gudiksen

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Author name code: gudiksen
ADS astronomy entries on 2022-09-14
author:"Gudiksen, Boris V."

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Title: Chromospheric emission from nanoflare heating in RADYN
simulations
Authors: Bakke, H.; Carlsson, M.; Rouppe van der Voort, L.; Gudiksen,
B. V.; Polito, V.; Testa, P.; De Pontieu, B.
2022A&A...659A.186B Altcode: 2022arXiv220111961B
Context. Heating signatures from small-scale magnetic reconnection
events in the solar atmosphere have proven to be difficult to
detect through observations. Numerical models that reproduce flaring
conditions are essential in understanding how nanoflares may act as a
heating mechanism of the corona. Aims: We study the effects of
non-thermal electrons in synthetic spectra from 1D hydrodynamic RADYN
simulations of nanoflare heated loops to investigate the diagnostic
potential of chromospheric emission from small-scale events. 
Methods: The Mg II h and k, Ca II H and K, Ca II 854.2 nm, and Hα and
Hβ chromospheric lines were synthesised from various RADYN models of
coronal loops subject to electron beams of nanoflare energies. The
contribution function to the line intensity was computed to better
understand how the atmospheric response to the non-thermal electrons
affects the formation of spectral lines and the detailed shape of
their spectral profiles. Results: The spectral line signatures
arising from the electron beams highly depend on the density of the
loop and the lower cutoff energy of the electrons. Low-energy (5 keV)
electrons deposit their energy in the corona and transition region,
producing strong plasma flows that cause both redshifts and blueshifts
of the chromospheric spectra. Higher-energy (10 and 15 keV) electrons
deposit their energy in the lower transition region and chromosphere,
resulting in increased emission from local heating. Our results indicate
that effects from small-scale events can be observed with ground-based
telescopes, expanding the list of possible diagnostics for the presence
and properties of nanoflares.

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Title: Accelerated particle beams in a 3D simulation of the quiet Sun
Authors: Frogner, L.; Gudiksen, B. V.; Bakke, H.
2020A&A...643A..27F Altcode: 2020arXiv200514483F
Context. Observational and theoretical evidence suggest that beams
of accelerated particles are produced in flaring events of all sizes
in the solar atmosphere, from X-class flares to nanoflares. Current
models of these types of particles in flaring loops assume an
isolated 1D atmosphere. Aims: A more realistic environment
for modelling accelerated particles can be provided by 3D radiative
magnetohydrodynamics codes. Here, we present a simple model for particle
acceleration and propagation in the context of a 3D simulation of
the quiet solar atmosphere, spanning from the convection zone to the
corona. We then examine the additional transport of energy introduced
by the particle beams. Methods: The locations of particle
acceleration associated with magnetic reconnection were identified by
detecting changes in magnetic topology. At each location, the parameters
of the accelerated particle distribution were estimated from local
conditions. The particle distributions were then propagated along the
magnetic field, and the energy deposition due to Coulomb collisions
with the ambient plasma was computed. Results: We find that
particle beams originate in extended acceleration regions that are
distributed across the corona. Upon reaching the transition region,
they converge and produce strands of intense heating that penetrate the
chromosphere. Within these strands, beam heating consistently dominates
conductive heating below the bottom of the transition region. This
indicates that particle beams qualitatively alter the energy transport
even outside of active regions.

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Title: A comprehensive three-dimensional radiative magnetohydrodynamic
simulation of a solar flare
Authors: Cheung, M. C. M.; Rempel, M.; Chintzoglou, G.; Chen, F.;
Testa, P.; Martínez-Sykora, J.; Sainz Dalda, A.; DeRosa, M. L.;
Malanushenko, A.; Hansteen, V.; De Pontieu, B.; Carlsson, M.; Gudiksen,
B.; McIntosh, S. W.
2019NatAs...3..160C Altcode: 2018NatAs...3..160C
Solar and stellar flares are the most intense emitters of X-rays and
extreme ultraviolet radiation in planetary systems1,2. On
the Sun, strong flares are usually found in newly emerging sunspot
regions3. The emergence of these magnetic sunspot groups
leads to the accumulation of magnetic energy in the corona. When
the magnetic field undergoes abrupt relaxation, the energy released
powers coronal mass ejections as well as heating plasma to temperatures
beyond tens of millions of kelvins. While recent work has shed light
on how magnetic energy and twist accumulate in the corona4
and on how three-dimensional magnetic reconnection allows for rapid
energy release5,6, a self-consistent model capturing how
such magnetic changes translate into observable diagnostics has remained
elusive. Here, we present a comprehensive radiative magnetohydrodynamics
simulation of a solar flare capturing the process from emergence to
eruption. The simulation has sufficient realism for the synthesis of
remote sensing measurements to compare with observations at visible,
ultraviolet and X-ray wavelengths. This unifying model allows us to
explain a number of well-known features of solar flares7,
including the time profile of the X-ray flux during flares, origin
and temporal evolution of chromospheric evaporation and condensation,
and sweeping of flare ribbons in the lower atmosphere. Furthermore,
the model reproduces the apparent non-thermal shape of coronal X-ray
spectra, which is the result of the superposition of multi-component
super-hot plasmas8 up to and beyond 100 million K.

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

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Title: On the Origin of the Magnetic Energy in the Quiet Solar
Chromosphere
Authors: Martínez-Sykora, Juan; Hansteen, Viggo H.; Gudiksen, Boris;
Carlsson, Mats; De Pontieu, Bart; Gošić, Milan
2019ApJ...878...40M Altcode: 2019arXiv190404464M
The presence of magnetic field is crucial in the transport of energy
through the solar atmosphere. Recent ground-based and space-borne
observations of the quiet Sun have revealed that magnetic field
accumulates at photospheric heights, via a local dynamo or from
small-scale flux emergence events. However, most of this small-scale
magnetic field may not expand into the chromosphere due to the entropy
drop with height at the photosphere. Here we present a study that uses
a high-resolution 3D radiative MHD simulation of the solar atmosphere
with non-gray and non-LTE radiative transfer and thermal conduction
along the magnetic field to reveal that (1) the net magnetic flux
from the simulated quiet photosphere is not sufficient to maintain a
chromospheric magnetic field (on average), (2) processes in the lower
chromosphere, in the region dominated by magnetoacoustic shocks,
are able to convert kinetic energy into magnetic energy, (3) the
magnetic energy in the chromosphere increases linearly in time until
the rms of the magnetic field strength saturates at roughly 4-30 G
(horizontal average) due to conversion from kinetic energy, (4) and
that the magnetic features formed in the chromosphere are localized
to this region.

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Title: Emission of Joule heating events in simulations of the
solar corona
Authors: Kanella, Charalambos; Gudiksen, Boris V.
2019A&A...621A..95K Altcode:
Context. Nanoscale events in cooperation with steady heating from a
slow heating mechanism, such as slow-burning current-sheets, could be
able to heat the corona; however, their observational traces are hard
to detect via current instrumentation. After we locate heating events
in magnetohydrodynamic (MHD) simulations and synthesise observational
data, we extract observational signatures of small-scale events. Aims: Our mission is threefold. The first goal is to observe
the manifestation of small-scale events via three observational
tools: intensity maps of three extreme ultraviolet (EUV) filters in
the Atmospheric Imaging Assembly (AIA) instrument with resolution
better than that in AIA images, emission measure (EM) analysis, and
time-lag maps. The second goal is to identify the reason why we cannot
quantify the energy release from observed events. The third goal is
to study the differences between the radiation from isolated heating
events and that from the whole corona. Methods: We employed a
three-dimensional magnetohydrodynamic (3D-MHD) simulation using the
Bifrost code. We simulated the atmosphere of a network embedded in the
quiet Sun (QS), and we identified 3D heating events in the corona in
several time-steps. Then we synthesised the three observational tools
for two cases. First, we considered information from the total column
mass in the corona, and then we considered only regions that exhibit
heating events. Results: We report on the differences between
the two regions of investigation, which also consist of the evidence
to justify why observers cannot identify small-scale heating events
in observations. We found that the combination of multiple heating
events at different cooling phases along the line of sight gives the
impression of thin elongated threads of events. For this reason, the
EM as a function of temperature has a multi-thermal distribution. Both
the radiation and the emission measure of the isolated heating events
have values at least ten times lower than the signal calculated from
the total corona. We also found that heating events move together
with diffuse emission from the slow heating mechanism, and for
this reason we cannot differentiate between the two. In addition,
we find that the frequency of heating events and their intensity
affect the EM distribution as a function of temperature. We also
find that the filter's intensity, EM, and time-lag maps of heating
events are different to those incorporating information from the
total column mass of the corona. However, the two regions have,
on average, comparable values, which are slightly smaller than the
analytical cooling timescales calculated for an optically thin and
radiation-dominated atmosphere.

