental small scale features present in the
solar atmosphere. They are ubiquitous at photospheric and chromospheric
levels and are often tightly coupled to small scale surface magnetic
fields. Therefore, they potentially contribute to the transport of
energy towards the upper layers of the solar atmosphere. We study
the dynamics and the statistical properties of small scale swirls in
realistic radiative magneto-hydrodynamical simulations of the solar
atmosphere realized with the CO5BOLD code. To this aim, we employ the
swirling strength criterion and its evolution equation to investigate
the properties and dynamics of 9 identified swirl events. It is found
that the studied photospheric and chromospheric swirls are the plasma
response to self-consistently launched torsional Alfvén pulses that
propagate vertically in the solar atmosphere. To infer the statistical
properties of the swirl population in the numerical simulations, we
apply a new algorithm for the automatic identification of vortices. This
algorithm is based on a state-of-the-art method that combines the rigor
of mathematical criteria and the global perspective of morphological
techniques. Compared to previous studies, our analysis reveals more and
smaller vortical motions in the simulated solar atmosphere. Moreover,
it is found that the large majority of the identified swirls in the
photosphere show twists in the magnetic field lines compatible with
torsional Alfvén waves. Therefore we confirm that the small scale
vortices in the lower solar atmosphere are Alfvénic in nature.
Title: Methodology for estimating the magnetic Prandtl number and
application to solar surface small-scale dynamo simulations
Authors: Riva, F.; Steiner, O.
Bibcode: 2022A&A...660A.115R
Altcode: 2022arXiv220212115R
Context. A crucial step in the numerical investigation of small-scale
dynamos in the solar atmosphere consists of an accurate determination
of the magnetic Prandtl number, Prm, stemming from radiative
magneto-hydrodynamic (MHD) simulations.
Aims: The aims are to
provide a reliable methodology for estimating the effective Reynolds
and magnetic Reynolds numbers, Re and Rem, and their ratio
Prm = Rem/Re (the magnetic Prandlt number),
that characterise MHD simulations and to categorise small-scale
dynamo simulations in terms of these dimensionless parameters.
Methods: The methodology proposed for computing Re and Rem
is based on the method of projection on proper elements and it relies
on a post-processing step carried out using higher order accurate
numerical operators than the ones in the simulation code. A number
of radiative MHD simulations with different effective viscosities and
plasma resistivities were carried out with the CO5BOLD code,
and the resulting growth rate of the magnetic energy and saturated
magnetic field strengths were characterised in terms of Re and
Rem.
Results: Overall, the proposed methodology
provides a solid estimate of the dissipation coefficients affecting
the momentum and induction equations of MHD simulation codes, and
consequently also a reliable evaluation of the magnetic Prandtl number
characterising the numerical results. Additionally, it is found that
small-scale dynamos are active and can amplify a small seed magnetic
field up to significant values in CO5BOLD simulations with
a grid spacing smaller than h = 12 km, even at Prm ≃
0.65. However, it is also evident that it is difficult to categorise
dynamo simulations in terms of Prm alone, because it
is not only important to estimate the amplitude of the dissipation
coefficients, but also at which scales energy dissipation takes place.
Title: Final Report for SAG 21: The Effect of Stellar Contamination
on Space-based Transmission Spectroscopy
Authors: Rackham, Benjamin V.; Espinoza, Néstor; Berdyugina, Svetlana
V.; Korhonen, Heidi; MacDonald, Ryan J.; Montet, Benjamin T.; Morris,
Brett M.; Oshagh, Mahmoudreza; Shapiro, Alexander I.; Unruh, Yvonne C.;
Quintana, Elisa V.; Zellem, Robert T.; Apai, Dániel; Barclay, Thomas;
Barstow, Joanna K.; Bruno, Giovanni; Carone, Ludmila; Casewell, Sarah
L.; Cegla, Heather M.; Criscuoli, Serena; Fischer, Catherine; Fournier,
Damien; Giampapa, Mark S.; Giles, Helen; Iyer, Aishwarya; Kopp, Greg;
Kostogryz, Nadiia M.; Krivova, Natalie; Mallonn, Matthias; McGruder,
Chima; Molaverdikhani, Karan; Newton, Elisabeth R.; Panja, Mayukh;
Peacock, Sarah; Reardon, Kevin; Roettenbacher, Rachael M.; Scandariato,
Gaetano; Solanki, Sami; Stassun, Keivan G.; Steiner, Oskar; Stevenson,
Kevin B.; Tregloan-Reed, Jeremy; Valio, Adriana; Wedemeyer, Sven;
Welbanks, Luis; Yu, Jie; Alam, Munazza K.; Davenport, James R. A.;
Deming, Drake; Dong, Chuanfei; Ducrot, Elsa; Fisher, Chloe; Gilbert,
Emily; Kostov, Veselin; López-Morales, Mercedes; Line, Mike; Močnik,
Teo; Mullally, Susan; Paudel, Rishi R.; Ribas, Ignasi; Valenti, Jeff A.
Bibcode: 2022arXiv220109905R
Altcode:
Study Analysis Group 21 (SAG21) of the Exoplanet Exploration Program
Analysis Group (ExoPAG) was organized to study the effect of stellar
contamination on space-based transmission spectroscopy, a method for
studying exoplanetary atmospheres by measuring the wavelength-dependent
radius of a planet as it transits its star. Transmission spectroscopy
relies on a precise understanding of the spectrum of the star being
occulted. However, stars are not homogeneous, constant light sources
but have temporally evolving photospheres and chromospheres with
inhomogeneities like spots, faculae, and plages. This SAG has brought
together an interdisciplinary team of more than 100 scientists, with
observers and theorists from the heliophysics, stellar astrophysics,
planetary science, and exoplanetary atmosphere research communities,
to study the current needs that can be addressed in this context to
make the most of transit studies from current NASA facilities like
HST and JWST. The analysis produced 14 findings, which fall into
three Science Themes encompassing (1) how the Sun is used as our best
laboratory to calibrate our understanding of stellar heterogeneities
("The Sun as the Stellar Benchmark"), (2) how stars other than the Sun
extend our knowledge of heterogeneities ("Surface Heterogeneities of
Other Stars") and (3) how to incorporate information gathered for the
Sun and other stars into transit studies ("Mapping Stellar Knowledge
to Transit Studies").
Title: A novel fourth-order WENO interpolation technique. A possible
new tool designed for radiative transfer
Authors: Janett, Gioele; Steiner, Oskar; Alsina Ballester, Ernest;
Belluzzi, Luca; Mishra, Siddhartha
Bibcode: 2021arXiv211011885J
Altcode:
Context. Several numerical problems require the interpolation of
discrete data that present various types of discontinuities. The
radiative transfer is a typical example of such a problem. This
calls for high-order well-behaved techniques to interpolate both
smooth and discontinuous data. Aims. The final aim is to propose
new techniques suitable for applications in the context of numerical
radiative transfer. Methods. We have proposed and tested two different
techniques. Essentially non-oscillatory (ENO) techniques generate
several candidate interpolations based on different substencils. The
smoothest candidate interpolation is determined from a measure for
the local smoothness, thereby enabling the essential non-oscillatory
property. Weighted ENO (WENO) techniques use a convex combination of
all candidate substencils to obtain high-order accuracy in smooth
regions while keeping the essentially non-oscillatory property. In
particular, we have outlined and tested a novel well-performing
fourth-order WENO interpolation technique for both uniform and
nonuniform grids. Results. Numerical tests prove that the fourth-order
WENO interpolation guarantees fourth-order accuracy in smooth regions
of the interpolated functions. In the presence of discontinuities, the
fourth-order WENO interpolation enables the non-oscillatory property,
avoiding oscillations. Unlike Bézier and monotonic high-order Hermite
interpolations, it does not degenerate to a linear interpolation near
smooth extrema of the interpolated function. Conclusions. The novel
fourth-order WENO interpolation guarantees high accuracy in smooth
regions, while effectively handling discontinuities. This interpolation
technique might be particularly suitable for several problems, including
a number of radiative transfer applications such as multidimensional
problems, multigrid methods, and formal solutions.
Title: The Alfvénic nature of chromospheric swirls
Authors: Battaglia, Andrea Francesco; Canivete Cuissa, José Roberto;
Calvo, Flavio; Bossart, Aleksi Antoine; Steiner, Oskar
Bibcode: 2021A&A...649A.121B
Altcode: 2021arXiv210307366B
Context. Observations show that small-scale vortical plasma motions
are ubiquitous in the quiet solar atmosphere. They have received
increasing attention in recent years because they are a viable
candidate mechanism for the heating of the outer solar atmospheric
layers. However, the true nature and the origin of these swirls, and
their effective role in the energy transport, are still unclear.
Aims: We investigate the evolution and origin of chromospheric swirls
by analyzing numerical simulations of the quiet solar atmosphere. In
particular, we are interested in finding their relation with magnetic
field perturbations and in the processes driving their evolution.
Methods: The radiative magnetohydrodynamic code CO5BOLD is used
to perform realistic numerical simulations of a small portion of the
solar atmosphere, ranging from the top layers of the convection zone
to the middle chromosphere. For the analysis, the swirling strength
criterion and its evolution equation are applied in order to identify
vortical motions and to study their dynamics. As a new criterion,
we introduce the magnetic swirling strength, which allows us to
recognize torsional perturbations in the magnetic field.
Results: We find a strong correlation between swirling strength and
magnetic swirling strength, in particular in intense magnetic flux
concentrations, which suggests a tight relation between vortical
motions and torsional magnetic field perturbations. Furthermore,
we find that swirls propagate upward with the local Alfvén speed as
unidirectional swirls driven by magnetic tension forces alone. In the
photosphere and low chromosphere, the rotation of the plasma co-occurs
with a twist in the upwardly directed magnetic field that is in the
opposite direction of the plasma flow. All together, these are clear
characteristics of torsional Alfvén waves. Yet, the Alfvén wave is
not oscillatory but takes the form of a unidirectional pulse. The
novelty of the present work is that these Alfvén pulses naturally
emerge from realistic numerical simulations of the solar atmosphere. We
also find indications of an imbalance between the hydrodynamic and
magnetohydrodynamic baroclinic effects being at the origin of the
swirls. At the base of the chromosphere, we find a mean net upwardly
directed Poynting flux of 12.8 ± 6.5 kW m−2, which is
mainly due to swirling motions. This energy flux is mostly associated
with large and complex swirling structures, which we interpret as the
superposition of various small-scale vortices.
Conclusions:
We conclude that the ubiquitous swirling events observed in numerical
simulations are tightly correlated with perturbations of the magnetic
field. At photospheric and chromospheric levels, they form Alfvén
pulses that propagate upward and may contribute to chromospheric
heating.
Movie associated to Fig. C.1 is available at https://www.aanda.org
Title: On the effect of oscillatory phenomena on Stokes inversion
results
Authors: Keys, P. H.; Steiner, O.; Vigeesh, G.
Bibcode: 2021RSPTA.37900182K
Altcode: 2020arXiv200805539K
Stokes inversion codes are crucial in returning properties of the solar
atmosphere, such as temperature and magnetic field strength. However,
the success of such algorithms to return reliable values can be
hindered by the presence of oscillatory phenomena within magnetic
wave guides. Returning accurate parameters is crucial to both
magnetohydrodynamics (MHD) studies and solar physics in general. Here,
we employ a simulation featuring propagating MHD waves within a flux
tube with a known driver and atmospheric parameters. We invert the
Stokes profiles for the 6301 Å and 6302 Å line pair emergent from
the simulations using the well-known Stokes Inversions from Response
functions code to see if the atmospheric parameters can be returned
for typical spatial resolutions at ground-based observatories. The
inversions return synthetic spectra comparable to the original input
spectra, even with asymmetries introduced in the spectra from wave
propagation in the atmosphere. The output models from the inversions
match closely to the simulations in temperature, line-of-sight magnetic
field and line-of-sight velocity within typical formation heights of the
inverted lines. Deviations from the simulations are seen away from these
height regions. The inversions results are less accurate during passage
of the waves within the line formation region. The original wave period
could be recovered from the atmosphere output by the inversions, with
empirical mode decomposition performing better than the wavelet approach
in this task. This article is part of the Theo Murphy meeting issue
`High-resolution wave dynamics in the lower solar atmosphere'.
Title: Acoustic-gravity wave propagation characteristics in
three-dimensional radiation hydrodynamic simulations of the solar
atmosphere
Authors: Fleck, B.; Carlsson, M.; Khomenko, E.; Rempel, M.; Steiner,
O.; Vigeesh, G.
Bibcode: 2021RSPTA.37900170F
Altcode: 2020arXiv200705847F
There has been tremendous progress in the degree of realism of
three-dimensional radiation magneto-hydrodynamic simulations of the
solar atmosphere in the past decades. Four of the most frequently
used numerical codes are Bifrost, CO5BOLD, MANCHA3D and MURaM. Here
we test and compare the wave propagation characteristics in model
runs from these four codes by measuring the dispersion relation of
acoustic-gravity waves at various heights. We find considerable
differences between the various models. The height dependence of
wave power, in particular of high-frequency waves, varies by up to
two orders of magnitude between the models, and the phase difference
spectra of several models show unexpected features, including ±180°
phase jumps. This article is part of the Theo Murphy meeting issue
`High-resolution wave dynamics in the lower solar atmosphere'.
Title: On the influence of magnetic topology on the propagation of
internal gravity waves in the solar atmosphere
Authors: Vigeesh, G.; Roth, M.; Steiner, O.; Fleck, B.
Bibcode: 2021RSPTA.37900177V
Altcode: 2020arXiv201006926V
The solar surface is a continuous source of internal gravity waves
(IGWs). IGWs are believed to supply the bulk of the wave energy for
the lower solar atmosphere, but their existence and role for the energy
balance of the upper layers is still unclear, largely due to the lack
of knowledge about the influence of the Sun's magnetic fields on
their propagation. In this work, we look at naturally excited IGWs
in realistic models of the solar atmosphere and study the effect
of different magnetic field topographies on their propagation. We
carry out radiation-magnetohydrodynamic simulations of a magnetic
field free and two magnetic models-one with an initial, homogeneous,
vertical field of 100 G magnetic flux density and one with an initial
horizontal field of 100 G flux density. The propagation properties
of IGWs are studied by examining the phase-difference and coherence
spectra in the kh - ω diagnostic diagram. We find that IGWs
in the upper solar atmosphere show upward propagation in the model with
predominantly horizontal field similar to the model without magnetic
field. In contrast to that the model with predominantly vertical fields
show downward propagation. This crucial difference in the propagation
direction is also revealed in the difference in energy transported by
waves for heights below 0.8 Mm. Higher up, the propagation properties
show a peculiar behaviour, which require further study. Our analysis
suggests that IGWs may play a significant role in the heating of
the chromospheric layers of the internetwork region where horizontal
fields are thought to be prevalent. This article is part of the
Theo Murphy meeting issue `High-resolution wave dynamics in the lower
solar atmosphere'.
Title: Interaction of Magnetic Fields with a Vortex Tube at Solar
Subgranular Scale
Authors: Fischer, C. E.; Vigeesh, G.; Lindner, P.; Borrero, J. M.;
Calvo, F.; Steiner, O.
Bibcode: 2020ApJ...903L..10F
Altcode: 2020arXiv201005577F
Using high-resolution spectropolarimetric data recorded with the
Swedish 1 m Solar Telescope, we have identified several instances of
granular lanes traveling into granules. These are believed to be the
observational signature of underlying tubes of vortical flow with
their axis oriented parallel to the solar surface. Associated with
these horizontal vortex tubes, we detect in some cases a significant
signal in linear polarization, located at the trailing dark edge of
the granular lane. The linear polarization appears at a later stage of
the granular lane development, and is flanked by patches of circular
polarization. Stokes inversions show that the elongated patch of linear
polarization signal arises from the horizontal magnetic field aligned
with the granular lane. We analyze snapshots of a magnetohydrodynamic
numerical simulation and find cases in which the horizontal vortex
tube of the granular lane redistributes and transports the magnetic
field to the solar surface causing a polarimetric signature similar to
what is observed. We thus witness a mechanism capable of transporting
magnetic flux to the solar surface within granules. This mechanism is
probably an important component of the small-scale dynamo supposedly
acting at the solar surface and generating the quiet-Sun magnetic field.
Title: Vortices evolution in the solar atmosphere. A dynamical
equation for the swirling strength
Authors: Canivete Cuissa, José R.; Steiner, Oskar
Bibcode: 2020A&A...639A.118C
Altcode: 2020arXiv200512871C
Aims: We study vortex dynamics in the solar atmosphere by
employing and deriving the analytical evolution equations of two
vortex identification criteria.
Methods: The two criteria used
are vorticity and the swirling strength. Vorticity can be biased
in the presence of shear flows, but its dynamical equation is well
known; the swirling strength is a more precise criterion for the
identification of vortical flows, but its evolution equation is not
known yet. Therefore, we explore the possibility of deriving a dynamical
equation for the swirling strength. We then apply the two equations
to analyze radiative magneto-hydrodynamical simulations of the solar
atmosphere produced with the CO5BOLD code.
Results:
We present a detailed review of the swirling strength criterion and
the mathematical derivation of its evolution equation. This equation
did not exist in the literature before and it constitutes a novel
tool that is suitable for the analysis of a wide range of problems in
(magneto-)hydrodynamics. By applying this equation to numerical models,
we find that hydrodynamical and magnetic baroclinicities are the
driving physical processes responsible for vortex generation in the
convection zone and the photosphere. Higher up in the chromosphere,
the magnetic terms alone dominate. Moreover, we find that the
swirling strength is produced at small scales in a chaotic fashion,
especially inside magnetic flux concentrations.
Conclusions:
The swirling strength represents an appropriate criterion for the
identification of vortices in turbulent flows, such as those in the
solar atmosphere. Moreover, its evolution equation, which is derived
in this paper, is pivotal for obtaining precise information about the
dynamics of these vortices and the physical mechanisms responsible
for their production and evolution. Since this equation is available,
the swirling strength is now the ideal quantity to study the dynamics
of vortices in (magneto-)hydrodynamics.
Title: Numerical Methods for the Radiative Transfer Equation of
Polarized Light
Authors: Janett, G.; Steiner, O.; Belluzzi, L.
Bibcode: 2019ASPC..526..133J
Altcode:
The quest of the "best formal solver" available is still open and the
lack of a clear comparison between the different numerical methods
proposed by the community does not facilitate a conclusion. This work
presents a reference paradigm for the characterization of formal
solvers, based on the concepts of order of accuracy, stability and
computational cost. This overview aims to facilitate the comprehension
of the advantages and weaknesses of the already existing formal solvers
and of those yet to come.
Title: A novel fourth-order WENO interpolation technique. A possible
new tool designed for radiative transfer
Authors: Janett, Gioele; Steiner, Oskar; Alsina Ballester, Ernest;
Belluzzi, Luca; Mishra, Siddhartha
Bibcode: 2019A&A...624A.104J
Altcode:
Context. Several numerical problems require the interpolation of
discrete data that present at the same time (i) complex smooth
structures and (ii) various types of discontinuities. The radiative
transfer in solar and stellar atmospheres is a typical example of such
a problem. This calls for high-order well-behaved techniques that are
able to interpolate both smooth and discontinuous data.
Aims:
This article expands on different nonlinear interpolation techniques
capable of guaranteeing high-order accuracy and handling discontinuities
in an accurate and non-oscillatory fashion. The final aim is to propose
new techniques which could be suitable for applications in the context
of numerical radiative transfer.