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Title: Non-thermal electrons from solar nanoflares
Authors: Bakke, Helle; Frogner, Lars; Gudiksen, Boris Vilhelm
2018arXiv181112404B Altcode:
Context. We introduce a model for including accelerated particles
in pure magnetohydrodynamics (MHD) simulations of the solar
atmosphere. Aims. We show that the method is viable and produces
results that enhance the realism of MHD simulations of the solar
atmosphere. Methods. The acceleration of high-energy electrons in
solar flares is an accepted fact, but is not included in the most
advanced 3D simulations of the solar atmosphere. The effect of the
acceleration is not known, and here we introduce a simple method to
account for the ability of the accelerated electrons to move energy
from the reconnection sites and into the dense transition zone and
chromosphere. Results. The method was only run for a short time and with
low reconnection energies, but this showed that the reconnection process
itself changes, and that there is a clear effect on the observables
at the impact sites of the accelerated electrons. Further work will
investigate the effect on the reconnection sites and the impact sites
in detail.

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Title: Non-thermal electrons from solar nanoflares. In a 3D radiative
MHD simulation
Authors: Bakke, H.; Frogner, L.; Gudiksen, B. V.
2018A&A...620L...5B Altcode:
Context. We introduce a model for including accelerated particles in
pure magnetohydrodynamics (MHD) simulations of the solar atmosphere. Aims: We show that the method is viable and produces results that
enhance the realism of MHD simulations of the solar atmosphere. 
Methods: The acceleration of high-energy electrons in solar flares is an
accepted fact, but is not included in the most advanced 3D simulations
of the solar atmosphere. The effect of the acceleration is not known,
and here we introduce a simple method to account for the ability of
the accelerated electrons to move energy from the reconnection sites
and into the dense transition zone and chromosphere. Results:
The method was only run for a short time and with low reconnection
energies, but this showed that the reconnection process itself changes,
and that there is a clear effect on the observables at the impact sites
of the accelerated electrons. Further work will investigate the effect
on the reconnection sites and the impact sites in detail.

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Title: Investigating 4D coronal heating events in magnetohydrodynamic
simulations
Authors: Kanella, Charalambos; Gudiksen, Boris V.
2018A&A...617A..50K Altcode: 2018arXiv180604495K
Context. One candidate model for heating the solar corona is magnetic
reconnection that embodies Ohmic dissipation of current sheets. When
numerous small-scale magnetic reconnection events occur, then it is
possible to heat the corona; if ever observed, these events would have
been the speculated nanoflares. Aims: Because of the limitations
of current instrumentation, nanoflares cannot be resolved. But their
importance is evaluated via statistics by finding the power-law index
of energy distribution. This method is however biased for technical
and physical reasons. We aim to overcome limitations imposed by
observations and statistical analysis. This way, we identify, and
study these small-scale impulsive events. Methods: We employed
a three-dimensional magnetohydrodynamic (3D MHD) simulation using
the Bifrost code. We also employed a new technique to identify the
evolution of 3D joule heating events in the corona. Then, we derived
parameters describing the heating events in these locations, studied
their geometrical properties and where they occurred with respect to
the magnetic field. Results: We report on the identification
of heating events. We obtain the distribution of duration, released
energy, and volume. We also find weak power-law correlation between
these parameters. In addition, we extract information about geometrical
parameters of 2D slices of 3D events, and about the evolution of
resolved joule heating compared to the total joule heating and magnetic
energy in the corona. Furthermore, we identify relations between the
location of heating events and the magnetic field. Conclusions:
Even though the energy power index is less than 2, when classifying the
energy release into three categories with respect to the energy release
(pico-, nano-, and micro-events), we find that nano-events release
82% of the resolved energy. This percentage corresponds to an energy
flux larger than that needed to heat the corona. Although no direct
conclusions can be drawn, it seems that the most popular population
among small-scale events is the one that contains nano-scale energetic
events that are short lived with small spatial extend. Generally, the
locations and size of heating events are affected by the magnitude of
the magnetic field.

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

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Title: Two-dimensional Radiative Magnetohydrodynamic Simulations of
Partial Ionization in the Chromosphere. II. Dynamics and Energetics
of the Low Solar Atmosphere
Authors: Martínez-Sykora, Juan; De Pontieu, Bart; Carlsson, Mats;
Hansteen, Viggo H.; Nóbrega-Siverio, Daniel; Gudiksen, Boris V.
2017ApJ...847...36M Altcode: 2017arXiv170806781M
We investigate the effects of interactions between ions and
neutrals on the chromosphere and overlying corona using 2.5D
radiative MHD simulations with the Bifrost code. We have extended
the code capabilities implementing ion-neutral interaction effects
using the generalized Ohm’s law, I.e., we include the Hall term
and the ambipolar diffusion (Pedersen dissipation) in the induction
equation. Our models span from the upper convection zone to the corona,
with the photosphere, chromosphere, and transition region partially
ionized. Our simulations reveal that the interactions between ionized
particles and neutral particles have important consequences for the
magnetothermodynamics of these modeled layers: (1) ambipolar diffusion
increases the temperature in the chromosphere; (2) sporadically the
horizontal magnetic field in the photosphere is diffused into the
chromosphere, due to the large ambipolar diffusion; (3) ambipolar
diffusion concentrates electrical currents, leading to more violent
jets and reconnection processes, resulting in (3a) the formation of
longer and faster spicules, (3b) heating of plasma during the spicule
evolution, and (3c) decoupling of the plasma and magnetic field in
spicules. Our results indicate that ambipolar diffusion is a critical
ingredient for understanding the magnetothermodynamic properties in the
chromosphere and transition region. The numerical simulations have been
made publicly available, similar to previous Bifrost simulations. This
will allow the community to study realistic numerical simulations with
a wider range of magnetic field configurations and physics modules
than previously possible.

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Title: Realistic radiative MHD simulation of a solar flare
Authors: Rempel, Matthias D.; Cheung, Mark; Chintzoglou, Georgios;
Chen, Feng; Testa, Paola; Martinez-Sykora, Juan; Sainz Dalda, Alberto;
DeRosa, Marc L.; Viktorovna Malanushenko, Anna; Hansteen, Viggo H.;
De Pontieu, Bart; Carlsson, Mats; Gudiksen, Boris; McIntosh, Scott W.
2017SPD....4840001R Altcode:
We present a recently developed version of the MURaM radiative
MHD code that includes coronal physics in terms of optically thin
radiative loss and field aligned heat conduction. The code employs
the "Boris correction" (semi-relativistic MHD with a reduced speed
of light) and a hyperbolic treatment of heat conduction, which allow
for efficient simulations of the photosphere/corona system by avoiding
the severe time-step constraints arising from Alfven wave propagation
and heat conduction. We demonstrate that this approach can be used
even in dynamic phases such as a flare. We consider a setup in which
a flare is triggered by flux emergence into a pre-existing bipolar
active region. After the coronal energy release, efficient transport
of energy along field lines leads to the formation of flare ribbons
within seconds. In the flare ribbons we find downflows for temperatures
lower than ~5 MK and upflows at higher temperatures. The resulting
soft X-ray emission shows a fast rise and slow decay, reaching a peak
corresponding to a mid C-class flare. The post reconnection energy
release in the corona leads to average particle energies reaching 50
keV (500 MK under the assumption of a thermal plasma). We show that
hard X-ray emission from the corona computed under the assumption of
thermal bremsstrahlung can produce a power-law spectrum due to the
multi-thermal nature of the plasma. The electron energy flux into the
flare ribbons (classic heat conduction with free streaming limit) is
highly inhomogeneous and reaches peak values of about 3x1011
erg/cm2/s in a small fraction of the ribbons, indicating
regions that could potentially produce hard X-ray footpoint sources. We
demonstrate that these findings are robust by comparing simulations
computed with different values of the saturation heat flux as well as
the "reduced speed of light".