Methods: We have proposed
and tested two different techniques. Essentially non-oscillatory
(ENO) techniques generate several candidate interpolations based
on different substencils. The smoothest candidate interpolation is
determined from a measure for the local smoothness, thereby enabling the
essentially non-oscillatory property. Weighted ENO (WENO) techniques
use a convex combination of all candidate substencils to obtain
high-order accuracy in smooth regions while keeping the essentially
non-oscillatory property. In particular, we have outlined and tested a
novel well-performing fourth-order WENO interpolation technique for both
uniform and nonuniform grids.
Results: Numerical tests prove that
the fourth-order WENO interpolation guarantees fourth-order accuracy
in smooth regions of the interpolated functions. In the presence
of discontinuities, the fourth-order WENO interpolation enables the
non-oscillatory property, avoiding oscillations. Unlike Bézier and
monotonic high-order Hermite interpolations, it does not degenerate
to a linear interpolation near smooth extrema of the interpolated
function. Conclusion. The novel fourth-order WENO interpolation
guarantees high accuracy in smooth regions, while effectively handling
discontinuities. This interpolation technique might be particularly
suitable for several problems, including a number of radiative transfer
applications such as multidimensional problems, multigrid methods,
and formal solutions.
Title: Internal Gravity Waves in the Magnetized Solar
Atmosphere. II. Energy Transport
Authors: Vigeesh, G.; Roth, M.; Steiner, O.; Jackiewicz, J.
Bibcode: 2019ApJ...872..166V
Altcode: 2019arXiv190108871V
In this second paper of the series on internal gravity waves (IGWs),
we present a study of the generation and propagation of IGWs in a
model solar atmosphere with diverse magnetic conditions. A magnetic
field-free and three magnetic models that start with an initial,
vertical, homogeneous field of 10, 50, and 100 G magnetic flux density,
are simulated using the CO5BOLD code. We find that the
IGWs are generated in similar manner in all four models in spite of
the differences in the magnetic environment. The mechanical energy
carried by IGWs is significantly larger than that of the acoustic
waves in the lower part of the atmosphere, making them an important
component of the total wave energy budget. The mechanical energy flux
(106-103 W m-2) is a few orders of
magnitude larger than the Poynting flux (103-101
W m-2). The Poynting fluxes show a downward component in
the frequency range corresponding to the IGWs, which confirm that
these waves do not propagate upward in the atmosphere when the fields
are predominantly vertical and strong. We conclude that, in the upper
photosphere, the propagation properties of IGWs depend on the average
magnetic field strength and therefore these waves can be potential
candidates for magnetic field diagnostics of these layers. However,
their subsequent coupling to Alfvénic waves is unlikely in a magnetic
environment permeated with predominantly vertical fields, and therefore
they may not directly or indirectly contribute to the heating of layers
above plasma-β less than 1.
Title: Formal Solutions for Polarized Radiative
Transfer. IV. Numerical Performances in Practical Problems
Authors: Janett, Gioele; Steiner, Oskar; Belluzzi, Luca
Bibcode: 2018ApJ...865...16J
Altcode: 2018arXiv180906604J
The numerical computation of reliable and accurate Stokes profiles
is of great relevance in solar physics. In the synthesis process,
many actors play a relevant role: among them the formal solver, the
discrete atmospheric model, and the spectral line. This paper tests
the performances of different numerical schemes in the synthesis
of polarized spectra for different spectral lines and atmospheric
models. The hierarchy between formal solvers is enforced, stressing the
peculiarities of high-order and low-order formal solvers. The density
of grid points necessary for reaching a given accuracy requirement is
quantitatively described for specific situations.
Title: Structure of the Balmer jump. The isolated hydrogen atom
Authors: Calvo, F.; Belluzzi, L.; Steiner, O.
Bibcode: 2018A&A...613A..55C
Altcode: 2019arXiv190110241C
Context. The spectrum of the hydrogen atom was explained by Bohr
more than one century ago. We revisit here some of the aspects of the
underlying quantum structure, with a modern formalism, focusing on the
limit of the Balmer series.
Aims: We investigate the behaviour
of the absorption coefficient of the isolated hydrogen atom in the
neighbourhood of the Balmer limit.
Methods: We analytically
computed the total cross-section arising from bound-bound and bound-free
transitions in the isolated hydrogen atom at the Balmer limit, and
established a simplified semi-analytical model for the surroundings of
that limit. We worked within the framework of the formalism of Landi
Degl'Innocenti & Landolfi (2004, Astrophys. Space Sci. Lib.,
307), which permits an almost straight-forward generalization of
our results to other atoms and molecules, and which is perfectly
suitable for including polarization phenomena in the problem.
Results: We analytically show that there is no discontinuity at the
Balmer limit, even though the concept of a "Balmer jump" is still
meaningful. Furthermore, we give a possible definition of the location
of the Balmer jump, and we check that this location is dependent
on the broadening mechanisms. At the Balmer limit, we compute the
cross-section in a fully analytical way.
Conclusions: The Balmer
jump is produced by a rapid drop of the total Balmer cross-section,
yet this variation is smooth and continuous when both bound-bound and
bound-free processes are taken into account, and its shape and location
is dependent on the broadening mechanisms.
Title: Simulation of the small-scale magnetism in main-sequence
stellar atmospheres
Authors: Salhab, R. G.; Steiner, O.; Berdyugina, S. V.; Freytag, B.;
Rajaguru, S. P.; Steffen, M.
Bibcode: 2018A&A...614A..78S
Altcode:
Context. Observations of the Sun tell us that its granular and
subgranular small-scale magnetism has significant consequences for
global quantities such as the total solar irradiance or convective
blueshift of spectral lines.
Aims: In this paper, properties
of the small-scale magnetism of four cool stellar atmospheres,
including the Sun, are investigated, and in particular its effects
on the radiative intensity and flux.
Methods: We carried out
three-dimensional radiation magnetohydrodynamic simulations with the
CO5BOLD code in two different settings: with and without
a magnetic field. These are thought to represent states of high and
low small-scale magnetic activity of a stellar magnetic cycle.
Results: We find that the presence of small-scale magnetism
increases the bolometric intensity and flux in all investigated
models. The surplus in radiative flux of the magnetic over the magnetic
field-free atmosphere increases with increasing effective temperature,
Teff, from 0.47% for spectral type K8V to 1.05% for the solar
model, but decreases for higher effective temperatures than solar. The
degree of evacuation of the magnetic flux concentrations monotonically
increases with Teff as does their depression of the visible
optical surface, that is the Wilson depression. Nevertheless, the
strength of the field concentrations on this surface stays remarkably
unchanged at ≈1560 G throughout the considered range of spectral
types. With respect to the surrounding gas pressure, the field strength
is close to (thermal) equipartition for the Sun and spectral type F5V
but is clearly sub-equipartition for K2V and more so for K8V. The
magnetic flux concentrations appear most conspicuous for model K2V
owing to their high brightness contrast.
Conclusions: For mean
magnetic flux densities of approximately 50 G, we expect the small-scale
magnetism of stars in the spectral range from F5V to K8V to produce a
positive contribution to their bolometric luminosity. The modulation
seems to be most effective for early G-type stars.
Title: Formal Solutions for Polarized Radiative
Transfer. II. High-order Methods
Authors: Janett, Gioele; Steiner, Oskar; Belluzzi, Luca
Bibcode: 2017ApJ...845..104J
Altcode: 2017arXiv170901280J
When integrating the radiative transfer equation for polarized light,
the necessity of high-order numerical methods is well known. In fact,
well-performing high-order formal solvers enable higher accuracy and
the use of coarser spatial grids. Aiming to provide a clear comparison
between formal solvers, this work presents different high-order
numerical schemes and applies the systematic analysis proposed by
Janett et al., emphasizing their advantages and drawbacks in terms of
order of accuracy, stability, and computational cost.
Title: On the effect of vorticity on the propagation of internal
gravity waves.
Authors: Vigeesh, G.; Steiner, O.; Calvo, F.; Roth, M.
Bibcode: 2017MmSAI..88...54V
Altcode:
We compare different models of solar surface convection to study
vorticity and how it can influence the propagation of internal
gravity waves. We conclude that simulations performed with higher grid
resolution may have a reduced gravity wave flux in the lower part of
the atmosphere due to strong vorticity. We also show that the vertical
extent of the allowed region of propagation depends on the magnetic
field inclination.
Title: Formal Solutions for Polarized Radiative Transfer. I. The
DELO Family
Authors: Janett, Gioele; Carlin, Edgar S.; Steiner, Oskar; Belluzzi,
Luca
Bibcode: 2017ApJ...840..107J
Altcode: 2017arXiv170901274J
The discussion regarding the numerical integration of the polarized
radiative transfer equation is still open and the comparison between
the different numerical schemes proposed by different authors in the
past is not fully clear. Aiming at facilitating the comprehension of
the advantages and drawbacks of the different formal solvers, this
work presents a reference paradigm for their characterization based
on the concepts of order of accuracy, stability, and computational
cost. Special attention is paid to understand the numerical methods
belonging to the Diagonal Element Lambda Operator family, in an attempt
to highlight their specificities.
Title: High-frequency Oscillations in Small Magnetic Elements Observed
with Sunrise/SuFI
Authors: Jafarzadeh, S.; Solanki, S. K.; Stangalini, M.; Steiner,
O.; Cameron, R. H.; Danilovic, S.
Bibcode: 2017ApJS..229...10J
Altcode: 2016arXiv161109302J
We characterize waves in small magnetic elements and investigate
their propagation in the lower solar atmosphere from observations at
high spatial and temporal resolution. We use the wavelet transform to
analyze oscillations of both horizontal displacement and intensity
in magnetic bright points found in the 300 nm and the Ca II H 396.8
nm passbands of the filter imager on board the Sunrise balloon-borne
solar observatory. Phase differences between the oscillations at the
two atmospheric layers corresponding to the two passbands reveal
upward propagating waves at high frequencies (up to 30 mHz). Weak
signatures of standing as well as downward propagating waves are also
obtained. Both compressible and incompressible (kink) waves are found
in the small-scale magnetic features. The two types of waves have
different, though overlapping, period distributions. Two independent
estimates give a height difference of approximately 450 ± 100 km
between the two atmospheric layers sampled by the employed spectral
bands. This value, together with the determined short travel times of
the transverse and longitudinal waves provide us with phase speeds of 29
± 2 km s-1 and 31 ± 2 km s-1, respectively. We
speculate that these phase speeds may not reflect the true propagation
speeds of the waves. Thus, effects such as the refraction of fast
longitudinal waves may contribute to an overestimate of the phase speed.
Title: Internal Gravity Waves in the Magnetized Solar
Atmosphere. I. Magnetic Field Effects
Authors: Vigeesh, G.; Jackiewicz, J.; Steiner, O.
Bibcode: 2017ApJ...835..148V
Altcode: 2016arXiv161204729V
Observations of the solar atmosphere show that internal gravity
waves are generated by overshooting convection, but are suppressed
at locations of magnetic flux, which is thought to be the result of
mode conversion into magnetoacoustic waves. Here, we present a study
of the acoustic-gravity wave spectrum emerging from a realistic,
self-consistent simulation of solar (magneto)convection. A magnetic
field free, hydrodynamic simulation and a magnetohydrodynamic (MHD)
simulation with an initial, vertical, homogeneous field of 50 G flux
density were carried out and compared with each other to highlight the
effect of magnetic fields on the internal gravity wave propagation
in the Sun’s atmosphere. We find that the internal gravity waves
are absent or partially reflected back into the lower layers in the
presence of magnetic fields and argue that the suppression is due to
the coupling of internal gravity waves to slow magnetoacoustic waves
still within the high-β region of the upper photosphere. The conversion
to Alfvén waves is highly unlikely in our model because there is no
strongly inclined magnetic field present. We argue that the suppression
of internal waves observed within magnetic flux concentrations may also
be due to nonlinear breaking of internal waves due to vortex flows that
are ubiquitously present in the upper photosphere and the chromosphere.
Title: CO5BOLD for MHD: progresses and deficiencies .
Authors: Steiner, O.; Calvo, F.; Salhab, R.; Vigeesh, G.
Bibcode: 2017MmSAI..88...37S
Altcode:
The magnetohydrodynamics module of CO5BOLD has been steadily improved
over the past decade and has been used for various solar and stellar
physical applications. We give an overview of current work with it
and of remaining and newly emerged shortcomings.
Title: Sub-photosphere to Solar Atmosphere Connection
Authors: Komm, Rudolf; De Moortel, Ineke; Fan, Yuhong; Ilonidis,
Stathis; Steiner, Oskar
Bibcode: 2017hdsi.book..173K
Altcode:
No abstract at ADS
Title: Non-magnetic photospheric bright points in 3D simulations of
the solar atmosphere
Authors: Calvo, F.; Steiner, O.; Freytag, B.
Bibcode: 2016A&A...596A..43C
Altcode: 2016arXiv161204278C
Context. Small-scale bright features in the photosphere of the Sun,
such as faculae or G-band bright points, appear in connection with
small-scale magnetic flux concentrations.
Aims: Here we report
on a new class of photospheric bright points that are free of magnetic
fields. So far, these are visible in numerical simulations only. We
explore conditions required for their observational detection.
Methods: Numerical radiation (magneto-)hydrodynamic simulations of the
near-surface layers of the Sun were carried out. The magnetic field-free
simulations show tiny bright points, reminiscent of magnetic bright
points, only smaller. A simple toy model for these non-magnetic bright
points (nMBPs) was established that serves as a base for the development
of an algorithm for their automatic detection. Basic physical properties
of 357 detected nMBPs were extracted and statistically evaluated. We
produced synthetic intensity maps that mimic observations with various
solar telescopes to obtain hints on their detectability.
Results:
The nMBPs of the simulations show a mean bolometric intensity contrast
with respect to their intergranular surroundings of approximately 20%, a
size of 60-80 km, and the isosurface of optical depth unity is at their
location depressed by 80-100 km. They are caused by swirling downdrafts
that provide, by means of the centripetal force, the necessary pressure
gradient for the formation of a funnel of reduced mass density that
reaches from the subsurface layers into the photosphere. Similar,
frequently occurring funnels that do not reach into the photosphere,
do not produce bright points.
Conclusions: Non-magnetic bright
points are the observable manifestation of vertically extending vortices
(vortex tubes) in the photosphere. The resolving power of 4-m-class
telescopes, such as the DKIST, is needed for an unambiguous detection
of them. The movie associated to Fig. 1 is available at http://www.aanda.org
Title: Chromospheric and Coronal Wave Generation in a Magnetic
Flux Sheath
Authors: Kato, Yoshiaki; Steiner, Oskar; Hansteen, Viggo; Gudiksen,
Boris; Wedemeyer, Sven; Carlsson, Mats
Bibcode: 2016ApJ...827....7K
Altcode: 2016arXiv160608826K
Using radiation magnetohydrodynamic simulations of the solar
atmospheric layers from the upper convection zone to the lower corona,
we investigate the self-consistent excitation of slow magneto-acoustic
body waves (slow modes) in a magnetic flux concentration. We
find that the convective downdrafts in the close surroundings of
a two-dimensional flux slab “pump” the plasma inside it in
the downward direction. This action produces a downflow inside the
flux slab, which encompasses ever higher layers, causing an upwardly
propagating rarefaction wave. The slow mode, excited by the adiabatic
compression of the downflow near the optical surface, travels along the
magnetic field in the upward direction at the tube speed. It develops
into a shock wave at chromospheric heights, where it dissipates,
lifts the transition region, and produces an offspring in the form
of a compressive wave that propagates further into the corona. In the
wake of downflows and propagating shock waves, the atmosphere inside
the flux slab in the chromosphere and higher tends to oscillate with a
period of ν ≈ 4 mHz. We conclude that this process of “magnetic
pumping” is a most plausible mechanism for the direct generation
of longitudinal chromospheric and coronal compressive waves within
magnetic flux concentrations, and it may provide an important heat
source in the chromosphere. It may also be responsible for certain
types of dynamic fibrils.
Title: Polarized radiative transfer in discontinuous media
Authors: Steiner, O.; Züger, F.; Belluzzi, L.
Bibcode: 2016A&A...586A..42S
Altcode:
Context. Observations of the solar atmosphere of ever increasing
spatial resolution reveal steep gradients in the magnetic field
and in thermal states. Likewise, numerical simulations of the solar
atmosphere show contact discontinuities and shock fronts. This asks
for the development of robust methods for computing the radiative
transfer of polarized light in discontinuous media.
Aims:
Here, we propose a new concept for dealing with discontinuities in
the radiative transfer of polarized light and carry out a few basic
test calculations. While in the past, the focus was on interpolating
the source function with ever-increasing accuracy and smoothness,
we propose to take the opposite approach by reconstructing it with
piecewise continuous functions, taking discontinuities on purpose into
account. This concept is known from computational fluid dynamics.
Methods: Test calculations were carried out for (I) a Milne-Eddington
atmosphere; (II) an atmosphere featuring a single discontinuity that
is shifted across one grid cell; and (III) a two-layered atmosphere
with discontinuities in the source function, the velocity, and the
magnetic field.
Results: It is shown that the method of piecewise
continuous reconstruction is a viable approach to solving the radiative
transfer equation for polarized light. In the special case where a
discontinuity coincides with a computational cell interface, the method
is capable of producing the exact solution. Overall, the assessment
of the piecewise continuous reconstruction method turns out to be
cautiously positive, but it does not lead to an order-of-magnitude
improvement in accuracy over conventional methods for the examples
considered here. More realistic model atmospheres need to be considered
for judging practical applicability.
Title: Sub-photosphere to Solar Atmosphere Connection
Authors: Komm, Rudolf; De Moortel, Ineke; Fan, Yuhong; Ilonidis,
Stathis; Steiner, Oskar
Bibcode: 2015SSRv..196..167K
Altcode: 2013SSRv..tmp...93K
Magnetic fields extend from the solar interior through the
atmosphere. The formation and evolution of active regions can be studied
by measuring subsurface flows with local helioseismology. The emergence
of magnetic flux from the solar convection zone is associated with
acoustic perturbation signatures. In near-surface layers, the average
dynamics can be determined for emerging regions. MHD simulations
of the emergence of a twisted flux tube show how magnetic twist
and free energy are transported from the interior into the corona
and the dynamic signatures associated with such transport in the
photospheric and sub-photospheric layers. The subsurface twisted flux
tube does not emerge into the corona as a whole in emerging active
regions. Shear flows at the polarity inversion line and coherent
vortical motions in the subsurface flux tubes are the major means by
which twist is transported into the corona, leading to the formation
of sigmoid-shaped coronal magnetic fields capable of driving solar
eruptions. The transport of twist can be followed from the interior
by using the kinetic helicity of subsurface flows as a proxy of
magnetic helicity; this quantity holds great promise for improving
the understanding of eruptive phenomena. Waves are not only vital for
studying the link between the solar interior and the surface but for
linking the photosphere with the corona as well. Acoustic waves that
propagate from the surface into the magnetically structured, dynamic
atmosphere undergo mode conversion and refraction. These effects
enable atmospheric seismology to determine the topography of magnetic
canopies in the solar atmosphere. Inclined magnetic fields lower
the cut-off frequency so that low frequency waves can leak into the
outer atmosphere. Recent high resolution, high cadence observations of
waves and oscillations in the solar atmosphere, have lead to a renewed
interest in the potential role of waves as a heating mechanism. In light
of their potential contribution to the heating of the solar atmosphere,
some of the recent observations of waves and oscillations and ongoing
modelling efforts are reviewed.