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Title: Identification of coronal heating events in 3D simulations
Authors: Kanella, Charalambos; Gudiksen, Boris V.
2017A&A...603A..83K Altcode: 2017arXiv170302808K
Context. The solar coronal heating problem has been an open question
in the science community since 1939. One of the proposed models
for the transport and release of mechanical energy generated
in the sub-photospheric layers and photosphere is the magnetic
reconnection model that incorporates Ohmic heating, which
releases a part of the energy stored in the magnetic field. In
this model many unresolved flaring events occur in the solar
corona, releasing enough energy to heat the corona. Aims:
The problem with the verification and quantification of this model
is that we cannot resolve small scale events due to limitations
of the current observational instrumentation. Flaring events have
scaling behavior extending from large X-class flares down to the so
far unobserved nanoflares. Histograms of observable characteristics
of flares show powerlaw behavior for energy release rate, size, and
total energy. Depending on the powerlaw index of the energy release,
nanoflares might be an important candidate for coronal heating; we seek
to find that index. Methods: In this paper we employ a numerical
three-dimensional (3D)-magnetohydrodynamic (MHD) simulation produced
by the numerical code Bifrost, which enables us to look into smaller
structures, and a new technique to identify the 3D heating events at
a specific instant. The quantity we explore is the Joule heating, a
term calculated directly by the code, which is explicitly correlated
with the magnetic reconnection because it depends on the curl of the
magnetic field. Results: We are able to identify 4136 events
in a volume 24 × 24 × 9.5 Mm3 (I.e., 768 × 786 × 331
grid cells) of a specific snapshot. We find a powerlaw slope of the
released energy per second equal to αP = 1.5 ± 0.02, and
two powerlaw slopes of the identified volume equal to αV
= 1.53 ± 0.03 and αV = 2.53 ± 0.22. The identified
energy events do not represent all the released energy, but of the
identified events, the total energy of the largest events dominate
the energy release. Most of the energy release happens in the lower
corona, while heating drops with height. We find that with a specific
identification method large events can be resolved into smaller ones,
but at the expense of the total identified energy releases. The
energy release that cannot be identified as an event favors a low
energy release mechanism. Conclusions: This is the first step
to quantitatively identify magnetic reconnection sites and measure
the energy released by current sheet formation.

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Title: Chromospheric and Coronal Wave Generation in a Magnetic
Flux Sheath
Authors: Kato, Yoshiaki; Steiner, Oskar; Hansteen, Viggo; Gudiksen,
Boris; Wedemeyer, Sven; Carlsson, Mats
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.

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

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Title: A publicly available simulation of an enhanced network region
of the Sun
Authors: Carlsson, Mats; Hansteen, Viggo H.; Gudiksen, Boris V.;
Leenaarts, Jorrit; De Pontieu, Bart
2016A&A...585A...4C Altcode: 2015arXiv151007581C
Context. The solar chromosphere is the interface between the
solar surface and the solar corona. Modelling of this region is
difficult because it represents the transition from optically
thick to thin radiation escape, from gas-pressure domination to
magnetic-pressure domination, from a neutral to an ionised state,
from MHD to plasma physics, and from near-equilibrium (LTE) to
non-equilibrium conditions. Aims: Our aim is to provide the
community with realistic simulations of the magnetic solar outer
atmosphere. This will enable detailed comparison of existing and
upcoming observations with synthetic observables from the simulations,
thereby elucidating the complex interactions of magnetic fields and
plasma that are crucial for our understanding of the dynamic outer
atmosphere. Methods: We used the radiation magnetohydrodynamics
code Bifrost to perform simulations of a computational volume
with a magnetic field topology similar to an enhanced network
area on the Sun. Results: The full simulation cubes are
made available from the Hinode Science Data Centre Europe. The
general properties of the simulation are discussed, and limitations
are discussed. The Hinode Science Data Centre Europe (<A
href="http://www.sdc.uio.no/search/simulations">http://www.sdc.uio.no/search/simulations</A>).

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Title: Time Dependent Nonequilibrium Ionization of Transition Region
Lines Observed with IRIS
Authors: Martínez-Sykora, Juan; De Pontieu, Bart; Hansteen, Viggo H.;
Gudiksen, Boris
2016ApJ...817...46M Altcode: 2015arXiv151200865M
The properties of nonstatistical equilibrium ionization of silicon
and oxygen ions are analyzed in this work. We focus on five solar
targets (quiet Sun; coronal hole; plage; quiescent active region,
AR; and flaring AR) as observed with the Interface Region Imaging
Spectrograph (IRIS). IRIS is best suited for this work owing to the
high cadence (up to 0.5 s), high spatial resolution (up to 0.″32),
and high signal-to-noise ratios for O IV λ1401 and Si IV λ1402. We
find that the observed intensity ratio between lines of three times
ionized silicon and oxygen ions depends on their total intensity
and that this correlation varies depending on the region observed
(quiet Sun, coronal holes, plage, or active regions) and on the
specific observational objects present (spicules, dynamic loops, jets,
microflares, or umbra). In order to interpret the observations, we
compare them with synthetic profiles taken from 2D self-consistent
radiative MHD simulations of the solar atmosphere, where the
statistical equilibrium or nonequilibrium treatment of silicon and
oxygen is applied. These synthetic observations show vaguely similar
correlations to those in the observations, I.e., between the intensity
ratios and their intensities, but only in the nonequilibrium case do
we find that (some of) the observations can be reproduced. We conclude
that these lines are formed out of statistical equilibrium. We use
our time-dependent nonequilibrium ionization simulations to describe
the physical mechanisms behind these observed properties.

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Title: Numerical Modeling of the Solar Chromosphere and Corona:
What Has Been Done? What Should Be Done?
Authors: Hansteen, V.; Carlsson, M.; Gudiksen, B.
2015ASPC..498..141H Altcode:
A number of increasingly sophisticated numerical simulations spanning
the solar atmosphere from below the photosphere in the convection
zone to far above in the corona have shed considerable insight into
the role of the magnetic field in the structure and energetics of the
Sun's outer layers. This development is strengthened by the wealth of
observational data now coming on-line from both ground and space based
observatories. In this talk we will concentrate on the successes and
failures of the modeling effort thus far and discuss the inclusion of
various effects not traditionally considered in the MHD description
such as time dependent ionization, non-LTE radiative transfer, and
generalized Ohm's law.

---------------------------------------------------------
Title: IRIS observations and 3D `realistic' MHD models of the solar
chromosphere
Authors: Hansteen, V.; Carlsson, M.; Gudiksen, B.
2015hsa8.conf...19H Altcode:
The Interface Region Imaging Spectrograph (IRIS) is a NASA “Small
Explorer” mission. It was launched in late June 2013 and since then
it has obtained spectra and images from the outer solar atmosphere at
unprecedented spatial and temporal resolution. Its primary goal is to
probe the photosphere-corona interface: the source region of outer
atmosphere heating and dynamics and a region that has an extremely
complicated interplay between plasma, radiation and magnetic field. The
scientific justification for IRIS hinges on the capabilities of 3D
magnetohydrodynamic models to allow the confident interpretation of
observed data. The interplay between observations and modeling is
discussed, illustrated with examples from recent IRIS observations.

---------------------------------------------------------
Title: Structures in the Outer Solar Atmosphere
Authors: Fletcher, L.; Cargill, P. J.; Antiochos, S. K.; Gudiksen,
B. V.
2015SSRv..188..211F Altcode: 2014SSRv..tmp...52F; 2014arXiv1412.7378F
The structure and dynamics of the outer solar atmosphere are reviewed
with emphasis on the role played by the magnetic field. Contemporary
observations that focus on high resolution imaging over a range
of temperatures, as well as UV, EUV and hard X-ray spectroscopy,
demonstrate the presence of a vast range of temporal and spatial scales,
mass motions, and particle energies present. By focusing on recent
developments in the chromosphere, corona and solar wind, it is shown
that small scale processes, in particular magnetic reconnection, play
a central role in determining the large-scale structure and properties
of all regions. This coupling of scales is central to understanding
the atmosphere, yet poses formidable challenges for theoretical models.

---------------------------------------------------------
Title: Synthesized Spectra of Optically Thin Emission Lines
Authors: Olluri, K.; Gudiksen, B. V.; Hansteen, V. H.; De Pontieu, B.
2015ApJ...802....5O Altcode:
In recent years realistic 3D numerical models of the solar atmosphere
have become available. The models attempt to recreate the solar
atmosphere and mimic observations in the best way, in order to make it
possible to couple complicated observations with physical properties
such as the temperatures, densities, velocities, and magnetic fields. We
here present a study of synthetic spectra created using the Bifrost code
in order to assess how well they fit with previously taken solar data. A
study of the synthetic intensity, nonthermal line widths, Doppler
shifts, and correlations between any two of these three components of
the spectra first assuming statistical equilibrium is made, followed by
a report on some of the effects nonequilibrium ionization will have on
the synthesized spectra. We find that the synthetic intensities compare
well with the observations. The synthetic observations depend on the
assumed resolution and point-spread function (PSF) of the instrument,
and we find a large effect on the results, especially for intensity
and nonthermal line width. The Doppler shifts produce the reported
persistent redshifts for the transition region (TR) lines and blueshifts
for the upper TR and corona lines. The nonthermal line widths reproduce
the well-known turnoff point around (2-3) × 105 K, but
with much lower values than those observed. The nonthermal line widths
tend to increase with decreasing assumed instrumental resolution, also
when nonequilibrium ionization is included. Correlations between the
nonthermal line width of any two TR line studies as reported by Chae et
al. are reproduced, while the correlations of intensity to line width
are reproduced only after applying a PSF to the data. Doppler shift
correlations reported by Doschek for the TR lines and correlations of
Doppler shift to nonthermal line width of the Fe xii 19.5
line reported by Doschek et al. are reproduced.

---------------------------------------------------------
Title: Non-equilibrium Ionization Effects on the Density Line Ratio
Diagnostics of O IV
Authors: Olluri, K.; Gudiksen, B. V.; Hansteen, V. H.
2013ApJ...767...43O Altcode:
The dynamic timescales in the solar atmosphere are shorter than the
ionization and recombination times of many ions used for line ratio
diagnostics of the transition region and corona. The long ionization
and recombination times for these ions imply that they can be found far
from their equilibrium temperatures, and spectroscopic investigations
require more care before being trusted in giving correct information
on local quantities, such as density and temperature. By solving
the full time-dependent rate equations for an oxygen model atom
in the three-dimensional numerical model of the solar atmosphere
generated by the Bifrost code, we are able to construct synthetic
intensity maps and study the emergent emission. We investigate the
method of electron density diagnostics through line ratio analysis
of the O IV 140.1 nm to the 140.4 nm ratio, the assumptions made
in carrying out the diagnostics, and the different interpretations
of the electron density. The results show big discrepancies between
emission in statistical equilibrium and emission where non-equilibrium
(NEQ) ionization is treated. Deduced electron densities are up to an
order of magnitude higher when NEQ effects are accounted for. The
inferred electron density is found to be a weighted mean average
electron density along the line of sight and has no relation to the
temperature of emission. This study shows that numerical modeling is
essential for electron density diagnostics and is a valuable tool when
the ions used for such studies are expected to be out of ionization
equilibrium. Though this study has been performed on the O IV ion,
similar results are also expected for other transition region ions.

---------------------------------------------------------
Title: Non-equilibrium Ionization in the Bifrost Stellar Atmosphere
Code
Authors: Olluri, K.; Gudiksen, B. V.; Hansteen, V. H.
2013AJ....145...72O Altcode:
The chromosphere and transition region have for the last 20 years been
known to be quite dynamic layers of the solar atmosphere, characterized
by timescales shorter than the ionization equilibrium timescales of
many of the ions dominating emission in these regions. Due to the
fast changes in the properties of the atmosphere, long ionization and
recombination times can lead these ions to being found far from their
equilibrium temperatures. A number of the spectral lines that we observe
can therefore not be expected a priori to reflect information about
local quantities such as the density or temperature, and interpreting
observations requires numerical modeling. Modeling the ionization
balance is computationally expensive and has earlier only been done
in one dimension. However, one-dimensional models can primarily be
used to investigate the possible importance of a physical effect, but
cannot verify or disprove the importance of that effect in the fully
three-dimensional solar atmosphere. Here, using the atomic database
package DIPER, we extend one-dimensional methods and implement a solver
for the rate equations of the full three-dimensional problem, using
the numerical code Bifrost. We present our implementation and report
on a few test cases. We also report on studies of the important C IV
and Fe XII ions in a semi-realistic two-dimensional solar atmosphere
model, focusing on differences between statistical equilibrium and
non-equilibrium ionization results.

---------------------------------------------------------
Title: Non-equilibrium ionization in 3D numerical models
Authors: Olluri, Kosovare; Gudiksen, Boris; Hansteen, Viggo
2012decs.confE.118O Altcode:
The dynamic timescales in the chromosphere and transition region have
been observed to be much smaller then the ionization equilibration
timescales of many ions found in the region. Due to the fast changes in
the properties of the atmosphere, long ionization- and recombination
times may lead to ions being found far from their equilibrium
temperatures. Spectroscopic investigations therefore needs to be
interpreted with the help of numerical modeling in order to produce
reliable results. By solving the rate equations within a realistic MHD
simulation of the solar atmosphere, we are able to follow the ionization
balance, and study the non equilibrium effects of the emitting gas. Due
top lack of computation power, this has previously been done in simple
1D, but because of the many free parameters in these models, their
conclusions are not free of uncertainties. The resent development in
computing technology and atmospheric modeling makes it possible to
study the full 3D effect of non equilibrium ionization. With the solar
atmosphere model Bifrost, we have a 3D platform for calculating and
following the ionization degree of important atoms of high abundances
in the solar atmosphere. We will present our implementation, and a
study of the carbon IV 1549 Å , Iron XII 195 Å, Oxygen IV 1399 Å
and 1401 Å lines in 2D.

---------------------------------------------------------
Title: State of the art single fluid MHD numerical modeling of the
coupled solar atmosphere
Authors: Gudiksen, Boris
2012decs.confE.115G Altcode:
Modeling the solar atmosphere has for a long time been known to be a
very complex problem. The wealth of observational features identified in
solar observations have multiplied with increasing spatial, temporal
and spectral resolution. To explain the high quality of space and
ground based observations, models must be very sophisticated and be
able to treat a number of physical regimes, where the dominating terms
in the equations change drastically. Numerical simulations are now able
to explain some, but certainly not all of the observed features. The
numerical complexity of solving the equations governing the physics of
the solar atmosphere is very high, and a number of different numerical
techniques must be used in order to create a coherent picture of the
connected solar atmosphere. We are now at a level where simulations
have to include a much larger range in vertical extend than has been
previously done. The wealth of numerical problems arising when doing so
has lead to a number of numerical codes that are specialized to deal
with a specific problem, and which now are being augmented to handle
a larger range of problems. Hopefully with time we will have a number
of numerical codes that are sophisticated enough to deal reliably with
the whole solar atmosphere. I will give a review of some the codes that
have been able to produce results from a fully connected solar model.