Title: On the Evolution of Magnetic White Dwarfs
Authors: Tremblay, P. -E.; Fontaine, G.; Freytag, B.; Steiner, O.;
Ludwig, H. -G.; Steffen, M.; Wedemeyer, S.; Brassard, P.
Bibcode: 2015ApJ...812...19T
Altcode: 2015arXiv150905398T
We present the first radiation magnetohydrodynamic simulations of the
atmosphere of white dwarf stars. We demonstrate that convective energy
transfer is seriously impeded by magnetic fields when the plasma-β
parameter, the thermal-to-magnetic-pressure ratio, becomes smaller
than unity. The critical field strength that inhibits convection
in the photosphere of white dwarfs is in the range B = 1-50 kG,
which is much smaller than the typical 1-1000 MG field strengths
observed in magnetic white dwarfs, implying that these objects have
radiative atmospheres. We have employed evolutionary models to study the
cooling process of high-field magnetic white dwarfs, where convection
is entirely suppressed during the full evolution (B ≳ 10 MG). We
find that the inhibition of convection has no effect on cooling rates
until the effective temperature (Teff) reaches a value of
around 5500 K. In this regime, the standard convective sequences start
to deviate from the ones without convection due to the convective
coupling between the outer layers and the degenerate reservoir of
thermal energy. Since no magnetic white dwarfs are currently known
at the low temperatures where this coupling significantly changes the
evolution, the effects of magnetism on cooling rates are not expected
to be observed. This result contrasts with a recent suggestion
that magnetic white dwarfs with Teff ≲ 10,000 K cool
significantly slower than non-magnetic degenerates.
Title: The statistical properties of vortex flows in the solar
atmosphere
Authors: Wedemeyer, Sven; Kato, Yoshiaki; Steiner, Oskar
Bibcode: 2015IAUGA..2256852W
Altcode:
Rotating magnetic field structures associated with vortex flows
on the Sun, also known as “magnetic tornadoes”, may serve
as waveguides for MHD waves and transport mass and energy upwards
through the atmosphere. Magnetic tornadoes may therefore potentially
contribute to the heating of the upper atmospheric layers in quiet
Sun regions.Magnetic tornadoes are observed over a large range
of spatial and temporal scales in different layers in quiet Sun
regions. However, their statistical properties such as size, lifetime,
and rotation speed are not well understood yet because observations
of these small-scale events are technically challenging and limited
by the spatial and temporal resolution of current instruments. Better
statistics based on a combination of high-resolution observations and
state-of-the-art numerical simulations is the key to a reliable estimate
of the energy input in the lower layers and of the energy deposition
in the upper layers. For this purpose, we have developed a fast and
reliable tool for the determination and visualization of the flow
field in (observed) image sequences. This technique, which combines
local correlation tracking (LCT) and line integral convolution (LIC),
facilitates the detection and study of dynamic events on small scales,
such as propagating waves. Here, we present statistical properties
of vortex flows in different layers of the solar atmosphere and try
to give realistic estimates of the energy flux which is potentially
available for heating of the upper solar atmosphere
Title: Properties of small-scale magnetism of stellar atmospheres
Authors: Steiner, Oskar; Salhab, René; Freytag, Bernd; Rajaguru,
Paul; Schaffenberger, Werner; Steffen, Matthias
Bibcode: 2014PASJ...66S...5S
Altcode: 2014PASJ..tmp...95S
The magnetic field outside of sunspots is concentrated in the
intergranular space, where it forms a delicate filigree of bright
ribbons and dots as seen on broad band images of the Sun. We expect this
small-scale magnetic field to exhibit a similar behavior in stellar
atmospheres. In order to find out more about it, we perform numerical
simulations of the surface layers of stellar atmospheres. Here, we
report on preliminary results from simulations in the range between
4000 K and 6500 K effective temperature with an initial vertical,
homogeneous magnetic field of 50 G strength. We find that the field
strength of the strongest magnetic flux concentrations increases with
decreasing effective temperature at the height level where the average
Rosseland optical depth is one. On the other hand, at the same level,
the field is less strong than the thermal equipartition value in the
coolest model but assumes superequipartition in the models hotter
than 5000 K. While the Wilson depression of the strongest field
concentrations is about one pressure scale height in the coolest
model, it is more than four times the pressure scale height in the
hottest one. We also find that the relative contribution of the bright
filigree to the bolometric, vertically directed radiative intensity is
most significant for the Teff = 5000 K model (0.6%-0.79%)
and least significant for the hottest and coolest models (0.1%-0.46%
and 0.14%-0.32%, respectively). This behavior suggests that the effect
of the small-scale magnetic field on the photometric variability is more
significant for K dwarf stars than for F-type and also M-type stars.
Title: On the plasma flow inside magnetic tornadoes on the Sun
Authors: Wedemeyer, Sven; Steiner, Oskar
Bibcode: 2014PASJ...66S..10W
Altcode: 2014PASJ..tmp...98W; 2014arXiv1406.7270W
High-resolution observations with the Swedish 1-m Solar Telescope (SST)
and the Solar Dynamics Observatory (SDO) reveal rotating magnetic field
structures that extend from the solar surface into the chromosphere
and the corona. These so-called magnetic tornadoes are primarily
detected as rings or spirals of rotating plasma in the Ca II 854.2 nm
line core (also known as chromospheric swirls). Detailed numerical
simulations show that the observed chromospheric plasma motion is
caused by the rotation of magnetic field structures, which again
are driven by photospheric vortex flows at their footpoints. Under
the right conditions, two vortex flow systems are stacked on top of
each other. We refer to the lower vortex, which extends from the low
photosphere into the convection zone, as intergranular vortex flow
(IVF). Once a magnetic field structure is co-located with an IVF,
the rotation is mediated into the upper atmospheric layers and an
atmospheric vortex flow (AVF, or magnetic tornado) is generated. In
contrast to the recent work by Shelyag et al. (2013, ApJ, 776, L4),
we demonstrate that particle trajectories in a simulated magnetic
tornado indeed follow spirals and argue that the properties of the
trajectories decisively depend on the location in the atmosphere and
the strength of the magnetic field.
Title: Magnetic tornadoes and chromospheric swirls - Definition
and classification
Authors: Wedemeyer, Sven; Scullion, Eamon; Steiner, Oskar; de la Cruz
Rodriguez, Jaime; Rouppe van der Voort, L. H. M.
Bibcode: 2013JPhCS.440a2005W
Altcode: 2013arXiv1303.0179W
Chromospheric swirls are the observational signatures of rotating
magnetic field structures in the solar atmosphere, also known as
magnetic tornadoes. Swirls appear as dark rotating features in the core
of the spectral line of singly ionized calcium at a wavelength of 854.2
nm. This signature can be very subtle and difficult to detect given
the dynamic changes in the solar chromosphere. Important steps towards
a systematic and objective detection method are the compilation and
characterization of a statistically significant sample of observed
and simulated chromospheric swirls. Here, we provide a more exact
definition of the chromospheric swirl phenomenon and also present a
first morphological classification of swirls with three types: (I) Ring,
(II) Split, (III) Spiral. We also discuss the nature of the magnetic
field structures connected to tornadoes and the influence of limited
spatial resolution on the appearance of their photospheric footpoints.
Title: On the Effects of the SDO Orbital Motion on the HMI Vector
Magnetic Field Measurements
Authors: Fleck, B.; Centeno, R.; Cheung, M.; Couvidat, S.; Hayashi,
K.; Rezaei, R.; Steiner, O.; Straus, T.
Bibcode: 2013enss.confE.145F
Altcode:
In a previous study we have investigated the magnetic field diagnostics
potential of SDO/HMI. We have used the output of high-resolution
3D, time-dependent, radiative magneto-hydrodynamics simulations to
calculate Stokes profiles for the Fe I 6173 Å line. From these we
constructed Stokes filtergrams using a representative set of HMI filter
response functions. The magnetic field vector (x,y) and line-of-sight
Doppler velocities V(x,y) were determined from these filtergrams using
a simplified version of the HMI magnetic field processing pipeline,
and the reconstructed magnetic field (x,y) and line-of-sight velocity
V(x,y) were compared to the actual magnetic field (x,y,z) and vertical
velocity V0(x,y,z) in the simulations. The present investigation expands
this analysis to include the effects of the significant orbital motions
of SDO, which, given the limited wavelength range of the HMI filter
profiles, affects the outer wing measurements and therefore might impact
the magnetic field measurements. We find that the effects of the orbital
movement of SDO are noticeable, in particular for the strongest fields
(B > 3 kG) and the maximum wavelength shift of 5.5 km/s (3.5 km/s
orbital movement + 2 km/s solar rotation). Saturation effects for strong
fields (B > 3 kG) are already visible for wavelength shifts of 3.2
km/s (orbital movement, disk center). The measurements of inclination
and vertical velocity are more robust. Compared to other factors of
uncertainty in the inversion of HMI Stokes measurements the orbital
movement is not a major concern or source of error.
Title: Three-dimensional magnetohydrodynamic simulations of M-dwarf
chromospheres
Authors: Wedemeyer, S.; Ludwig, H. -G.; Steiner, O.
Bibcode: 2013AN....334..137W
Altcode: 2013csss...17..137W; 2012arXiv1207.2342W
We present first results from three-dimensional radiation
magnetohydrodynamic simulations of M-type dwarf stars with
CO5BOLD. The local models include the top of the convection
zone, the photosphere, and the chromosphere. The results are illustrated
for models with an effective temperature of 3240 K and a gravitational
acceleration of {log g = 4.5}, which represent analogues of AD Leo. The
models have different initial magnetic field strengths and field
topologies. This first generation of models demonstrates that the
atmospheres of M dwarfs are highly dynamic and intermittent. Magnetic
fields and propagating shock waves produce a complicated fine-structure,
which is clearly visible in synthetic intensity maps in the core of the
Ca II K spectral line and also at millimeter wavelengths. The dynamic
small-scale pattern cannot be described by means of one-dimensional
models, which has important implications for the construction of
semi-empirical model atmospheres and thus for the interpretation
of observations in general. Detailed three-dimensional numerical
simulations are valuable in this respect. Furthermore, such models
facilitate the analysis of small-scale processes, which cannot be
observed on stars but nevertheless might be essential for understanding
M-dwarf atmospheres and their activity. An example are so-called
``magnetic tornadoes'', which have recently been found on the Sun and
are presented here in M-dwarf models for the first time.
Title: First steps with CO5BOLD using HLLMHD and PP reconstruction .
Authors: Steiner, O.; Rajaguru, S. P.; Vigeesh, G.; Steffen, M.;
Schaffenberger, W.; Freytag, B.
Bibcode: 2013MSAIS..24..100S
Altcode:
We report on first experiences with real-life applications using
the MHD-module of CO5BOLD together with the piecewise parabolic
reconstruction scheme and present preliminary results of stellar
magnetic models with Teff = 4000 K to Teff =
5770 K.
Title: The science challenges for large solar telescopes
Authors: Steiner, O.
Bibcode: 2012IAUSS...6E.101S
Altcode:
Looking at numerical simulations of highest spatial resolution
and highest resolution observations, we endeavor extrapolating
what science questions can be tackled with future large solar
telescopes. By means of selected examples we shall try to determine the
instrumental requirements for answering these questions. For example,
what polarimetric accuracy do we need to make progress regarding the
topography of the internetwork magnetic field? What spatial and temporal
resolution is needed to track the driving forces of dynamic fibrils,
spicules and other jet-like features of the solar chromosphere, what to
asses the role of vortical flows in the photosphere and its impact on
the chromosphere and corona, or what to shed light on the turbulent
dynamo supposedly working in the surface layers of the convection
zone. Will large solar telescopes help us resolving the remaining
puzzles of the sunspot penumbra and the still largely enigmatic
formation process of sunspots? Besides such foreseeable science
questions however, we should be aware that the best discoveries come
unexpectedly and therefore, it may be worthwhile to wonder about what
instrumental capabilities may be best conducive to the unexpected.
Title: Revealing the nature of magnetic shadows with numerical
3D-MHD simulations
Authors: Nutto, C.; Steiner, O.; Roth, M.
Bibcode: 2012A&A...542L..30N
Altcode: 2012arXiv1205.5308N
Aims: We investigate the interaction of magneto-acoustic waves
with magnetic network elements with the aim of finding possible
signatures of the magnetic shadow phenomenon in the vicinity of
network elements.
Methods: We carried out three-dimensional
numerical simulations of magneto-acoustic wave propagation in a model
solar atmosphere that is threaded by a complexly structured magnetic
field, resembling that of a typical magnetic network element and of
internetwork regions. High-frequency waves of 10 mHz are excited at
the bottom of the simulation domain. On their way through the upper
convection zone and through the photosphere and the chromosphere they
become perturbed, refracted, and converted into different mode types. We
applied a standard Fourier analysis to produce oscillatory power-maps
of the line-of-sight velocity.
Results: In the power maps of
the upper photosphere and the lower chromosphere, we clearly see the
magnetic shadow: a seam of suppressed power surrounding the magnetic
network elements. We demonstrate that this shadow is linked to the
mode conversion process and that power maps at these height levels
show the signature of three different magneto-acoustic wave modes.
Title: Magnetic tornadoes as energy channels into the solar corona
Authors: Wedemeyer-Böhm, Sven; Scullion, Eamon; Steiner, Oskar;
Rouppe van der Voort, Luc; de La Cruz Rodriguez, Jaime; Fedun, Viktor;
Erdélyi, Robert
Bibcode: 2012Natur.486..505W
Altcode:
Heating the outer layers of the magnetically quiet solar atmosphere to
more than one million kelvin and accelerating the solar wind requires
an energy flux of approximately 100 to 300 watts per square metre,
but how this energy is transferred and dissipated there is a puzzle and
several alternative solutions have been proposed. Braiding and twisting
of magnetic field structures, which is caused by the convective flows
at the solar surface, was suggested as an efficient mechanism for
atmospheric heating. Convectively driven vortex flows that harbour
magnetic fields are observed to be abundant in the photosphere
(the visible surface of the Sun). Recently, corresponding swirling
motions have been discovered in the chromosphere, the atmospheric
layer sandwiched between the photosphere and the corona. Here we
report the imprints of these chromospheric swirls in the transition
region and low corona, and identify them as observational signatures
of rapidly rotating magnetic structures. These ubiquitous structures,
which resemble super-tornadoes under solar conditions, reach from
the convection zone into the upper solar atmosphere and provide an
alternative mechanism for channelling energy from the lower into the
upper solar atmosphere.
Title: First Results from the SUNRISE Mission
Authors: Solanki, S. K.; Barthol, P.; Danilovic, S.; Feller, A.;
Gandorfer, A.; Hirzberger, J.; Jafarzadeh, S.; Lagg, A.; Riethmüller,
T. L.; Schüssler, M.; Wiegelmann, T.; Bonet, J. A.; González,
M. J. M.; Pillet, V. M.; Khomenko, E.; Yelles Chaouche, L.; Iniesta,
J. C. d. T.; Domingo, V.; Palacios, J.; Knölker, M.; González,
N. B.; Borrero, J. M.; Berkefeld, T.; Franz, M.; Roth, M.; Schmidt,
W.; Steiner, O.; Title, A. M.
Bibcode: 2012ASPC..455..143S
Altcode:
The SUNRISE balloon-borne solar observatory consists of a 1m aperture
Gregory telescope, a UV filter imager, an imaging vector polarimeter,
an image stabilization system, and further infrastructure. The first
science flight of SUNRISE yielded high-quality data that reveal the
structure, dynamics, and evolution of solar convection, oscillations,
and magnetic fields at a resolution of around 100 km in the quiet
Sun. Here we describe very briefly the mission and the first results
obtained from the SUNRISE data, which include a number of discoveries.
Title: Excitation of Slow-Modes in Network Magnetic Elements
Authors: Kato, Y.; Steiner, O.; Steffen, M.; Suematsu, Y.
Bibcode: 2012ASPC..455..237K
Altcode:
From radiation magnetohydrodynamic (RMHD) simulations of the solar
atmosphere we have found a new mechanism for the excitation of
longitudinal slow modes within magnetic flux concentrations. It is
found that the convective downdrafts in the immediate surroundings of
magnetic elements are responsible for the excitation of slow modes. The
coupling between the external downdraft and the plasma motion internal
to the flux concentration is mediated by the inertial forces of the
downdraft that act on the magnetic flux concentration. These forces
pump the internal atmosphere in the downward direction, which entails
a fast downflow in the photospheric and chromospheric layers of the
magnetic element. Subsequent to the transient pumping phase, the
atmosphere rebounds, causing a slow mode traveling along the magnetic
flux concentration in the upward direction and developing into a shock
wave in chromospheric heights, possibly capable of producing some kind
of dynamic fibril. This event occurs recurrently. We compare the power
spectra of the temperature and velocity of the flux-sheet atmosphere
to the corresponding spectra of the unmagnetized atmosphere.
Title: Recent Advances in the Exploration of the Small-Scale Structure
of the Quiet Solar Atmosphere: Vortex Flows, the Horizontal Magnetic
Field, and the Stokes- V Line-Ratio Method
Authors: Steiner, O.; Rezaei, R.
Bibcode: 2012ASPC..456....3S
Altcode: 2012arXiv1202.4040S
We review (i) observations and numerical simulations of vortical flows
in the solar atmosphere and (ii) measurements of the horizontal magnetic
field in quiet Sun regions. First, we discuss various manifestations of
vortical flows and emphasize the role of magnetic fields in mediating
swirling motion created near the solar surface to the higher layers
of the photosphere and to the chromosphere. We reexamine existing
simulation runs of solar surface magnetoconvection with regard to
vortical flows and compare to previously obtained results. Second,
we reviews contradictory results and problems associated with
measuring the angular distribution of the magnetic field in quiet Sun
regions. Furthermore, we review the Stokes-V-amplitude ratio method
for the lines Fe i λλ 630.15 and 630.25 nm. We come to the conclusion
that the recently discovered two distinct populations in scatter plots
of this ratio must not bee interpreted in terms of “uncollapsed''
and “collapsed'' fields but stem from weak granular magnetic fields
and weak canopy fields located at the boundaries between granules and
the intergranular space. Based on new simulation runs, we reaffirm
earlier findings of a predominance of the horizontal field components
over the vertical one, particularly in the upper photosphere and at
the base of the chromosphere.
Title: On The Magnetic-Field Diagnostics Potential of SDO/HMI
Authors: Fleck, Bernard; Hayashi, K.; Rezaei, R.; Vitas, N.; Centeno,
R.; Cheung, M.; Couvidat, S.; Fischer, C.; Steiner, O.; Straus, T.;
Viticchie, B.
Bibcode: 2012AAS...22020701F
Altcode:
The Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics
Observatory (SDO) is designed to study oscillations and the magnetic
field in the solar photosphere. It observes the full solar disk
in the Fe I absorption line at 6173 Å. We use the output of three
high-resolution 3D, time-dependent, radiative magneto-hydrodynamics
simulations (two based on the MURaM code, one on the CO5BOLD
code) to calculate Stokes profiles for the Fe I 6173 Å line
for snapshots of a sunspot, a plage area and an enhanced network
region. Stokes filtergrams are constructed for the 6 nominal HMI
wavelengths by multiplying the Stokes profiles with a representative
set of HMI filter response functions. The magnetic field vector B(x,y)
and line-of-sight Doppler velocities V(x,y) are determined from these
filtergrams using a simplified version of the HMI magnetic field
processing pipeline. Finally, the reconstructed magnetic field B(x,y)
and line-of-sight velocity V(x,y) are compared to the actual magnetic
field B0(x,y,z) and vertical velocity V0(x,y,z)
in the simulations.