---------------------------------------------------------
Title: Non-equilibrium ionization in 3D numerical models
Authors: Olluri, Kosovare; Gudiksen, Boris; Hansteen, Viggoh
2012decs.confE.117O Altcode:
The dynamic timescales in the chromosphere and transition region have
been observed to be much smaller then the ionization equilibration
timescales of many ions found in the region. Due to the fast changes in
the properties of the atmosphere, long ionization- and recombination
times may lead to ions being found far from their equilibrium
temperatures. Spectroscopic investigations therefore needs to be
interpreted with the help of numerical modeling in order to produce
reliable results. By solving the rate equations within a realistic MHD
simulation of the solar atmosphere, we are able to follow the ionization
balance, and study the non equilibrium effects of the emitting gas. Due
top lack of computation power, this has previously been done in simple
1D, but because of the many free parameters in these models, their
conclusions are not free of uncertainties. The resent development in
computing technology and atmospheric modeling makes it possible to
study the full 3D effect of non equilibrium ionization. With the solar
atmosphere model Bifrost, we have a 3D platform for calculating and
following the ionization degree of important atoms of high abundances
in the solar atmosphere. We will present our implementation, and a
study of the carbon IV 1549 Å , Iron XII 195 Å, Oxygen IV 1399 Å
and 1401 Å lines in 2D.

---------------------------------------------------------
Title: The stellar atmosphere simulation code Bifrost. Code
description and validation
Authors: Gudiksen, B. V.; Carlsson, M.; Hansteen, V. H.; Hayek, W.;
Leenaarts, J.; Martínez-Sykora, J.
2011A&A...531A.154G Altcode: 2011arXiv1105.6306G
Context. Numerical simulations of stellar convection and photospheres
have been developed to the point where detailed shapes of observed
spectral lines can be explained. Stellar atmospheres are very complex,
and very different physical regimes are present in the convection zone,
photosphere, chromosphere, transition region and corona. To understand
the details of the atmosphere it is necessary to simulate the whole
atmosphere since the different layers interact strongly. These physical
regimes are very diverse and it takes a highly efficient massively
parallel numerical code to solve the associated equations. Aims:
The design, implementation and validation of the massively parallel
numerical code Bifrost for simulating stellar atmospheres from the
convection zone to the corona. Methods: The code is subjected
to a number of validation tests, among them the Sod shock tube test,
the Orzag-Tang colliding shock test, boundary condition tests and
tests of how the code treats magnetic field advection, chromospheric
radiation, radiative transfer in an isothermal scattering atmosphere,
hydrogen ionization and thermal conduction. Results.Bifrost completes
the tests with good results and shows near linear efficiency scaling
to thousands of computing cores.

---------------------------------------------------------
Title: On the minimum temperature of the quiet solar chromosphere
Authors: Leenaarts, J.; Carlsson, M.; Hansteen, V.; Gudiksen, B. V.
2011A&A...530A.124L Altcode: 2011arXiv1104.5081L
 Aims: We aim to provide an estimate of the minimum temperature
of the quiet solar chromosphere. Methods: We perform a 2D
radiation-MHD simulation spanning the upper convection zone to the
lower corona. The simulation includes non-LTE radiative transfer
and an equation-of-state that includes non-equilibrium ionization
of hydrogen and non-equilibrium H2 molecule formation. We
analyze the reliability of the various assumptions made in our model
in order to assess the realism of the simulation. Results:
Our simulation contains pockets of cool gas with down to 1660 K from
1 Mm up to 3.2 Mm height. It overestimates the radiative heating,
and contains non-physical heating below 1660 K. Therefore we conclude
that cool pockets in the quiet solar chromosphere might have even
lower temperatures than in the simulation, provided that there exist
areas in the chromosphere without significant magnetic heating. We
suggest off-limb molecular spectroscopy to look for such cool pockets
and 3D simulations including a local dynamo and a magnetic carpet to
investigate Joule heating in the quiet chromosphere.

---------------------------------------------------------
Title: Three-dimensional surface convection simulations of metal-poor
stars. The effect of scattering on the photospheric temperature
stratification
Authors: Collet, R.; Hayek, W.; Asplund, M.; Nordlund, Å.; Trampedach,
R.; Gudiksen, B.
2011A&A...528A..32C Altcode: 2011arXiv1101.3265C
Context. Three-dimensional (3D) radiative hydrodynamic model atmospheres
of metal-poor late-type stars are characterized by cooler upper
photospheric layers than their one-dimensional counterparts. This
property of 3D model atmospheres can dramatically affect the
determination of elemental abundances from temperature-sensitive
spectral features, with profound consequences on galactic chemical
evolution studies. Aims: We investigate whether the cool surface
temperatures predicted by 3D model atmospheres of metal-poor stars
can be ascribed to approximations in the treatment of scattering
during the modelling phase. Methods: We use the Bifrost
code to construct 3D model atmospheres of metal-poor stars and test
three different ways to handle scattering in the radiative transfer
equation. As a first approach, we solve iteratively the radiative
transfer equation for the general case of a source function with
a coherent scattering term, treating scattering in a correct and
consistent way. As a second approach, we solve the radiative transfer
equation in local thermodynamic equilibrium approximation, neglecting
altogether the contribution of continuum scattering to extinction in the
optically thin layers; this has been the default mode in our previous
3D modelling as well as in present Stagger-Code models. As our third
and final approach, we treat continuum scattering as pure absorption
everywhere, which is the standard case in the 3D modelling by the
CO5BOLD collaboration. Results: For all simulations,
we find that the second approach produces temperature structures
with cool upper photospheric layers very similar to the case in which
scattering is treated correctly. In contrast, treating scattering as
pure absorption leads instead to significantly hotter and shallower
temperature stratifications. The main differences in temperature
structure between our published models computed with the Stagger-
and Bifrost codes and those generated with the CO5BOLD
code can be traced to the different treatments of scattering. 
Conclusions: Neglecting the contribution of continuum scattering to
extinction in optically thin layers provides a good approximation
to the full, iterative solution of the radiative transfer equation
in metal-poor stellar surface convection simulations, and at a much
lower computational cost. Our results also demonstrate that the cool
temperature stratifications predicted for metal-poor late-type stars
by previous models by our collaboration are not an artifact of the
approximated treatment of scattering.

---------------------------------------------------------
Title: Radiative transfer with scattering for domain-decomposed 3D
MHD simulations of cool stellar atmospheres. Numerical methods and
application to the quiet, non-magnetic, surface of a solar-type star
Authors: Hayek, W.; Asplund, M.; Carlsson, M.; Trampedach, R.; Collet,
R.; Gudiksen, B. V.; Hansteen, V. H.; Leenaarts, J.
2010A&A...517A..49H Altcode: 2010arXiv1007.2760H
 Aims: We present the implementation of a radiative
transfer solver with coherent scattering in the new BIFROST
code for radiative magneto-hydrodynamical (MHD) simulations of
stellar surface convection. The code is fully parallelized using
MPI domain decomposition, which allows for large grid sizes and
improved resolution of hydrodynamical structures. We apply the code
to simulate the surface granulation in a solar-type star, ignoring
magnetic fields, and investigate the importance of coherent scattering
for the atmospheric structure. Methods: A scattering term
is added to the radiative transfer equation, requiring an iterative
computation of the radiation field. We use a short-characteristics-based
Gauss-Seidel acceleration scheme to compute radiative flux divergences
for the energy equation. The effects of coherent scattering are
tested by comparing the temperature stratification of three 3D
time-dependent hydrodynamical atmosphere models of a solar-type star:
without scattering, with continuum scattering only, and with both
continuum and line scattering. Results: We show that continuum
scattering does not have a significant impact on the photospheric
temperature structure for a star like the Sun. Including scattering in
line-blanketing, however, leads to a decrease of temperatures by about
350 K below log10 τ5000 ⪉ -4. The effect is
opposite to that of 1D hydrostatic models in radiative equilibrium,
where scattering reduces the cooling effect of strong LTE lines in
the higher layers of the photosphere. Coherent line scattering also
changes the temperature distribution in the high atmosphere, where
we observe stronger fluctuations compared to a treatment of lines as
true absorbers.

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

---------------------------------------------------------
Title: Chromospheric heating and structure as determined from high
resolution 3D simulations .
Authors: Carlsson, M.; Hansteen, V. H.; Gudiksen, B. V.
2010MmSAI..81..582C Altcode: 2010arXiv1001.1546C
We have performed 3D radiation MHD simulations extending from the
convection zone to the corona covering a box 16 Mm3 at 32
km spatial resolution. The simulations show very fine structure in
the chromosphere with acoustic shocks interacting with the magnetic
field. Magnetic flux concentrations have a temperature lower than the
surroundings in the photosphere but higher in the low chromosphere. The
heating is there mostly through ohmic dissipation preferentially at
the edges of the flux concentrations. The magnetic field is often
wound up around the flux concentrations. When acoustic waves travel
up along the field this topology leads to swirling motions seen in
chromospheric diagnostic lines such as the calcium infrared triplet.

---------------------------------------------------------
Title: Photospheric Motions and Their Effects on the Corona: A
Numerical Approach
Authors: Gomes de Jesus, Leandro Filipe; Gudiksen, Boris Vilhelm
2009ApJ...704..705G Altcode: 2009arXiv0908.4174G; 2009ApJ...704..705F
We perform a number of numerical simulations of the solar corona with
the aim of understanding how it responds to different conditions in
the photosphere. By changing parameters which govern the motion of
the plasma at the photosphere, we study the behavior of the corona,
in particular, the effects on the current density generated. A
magnetohydrodynamics code is used to run simulations, using a 20 × 20
× 20 Mm3 box with timespans ranging from one hundred to
several hundreds of minutes. All the experiments show a fast initial
increase of the current density, followed by a stabilization around an
asymptotic value which depends on the photospheric conditions. These
asymptotic average current densities as well as the turnover points
are discussed.

---------------------------------------------------------
Title: Coronal dynamics and heating theories
Authors: Gudiksen, Boris V.
2009AdSpR..43..108G Altcode:
A large number of coronal heating theories have been proposed and
most of them can be labeled as AC or DC heating theories. Here a short
description of these two main theories is given, and the results of a
DC heating model simulation is explained in more detail. As a result of
that model, arguments are given for putting emphasis on launching a very
fast spectrograph, ideally a imaging spectrograph as soon as possible.

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

---------------------------------------------------------
Title: On the nature of coronal loops
Authors: Peter, H.; Bingert, S.; Gudiksen, B. V.
2008AGUSMSP41C..05P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Topological Dissipation &amp; The Solar Corona
Authors: Gudiksen, B. V.
2007ASPC..369..269G Altcode:
Reconnection in the solar corona has to take place, as was convincingly
shown by tet{Parker72}, the question remains if it is sufficient
to heat the corona. One of the major problems in coronal physics,
is modeling reconnection. Reconnection is the basis of most heating
models, in spite the fact that we really don't know how reconnection
works. Simulating reconnection with realistic parameters is highly
problematic and the solar corona has a parameter space not well
explored. Here I try to give a hint of what conclusions one can reach
about reconnection from large scale simulations of the solar corona. A
model of the solar corona with a numerical diffusion reproduces a
number of observables, and seem to reproduce the corona well, only
using minimal assumptions. The overall well reproduced corona, means
that it is highly likely that reconnection does not differ much from the
diffusion scheme of the numerical code. That means that reconnection in
the solar corona transfers most of the liberated magnetic energy into
heat locally and mechanisms such as waves and high energy particles can
not carry the a significant part of the energy released in reconnection.

---------------------------------------------------------
Title: 3D Numerical Models of the Chromosphere, Transition Region,
and Corona
Authors: Hansteen, V. H.; Carlsson, M.; Gudiksen, B.
2007ASPC..368..107H Altcode: 2007arXiv0704.1511H
A major goal in solar physics has during the last five decades
been to find how energy flux generated in the solar convection zone
is transported and dissipated in the outer solar layers. Progress
in this field has been slow and painstaking. However, advances in
computer hardware and numerical methods, vastly increased observational
capabilities and growing physical insight seem finally to be leading
towards understanding. Here we present exploratory numerical MHD models
that span the entire solar atmosphere from the upper convection zone
to the lower corona. These models include non-grey, non-LTE radiative
transport in the photosphere and chromosphere, optically thin radiative
losses as well as magnetic field-aligned heat conduction in the
transition region and corona.

---------------------------------------------------------
Title: Heating the solar corona.
Authors: Gudiksen, B. V.
2007MmSAI..78..293G Altcode:
The heating mechanism at work in the solar corona has been unknown
since the temperature of the corona was discovered in the late nineteen
thirties. Here I will present results from a model which we believe
is the first model which allows forward modeling of observational
signatures, and can fit several observational features of the solar
corona. If this model proves to be a correct representation of the
solar corona, the question of the coronal heating mechanism will
finally be solved.

---------------------------------------------------------
Title: Connections: Photosphere -- Chromosphere - Corona
Authors: Gudiksen, B. V.
2006ASPC..354..331G Altcode:
The chromosphere is not only the region where the atmosphere goes from
being optically thick to optically thin, but also the region where
the dynamics changes from being controlled by the plasma to being
controlled by the magnetic field. The magnetic field changes from
being concentrated in small regions to being space filling. This
expansion has traditionally been modeled by the magnetic funnel
or wine-glass picture. For several reasons it is hard to gain any
information about the magnetic field in this region, so this model
remains unconfirmed. Three recent methods to acquire magnetic field
information from this interesting region will be reviewed, and I will
argue that the results from such investigations will require that we
heavily revise the simplistic magnetic funnel picture.

---------------------------------------------------------
Title: Forward Modeling of the Corona of the Sun and Solar-like Stars:
From a Three-dimensional Magnetohydrodynamic Model to Synthetic
Extreme-Ultraviolet Spectra
Authors: Peter, Hardi; Gudiksen, Boris V.; Nordlund, Åke
2006ApJ...638.1086P Altcode: 2005astro.ph..3342P
A forward model is described in which we synthesize spectra from an ab
initio three-dimensional MHD simulation of an outer stellar atmosphere,
where the coronal heating is based on braiding of magnetic flux due to
photospheric footpoint motions. We discuss the validity of assumptions
such as ionization equilibrium and investigate the applicability of
diagnostics like the differential emission measure inversion. We find
that the general appearance of the synthesized corona is similar to
the solar corona and that, on a statistical basis, integral quantities
such as average Doppler shifts or differential emission measures are
reproduced remarkably well. The persistent redshifts in the transition
region, which have puzzled theorists since their discovery, are
explained by this model as caused by the flows induced by the heating
through braiding of magnetic flux. While the model corona is only
slowly evolving in intensity, as is observed, the amount of structure
and variability in Doppler shift is very large. This emphasizes the need
for fast coronal spectroscopic observations, as the dynamical response
of the corona to the heating process manifests itself in a comparably
slow evolving coronal intensity but rapid changes in Doppler shift.