Title: Detection of Vortex Tubes in Solar Granulation from
Observations SUNRISE
Authors: Steiner, O.; Franz, M.; González, N. B.; Nutto, C.; Rezaei,
R.; Pillet, V. M.; Bonet, J. A.; Iniesta, J. C. d. T.; Domingo, V.;
Solanki, S. K.; Knölker, M.; Schmidt, W.; Barthol, P.; Gandorfer, A.
Bibcode: 2012ASPC..455...35S
Altcode:
We investigated a time series of continuum intensity maps and
Dopplergrams of granulation in a very quiet solar region at the disk
center, recorded with the Imaging Magnetograph eXperiment (IMaX)
on board the balloon-borne solar observatory SUNRISE. We find that
granules frequently show substructure in the form of lanes composed of
a leading bright rim and a trailing dark edge, which move together
from the boundary of a granule into the granule itself. We find
strikingly similar events in synthesized intensity maps from an ab
initio numerical simulation of solar surface convection. We conclude
that these granular lanes are the visible signature of (horizontally
oriented) vortex tubes. The characteristic optical appearance of vortex
tubes at the solar surface is explained. This paper is a summary and
update of the results previously presented in Steiner et al. (2010).
Title: Small-scale rotating magnetic flux structures as alternative
energy channels into the low corona
Authors: Wedemeyer-Böhm; , Sven; Scullion; , Eamon; Steiner; , Oskar;
Rouppe van der Voort, Luc; de la Cruz Rodriguez, Jaime; Erdelyi,
Robertus; Fedun, Viktor
Bibcode: 2012decs.confE..67W
Altcode:
Vortex flows are frequently observed in the downflow areas in the lanes
between granules. The magnetic field is advected and trapped by these
flows in the low photosphere. Consequently, the rotation of a vortex
flow is transferred to the atmospheric layers above by means of the
magnetic flux structure. This effect results in so-called swirls, which
are observed in the chromosphere. New simultaneous observations with
the Swedish Solar Telescope and the Solar Dynamics Observatory reveal
that chromospheric swirls can have a coronal counterpart. This finding
implies that the rotating flux structure couples the layers of the solar
atmosphere from the photosphere to the (low) corona. Three-dimensional
numerical simulations confirm this picture and reproduce the swirl
signature. A combined analysis of the simulations and observations
implies that such small-scale rotating flux structures could provide
an alternative mechanism for channeling substantial energy from the
photosphere into the upper solar atmosphere.
Title: On the Magnetic-Field Diagnostics Potential of SDO/HMI
Authors: Fleck, B.; Hayashi, K.; Rezaei, R.; Vitas, N.; Centeno,
R.; Cheung, M.; Couvidat, S.; Fischer, C.; Steiner, O.; Straus, T.;
Viticchie, B.
Bibcode: 2012decs.confE.104F
Altcode:
The Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics
Observatory (SDO) is designed to study oscillations and the magnetic
field in the solar photosphere. It observes the full solar disk
in the Fe I absorption line at 6173 Å. We use the output of three
high-resolution 3D, time-dependent, radiative magneto-hydrodynamics
simulations (two based on the MURaM code, one on the CO5BOLD code)
to calculate Stokes profiles Fi(λ,x,y; i=I, V, Q, U) for the Fe I
6173 Å line for snapshots of a sunspot, a plage area and an enhanced
network region. Stokes filtergrams are constructed for the 6 nominal
HMI wavelengths by multiplying the Stokes profiles with a representative
set of HMI filter response functions. The magnetic field vector B(x,y)
and line-of-sight Doppler velocities V(x,y) are determined from these
filtergrams using a simplified version of the HMI magnetic field
processing pipeline. Finally, the reconstructed magnetic field B(x,y)
and line-of-sight velocity V(x,y) are compared to the actual magnetic
field B0(x,y,z) and vertical velocity V0(x,y,z) in the simulations.
Title: The generation of shock waves traveling from the photosphere
to the transition region within network magnetic elements
Authors: Kato, Y.; Hansteen, V.; Steiner, O.; Carlsson, M.
Bibcode: 2012decs.confE..54K
Altcode:
We investigate the generation of shock waves near the photosphere by
convective downdrafts in the immediate surroundings of the magnetic
flux concentration, using radiation magnetohydrodynamic (RMHD) 2D
simulations of the solar atmosphere. The simulations comprise the layers
from the upper convection zone to the lower corona. We call this the
"magnetic pumping process". We find that the generated slow modes via
magnetic pumping travel upward along the magnetic flux concentration,
developing into a shock wave in chromospheric heights. The waves
continue to propagate further up through the transition region and into
the corona. In the course of propagation through the transition layer,
a small fraction of the longitudinal slow mode is converted into a
transverse wave mode. We report on how much energy is deposited by
propagating shock waves through the transition region and we discuss
the the dissipation process above the photosphere within the magnetic
flux concentration..
Title: Simulations of stellar convection with CO5BOLD
Authors: Freytag, B.; Steffen, M.; Ludwig, H. -G.; Wedemeyer-Böhm,
S.; Schaffenberger, W.; Steiner, O.
Bibcode: 2012JCoPh.231..919F
Altcode: 2011arXiv1110.6844F
High-resolution images of the solar surface show a granulation
pattern of hot rising and cooler downward-sinking material - the
top of the deep-reaching solar convection zone. Convection plays a
role for the thermal structure of the solar interior and the dynamo
acting there, for the stratification of the photosphere, where most
of the visible light is emitted, as well as for the energy budget of
the spectacular processes in the chromosphere and corona. Convective
stellar atmospheres can be modeled by numerically solving the coupled
equations of (magneto)hydrodynamics and non-local radiation transport
in the presence of a gravity field. The CO5BOLD code described in this
article is designed for so-called "realistic" simulations that take
into account the detailed microphysics under the conditions in solar
or stellar surface layers (equation-of-state and optical properties of
the matter). These simulations indeed deserve the label "realistic"
because they reproduce the various observables very well - with only
minor differences between different implementations. The agreement
with observations has improved over time and the simulations are now
well-established and have been performed for a number of stars. Still,
severe challenges are encountered when it comes to extending these
simulations to include ideally the entire star or substellar object:
the strong stratification leads to completely different conditions in
the interior, the photosphere, and the corona. Simulations have to cover
spatial scales from the sub-granular level to the stellar diameter and
time scales from photospheric wave travel times to stellar rotation
or dynamo cycle periods. Various non-equilibrium processes have to be
taken into account. Last but not least, realistic simulations are based
on detailed microphysics and depend on the quality of the input data,
which can be the actual accuracy limiter. This article provides an
overview of the physical problem and the numerical solution and the
capabilities of CO5BOLD, illustrated with a number of applications.
Title: Modification of wave propagation and wave travel-time by the
presence of magnetic fields in the solar network atmosphere
Authors: Nutto, C.; Steiner, O.; Schaffenberger, W.; Roth, M.
Bibcode: 2012A&A...538A..79N
Altcode:
Context. Observations of waves at frequencies above the acoustic cut-off
frequency have revealed vanishing wave travel-times in the vicinity of
strong magnetic fields. This detection of apparently evanescent waves,
instead of the expected propagating waves, has remained a riddle.
Aims: We investigate the influence of a strong magnetic field on the
propagation of magneto-acoustic waves in the atmosphere of the solar
network. We test whether mode conversion effects can account for the
shortening in wave travel-times between different heights in the solar
atmosphere.
Methods: We carry out numerical simulations of the
complex magneto-atmosphere representing the solar magnetic network. In
the simulation domain, we artificially excite high frequency waves
whose wave travel-times between different height levels we then
analyze.
Results: The simulations demonstrate that the wave
travel-time in the solar magneto-atmosphere is strongly influenced by
mode conversion. In a layer enclosing the surface sheet defined by the
set of points where the Alfvén speed and the sound speed are equal,
called the equipartition level, energy is partially transferred from the
fast acoustic mode to the fast magnetic mode. Above the equipartition
level, the fast magnetic mode is refracted due to the large gradient
of the Alfvén speed. The refractive wave path and the increasing phase
speed of the fast mode inside the magnetic canopy significantly reduce
the wave travel-time, provided that both observing levels are above
the equipartition level.
Conclusions: Mode conversion and the
resulting excitation and propagation of fast magneto-acoustic waves is
responsible for the observation of vanishing wave travel-times in the
vicinity of strong magnetic fields. In particular, the wave propagation
behavior of the fast mode above the equipartition level may mimic
evanescent behavior. The present wave propagation experiments provide an
explanation of vanishing wave travel-times as observed with multi-line
high-cadence instruments. Movies are available in electronic form
at http://www.aanda.org
Title: Stokes Diagnostics of Magneto-Acoustic Wave Propagation in
the Magnetic Network on the Sun
Authors: Vigeesh, G.; Steiner, O.; Hasan, S. S.
Bibcode: 2011SoPh..273...15V
Altcode: 2011SoPh..tmp..349V; 2011arXiv1104.4069V
The solar atmosphere is magnetically structured and highly
dynamic. Owing to the dynamic nature of the regions in which the
magnetic structures exist, waves can be excited in them. Numerical
investigations of wave propagation in small-scale magnetic flux
concentrations in the magnetic network on the Sun have shown that
the nature of the excited modes depends on the value of plasma β
(the ratio of gas to magnetic pressure) where the driving motion
occurs. Considering that these waves should give rise to observable
characteristic signatures, we have attempted a study of synthesised
emergent spectra from numerical simulations of magneto-acoustic
wave propagation. We find that the signatures of wave propagation
in a magnetic element can be detected when the spatial resolution
is sufficiently high to clearly resolve it, enabling observations in
different regions within the flux concentration. The possibility to
probe various lines of sight around the flux concentration bears the
potential to reveal different modes of the magnetohydrodynamic waves
and mode conversion. We highlight the feasibility of using the Stokes-V
asymmetries as a diagnostic tool to study the wave propagation within
magnetic flux concentrations. These quantities can possibly be compared
with existing and new observations in order to place constraints on
different wave excitation mechanisms.
Title: On the Magnetic-Field Diagnostics Potential of SDO/HMI
Authors: Fleck, B.; Hayashi, K.; Rezaei, R.; Vitas, N.; Centeno, R.;
Couvidat, S.; Fischer, C.; Steiner, O.; Straus, T.; Viticchie, B.
Bibcode: 2011sdmi.confE..74F
Altcode:
The Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics
Observatory (SDO) is designed to study oscillations and the magnetic
field in the solar photosphere. It observes the full solar disk in the
Fe I 6173 absorption line. We use the output of two high-resolution 3D,
time-dependent, radiative magneto-hydrodynamics simulations (one based
on the MURAM code, the other one on the COBOLD code) to calculate
Stokes profiles for the Fe I 6173 line for a snapshot of a plage
region and a snapshot of an enhanced network region. After spatially
degrading the Stokes profiles to HMI resolution, they are multiplied
by a representative set of HMI filter response functions and Stokes
filtergrams are constructed for the 6 nominal HMI wavelengths. The
magnetic field vector and line-of-sight Doppler velocities are
determined from these filtergrams using a simplified version of the HMI
magnetic field processing pipeline. Finally, the reconstructed magnetic
field is compared to the actual magnetic field in the simulation.
Title: The Sun at high resolution: first results from the Sunrise
mission
Authors: Solanki, S. K.; Barthol, P.; Danilovic, S.; Feller,
A.; Gandorfer, A.; Hirzberger, J.; Lagg, A.; Riethmüller, T. L.;
Schüssler, M.; Wiegelmann, T.; Bonet, J. A.; Pillet, V. Martínez;
Khomenko, E.; del Toro Iniesta, J. C.; Domingo, V.; Palacios, J.;
Knölker, M.; González, N. Bello; Borrero, J. M.; Berkefeld, T.;
Franz, M.; Roth, M.; Schmidt, W.; Steiner, O.; Title, A. M.
Bibcode: 2011IAUS..273..226S
Altcode:
The Sunrise balloon-borne solar observatory consists of a 1m aperture
Gregory telescope, a UV filter imager, an imaging vector polarimeter,
an image stabilization system and further infrastructure. The first
science flight of Sunrise yielded high-quality data that reveal the
structure, dynamics and evolution of solar convection, oscillations
and magnetic fields at a resolution of around 100 km in the quiet
Sun. Here we describe very briefly the mission and the first results
obtained from the Sunrise data, which include a number of discoveries.
Title: On the Origin of Intergranular Jets
Authors: Yurchyshyn, V. B.; Goode, P. R.; Abramenko, V. I.; Steiner, O.
Bibcode: 2011ApJ...736L..35Y
Altcode: 2011arXiv1106.5535Y
We observe that intergranular jets, originating in the intergranular
space surrounding individual granules, tend to be associated with
granular fragmentation, in particular, with the formation and evolution
of a bright granular lane (BGL) within individual granules. The BGLs
have recently been identified as vortex tubes by Steiner et al. We
further discover the development of a well-defined bright grain
located between the BGL and the dark intergranular lane to which it
is connected. Signatures of a BGL may reach the lower chromosphere
and can be detected in off-band Hα images. Simulations also indicate
that vortex tubes are frequently associated with small-scale magnetic
fields. We speculate that the intergranular jets detected in the New
Solar Telescope (NST) data may result from the interaction between
the turbulent small-scale fields associated with the vortex tube
and the larger-scale fields existing in the intergranular lanes. The
intergranular jets are much smaller and weaker than all previously known
jet-like events. At the same time, they appear much more numerous than
the larger events, leading us to the speculation that the total energy
release and mass transport by these tiny events may not be negligible in
the energy and mass-flux balance near the temperature minimum atop the
photosphere. The study is based on the photospheric TiO broadband (1.0
nm) filter data acquired with the 1.6 m NST operating at the Big Bear
Solar Observatory. The data set also includes NST off-band Hα images
collected through a Zeiss Lyot filter with a passband of 0.025 nm.
Title: Excitation of magneto-acoustic waves in network magnetic
elements
Authors: Kato, Yoshiaki; Steiner, Oskar; Steffen, Matthias; Suematsu,
Yoshinori
Bibcode: 2011IAUS..273..442K
Altcode:
From radiation magnetohydrodynamic (RMHD) simulations we track the
temporal evolution of a vertical magnetic flux sheet embedded in a
two-dimensional non-stationary atmosphere that reaches all the way
from the upper convection zone to the low chromosphere. Examining its
temporal behavior near the interface between the convection zone and
the photosphere, we describe the excitation of propagating longitudinal
waves within the magnetic element as a result of convective motion in
its surroundings.
Title: Flux Tube Model
Authors: Steiner, O.
Bibcode: 2011ascl.soft05008S
Altcode:
This Fortran code computes magnetohydrostatic flux tubes and sheets
according to the method of Steiner, Pneuman, & Stenflo (1986)
A&A 170, 126-137. The code has many parameters contained in one
input file that are easily modified. Extensive documentation is provided
in README files.
Title: Excitation of Slow Modes in Network Magnetic Elements Through
Magnetic Pumping
Authors: Kato, Yoshiaki; Steiner, Oskar; Steffen, Matthias; Suematsu,
Yoshinori
Bibcode: 2011ApJ...730L..24K
Altcode: 2011arXiv1102.5164K
From radiation magnetohydrodynamic simulations of the solar atmosphere,
we find a new mechanism for the excitation of longitudinal slow modes
within magnetic flux concentrations. We find that the convective
downdrafts in the immediate surroundings of magnetic elements are
responsible for the excitation of slow modes. The coupling between
the external downdraft and the plasma motion internal to the flux
concentration is mediated by the inertial forces of the downdraft that
act on the magnetic flux concentration. These forces, in conjunction
with the downward movement, pump the internal atmosphere in the
downward direction, which entails a fast downdraft in the photospheric
and chromospheric layers of the magnetic element. Subsequent to the
transient pumping phase, the atmosphere rebounds, causing a slow
mode traveling along the magnetic flux concentration in the upward
direction. It develops into a shock wave in chromospheric heights,
possibly capable of producing some kind of dynamic fibril. We propose
an observational detection of this process.
Title: Magneto-acoustic wave propagation and mode conversion
in a magnetic solar atmosphere: Comparing results from the
CO5BOLD code with ray theory
Authors: Nutto, C.; Steiner, O.; Roth, M.
Bibcode: 2010AN....331..915N
Altcode: 2010arXiv1009.5586N
We present simulations of magneto-acoustic wave propagation in
a magnetic, plane-parallel stratified solar model atmosphere,
employing the CO5BOLD-code. The tests are carried out for
two models of the solar atmosphere, which are similar to the ones used
by \citet{nutto_cally07} and \citet{nutto_schunker06}. The two models
differ only in the orientation of the magnetic field. A qualitative
comparison shows good agreement between the numerical results and the
results from ray theory. The tests are done in view of the application
of the present numerical code for the computation of energy fluxes
of propagating acoustic waves into a dynamically evolving magnetic
solar atmosphere. For this, we consider waves with frequencies above
the acoustic cut-off frequency.
Title: Detection of Vortex Tubes in Solar Granulation from
Observations with SUNRISE
Authors: Steiner, O.; Franz, M.; Bello González, N.; Nutto, Ch.;
Rezaei, R.; Martínez Pillet, V.; Bonet Navarro, J. A.; del Toro
Iniesta, J. C.; Domingo, V.; Solanki, S. K.; Knölker, M.; Schmidt,
W.; Barthol, P.; Gandorfer, A.
Bibcode: 2010ApJ...723L.180S
Altcode: 2010arXiv1009.4723S
We have investigated a time series of continuum intensity maps and
corresponding Dopplergrams of granulation in a very quiet solar region
at the disk center, recorded with the Imaging Magnetograph eXperiment
(IMaX) on board the balloon-borne solar observatory SUNRISE. We
find that granules frequently show substructure in the form of lanes
composed of a leading bright rim and a trailing dark edge, which move
together from the boundary of a granule into the granule itself. We
find strikingly similar events in synthesized intensity maps from an
ab initio numerical simulation of solar surface convection. From cross
sections through the computational domain of the simulation, we conclude
that these granular lanes are the visible signature of (horizontally
oriented) vortex tubes. The characteristic optical appearance of vortex
tubes at the solar surface is explained. We propose that the observed
vortex tubes may represent only the large-scale end of a hierarchy of
vortex tubes existing near the solar surface.
Title: CO5BOLD: COnservative COde for the COmputation of COmpressible
COnvection in a BOx of L Dimensions with l=2,3
Authors: Freytag, Bernd; Steffen, Matthias; Wedemeyer-Böhm, Sven;
Ludwig, Hans-Günter; Leenaarts, Jorrit; Schaffenberger, Werner;
Allard, France; Chiavassa, Andrea; Höfner, Susanne; Kamp, Inga;
Steiner, Oskar
Bibcode: 2010ascl.soft11014F
Altcode:
CO5BOLD - nickname COBOLD - is the short form of "COnservative
COde for the COmputation of COmpressible COnvection in a BOx of L
Dimensions with l=2,3". It is used to model solar and stellar
surface convection. For solar-type stars only a small fraction of the
stellar surface layers are included in the computational domain. In
the case of red supergiants the computational box contains the entire
star. Recently, the model range has been extended to sub-stellar objects
(brown dwarfs). CO5BOLD solves the coupled non-linear equations
of compressible hydrodynamics in an external gravity field together
with non-local frequency-dependent radiation transport. Operator
splitting is applied to solve the equations of hydrodynamics (including
gravity), the radiative energy transfer (with a long-characteristics
or a short-characteristics ray scheme), and possibly additional 3D
(turbulent) diffusion in individual sub steps. The 3D hydrodynamics
step is further simplified with directional splitting (usually). The 1D
sub steps are performed with a Roe solver, accounting for an external
gravity field and an arbitrary equation of state from a table. The radiation transport is computed with either one of three
modules: MSrad module: It uses long characteristics. The lateral
boundaries have to be periodic. Top and bottom can be closed or open
("solar module"). LHDrad module: It uses long characteristics
and is restricted to an equidistant grid and open boundaries at all
surfaces (old "supergiant module"). SHORTrad module: It uses
short characteristics and is restricted to an equidistant grid and
open boundaries at all surfaces (new "supergiant module"). The
code was supplemented with an (optional) MHD version [Schaffenberger
et al. (2005)] that can treat magnetic fields. There are also modules
for the formation and advection of dust available. The current version
now contains the treatment of chemical reaction networks, mostly used
for the formation of molecules [Wedemeyer-Böhm et al. (2005)], and
hydrogen ionization [Leenaarts & Wedemeyer-Böhm (2005)], too. CO5BOLD is written in Fortran90. The parallelization is done with
OpenMP directives.
Title: A Chromospheric Conundrum?
Authors: Judge, Philip; Knölker, Michael; Schmidt, Wolfgang;
Steiner, Oskar
Bibcode: 2010ApJ...720..776J
Altcode: 2010arXiv1007.1203J
We examine spectra of the Ca II H line, obtained under good seeing
conditions with the VTT Echelle Spectrograph in 2007 June, and
higher resolution data of the Ca II λ8542 line from Fabry-Pérot
instruments. The VTT targets were areas near disk center which included
quiet Sun and some dispersed plage. The infrared data included quiet
Sun and plage associated with small pores. Bright chromospheric network
emission patches expand little with wavelength from line wing to line
center, i.e., with increasing line opacity and height. We argue that
this simple observation has implications for the force and energy
balance of the chromosphere, since bright chromospheric network
emission is traditionally associated with enhanced local mechanical
heating which increases temperatures and pressures. Simple physical
considerations then suggest that the network chromosphere may not be
able to reach horizontal force balance with its surroundings, yet the
network is a long-lived structure. We speculate on possible reasons for
the observed behavior. By drawing attention to a potential conundrum,
we hope to contribute to a better understanding of a long-standing
unsolved problem: the heating of the chromospheric network.
Title: Magnetic Coupling in the Quiet Solar Atmosphere
Authors: Steiner, O.
Bibcode: 2010ASSP...19..166S
Altcode: 2010mcia.conf..166S; 2009arXiv0904.2026S
Three kinds of magnetic couplings in the quiet solar atmosphere are
highlighted and discussed, all fundamentally connected to the Lorentz
force: first, the coupling of the convecting and overshooting fluid
in the surface layers of the Sun with the magnetic field. Here, the
plasma motion provides the dominant force, which shapes the magnetic
field and drives the surface dynamo. Progress in the understanding of
the horizontal magnetic field is summarized and discussed. Second, the
coupling between acoustic waves and the magnetic field, in particular
the phenomenon of wave conversion and wave refraction. It is described
how measurements of wave travel times in the atmosphere can provide
information about the topography of the wave conversion zone, that
is, the surface of equal Alfvéen and sound speed. In quiet regions,
this surface separates a highly dynamic magnetic field with fast moving
magnetosonic waves and shocks around and above it from the more slowly
evolving field of high-beta plasma below it. Third, the magnetic
field also couples to the radiation field, which leads to radiative
flux channeling and increased anisotropy in the radiation field. It
is shown how faculae can be understood in terms of this effect. The
article starts with an introduction to the magnetic field of the quiet
Sun in the light of new results from the Hinode space observatory and
with a brief survey of measurements of the turbulent magnetic field
with the help of the Hanle effect.
Title: Numerical simulations of wave propagation in the solar
chromosphere .
Authors: Nutto, C.; Steiner, O.; Roth, M.
Bibcode: 2010MmSAI..81..744N
Altcode: 2010arXiv1009.5607N
We present two-dimensional simulations of wave propagation in a
realistic, non-stationary model of the solar atmosphere. This model
shows a granular velocity field and magnetic flux concentrations in
the intergranular lanes similar to observed velocity and magnetic
structures on the Sun and takes radiative transfer into account. We present three cases of magneto-acoustic wave propagation through
the model atmosphere, where we focus on the interaction of different
magneto-acoustic wave modes at the layer of similar sound and Alfvén
speeds, which we call the equipartition layer. At this layer acoustic
and magnetic mode can exchange energy depending on the angle between the
wave vector and the magnetic field vector. Our results show that
above the equipartition layer and in all three cases the fast magnetic
mode is refracted back into the solar atmosphere. Thus, the magnetic
wave shows an evanescent behavior in the chromosphere. The acoustic
mode, which travels along the magnetic field in the low plasma-beta
regime, can be a direct consequence of an acoustic source within or
outside the low-beta regime, or it can result from conversion of the
magnetic mode, possibly from several such conversions when the wave
travels across a series of equipartition layers.
Title: Wave propagation and energy transport in the magnetic network
of the Sun
Authors: Vigeesh, G.; Hasan, S. S.; Steiner, O.
Bibcode: 2009A&A...508..951V
Altcode: 2009arXiv0909.2325V
Aims. We investigate wave propagation and energy transport in
magnetic elements, which are representatives of small scale magnetic
flux concentrations in the magnetic network on the Sun. This is
a continuation of earlier work by Hasan et al. (2005, ApJ, 631,
1270). The new features in the present investigation include
a quantitative evaluation of the energy transport in the various
modes and for different field strengths, as well as the effect of the
boundary-layer thickness on wave propagation.
Methods: We carry
out 2D MHD numerical simulations of magnetic flux concentrations for
strong and moderate magnetic fields for which β (the ratio of gas to
magnetic pressure) on the tube axis at the photospheric base is 0.4 and
1.7, respectively. Waves are excited in the tube and ambient medium by
a transverse impulsive motion of the lower boundary.
Results: The
nature of the modes excited depends on the value of β. Mode conversion
occurs in the moderate field case when the fast mode crosses the β =
1 contour. In the strong field case the fast mode undergoes conversion
from predominantly magnetic to predominantly acoustic when waves are
leaking from the interior of the flux concentration to the ambient
medium. We also estimate the energy fluxes in the acoustic and magnetic
modes and find that in the strong field case, the vertically directed
acoustic wave fluxes reach spatially averaged, temporal maximum values
of a few times 106 erg cm-2 s-1 at
chromospheric height levels.
Conclusions: The main conclusions
of our work are twofold: firstly, for transverse, impulsive excitation,
flux tubes/sheets with strong fields are more efficient than those with
weak fields in providing acoustic flux to the chromosphere. However,
there is insufficient energy in the acoustic flux to balance the
chromospheric radiative losses in the network, even for the strong
field case. Secondly, the acoustic emission from the interface between
the flux concentration and the ambient medium decreases with the width
of the boundary layer.
Title: The Horizontal Magnetic Field of the Quiet Sun: Numerical
Simulations in Comparison to Observations with Hinode
Authors: Steiner, O.; Rezaei, R.; Schlichenmaier, R.; Schaffenberger,
W.; Wedemeyer-Böhm, S.
Bibcode: 2009ASPC..415...67S
Altcode: 2009arXiv0904.2030S
Three-dimensional magnetohydrodynamic simulations of the surface layers
of the Sun intrinsically produce a predominantly horizontal magnetic
field in the photosphere. This is a robust result in the sense that it
arises from simulations with largely different initial and boundary
conditions for the magnetic field. While the disk-center synthetic
circular and linear polarization signals agree with measurements from
Hinode, their center-to-limb variation sensitively depends on the
height variation of the horizontal and the vertical field component
and they seem to be at variance with the observed behavior.
Title: Near-surface stellar magneto-convection: simulations for the
Sun and a metal-poor solar analog
Authors: Steffen, Matthias; Ludwig, H. -G.; Steiner, O.
Bibcode: 2009IAUS..259..233S
Altcode: 2009arXiv0902.2753S
We present 2D local box simulations of near-surface radiative
magneto-convection with prescribed magnetic flux, carried out with the
MHD version of the CO5BOLD code for the Sun and a solar-like
star with a metal-poor chemical composition (metal abundances reduced by
a factor 100, [M/H] = -2). The resulting magneto-hydrodynamical models
can be used to study the influence of the metallicity on the properties
of magnetized stellar atmospheres. A preliminary analysis indicates
that the horizontal magnetic field component tends to be significantly
stronger in the optically thin layers of metal-poor stellar atmospheres.
Title: Numerical simulation of wave propagation in magnetic network
Authors: Vigeesh, G.; Hasan, S. S.; Steiner, O.
Bibcode: 2009IAUS..257..185V
Altcode:
We present 2-D numerical simulations of wave propagation in the magnetic
network. The network is modelled as consisting of individual magnetic
flux sheets located in intergranular lanes. They have a typical
horizontal size of about 150 km at the base of the photosphere and
expand upward and become uniform. We consider flux sheets of different
field strengths. Waves are excited by means of transverse motions at
the lower boundary, to simulate the effect of granular buffeting. We
look at the magneto-acoustic waves generated within the flux sheet
and the acoustic waves generated in the ambient medium due to the
excitation. We calculate the wave energy fluxes separating them into
contributions from the acoustic and the Poynting part and study the
effect of the different field strengths.
Title: Observational Evidence for Shocks in the Solar Photosphere -
New TESOS/VTT Results
Authors: Rybak, J.; Kucera, A.; Hanslmeier, A.; Woehl, H.;
Wedemeyer-Boehm, S.; Steiner, O.
Bibcode: 2008ESPM...12.2.36R
Altcode:
High-resolution spectroscopic observations recently acquired with the
TESOS spectrometer at the Vacuum Tower Telescope (VTT, Observatorio
del Teide, Tenerife) are used to test predictions regarding strongly
dynamic events in the photosphere as obtained from three-dimensional
numerical simulations with the CO5BOLD-code. Time series of
two-dimensional maps of the Fe I 543.4 nm spectral line profile at
different centre-to-limb positions are investigated in a statistical
sense by comparing the distributions of individual spectral parameters
derived from observations with the corresponding distributions
from synthesized spectra calculated with the LINFOR3D code from
the simulations. Appropriate degradation of the synthesized spectra
was applied in order to take the limited spatial resolution of the
telescope, seeing effects, and the scattered instrumental light into
account. At the actual spatial resolution of 0.5 arc sec, the
statistics show that signatures of the photospheric dynamics, including
the most dynamical events like occasional supersonic flows of plasma in
the nearly horizontal direction, are very similar in both observations
and simulations. Discrepancies are found only for those spectral
parameters (residual line intensity, Doppler line core shifts), which
are affected by non-LTE effects, since non-LTE effects are not taken
into account in the synthesis of the Fe I 543.4nm spectral line.
Title: Numerical Experiments with Magnetoacoustic Waves in the
Solar Atmosphere
Authors: Nutto, C.; Schaffenberger, W.; Steiner, O.
Bibcode: 2008ESPM...12.3.23N
Altcode:
With numerical experiments we explore the feasibility of using high
frequency waves for probing the magnetic field in the photosphere
and the chromosphere of the Sun. We track monochromatic wave trains
that propagates through a magnetically structured, realistic solar
atmosphere. When entering the magnetically dominated chromosphere,
the waves undergo partial mode conversion and get refracted and
reflected. We explore the relationship between wave travel times and
the topography of the surface of equal Alfven and sound speeds, viz.,
the magnetic canopy.
Title: Numerical simulation of wave propagation in the presence of
a magnetic flux sheet
Authors: Vigeesh, G.; Steiner, O.; Hasan, S. S.
Bibcode: 2008ESPM...12.3.24V
Altcode:
We model network magnetic fields as consisting of individual magnetic
flux sheets located in intergranular lanes. With a typical horizontal
size of about 150 km at the base of the photosphere, they expand upward
and merge with their neighbors at a height of about 600 km. Above
a height of approximately 1000 km the magnetic field starts to
become uniform. Granular buffeting is thought to excite waves in this
medium, which is modeled by means of transversal motions at the lower
boundary. The transverse driving, generates both fast and slow waves
within the flux sheet and acoustic waves in the ambient medium. We
consider flux sheets of different field strengths and different
boundary-layer widths. Separating the energy flux of the waves into
contributions due to the acoustic flux and the Poynting flux, we show
the longitudinal and transversal components of both and study their
temporal evolution.
Title: The Horizontal Internetwork Magnetic Field: Numerical
Simulations in Comparison to Observations with Hinode
Authors: Steiner, O.; Rezaei, R.; Schaffenberger, W.; Wedemeyer-Böhm,
S.
Bibcode: 2008ESPM...12.3.22S
Altcode:
Observations with the Hinode space observatory led to the discovery
of predominantly horizontal magnetic fields in the photosphere of the
quiet internetwork region. Here we investigate realistic numerical
simulations of the surface layers of the Sun with respect to horizontal
magnetic fields and compute the corresponding polarimetric response
in the Fe I 630 nm line pair. We find a local maximum in the mean
strength of the horizontal field component at a height of around 500
km in the photosphere, where, depending on the initial state or the
boundary condition, it surpasses the vertical component by a factor
of 2.0 or 5.6. From the synthesized Stokes profiles, we derive a mean
horizontal field component that is 1.6 or 4.3 times stronger than
the vertical component, depending on the initial state or the boundary
condition. This is a consequence of both the intrinsically stronger flux
density of and the larger area occupied by the horizontal fields. We
find that convective overshooting expels horizontal fields to the upper
photosphere, making the Poynting flux positive in the photosphere,
whereas it is negative in the convectively unstable layer below it.
Title: The Horizontal Internetwork Magnetic Field: Numerical
Simulations in Comparison to Observations with Hinode
Authors: Steiner, O.; Rezaei, R.; Schaffenberger, W.; Wedemeyer-Böhm,
S.
Bibcode: 2008ApJ...680L..85S
Altcode: 2008arXiv0801.4915S
Observations with the Hinode space observatory led to the discovery
of predominantly horizontal magnetic fields in the photosphere of the
quiet internetwork region. Here we investigate realistic numerical
simulations of the surface layers of the Sun with respect to horizontal
magnetic fields and compute the corresponding polarimetric response
in the Fe I 630 nm line pair. We find a local maximum in the mean
strength of the horizontal field component at a height of around 500
km in the photosphere, where, depending on the initial state or the
boundary condition, it surpasses the vertical component by a factor
of 2.0 or 5.6. From the synthesized Stokes profiles, we derive a mean
horizontal field component that is 1.6 or 4.3 times stronger than
the vertical component, depending on the initial state or the boundary
condition. This is a consequence of both the intrinsically stronger flux
density of and the larger area occupied by the horizontal fields. We
find that convective overshooting expels horizontal fields to the upper
photosphere, making the Poynting flux positive in the photosphere,
whereas the Poynting flux is negative in the convectively unstable
layer below it.
Title: Wave propagation in multiple flux tubes and chromospheric
heating
Authors: Hasan, S. S.; van Ballegooijen, A.; Steiner, O.
Bibcode: 2008IAUS..247...82H
Altcode: 2007IAUS..247...82H
This investigation is a continuation of earlier work on the dynamics of
the magnetic network. In a previous calculation (Hasan et al. 2005),
we examined the response of a single flux tube to transverse motions
of its footpoints. We now extend this analysis to a more realistic
model of the network consisting of multiple flux tubes. We apply a
transverse velocity perturbation uniformly along the lower boundary
located at the base of the photosphere. Our 2-D MHD simulations enable
us to study the complex wave pattern due to waves generated in the
individual tubes as well as their interaction with those emanating
from adjacent tubes. Our results show that the dominant heating of the
chromosphere occurs due to slow magnetoacoustic waves in a region that
is close to the central region of the flux tube.
Title: Inferring the chromospheric magnetic topology through waves
Authors: Hasan, S. S.; Steiner, O.; van Ballegooijen, A.
Bibcode: 2008IAUS..247...78H
Altcode: 2007IAUS..247...78H
The aim of this work is to examine the hypothesis that the wave
propagation time in the solar atmosphere can be used to infer the
magnetic topography in the chromosphere as suggested by Finsterle et
al. (2004). We do this by using an extension of our earlier 2-D MHD
work on the interaction of acoustic waves with a flux sheet. It is well
known that these waves undergo mode transformation due to the presence
of a magnetic field which is particularly effective at the surface
of equipartition between the magnetic and thermal energy density, the
β = 1 surface. This transformation depends sensitively on the angle
between the wave vector and the local field direction. At the β =
1 interface, the wave that enters the flux sheet, (essentially the
fast mode) has a higher phase speed than the incident acoustic wave. A
time correlation between wave motions in the non-magnetic and magnetic
regions could therefore provide a powerful diagnostic for mapping the
magnetic field in the chromospheric network.
Title: Hinode observations reveal boundary layers of magnetic elements
in the solar photosphere
Authors: Rezaei, R.; Steiner, O.; Wedemeyer-Böhm, S.; Schlichenmaier,
R.; Schmidt, W.; Lites, B. W.
Bibcode: 2007A&A...476L..33R
Altcode: 2007arXiv0711.0408R
Aims:We study the structure of the magnetic elements in network-cell
interiors.
Methods: A quiet Sun area close to the disc centre was
observed with the spectro-polarimeter of the Solar Optical Telescope
on board the Hinode space mission, which yielded the best spatial
resolution ever achieved in polarimetric data of the Fe I 630 nm line
pair. For comparison and interpretation, we synthesize a similar data
set from a three-dimensional magneto-hydrodynamic simulation.
Results: We find several examples of magnetic elements, either
roundish (tube) or elongated (sheet), which show a central area of
negative Stokes-V area asymmetry framed or surrounded by a peripheral
area with larger positive asymmetry. This pattern was predicted
some eight years ago on the basis of numerical simulations. Here,
we observationally confirm its existence for the first time.
Conclusions: We gather convincing evidence that this pattern of
Stokes-V area asymmetry is caused by the funnel-shaped boundary of
magnetic elements that separates the flux concentration from the
weak-field environment. On this basis, we conclude that electric
current sheets induced by such magnetic boundary layers are common in
the photosphere.
Title: How to Reach Superequipartition Field Strengths in Solar
Magnetic Flux Tubes
Authors: Ferriz-Mas, A.; Steiner, O.
Bibcode: 2007SoPh..246...31F
Altcode:
A number of independent arguments indicate that the toroidal flux
system responsible for the sunspot cycle is stored at the base of the
convection zone in the form of flux tubes with field strength close
to 105 G. Although the evidence for such strong fields is
quite compelling, how such field strength can be reached is still a
topic of debate. Flux expulsion by convection should lead to about
the equipartition field strength, but the magnetic energy density of
a 105-G field is two orders of magnitude larger than the
mean kinetic energy density of convective motions. Line stretching
by differential rotation (i.e., the "Ω effect" in the classical
mean-field dynamo approach) probably plays an important role, but
arguments based on energy considerations show that it does not seem
feasible that a 105-G field can be produced in this way. An
alternative scenario for the intensification of the toroidal flux
system in the overshoot layer is related to the explosion of rising,
buoyantly unstable magnetic flux tubes, which opens a complementary
mechanism for magnetic-field intensification. A parallelism is pointed
out with the mechanism of "convective collapse" for the intensification
of photospheric magnetic flux tubes up to field strengths well above
equipartition; both mechanisms, which are fundamentally thermal
processes, are reviewed.