---------------------------------------------------------
Title: Coronal dynamics and heating theories
Authors: Gudiksen, B. V.; Nordlund, Aa.
2006cosp...36.3545G Altcode: 2006cosp.meet.3545G
The solar corona has been modeled as a collection of single hydrostatic
loops and as a hydrostatic plane parallel atmosphere but these models
have had serious problems reproducing the observations made of the
solar corona The favored coronal heating models rely on large gradients
in wave speed for AC models and gradients in the magnetic field for
DC models and both provide an energy release that has no reason to
be uniform across the magnetic field and so the heating function at
play in the solar corona is most likely intermittent in both space and
time and we must therefore embrace a dynamic model of the corona This
conclusion is supported by high cadence high resolution observations
of the chromosphere and transition region which show a very dynamic
atmosphere I will concentrate on a 3D MHD simulation of the solar
atmosphere from the photosphere to the low corona and will show
that even for a quiescent active region and even though the observed
intensity of the loops is close to constant the loops are not in a
static equilibrium The corona is very dynamic and images made from
the simulation of doppler shifts show much more time dependence and
intermittency than does images showing only intensity

---------------------------------------------------------
Title: Coronal Heating Through Braiding of Magnetic Field Lines
Synthesized Coronal EUV Emission and Magnetic Structure
Authors: Peter, H.; Gudiksen, B. V.; Nordlund, A.
2005ESASP.596E..14P Altcode: 2005ccmf.confE..14P
No abstract at ADS

---------------------------------------------------------
Title: EUV Emission from a 3D MHD Coronal Model: Temporal Variability
in a Synthesized Corona
Authors: Peter, H.; Gudiksen, B. V.; Nordlund, Å.
2005ESASP.592..527P Altcode: 2005soho...16E..98P; 2005ESASP.592E..98P
No abstract at ADS

---------------------------------------------------------
Title: The Structure of the Base of the Corona
Authors: Bingert, S.; Peter, H.; Gudiksen, B.; Nordlund, Ake
2005ESASP.592..471B Altcode: 2005ESASP.592E..84B; 2005soho...16E..84B
No abstract at ADS

---------------------------------------------------------
Title: 3D Numerical Models of Quiet Sun Coronal Heating
Authors: Hansteen, V. H.; Gudiksen, B.
2005ESASP.592..483H Altcode: 2005soho...16E..87H; 2005ESASP.592E..87H
No abstract at ADS

---------------------------------------------------------
Title: DC Heating - Is it Enough? (Invited)
Authors: Gudiksen, B. V.
2005ESASP.592..165G Altcode: 2005soho...16E..25G; 2005ESASP.592E..25G
No abstract at ADS

---------------------------------------------------------
Title: Erratum: “An AB Initio Approach to Solar Coronal Loops”
(<A href="/abs/2005ApJ...618.1031G">ApJ, 618, 1031 [2005]</A>)
Authors: Gudiksen, Boris Vilhelm; Nordlund, Åke
2005ApJ...623..597G Altcode:
Because of an error at the Press, incorrect versions of Figures 4 (top
and bottom panels), 5, 7, and 9 were published. In all these figures, a
dotted or dash-dotted line appeared as a solid line. The correct figures
appear below. Figures 4 (middle panel), 6, 8, and 10 are also reproduced
here for comparison purposes. The Press sincerely regrets these errors.

---------------------------------------------------------
Title: Erratum: “An AB Initio Approach to the Solar Coronal
Heating Problem” (<A href="/abs/2005ApJ...618.1020G">ApJ, 618, 1020
[2005]</A>)
Authors: Gudiksen, Boris Vilhelm; Nordlund, Åke
2005ApJ...623..600G Altcode:
Because of an error at the Press, an incorrect version of Figure 5
was published, in which what should be a dash-dotted line (showing
convective flux) appears as a solid line. The correct version appears
below. The Press sincerely regrets the error.

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Title: Tackling the coronal heating problem using 3D MHD coronal
simulations with spectral synthesis
Authors: Peter, H.; Gudiksen, B. V.; Nordlund, A.
2005ESASP.560...59P Altcode: 2005csss...13...59P
No abstract at ADS

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Title: Dynamo action in M-dwarfs
Authors: Dorch, S. B. F.; Gudiksen, B. V.; Ludwig, H. -G.
2005ESASP.560..515D Altcode: 2005csss...13..515D; 2004astro.ph..9219D
Magnetic activity in M-dwarfs present enigmatic questions: On the
one hand they have higher field strengths and larger filling factors
than the magnetic field on the Sun, on the other hand, they are fully
convective and their atmospheres are more neutral, hence they do not
have an undershoot layer for magnetic flux storage and as we show here,
cannot have small-scale dynamo action in their photospheres either. We
present a discussion of these facts and propose a new numerical model
to investigate M-dwarf magnetism.

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Title: An AB Initio Approach to Solar Coronal Loops
Authors: Gudiksen, Boris Vilhelm; Nordlund, Åke
2005ApJ...618.1031G Altcode: 2004astro.ph..7267G
Data from recent numerical simulations of the solar corona and
transition region are analyzed, and the magnetic field connections
between the low corona and the photosphere are found to be close to
those of a potential field. The field line-to-field line displacements
follow a power-law distribution with typical displacements of just a
few Mm. Three loops visible in simulated TRACE filters are analyzed
in detail and found to have significantly different heating rates
and distributions thereof, one of them showing a small-scale heating
event. The dynamical structure is complicated, even though all the
loops are visible in a single filter along most of their lengths. The
loops are nonstatic and are in the process of evolving into loops with
very different characteristics. Differential emission measure (DEM)
curves along one of the loops illustrate that DEM curves have to be
treated carefully if physical characteristics are to be extracted.

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Title: An Ab Initio Approach to the Solar Coronal Heating Problem
Authors: Gudiksen, Boris Vilhelm; Nordlund, Åke
2005ApJ...618.1020G Altcode: 2004astro.ph..7266G
We present an ab initio approach to the solar coronal heating problem by
modeling a small part of the solar corona in a computational box using
a three-dimensional MHD code including realistic physics. The observed
solar granular velocity pattern and its amplitude and vorticity power
spectra, as reproduced by a weighted Voronoi tessellation method,
are used as a boundary condition that generates a Poynting flux in
the presence of a magnetic field. The initial magnetic field is a
potential extrapolation of a SOHO/MDI high-resolution magnetogram,
and a standard stratified atmosphere is used as a thermal initial
condition. Except for the chromospheric temperature structure, which
is kept nearly fixed, the initial conditions are quickly forgotten
because the included Spitzer conductivity and radiative cooling
function have typical timescales much shorter than the time span of the
simulation. After a short initial start-up period, the magnetic field is
able to dissipate (3-4)×106ergscm-2s-1
in a highly intermittent corona, maintaining an average temperature
of ~106 K, at coronal density values for which simulated
images of the TRACE 171 and 195 Å passbands reproduce observed photon
count rates.

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Title: Analysis of Synthetic EUV Spectra from 3d Models of the Corona
Authors: Bingert, S.; Peter, H.; Gudiksen, B.; Nordlund, A.; Dobler, W.
2004ESASP.575..348B Altcode: 2004soho...15..348B
No abstract at ADS

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Title: Synthetic EUV Spectra from 3D MHD Coronal Simulations:
Coronal Heating Through Magnetic Braiding
Authors: Peter, H.; Gudiksen, B. V.; Nordlund, Å.
2004ESASP.575...50P Altcode: 2004soho...15...50P
No abstract at ADS

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Title: Coronal Heating through Braiding of Magnetic Field Lines
Authors: Peter, Hardi; Gudiksen, Boris V.; Nordlund, Åke
2004ApJ...617L..85P Altcode: 2004astro.ph..9504P
Cool stars such as our Sun are surrounded by a million degree hot outer
atmosphere, the corona. For more than 60 years, the physical nature
of the processes heating the corona to temperatures well in excess of
those on the stellar surface have remained puzzling. Recent progress in
observational techniques and numerical modeling now opens a new window
to approach this problem. We present the first coronal emission-line
spectra synthesized from three-dimensional numerical models describing
the evolution of the dynamics and energetics as well as of the magnetic
field in the corona. In these models the corona is heated through
motions on the stellar surface that lead to a braiding of magnetic
field lines inducing currents that are finally dissipated. These
forward models enable us to synthesize observed properties such as
(average) emission-line Doppler shifts or emission measures in the outer
atmosphere, which until now have not been understood theoretically,
even though many suggestions have been made in the past. As our model
passes these observational tests, we conclude that the flux braiding
mechanism is a prime candidate for being the dominant heating process
of the magnetically closed corona of the Sun and solar-like stars.