Title: Variation of the Stokes-V area asymmetry across magnetic
elements
Authors: Rezaei, R.; Steiner, O.; Wedemeyer-Böhm, S.; Schlichenmaier,
R.; Lites, B. W.
Bibcode: 2007AN....328..706R
Altcode:
No abstract at ADS
Title: Opposite magnetic polarity of two photospheric lines in single
spectrum of the quiet Sun
Authors: Rezaei, R.; Schlichenmaier, R.; Schmidt, W.; Steiner, O.
Bibcode: 2007A&A...469L...9R
Altcode: 2007arXiv0704.3135R
Aims:We study the structure of the photospheric magnetic field of the
quiet Sun by investigating weak spectro-polarimetric signals.
Methods: We took a sequence of Stokes spectra of the Fe I 630.15
nm and 630.25 nm lines in a region of quiet Sun near the disk
center, using the POLIS spectro-polarimeter at the German VTT on
Tenerife. The line cores of these two lines form at different heights
in the atmosphere. The 3σ noise level of the data is about 1.8
× 10-3 I_c.
Results: We present co-temporal and
co-spatial Stokes-V profiles of the Fe I 630 nm line pair, where
the two lines show opposite polarities in a single spectrum. We
compute synthetic line profiles and reproduce these spectra with a
two-component model atmosphere: a non-magnetic component and a magnetic
component. The magnetic component consists of two magnetic layers with
opposite polarity: the upper one moves upwards while the lower one moves
downward. In-between, there is a region of enhanced temperature.
Conclusions: The Stokes-V line pair of opposite polarity in a single
spectrum can be understood as a magnetic reconnection event in the
solar photosphere. We demonstrate that such a scenario is realistic,
but the solution may not be unique.
Title: Photospheric processes and magnetic flux tubes
Authors: Steiner, Oskar
Bibcode: 2007AIPC..919...74S
Altcode: 2007arXiv0709.0081S
New high-resolution observations reveal that small-scale magnetic
flux concentrations have a delicate substructure on a spatial scale
of 0.1''. Their basic structure can be interpreted in terms of a
magnetic flux sheet or tube that vertically extends through the
ambient weak-field or field-free atmosphere with which it is in
mechanical equilibrium. A more refined interpretation comes from new
three-dimensional magnetohydrodynamic simulations that are capable
of reproducing the corrugated shape of magnetic flux concentrations
and their signature in the visible continuum. Faculae are another
manifestation of small-scale magnetic flux concentrations. It is shown
that the characteristic asymmetric shape of the contrast profile
of faculae is an effect of radiative transfer across the rarefied
atmosphere of the magnetic flux concentration. Also discussed
are three-dimensional radiation magnetohydrodynamic simulations
of the integral layers from the top of the convection zone to the
mid-chromosphere. They show a highly dynamic chromospheric magnetic
field, marked by rapidly moving filaments of stronger than average
magnetic field that form in the compression zone downstream and along
propagating shock fronts. The simulations confirm the picture of flux
concentrations that strongly expand through the photosphere into a
more homogeneous, space filling chromospheric field. Future directions
in the simulation of small-scale magnetic fields are indicated with a
few examples from recent reports. The second part of these lecture
notes is devoted to a few basic properties of magnetic flux tubes that
can be considered to be an abstraction of the more complicated flux
concentrations known from observations and numerical simulations. By
analytical means we will find that an electrical current flows in a
sheet at the surface of a flux-tube for which location we also derive
the mechanical equilibrium condition. The equations for constructing a
magnetohydrostatic flux tube embedded in a gravitationally stratified
atmosphere are derived. It is shown that the expansion of a flux
tube with height sensibly depends on the difference in the thermal
structure between the atmosphere of the flux tube and the surrounding
atmosphere. Furthermore, we will find that radiative equilibrium
produces a smaller temperature gradient within the flux tube compared
to that in the surrounding atmosphere. The condition for interchange
stability is derived and it is shown that small-scale magnetic flux
concentrations are liable to the interchange instability.
Title: What is Heating the Quiet-Sun Chromosphere?
Authors: Wedemeyer-Böhm, S.; Steiner, O.; Bruls, J.; Rammacher, W.
Bibcode: 2007ASPC..368...93W
Altcode: 2006astro.ph.12627W
It is widely believed that the heating of the chromosphere in quiet-Sun
internetwork regions is provided by dissipation of acoustic waves
that are excited by the convective motions close to the top of
the convection zone and in the photospheric overshoot layer. This
view lately became challenged by observations suggesting that the
acoustic energy flux into the chromosphere is too low, by a factor
of at least ten. Based on a comparison of TRACE data with synthetic
image sequences for a three-dimensional simulation extending from
the top layers of the convection zone to the middle chromosphere,
we come to the contradicting conclusion that the acoustic flux in the
model provides sufficient energy for heating the solar chromosphere of
internetwork regions. The role of a weak magnetic field and associated
electric current sheets is also discussed.
Title: First local helioseismic experiments with CO5BOLD
Authors: Steiner, O.; Vigeesh, G.; Krieger, L.; Wedemeyer-Böhm, S.;
Schaffenberger, W.; Freytag, B.
Bibcode: 2007AN....328..323S
Altcode: 2007astro.ph..1029S
With numerical experiments we explore the feasibility of using high
frequency waves for probing the magnetic fields in the photosphere and
the chromosphere of the Sun. We track a plane-parallel, monochromatic
wave that propagates through a non-stationary, realistic atmosphere,
from the convection-zone through the photosphere into the magnetically
dominated chromosphere, where it gets refracted and reflected. We
compare the wave travel time between two fixed geometrical height levels
in the atmosphere (representing the formation height of two spectral
lines) with the topography of the surface of equal magnetic and thermal
energy density (the magnetic canopy or β=1 contour) and find good
correspondence between the two. We conclude that high frequency waves
indeed bear information on the topography of the `magnetic canopy'.
Title: Recent progresses in the simulation of small-scale magnetic
fields
Authors: Steiner, O.
Bibcode: 2007msfa.conf..321S
Altcode: 2007arXiv0705.1848S
New high-resolution observations reveal that small-scale magnetic
flux concentrations have a delicate substructure on a spatial scale of
0.1". Its basic structure can be interpreted in terms of a magnetic flux
sheet or tube that vertically extends through the ambient weak-field or
field-free atmosphere with which it is in mechanical equilibrium. A
more refined interpretation comes from new three-dimensional
magnetohydrodynamic simulations that are capable of reproducing the
corrugated shape of magnetic flux concentrations and their signature in
the visible continuum. Furthermore it is shown that the characteristic
asymmetric shape of the contrast profile of facular granules is
an effect of radiative transfer across the rarefied atmosphere of
the magnetic flux concentration. I also discuss three-dimensional
radiation magnetohydrodynamic simulations of the integral layers from
the top of the convection zone to the mid-chromosphere. They show a
highly dynamic chromospheric magnetic field, marked by rapidly moving
filaments of stronger than average magnetic field that form in the
compression zone downstream and along propagating shock fronts. The
simulations confirm the picture of flux concentrations that strongly
expand through the photosphere into a more homogeneous, space filling
chromospheric field. Future directions in the simulation of small-scale
magnetic fields are indicated by a few examples of very recent work.
Title: A New Method for Comparing Numerical Simulations with
Spectroscopic Observations of the Solar Photosphere
Authors: Rybák, J.; Kučera, A.; Wöhl, H.; Wedemeyer-Böhm, S.;
Steiner, O.
Bibcode: 2006ASPC..354...77R
Altcode:
A method for comparing high-resolution spectroscopic observations of
the solar photosphere with numerical simulations of convection in the
solar photosphere is presented. It is based on the comparison of
the granular continuum contrast obtained from both the observations and
the synthetic spectra, when the latter are calculated from numerical
simulations using a particular type of data degradation. This method
can be used post facto when a minimum of auxiliary information on
characteristics of the telescope/spectrograph and on seeing conditions
is available. Here, the method is applied to results of numerical
simulations computed with the CO5BOLD code and high-resolution
spectroscopic observations obtained with the VTT on Tenerife.
Title: Holistic MHD-Simulation from the Convection Zone to the
Chromosphere
Authors: Schaffenberger, W.; Wedemeyer-Böhm, S.; Steiner, O.;
Freytag, B.
Bibcode: 2006ASPC..354..345S
Altcode:
A three-dimensional magnetohydrodynamic simulation of the integral
layers from the convection zone to the chromosphere has been
carried out. The simulation represents magnetoconvection in a quiet
network-cell interior. The following preliminary new results are
obtained: The chromospheric magnetic field is very dynamic with a
continuous rearrangement of magnetic flux on a time scale of less than
one~minute. Rapidly moving magnetic filaments (rarely exceeding 40~G)
form in the compression zone downstream and along propagating shock
fronts that are present throughout the chromosphere. The magnetic
filaments rapidly move, form, and dissolve with the shock waves. Flux
concentrations strongly expand through the photosphere into a more
homogeneous, space filling chromospheric field. ``Canopy fields''
form on a granular scale above largely field-free granule centers
leading to a mesh-work of current sheets in a height range between
approximately 400 and 900~km.
Title: Recent Progresses in the Physics of Small-Scale Magnetic Fields
Authors: Steiner, O.
Bibcode: 2005ESASP.600E..10S
Altcode: 2005ESPM...11...10S; 2005dysu.confE..10S
No abstract at ADS
Title: Simulations of Magnetohydrodynamics and CO Formation from
the Convection Zone to the Chromosphere
Authors: Wedemeyer-Böhm, S.; Schaffenberger, W.; Steiner, O.; Steffen,
M.; Freytag, B.; Kamp, I.
Bibcode: 2005ESASP.596E..16W
Altcode: 2005ccmf.confE..16W
No abstract at ADS
Title: Magnetohydrodynamic Simulation from the Convection Zone to
the Chromosphere
Authors: Schaffenberger, W.; Wedemeyer-Böhm, S.; Steiner, O.;
Freytag, B.
Bibcode: 2005ESASP.596E..65S
Altcode: 2005ccmf.confE..65S
No abstract at ADS
Title: Dynamics of the Solar Magnetic Network: Two-dimensional
MHD Simulations
Authors: Hasan, S. S.; van Ballegooijen, A. A.; Kalkofen, W.;
Steiner, O.
Bibcode: 2005ApJ...631.1270H
Altcode: 2005astro.ph..3525H
The aim of this work is to identify the physical processes that occur
in the network and contribute to its dynamics and heating. We model the
network as consisting of individual flux tubes, each with a nonpotential
field structure, that are located in intergranular lanes. With a typical
horizontal size of about 150 km at the base of the photosphere, they
expand upward and merge with their neighbors at a height of about 600
km. Above a height of approximately 1000 km the magnetic field starts
to become uniform. Waves are excited in this medium by means of motions
at the lower boundary. We focus on transverse driving, which generates
both fast and slow waves within a flux tube and acoustic waves at
the interface of the tube and the ambient medium. The acoustic waves
at the interface are due to compression of the gas on one side of
the flux tube and expansion on the other. These longitudinal waves
are guided upward along field lines at the two sides of the flux
tube, and their amplitude increases with height due to the density
stratification. Being acoustic in nature, they produce a compression
and significant shock heating of the plasma in the chromospheric part of
the flux tube. For impulsive excitation with a time constant of 120 s,
we find that a dominant feature of our simulations is the creation of
vortical motions that propagate upward. We have identified an efficient
mechanism for the generation of acoustic waves at the tube edge, which
is a consequence of the sharp interface of the flux concentration. We
examine some broad implications of our results.
Title: Dynamics of the Magnetic Network on the Sun
Authors: Hasan, S.; van Ballegoiijen, A.; Kalkofen, W.; Steiner, O.
Bibcode: 2005AGUSMSH13C..08H
Altcode:
Observations have revealed the presence of a rich spectrum of waves
with different periods in regions of the solar atmosphere called the
"magnetic network" that are dominated by strong magnetic fields. This
network is believed to be heated by dissipation of magnetohydrodynamic
(MHD) waves, but the MHD processes involved in wave generation,
propagation and dissipation are poorly understood. In this work we
attempt to identify some of the processes that occur in the network and
which contribute to its dynamics and heating. We model the network as
consisting of individual magnetic elements or flux tubes, rooted in
intergranular lanes, with a typical horizontal size of 100 km. They
expand upward and merge with their neighbors at a height of about 600
km. Above this height the magnetic field becomes uniform. An equilibrium
configuration based on the above model is constructed by solving the
magnetostatic equations in 2-D. Waves are generated in this medium
by means of motions at the lower boundary. We focus on transverse
driving which generates fast waves within the flux tubes and acoustic
waves at the interface of the tubes and the field-free medium, but not
otherwise in the field-free gas. The acoustic waves at the interface
are due to compression of the gas on one side of the flux tube and
expansion on the other. These waves travel upward along the two sides
of the (2D) flux tube and enter it, where they become longitudinal
waves. For impulsive excitation with a time constant of 120 s, we
find that a dominant feature is the creation of vortical motions that
propagate upwards. We have identified a new and efficient mechanism
for the generation of longitudinal waves and shock formation in the
chromosphere. We examine the observational implications of our results
and their broad applications to chromospheric heating and activity.
Title: Connecting solar radiance variability to the solar dynamo
with the virial theorem
Authors: Steiner, O.; Ferriz-Mas, A.
Bibcode: 2005AN....326..190S
Altcode:
The variability of solar radiance over a solar cycle is thought to
result from a delicate balance between the radiative deficit of sunspots
and the extra contribution of plage and network regions. Although the
net effect is tiny, it must imply structural and thermal changes in
the Sun or in partial layers of it as an unavoidable consequence of
the virial theorem. Using the virial theorem for continua--including
the magnetic field--it can be shown how solar radiance variability
might be connected to a deeply seated flux-tube dynamo and how this
connection is established on a hydrodynamical time scale.
Title: Radiative properties of magnetic elements. II. Center to limb
variation of the appearance of photospheric faculae
Authors: Steiner, O.
Bibcode: 2005A&A...430..691S
Altcode:
For the understanding of more complex numerical simulation results,
a basic facular model is created consisting of a magnetic flux sheet
embedded in a plane parallel atmosphere. The atmosphere within the
flux sheet is similar to the external one but shifted in the downward
direction to result in a Wilson depression of 150 km. It is shown that
the horizontal spatial extension of the contrast enhancement produced
by this model increases from center to limb from a few tenths to up
to 1 arcsec as a consequence of enhanced radiation from the limbward
surface outside (behind) the magnetic flux concentration. For a
conceivable explanation of this radiative effect it is noted that
a plasma parcel on the solar surface sideways of the flux sheet
``sees'' a more transparent sky in the direction towards the flux
sheet compared to a direction away from it because of the rarefied
atmosphere within the flux sheet. This facular model also produces a
dark lane at the disk-center side (in front) of the flux sheet even
though no flow is present. It it is due to the deep layers of the
flux sheet that have a lower temperature gradient and are cooler
than the surrounding atmosphere at equal geometrical depth. This
implies that limb observations offer a glimpse of the ``cool bottom''
of magnetic elements. The center-to-limb variation of the size and
the dark-lane effect derived from this basic model is recovered in
a self-consistent, two-dimensional non-stationary simulation of a
magnetic flux concentration. These findings are in excellent agreement
with and provide an interpretation of recent observations of faculae
at very high resolution by Lites et al. (2004, \solphys, 221, 65).
Title: The deep roots of solar radiance variability .
Authors: Steiner, O.; Ferriz-Mas, A.
Bibcode: 2005MmSAI..76..789S
Altcode:
The variability of solar radiance over a solar cycle is thought to
result from a delicate balance between the radiative deficit of sunspots
and the extra contribution of plage and network regions. Although the
net effect is tiny, it implies structural and thermal changes in the
Sun or in partial layers of it as an unavoidable consequence of the
virial theorem. Using the virial theorem for continua--including the
magnetic field--it can be shown how solar radiance variability might be
connected to a deeply seated flux-tube dynamo and how this connection
is established on a hydrodynamical time scale.
Title: Indications of shock waves in the solar photosphere
Authors: Rybák, J.; Wöhl, H.; Kučera, A.; Hanslmeier, A.;
Steiner, O.
Bibcode: 2004A&A...420.1141R
Altcode:
High resolution observations of solar granulation near the solar limb
are used in a search for hydrodynamic shocks caused by an abrupt
braking of the fast (probably supersonic) horizontal flow of the
granular plasma towards the intergranular lane. Shock signatures in
the spectral line of Fe II 6456.38 Åof one particular observed shock
event are investigated in detail. Evolution, amplitude, and spatial
relation of the spectral line characteristics of the shock event are in
agreement with predictions from numerical simulations for such shock
phenomena in the solar photosphere. The dimensions and amplitudes of
the observed shock signatures are comparable to predicted values when
seeing and instrumental effects as well as a possible obliqueness of
the shock front with respect to the observer's line-of-sight are taken
into account. The temporal evolution of such an event is observed for
the first time. The stable and declining phase of the event were studied
for a time period of almost 2 min. A particular relationship was found
between the shock event and a nearby G-band bright point located 2''
from the shock event. It is suggestive that the observed shock is a
causal consequence of the magnetic flux concentration, traced by the
G-band bright point. Such a type of shock can appear outside the flux
concentrations as a consequence of a rapid flux-tube motion.
Title: Connecting solar radiance variability to the solar dynamo
with the virial theorem
Authors: Steiner, Oskar
Bibcode: 2004IAUS..223...77S
Altcode: 2005IAUS..223...77S
The variability of solar radiance over a solar cycle is thought to
be a delicate balance between the radiative deficit of sunspots and
the extra contribution of plage and network regions. Although the net
effect is tiny, it must imply structural and thermal changes in the
Sun or in partial layers of it as an unavoidable consequence of the
virial theorem. Using the virial theorem for continua including the
magnetic field it is shown, how solar radiance variability might be
connected to a deeply seated flux-tube dynamo and how this connection
is established on a hydrodynamical time-scale.
Title: Understanding facular granules and lanes
Authors: Steiner, Oskar
Bibcode: 2004IAUS..223..299S
Altcode: 2005IAUS..223..299S
Recent high resolution observations by Lites et al. (2004) show
details of facular granules at 0.12^{primeprime}, including dark
facular lanes. For an interpretation of these data a basic facular
model is constructed, consisting of a magnetic flux sheet embedded
in a plane parallel atmosphere. While the maximum contrast originates
from the "hot wall" of the flux-sheet depression, the model explains
the wide brightening limbward of the facular magnetic field as due
to a radiative transfer effect caused by the reduced opacity of the
rarefied flux-sheet atmosphere. This model produces a dark, narrow
lane centerward of the facular granule even in the absence of granular
flow as a consequence of the cool deep layers of the magnetic flux
sheet. These results carry over to a self-consistent simulation of
a flux concentration in dynamic interaction with convective motion,
where the dark lane deepens and broadens.
Title: Distribution of magnetic flux density at the solar
surface. Formulation and results from simulations
Authors: Steiner, O.