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Title: Self-Regulating Supernova Heating in Interstellar Medium
Simulations
Authors: Sarson, Graeme R.; Shukurov, Anvar; Nordlund, Åke; Gudiksen,
Boris; Brandenburg, Axel
2004Ap&SS.292..267S Altcode: 2003astro.ph..7013S
Numerical simulations of the multi-phase interstellar medium have been
carried out, using a 3D, nonlinear, magnetohydrodynamic, shearing-box
model, with random motions driven by supernova explosions. These
calculations incorporate the effects of magnetic fields and rotation
in 3D; these play important dynamical roles in the galaxy, but are
neglected in many other simulations. The supernovae driving the motions
are not arbitrarily imposed, but occur where gas accumulates into cold,
dense clouds; their implementation uses a physically motivated model
for the evolution of such clouds. The process is self-regulating, and
produces mean supernova rates as part of the solution. Simulations with
differing mean density show a power law relation between the supernova
rate and density, with exponent 1.7; this value is within the range
suggested from observations (taking star formation rate as a proxy for
supernova rate). The global structure of the supernova driven medium
is strongly affected by the presence of magnetic fields; e.g. for one
solution the filling factor of hot gas is found to vary from 0.19 (with
no field) to 0.12 (with initial mid-plane field B 0= 6 μG).

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Title: The effects of spiral arms on the multi-phase ISM
Authors: Shukurov, Anvar; Sarson, Graeme R.; Nordlund, Åke; Gudiksen,
Boris; Brandenburg, Axel
2004Ap&SS.289..319S Altcode: 2002astro.ph.12260S
Statistical parameters of the ISM driven by thermal energy
injectionsfrom supernova explosions have been obtained from 3D,
nonlinear,magnetohydrodynamic, shearing-box simulations for spiral
arm andinterarm regions. The density scale height obtained for the
interarm regionsis 50% larger than within the spiral arms because
of thehigher gas temperature. The filling factorof the hot gas is
also significantly larger between the armsand depends sensitively on
magnetic field strength.

---------------------------------------------------------
Title: An Ab Initio Approach to the Solar Coronal Heating Problem
Authors: Gudiksen, B. V.; Nordlund, Å.
2004IAUS..219..488G Altcode:
No abstract at ADS

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Title: The coronal heating problem
Authors: Gudiksen, Boris V.
2004PhDT.......169G Altcode:
The heating of the solar corona has been investigated during four
of decades and several mechanisms able to produce heating have been
proposed. It has until now not been possible to produce quantitative
estimates that would establish any of these heating mechanism as the
most important in the solar corona. In order to investigate which
heating mechanism is the most important, a more detailed approach
is needed. In this thesis, the heating problem is approached
"ab initio";, using well observed facts and including realistic
physics in a 3D magneto-hydrodynamic simulation of a small part
of the solar atmosphere. The "engine" of the heating mechanism is
the solar photospheric velocity field, that braids the magnetic
field into a configuration where energy has to be dissipated. The
initial magnetic field is taken from an observation of a typical
magnetic active region scaled down to fit inside the computational
domain. The driving velocity field is generated by an algorithm that
reproduces the statistical and geometrical fingerprints of solar
granulation. Using a standard model atmosphere as the thermal initial
condition, the simulation goes through a short startup phase, where
the initial thermal stratification is quickly forgotten, after which
the simulation stabilizes in statistical equilibrium. In this state,
the magnetic field is able to dissipate the same amount of energy as
is estimated to be lost through radiation, which is the main energy
loss mechanism in the solar corona. The simulation produces
heating that is intermittent on the smallest resolved scales and
hot loops similar to those observed through narrow band filters in
the ultra violet. Other observed characteristics of the heating are
reproduced, as well as a coronal temperature of roughly one million
K. Because of the ab initio approach, the amount of heating produced
in these simulations represents a lower limit to coronal heating and
the conclusion is that such heating of the corona is unavoidable.

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Title: Dark cores in sunspot penumbral filaments
Authors: Scharmer, Göran B.; Gudiksen, Boris V.; Kiselman, Dan;
Löfdahl, Mats G.; Rouppe van der Voort, Luc H. M.
2002Natur.420..151S Altcode:
Sunspot umbrae-the dark central regions of the spots-are surrounded
by brighter filamentary penumbrae, the existence of which remains
largely inexplicable. The penumbral filaments contain magnetic fields
with varying inclinations and are associated with flowing gas, but
discriminating between theoretical models has been difficult because
the structure of the filaments has not hitherto been resolved. Here
we report observations of penumbral filaments that reveal dark cores
inside them. We cannot determine the nature of these dark cores,
but their very existence provides a crucial test for any model of
penumbrae. Our images also reveal other very small structures, in line
with the view that many of the fundamental physical processes in the
solar photosphere occur on scales smaller than 100km.

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Title: Bulk Heating and Slender Magnetic Loops in the Solar Corona
Authors: Gudiksen, Boris Vilhelm; Nordlund, Åke
2002ApJ...572L.113G Altcode:
The heating of the solar corona and the puzzle of the slender high
reaching magnetic loops seen in observations from the Transition
Region and Coronal Explorer (TRACE) has been investigated through
three-dimensional numerical simulations and found to be caused by
the well-observed plasma flows in the photosphere displacing the
footpoints of magnetic loops in a nearly potential configuration. It
is found that even the small convective displacements cause magnetic
dissipation sufficient to heat the corona to temperatures of the
order of a million K. The heating is intermittent in both space and
time-at any one height and time it spans several orders of magnitude,
and localized heating causes transonic flows along field lines, which
explains the observed nonhydrostatic stratification of loops that are
bright in emission measure.

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Title: Bright loops in the solar corona
Authors: Gudiksen, Boris V.; Nordlund, Aake
2002astro.ph..3167G Altcode:
The heating of the solar corona and the puzzle of the slender high
reaching magnetic loops seen in observations from the Transition
Region And Coronal Explorer(TRACE) has been investigated through 3D
numerical simulations, and found to be caused by the well observed
plasma flows in the photosphere displacing the footpoints of magnetic
loops in a nearly potential configuration. It is found that even the
small convective displacements cause magnetic dissipation sufficient to
heat the corona to temperatures of the order of a million Kelvin. The
heating is intermittent in both space and time - at any one height
and time it spans several orders of magnitude, and localized heating
causes transonic flows along field lines, which explains the observed
non-hydrostatic equilibrium of loops that are bright in emission
measure.

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Title: Bulk and Loop Heating of the Solar Corona
Authors: Gudiksen, Boris
2002smra.progE...7G Altcode:
No abstract at ADS

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Title: Flux-loss of buoyant ropes interacting with convective flows
Authors: Dorch, S. B. F.; Gudiksen, B. V.; Abbett, W. P.; Nordlund, Å.
2001A&A...380..734D Altcode: 2001astro.ph.10205D
We present 3-d numerical magneto-hydrodynamic simulations of a buoyant,
twisted magnetic flux rope embedded in a stratified, solar-like model
convection zone. The flux rope is given an initial twist such that it
neither kinks nor fragments during its ascent. Moreover, its magnetic
energy content with respect to convection is chosen so that the flux
rope retains its basic geometry while being deflected from a purely
vertical ascent by convective flows. The simulations show that magnetic
flux is advected away from the core of the flux rope as it interacts
with the convection. The results thus support the idea that the amount
of toroidal flux stored at or near the bottom of the solar convection
zone may currently be underestimated.