Bibcode: 2003A&A...406.1083S
Altcode:
A formal description of the distribution of magnetic flux density
in a quiet Sun region supplemented by an example and an application
is presented. We define a flux-based probability density, which
is useful to reveal the presence of any strong-field component in
the region. The corresponding flux-based probability distribution
gives the fraction of the total absolute magnetic flux with a given
field-strength limit. Application to the simulations of convective field
intensification of Grossmann-Doerth et al. (1998) shows, that, depending
on the strength of the initial homogeneous vertical field, 1-50% of the
total magnetic flux within the computational domain of 3 arcsec width
is concentrated into flux fibrils with a flux density exceeding 0.1
T. It is shown that a low efficiency of the flux-concentration process
is compatible with new observations that suggest a large fraction of
the surface magnetic field to have a flux density below 0.1 T.
Title: Distribution of the Magnetic Flux Density at the Solar Surface
Authors: Steiner, Oskar
Bibcode: 2003ANS...324R..31S
Altcode: 2003ANS...324..D07S
No abstract at ADS
Title: Solar Radiance Variability as a Direct Consequence of the
Flux-tube Dynamo
Authors: Steiner, Oskar
Bibcode: 2003ANS...324..106S
Altcode: 2003ANS...324..P12S
No abstract at ADS
Title: Convergence of a Solenoidal Discrete Rot-operator
Authors: Steiner, Oskar
Bibcode: 2003ANS...324...75S
Altcode: 2003ANS...324..I20S
No abstract at ADS
Title: The contrast of magnetic elements across the solar spectrum
Authors: Steiner, O.; Hauschildt, P. H.; Bruls, J.
Bibcode: 2003AN....324..398S
Altcode:
No abstract at ADS
Title: Multi-Grid Radiative Transfer Revisited
Authors: Steiner, O.
Bibcode: 2003ASPC..288...83S
Altcode: 2003sam..conf...83S
Multi-grid radiation transfer is an efficient method for solving
a variety of radiation transfer problems, in particular problems
of multiple spatial dimensions on scalar computers. This advantage
is lost on massively parallel machines in which the computational
grid can be directly mapped onto the processor array. Contrary to
operator splitting methods, the convergence rate of the multi-grid
method does not deteriorate with increasing spatial resolution of the
computational grid. It is therefore well suited for high resolution
problems, while performance at low resolution is not better than the
best operator splitting methods. There exists a considerable
variety of basic multi-grid algorithms, which leave ample room for
improvements of the few multi-grid radiation-transfer calculations
that have been carried out so far. This poster is available under Poster
Title: Large-Scale Flow in Two-Dimensional Simulation of Solar
Convection
Authors: Steiner, O.
Bibcode: 2003IAUS..210P.C11S
Altcode:
No abstract at ADS
Title: Spectral signature of magnetic flux tubes in sunspot penumbrae
Authors: Müller, D. A. N.; Schlichenmaier, R.; Steiner, O.; Stix, M.
Bibcode: 2002A&A...393..305M
Altcode:
We study the polarization of spectral lines in the penumbra by
integrating the radiative transfer equation of polarized light for a
three-dimensional model atmosphere of a sunspot. In this model, the
Evershed flow is confined to magnetic flux tubes which are embedded
in a static background atmosphere, in accordance with the moving tube
model of Schlichenmaier et al. (\cite{Schlichenmaier1998apjl},b). The
gradients and/or discontinuities in the fluid velocity and the
magnetic field at the flux tube boundaries give rise to asymmetric
Stokes profiles. We concentrate on the Stokes-V profiles and study the
net circular polarization (NCP) of two photospheric spectral lines of
neutral iron, Fe I 630.25 nm and Fe I 1564.8 nm. The different behavior
of these two lines, which are exemplary for atomic spectral lines with
a large Landé factor and significantly different wavelength, is based
on the difference in excitation potential of the corresponding atomic
transitions and the fact that the wavelength dependence of the Doppler
shift is linear, while that of the Zeeman splitting is quadratic. We
find that the azimuthal variation of the NCP, N(psi, is a predominantly
antisymmetric function of psi with respect to the line connecting
disk center and spot center (line-of-symmetry) for the infrared line
of Fe I 1564.8 nm, while the variation is predominantly symmetric for
Fe I 630.25 nm. We show that the antisymmetric variation is caused
by anomalous dispersion (Faraday pulsation) and the discontinuity
in the azimuthal angle of the magnetic field, which is due to the
relative inclination between flux tube and background field. We
then compute synthetic NCP maps of a sunspot and compare them with
observational results. Finally, the center-to-limb variation of the
NCP, N(theta ), of these spectral lines is investigated. We show
that the location of the zero-crossing point of N(theta ) on the
center side of the line-of-symmetry represents a diagnostic tool to
determine the inclination angle of the Evershed flow: A vanishing NCP
on the center-side of the line-of-symmetry is an indirect evidence of
downflows in the penumbra.
Title: Net circular polarization of sunspot penumbrae - symmetry
breaking by anomalous dispersion
Authors: Müller, D. A. N.; Schlichenmaier, R.; Steiner, O.; Stix, M.
Bibcode: 2002ESASP.508..141M
Altcode: 2002soho...11..141M
We examine the polarization of spectral lines in the penumbra of
sunspots by solving the radiative transfer equation of polarized
light for a three-dimensional axially symmetric model atmosphere
of a sunspot. The Evershed flow is confined to horizontal magnetic
flux tubes obtained from MHD calculations. These are embedded in
an inclined background magnetic field. In this work, we concentrate
on the Stokes-V profiles and examine the net circular polarization
(NCP), N = ∫V(λ)dλ, of two photospheric spectral lines of neutral
iron, Fe I 630.25 nm and Fe I 1564.8 nm. Analyzing spectra at a fixed
distance from the spot center, we find that the azimuthal variation
of N, N(ψ), is an antisymmetric function of ψ w.r.t. to the line
connecting disk center and spot center for Fe I 1564.8 nm, while
the variation is predominantly symmetric for Fe I 630.25 nm. We show
that the antisymmetric variation is caused by anomalous dispersion
(rotation of the polarization vector in a magnetized plasma). The
different inclination angles lead to a discontinuity in the azimuth
of the magnetic field along the line-of-sight. We show that this
discontinuity together with the effect of anomalous dispersion produced
an antisymmetric component in N(ψ) which outweighs the symmetric
component from the discontinuity for Fe I 1564.8 nm, while it is
negligible for Fe I 630.25 nm. We finally compute synthetic NCP maps of
a sunspot which offer an explanation for recent observational results.
Title: Net circular polarization of sunspot penumbrae. Symmetry
breaking through anomalous dispersion
Authors: Schlichenmaier, R.; Müller, D. A. N.; Steiner, O.; Stix, M.
Bibcode: 2002A&A...381L..77S
Altcode:
The net circular polarization, N, is used as a measure for the
asymmetry of Stokes-V profiles: Nequiv int V(lambda ) d lambda ,
integrated over an absorption line. Exemplary for Fe I 630.2 nm and
Fe I 1564.8 nm, we synthesize penumbral V-profiles that stem from a
model atmosphere in which the Evershed flow is confined to horizontal
flux tubes which are embedded in a magnetic field that has the same
magnetic field strength as the flow channel, but is less inclined
w.r.t. the surface normal. At the two points where a line-of-sight
enters and leaves the flow channel, discontinuities in the inclination,
gamma , the velocity v, and the azimuth, phi , of the magnetic field
vector w.r.t. the plane perpendicular to the line-of-sight produce
V-asymmetries. Assuming an axially symmetric penumbra, we investigate
the azimuthal dependence N(psi ) for a mid-penumbral radius. We find:
(1) Without including anomalous dispersion, N(psi ) is symmetric
w.r.t. the line that connects disk center to the center of the spot. (2)
Including anomalous dispersion, this symmetry is broken. We demonstrate
that this is due to the difference in azimuth, triangle phi (psi ),
between the flow channel and the background that varies along the
penumbral circle. For Fe I 630.2 nm this effect is found to be of
minor relevance leading to essentially symmetric N-maps, whereas strong
asymmetries are predicted for Fe I 1564.8 nm. Our results provide an
explanation for recent observational findings.
Title: Time-slice diagrams of solar granulation
Authors: Müller, D. A. N.; Steiner, O.; Schlichenmaier, R.; Brandt,
P. N.
Bibcode: 2001SoPh..203..211M
Altcode:
From a series of 1400 white-light images of solar granulation spanning
a time period of 8.2 hours, skeletal plots of time-slice diagrams are
derived showing intergranular lane positions as a function of time. The
diagrams permit to automatically track, classify, and relate 42 186
granules. Recurrently fragmenting granules are found that survive
by means of their descendants for more than 3 hours. Such long-lived
active granules tend to have a mean spatial distance along the slice
of about 10 Mm. This distance decreases with decreasing minimal
required lifetime. Since active granules are expected to generate a
steadily divergent flow over a long period of time, it is suggested
to identify them as a source of the mesogranular flow. Deficiencies
of the time-slice analysis are discussed. The relative frequency of
different types of granules and the granule decay time as derived
from the time-slice diagrams are compared with corresponding results
of previous works.
Title: Radiative properties of magnetic elements. I. Why are vec
G-band bright points bright?
Authors: Steiner, O.; Hauschildt, P. H.; Bruls, J.
Bibcode: 2001A&A...372L..13S
Altcode:
Photospheric magnetic elements are most conspicuously visible in
high-resolution G-band filtergrams. We show that their enhanced
contrast in the G-band is due to a reduction of the CH abundance by
dissociation in the deep photospheric layers of the flux tube, where
it is hotter than in the surrounding atmosphere. As a consequence, the
CH-lines weaken, allowing more of the continuum to ``shine'' through
the forest of G-band CH-lines. We suggest that other molecular bands
or atomic lines may exhibit a similar behaviour.
Title: Understanding Small Solar Magnetic Structures: Comparing
Numerical Simulations to Observations
Authors: Leka, K. D.; Steiner, O.
Bibcode: 2001ApJ...552..354L
Altcode:
We present direct comparisons of small magnetic structures observed
in the solar photosphere with the results from numerical simulations
of those structures. We compare diagnostic signatures derived
from emergent Stokes polarization spectra from both the observed
and model atmospheres, the former recorded with the National Solar
Observatory/High Altitude Observatory Advanced Stokes Polarimeter,
the latter from a fully dynamic MHD simulation of a magnetic flux
sheet in a convective atmosphere. We focus on the asymmetries in the
Stokes V spectra and find, first and foremost, that the asymmetries
from the observed Stokes I and V in and around solar pores and azimuth
centers (ACs) are quantitatively comparable to those derived from
the simulation. We also find enhanced Stokes V asymmetry on the
periphery of pores and ACs. We interpret this as a consequence of
strong downdrafts in the surroundings of these magnetic structures,
accompanied by the expansion of the magnetic field lines with height
above these field-free downdrafts (the ``canopy effect''). The
magnetic canopy can be present whether or not there is a continuum
signature (i.e., a dark ``pore''). Not surprisingly, the patterns
and magnitudes of asymmetries scale with the size of the magnetic
element. In the interior of the pores and ACs, we find evidence for
mixed up- and downflows, with little spatial correlation between the
zero-crossing shift of the V profile and the V amplitude. Finally,
we report on asymmetries observed in the linear polarization
Plin(λ)=[Q(λ)2+U(λ)2]1/2,
finding further support for the presence of the magnetic canopy
from those diagnostics. We additionally present expectations for
spectropolarimetric observations at significantly higher spatial
resolution.
Title: Why are G-Band Bright Points Bright?
Authors: Steiner, O.; Bruls, J.; Hauschildt, P. H.
Bibcode: 2001ASPC..236..453S
Altcode: 2001aspt.conf..453S
No abstract at ADS
Title: The Formation of Asymmetric Stokes V Profiles in the Presence
of a Magnetopause
Authors: Steiner, O.
Bibcode: 2001ASPC..236..587S
Altcode: 2001aspt.conf..587S
No abstract at ADS
Title: Chromosphere: Magnetic Canopy
Authors: Steiner, O.
Bibcode: 2000eaa..bookE2264S
Altcode:
The magnetic canopy is a layer of magnetic field which is directed
parallel to the solar surface and located in the low CHROMOSPHERE,
overlying a field-free region of the SOLAR PHOTOSPHERE. It has a field
strength of the order of 0.01 T and covers a large area fraction of
the solar surface. The magnetic canopy can be compared with the canopy
of the rain forest: the tree-trunks correspond to the ...
Title: The formation of asymmetric Stokes V profiles in the presence
of a magnetopause
Authors: Steiner, Oskar
Bibcode: 2000SoPh..196..245S
Altcode:
A magnetopause that separates two regimes of different flow, additional
to the separation of a magnetic field from a field-free plasma, gives
rise to the formation of asymmetric Stokes profiles. Using a simple
two-layer model atmosphere, where one layer comprises a magnetic field,
the other being field-free, it is shown by analytical derivation that
a wide variety of Stokes V profiles can be produced, having amplitude
asymmetries δa in the range −∞≤δa≤∞. These include
two-humped V profiles, which have two lobes of equal sign. For the
most simple models, the asymmetry depends on the ratio of continuum
intensity to the Planck radiation intensity of the magnetic layer
at the wavelength of the spectral line under consideration, and
on the line depth. Two-humped profiles (|δa|>1) require the
temperature of the magnetic layer to surpass the temperature of the
line-core forming region, implying a temperature inversion, so that
the V profile is partially in emission. The confrontation of this
formation scenario with properties of observed one-lobe profiles of
quiet-Sun network regions is inconclusive due to insufficient spatial
resolution and lack of a sufficient sample of simultaneously recorded
Stokes spectral lines of varying line depths. It seems, however, to be
in good agreement with the observed frequent occurrence of abnormal
V profiles of the very strong Na i D2 and D2
spectral line. A possible observational verification for the present
formation scenario of abnormal Stokes V profiles and a novel method
of Stokes inversion are discussed.
Title: Strong Stokes V asymmetries of photospheric spectral lines:
What can they tell us about the magnetic field structure?
Authors: Grossmann-Doerth, U.; Schüssler, M.; Sigwarth, M.;
Steiner, O.
Bibcode: 2000A&A...357..351G
Altcode:
In an attempt to identify the mechanism responsible for the extremely
asymmetric Stokes V profiles which were recently observed we analyzed
several simple atmospheric configurations with separated layers of mass
flow and magnetic field. We found that under appropriate conditions
the models are capable of producing the observed one-lobe profiles.
Title: Flux Tube Dynamics
Authors: Steiner, O.
Bibcode: 1999ASPC..184...38S
Altcode:
We propose that the observed fragmentation of magnetic elements is
due to their inherent liability to the interchange instability. The
convective collapse and numerical simulations of the formation of
intense magnetic flux sheets including spectral signatures of the
formation process are discussed. We suggest that the mean downflow
measured in magnetic elements is due to a fraction of an ensemble of
magnetic elements being in the formation phase at any given time. It is
shown that observed Stokes V profiles of magnetic elements are actua
lly an average of spatially strongly varying profiles with a delicate
balance of positive and negative contributions to the area asymmetry.
Title: Understanding Small Solar Magnetic Elements: Comparing Models
and Observations
Authors: Leka, K. D.; Steiner, O.; Grossmann-Doerth, U.
Bibcode: 1999AAS...194.5507L
Altcode: 1999BAAS...31R.911L
We perform direct comparisons of high-resolution spectropolarimetric
observations with a full MHD model of the magnetized solar
atmosphere. In this manner we investigate the evolution and dynamics of
small magnetic elements by fully utilizing the diagnostics available
with Stokes spectropolarimetry, both computed and observed. The model
is a 2-D time-dependent numerical simulation of a small (~ 600 km
diameter) magnetic feature embedded in a non-magnetized atmosphere
(Steiner et al., 1998). At select time-steps, synthetic emergent
Stokes I and V profiles are computed using a polarized radiation
transfer code. The data consist of Stokes I and V spectra from the
Advanced Stokes Polarimeter for seventeen small magnetic elements
located near disk-center. For both the observed and computed Stokes
spectra, diagnostics are computed including the emergent continuum
intensity, V-crossing shift, and amplitude and area asymmetries of
the V-profile. We find that it is possible to differentiate between
salient processes occurring in the magnetic atmosphere (strong flows,
gradients, etc.) by their spectropolarimetric signature; from this,
we determine the dominant processes present in the observed magnetic
structures. The results are extremely encouraging. We find good
qualitative agreement between the amplitude and area asymmetries
and their spatial variation. Quantitatively, the agreement is
surprisingly good in many cases. While limitations exist for both
the model and observations, this stringent test allows us to comment
on the dynamics and possible evolutionary differences present in the
observed magnetic features. This work is funded in part by NSF grant
ATM-9710782. Reference: - Steiner, O., Grossmann-Doerth, U., Knolker,
M., Schussler, M.: 1998, ApJ 495, 468
Title: Small Scale Magnetic Flux Tubes in the Photosphere: A
Simulation Perspective
Authors: Steiner, O.
Bibcode: 1999ASPC..183...17S
Altcode: 1999hrsp.conf...17S
No abstract at ADS
Title: 2D radiative equilibrium models of magnetic flux tubes
Authors: Hasan, S. S.; Kalkofen, W.; Steiner, O.
Bibcode: 1999ASSL..243..409H
Altcode: 1999sopo.conf..409H
No abstract at ADS
Title: The formation of extremely asymmetric Stokes V profiles
Authors: Steiner, O.; Grossmann-Doerth, U.; Schüssler, M.; Sigwarth,
M.
Bibcode: 1999AGAb...15R..10S
Altcode: 1999AGM....15..A11S
Recent polarimetric observations at high spatial resolution (<
1 arcsec) and with high polarimetric accuracy (noise of Stokes V/I_c
< 3 cdot 10^{-4}) have revealed that about 10% of all Stokes V
profiles of a quiet Sun region have an abnormal shape that strongly
deviates from the more common, nearly antisymmetric profiles. 35% of the
abnormal Stokes V profiles are of one-wing type, where the profile shows
essentially one lobe only. We show, that a strongly asymmetric Stokes
V profile can be obtained when the atmosphere, in which the profile
is formed, is divided into two or more layers of different magnetic,
flow, and thermal properties. A simple configuration of that kind
(sometimes referred to as magnetic canopy) consists of a layer of
plasma at rest with a magnetic field component parallel to the line
of sight, located on top of a field-free layer with downdraft. The
Stokes V asymmetry in this case sensitively depends on the position
of the layers interface and may assume values up to about 80%. An
asymmetry of nearly 100% (corresponding to a one-wing profile) is
obtained when the plasma below the magnetic canopy is relatively cool,
giving rise to the formation of a strong, redshifted spectral line in
the field-free plasma beneath the canopy. If the core of this line is
formed at lower temperature than the temperature of the canopy layer,
the red wing of the resulting Stokes V profile is driven into emission,
leading to a ``pathological'' V profile.
Title: Meso and supergranulation in two-dimensional simulation of
solar convection
Authors: Steiner, O.
Bibcode: 1999AGAb...15...92S
Altcode: 1999AGM....15..P10S
A two-dimensional hydrodynamic simulation of a thin surface layer
of the solar convection zone, encompassing a horizontal span of 150
Mm and a depth of 1 Mm, is carried out. An additional 0.5 Mm height
includes the photospheric layer. Radiation transfer using the OPAL
Rosseland mean opacity is taken into account as well as Hydrogen
ionization. The simulation shows granule evolution of the well known
dichotomous character: granules either dissolve by shrinking or they
fragment into smaller granules. The maximum horizontal granule size
in the simulation is about 3.5 Mm. The Fourier decomposition of
the horizontal velocity near the solar surface (tau_c = 1) shows
a local maximum in amplitude at around 5-7 Mm, which we identify
with mesogranular flow. The maximum amplitude is found at 50 Mm,
corresponding to supergranular flow. Cork tracks visualize these
findings. The occurrence of supergranules in this shallow simulation
domain supports recent results from local helioseismology, suggesting
that supergranulation pertains to a thin surface layer only. We discuss
possible interpretation of the simulation results.
Title: Convective intensification of solar surface magnetic fields:
results of numerical experiments
Authors: Grossmann-Doerth, U.; Schuessler, M.; Steiner, O.
Bibcode: 1998A&A...337..928G
Altcode:
The concentration of magnetic flux by convective flows in the solar
surface layers is studied by means of two-dimensional numerical
simulations with radiative transfer. We follow the evolution of an
initially homogeneous, vertical magnetic field, starting from an
evolved state of simulated solar granulation. The results of three
simulation runs with initial field strengths, B_0, of 100 G, 200 G,
and 400 G, respectively, are shown. In all cases, horizontal convective
flows rapidly sweep magnetic flux into the intergranular downflow
channels. The field is further amplified up to kilogauss values by
partial evacuation due to a strongly accelerated downflow within
the magnetic structure. The value of the field strength reached at
a given depth and the size of the flux concentrations grows with the
initial field strength (i.e., the amount of magnetic flux within the
computional box). In the case of B_0 = 400 G, the downflow within the
flux concentration becomes so strong that it `bounces' off the high
density plasma in the deeper layers; the resulting upflow leads to a
strong, upward moving shock and to the dispersal of the flux sheet after
a lifetime of about 200 s. In the cases with less magnetic flux (B_0 100
G, 200 G), the downflow is less vigorous and the flux concentrations
persist to the end of the simulation (about 5 minutes). Radiation
diagnostics in the continuum and in spectral lines predicts observable
signatures of the intensification process. The accelerated downflow
leads to a conspicuous Doppler shift and a negative area asymmetry of
Stokes V-profiles of spectral lines, while the intensification of the
magnetic field may be detectable through the `magnetic line ratio'
method in the visible and by direct Zeeman splitting of magnetically
sensitive lines in the infrared wavelength ranges.
Title: On the reliability of Stokes diagnostics of magnetic elements
away from solar disc centre
Authors: Solanki, S. K.; Steiner, O.; Buente, M.; Murphy, G.; Ploner,
S. R. O.
Bibcode: 1998A&A...333..721S
Altcode:
{The spectropolarimetric signature of models of small-scale magnetic
features is well understood at the centre of the solar disc, but
has been little studied near the solar limb, mainly because the
detailed geometry of the flux tubes must then be taken into account
in a realistic analysis. We present multi-ray calculations of Stokes
profiles through arrays of 2-D magnetohydrostatic models of small flux
tubes. We compare the Stokes profile shapes and Stokes based diagnostics
(Q to V ratio, V amplitude, magnetic line ratio, centre of gravity
wavelengths, etc.) resulting from plane-parallel and 2-D flux-tube
models at different limb distances for two lines in the visible and
an infrared H-band line. For the visible lines around 5250 Angstroms
all the diagnostics we have studied, with the exception of the Q and U
to V ratio, are significantly affected by the finite size of the flux
tubes and the passage of the rays through non-magnetic material near
the limb. We show that magnetic filling factors and the global magnetic
flux may be underestimated using the usual calibration techniques. In
addition, near the solar limb the magnetic line ratio can move into
a regime that is forbidden according to simpler models. The spatially
averaged longitudinal field derived from the centre-of-gravity method
also becomes strongly model dependent. The thermal structure of the
non-magnetic atmosphere appears to play a particularly important role
in determining the values of these diagnostic parameters. The infrared
line at 1.5648 mu m, on the one hand, reacts little to the external
atmosphere and provides superior diagnostics of both the magnetic field
strength and filling factor. On the other hand, for a flux tube with
a sharp boundary this line shows little dependence on the flux-tube
diameter, in contrast to the earlier findings of Zayer et al. (1989). }
Title: Dynamical Interaction of Solar Magnetic Elements and Granular
Convection: Results of a Numerical Simulation
Authors: Steiner, O.; Grossmann-Doerth, U.; Knölker, M.; Schüssler,
M.
Bibcode: 1998ApJ...495..468S
Altcode:
Nonstationary convection in the solar photosphere and its
interaction with photospheric magnetic structures (flux sheets in
intergranular lanes) have been simulated using a numerical code
for two-dimensional MHD with radiative energy transfer. Dynamical
phenomena are identified in the simulations, which may contribute to
chromospheric and coronal heating. Among these are the bending and
horizontal displacement of a flux sheet by convective flows and the
excitation and propagation of shock waves both within and outside the
magnetic structure. Observational signatures of these phenomena are
derived from calculated Stokes profiles of Zeeman-sensitive spectral
lines. We suggest that the extended red wings of the observed Stokes
V profiles are due to downward coacceleration of magnetized material
in a turbulent boundary layer between the flux sheet and the strong
external downflow. Upward-propagating shocks in magnetic structures
should be detectable if a time resolution of about 10 s is achieved,
together with a spatial resolution that allows one to isolate individual
magnetic structures. Determination of the complicated internal dynamics
of magnetic elements requires observations with a spatial resolution
better than 100 km in the solar photosphere.
Title: Computational methods for astrophysical fluid flow
Authors: Steiner, O.; Gautschy, A.
Bibcode: 1998cmaf.conf.....S
Altcode:
No abstract at ADS
Title: Numerical simulations of magnetic flux sheets.
Authors: Steiner, O.; Knölker, M.; Schüssler, M.
Bibcode: 1997smf..conf...31S
Altcode:
Non-stationary convection in the solar photosphere and its interaction
with photospheric magnetic structures (flux sheets in intergranular
lanes) has been simulated using a numerical code for two-dimensional
MHD with radiative transfer. Dynamical phenomena, which may contribute
to chromospheric and coronal heating, like bending and horizontal
displacement of a flux sheet by convective flows as well as the
excitation and propagation of shock waves within and outside the
magnetic structure are found. Observational signature of transversal
displacement and shocks are derived. It is shown that upward propagating
shocks in magnetic structures should be detectable. The evolution of
an initially homogeneous vertical magnetic field is followed, starting
from an evolved state of a two-dimensional numerical simulation of
solar granulation.
Title: Convective intensification of magnetic fields at the solar
surface.
Authors: Steiner, O.
Bibcode: 1996NAWG.1996..185S
Altcode:
The evolution of an initially homogeneous vertical magnetic field is
followed, starting from an evolved state of a 2D numerical simulation of
solar granulation. The field is concentrated in the intergranular lane
by the combined action of flux expulsion and field intensification by
the downflow. A flux sheet forms with a maximum field strength of 1600
G at the solar surface, a value well above the equipartition field
strength of about 700 G. The rapid downflow within the flux sheet
bounces off the high density plasma in the deeper layers of the flux
sheet; the resulting upflow leads to a strong, upward moving shock and
to the destruction of the flux sheet after a lifetime of about 200 s.
Title: Polarized Radiation Diagnostics of Magnetohydrodynamic Models
of the Solar Atmosphere
Authors: Steiner, O.; Grossmann-Doerth, U.; Schüssler, M.; Knölker,
M.
Bibcode: 1996SoPh..164..223S
Altcode:
Solar magnetic elements and their dynamical interaction with
the convective surface layers of the Sun are numerically
simulated. Radiation transfer in the photosphere is taken into
account. A simulation run over 18.5 minutes real time shows that the
granular flow is capable of moving and bending a magnetic flux sheet
(the magnetic element). At times it becomes inclined by up to 30°
with respect to the vertical around the level τ5000 =
1 and it moves horizontally with a maximal velocity of 4 km/s. Shock
waves form outside and within the magnetic flux sheet. The latter
cause a distinctive signature in a time series of synthetic Stokes
V-profiles. Such shock events occur with a mean frequency of about
2.5 minutes. A time resolution of at least 10 seconds in Stokes V
recordings is needed to reveal an individual shock event by observation.
Title: Convective intensification of photospheric magnetic fields.
Authors: Schüssler, M.; Grossmann-Doerth, U.; Steiner, O.; Knölker,
M.
Bibcode: 1996AGAb...12...89S
Altcode:
No abstract at ADS
Title: Simulation of the Interaction of Convective Flow with Magnetic
Elements in the Solar Atmosphere.
Authors: Steiner, O.; Grossmann-Doerth, U.; Knoelker, M.; Schuessler,
M.
Bibcode: 1995RvMA....8...81S
Altcode:
No abstract at ADS
Title: Dynamic interaction of convection with magnetic flux sheets:
first results of a new MHD code
Authors: Steiner, O.; Knölker, M.; Schüssler, M.
Bibcode: 1994ASIC..433..441S
Altcode:
No abstract at ADS
Title: Flux Tube Shredding Its Infrared Signature
Authors: Bunte, M.; Steiner, O.; Solanki, S. K.; Pizzo, V. J.
Bibcode: 1994IAUS..154..459B
Altcode:
No abstract at ADS
Title: Theoretical Models of Magnetic Flux Tubes: Structure and
Dynamics
Authors: Steiner, O.
Bibcode: 1994IAUS..154..407S
Altcode:
No abstract at ADS
Title: The upper photosphere and lower chromosphere of small-scale
magnetic features
Authors: Solanki, S. K.; Bruls, J. H. M. J.; Steiner, O.; Ayres, T.;
Livingston, W.; Uitenbroek, H.
Bibcode: 1994ASIC..433...91S
Altcode:
No abstract at ADS
Title: MHD simulations with adaptive mesh refinement
Authors: Steiner, O.; Grossmann-Doerth, U.; Knölker, M.; Schüssler,
M.
Bibcode: 1994smf..conf..282S
Altcode:
No abstract at ADS
Title: Simulation of magneto-convection with radiative transfer
Authors: Steiner, O.; Grossmann-Doerth, U.; Knölker, M.; Schüssler,
M.
Bibcode: 1994smf..conf..286S
Altcode:
No abstract at ADS
Title: Centre-to-limb variation of the Stokes V asymmetry in solar
magnetic flux tubes
Authors: Buente, M.; Solanki, S. K.; Steiner, O.
Bibcode: 1993A&A...268..736B
Altcode:
The center-to-limb variation (CLV) of synthetic Stokes V line
profiles of the spectral lines Fe 15250.22 A and Fe I 5083.35 A is
computed and compared with observations. The basic elements of the
hydromagnetic model used to calculate the Stokes V profiles consist
of a vertical cylindrical flux tube surrounded by a field-free plasma
in stationary motion with a downflow along the tube boundary and
an upflow further away from it. It is shown that an array of such
magnetic flux tubes reproduces the peculiar observed CLV of the Stokes
V asymmetry, in particular the sign reversal of the asymmetry near the
limb. We determine a minimum number of model components needed for the
reproduction of the Stokes V area asymmetry and study the influence of
various free model parameters on it. Satisfactory agreement between
synthetic and observed Stokes V profiles can only be achieved by
including a considerable amount of realism into the model such as
a granular temperature-velocity correlation. This suggests that the
basic picture of magnetic structures in the quiet solar network and
active region plages is correct.
Title: On the interchange instability of solar magnetic flux tubes. I
- The influence of magnetic tension and internal gas pressure
Authors: Buente, M.; Steiner, O.; Pizzo, V. J.
Bibcode: 1993A&A...268..299B
Altcode:
Small magnetic flux tubes at the solar surface are known to be
interchange-unstable. Previous studies of this instability have focused
on evacuated tubes in the thin tube approximation, thereby neglecting
magnetic tension forces and internal atmospheres. We have extended the
analysis to study the influence of these two missing components. An
internal atmosphere can either have a stabilizing or destabilizing
effect, depending upon the details of the prescribed gas pressure
stratification. Magnetic tension forces in general stabilize the tubes
by reducing the curvature of the boundary. For evacuated numerical
models we find that the instability can be suppressed by a critical
whirl flow of 2.2 km/s surrounding the tubes. This is considerably
lower than previously found for evacuated thin tube structures. For
tubes in temperature equilibrium with their surroundings this critical
whirl velocity is even lower, while temperature differences at equal
geometric height increase its value. The sharp interface between
the magnetic field and the whirl flow is due to the Kelvin-Helmholtz
instability (KHI). However, the boundary layer of finite width at the
tube surface might suppress the KHI, and hence make the whirl flow
mechanism feasible, if the tube is highly evacuated at the relevant
height of the atmosphere.
Title: CA II K Line Diagnostics of Two Dimensional Models of the
Solar Chromosphere
Authors: Solanki, S. K.; Buente, M.; Steiner, O.; Uitenbroek, H.
Bibcode: 1992ASPC...26..294S
Altcode: 1992csss....7..294S
No abstract at ADS
Title: MHD simulations with adaptive mesh refinement.
Authors: Steiner, O.; Grossmann-Doerth, U.; Knölker, M.; Schüssler,
M.
Bibcode: 1992AGAb....7..213S
Altcode:
No abstract at ADS
Title: Two-dimensional models of the solar chromosphere. I - The CA
II K line as a diagnostic: 1.5-D radiative transfer
Authors: Solanki, S. K.; Steiner, O.; Uitenbroeck, H.
Bibcode: 1991A&A...250..220S
Altcode:
Ca II K line calculations for a 5-level atom and partial frequency
redistribution along multiple rays passing through 2D MHD models of
magnetic flux tubes in the solar atmosphere are presented. Temperature
stratifications corresponding to various empirical chromospheric
models are incorporated into the models. The influence of a number
of model parameters on individual and spatially averaged line
profiles is considered in detail. The profiles are also compared
with observations. It is found that the present models can, at
least qualitatively, reproduce a wide variety of observations if the
temperature within the magnetic component lies between that of model
F of Vernazza et al. (1981) and of model VALP of Ayres et al. (1986)
and if the temperature in the nonmagnetic part of the atmosphere
corresponds to that of the COOLC model of Ayres et al. Observations
that are consistent with this model are presented.
Title: Fast solution of radiative transfer problems using a method
of multiple grids
Authors: Steiner, O.
Bibcode: 1991A&A...242..290S
Altcode:
A method of multiple grids of the second kind is used to solve two
basic radiative transfer problems: the spectral line transfer in a
homogeneous slab of two-level atoms and a two-dimensional LTE model
atmosphere in radiative equilibrium. The employed multigrid method
(W-cycle) alternately performs coarse-grid corrections and smoothing
steps, the latter in the form of an accelerated Lambda-iteration. The
new iterative method preserves all the advantages of approximate
operator techniques, but convergence is considerably improved. In all
the relevant examples of line transfer calculations made, the multigrid
method is faster by a factor that varies from 4 to more than 20 with
respect to the CPU time needed by an accelerated Lambda-iteration based
on a nonlocal approximate operator. The two-dimensional slab problem
even shows speedup factors as high as 40. Grid refinement generally
improves the convergence rate.
Title: Center-to-limb variation of the Stokes V asymmetry in solar
magnetic flux tubes.
Authors: Bünte, M.; Steiner, O.; Solanki, S. K.
Bibcode: 1991sopo.work..468B
Altcode:
The center-to-limb-variation of synthetic Stokes V line profiles
of the spectral line Fe I 5250.22 Å is presented and compared with
observations. These synthetic profiles are calculated using models
that contain the main features of the current basic pictures of small
scale magnetic fields on the Sun.
Title: Fast Solution of Radiative Transfer Problems with a Multi-Grid
Method
Authors: Steiner, O.
Bibcode: 1991ASIC..341...19S
Altcode: 1991sabc.conf...19S
No abstract at ADS
Title: How magnetic is the solar chromosphere?
Authors: Solanki, S. K.; Steiner, O.
Bibcode: 1990A&A...234..519S
Altcode:
The lower solar chromosphere is thought to have a very inhomogeneous
temperature structure, with hot magnetic flux tubes surrounded by cool
(T less than 4000 K) nonmagnetic gas (Ayres et al., 1986). The effect
of such a thermally bifurcated atmosphere on the structure of the
magnetic field in the chromosphere is considered. It is shown that
magnetic flux tubes expand much more rapidly if the atmosphere is
thermally bifurcated than if it is homogeneous. They merge and form a
magnetic canopy with an almost horizontal base which does not exceed
approximately 800-1000 km above tau = 1, irrespective of the magnetic
filling factor. Hence the middle and upper chromosphere is filled with
a magnetic field almost everywhere on the sun. The consequences of this
result both for the sun and for other late type stars are discussed.
Title: Structure of solar magnetic fluxtubes from the inversion of
Stokes spectra at disk center
Authors: Keller, C. U.; Steiner, O.; Stenflo, J. O.; Solanki, S. K.
Bibcode: 1990A&A...233..583K
Altcode:
The paper presents an inversion procedure that derives the temperature
stratification, the turbulent velocity, and the magnetic field strength
of the photospheric layers of small-scale magnetic fields from observed
Stokes V spectra and the continuum intensity. The inversion is based
on the determination of a small number of model flux parameters by a
nonlinear least squares fitting algorithm. The minimization of the sum
of the squared differences between observed and synthetic observables
makes it possible to determine the temperature stratification and the
magnetic field strength.
Title: A rapidly converging temperature correction procedure using
operator perturbation
Authors: Steiner, O.
Bibcode: 1990A&A...231..278S
Altcode:
The Lambda-operator perturbation technique, known in NLTE line transfer
calculations, is used for improving the temperature correction
procedure. The approach presented here makes use of the formal
solution to the radiative transfer equation and is capable of treating
line blanketing effects by considering line opacity distribution
functions. The method is shown to overcome all the problems related
to the classical Lambda-iteration.
Title: Model calculations of solar magnetic fluxtubes and radiative
transfer
Authors: Steiner, Oskar Urs
Bibcode: 1990PhDT.......358S
Altcode:
No abstract at ADS
Title: Model Calculations of the Photospheric Layers of Solar
Magnetic Fluxtubes
Authors: Steiner, O.; Stenflo, J. O.
Bibcode: 1990IAUS..138..181S
Altcode:
No abstract at ADS
Title: A parametric survey of model solar fluxtubes
Authors: Steiner, O.; Pizzo, V. J.
Bibcode: 1989A&A...211..447S
Altcode:
A parametric survey of numerically-generated, axially-symmetric
two-dimensional magnetostatic fluxtubes is conducted to quantify the
sensitivity of the properties of fluxtube models. It is found that
heating of a static fluxtube atmosphere leads to an increase in the
gas pressure above the level where the heating occurs, causing the
fluxtube to fan out more rapidly with height. The heating also leads
an increase in the opacity, causing to an upward shift in the surfaces
of constant optical depth in the tube. The magnitude of these effects
depends upon the temperature excess in the tube relative to that in
the surrounding photosphere and upon the level in the tube atmosphere
where heating is introduced. The consequences of these results for
fluxtube models are discussed.
Title: Numerical models for solar magnetic fluxtubes
Authors: Steiner, O.; Pneuman, G. W.; Stenflo, J. O.
Bibcode: 1986A&A...170..126S
Altcode:
A fully self-consistent method for constructing magnetostatic solutions
for magnetic fluxtubes is presented, and the method is applied to three
untwisted fluxtube configurations, two in which a sheet current exists
at the surface of the tube, and one in which the the internal magnetic
field varies continuously as a Gaussian. Convergence was obtained in
all three cases, though the rate of convergence and accuracy of the
solution were superior for the continuous field distribution case. For
twisted fluxtubes, a maximum twist compatible with equilibrium is
found for a given configuration, and the merging height is shown to
decrease slightly with increased twist and to increase if the twist
is concentrated more toward the surface of the tube.