keppens

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Author name code: keppens
ADS astronomy entries on 2022-09-14
author:"Keppens, Rony"

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Title: Thermally enhanced tearing in solar current sheets: explosive
reconnection with plasmoid-trapped condensations
Authors: Sen, Samrat; Keppens, Rony
2022arXiv220804355S Altcode:
In flare-relevant current sheets, tearing instability may trigger
explosive reconnection and plasmoid formation. We explore how the
thermal and tearing modes reinforce each other in the fragmentation of
a current sheet in the solar corona through an explosive reconnection
process, characterized by the formation of plasmoids which interact
and trap condensing plasma. We use a resistive magnetohydrodynamic
(MHD) simulation of a 2D current layer, incorporating the non-adiabatic
effects of optically thin radiative energy loss and background heating
using \texttt{MPI-AMRVAC}. Our parametric survey explores different
resistivities and plasma-$\beta$ to quantify the instability growth rate
in the linear and nonlinear regimes. We notice that for dimensionless
resistivity values within $10^{-4} - 5 \times 10^{-3}$, we get explosive
behavior where thermal instability and tearing behavior reinforce
each other. This is clearly below the usual critical Lundquist number
range of pure resistive explosive plasmoid formation. The non-linear
growth rates follow weak power-law dependency with resistivity. The
fragmentation of the current sheet and the formation of the plasmoids
in the nonlinear phase of the evolution due to the thermal and tearing
instabilities are obtained. The formation of plasmoids is noticed
for the Lundquist number ($S_L$) range $4.6 \times 10^3 - 2.34 \times
10^5$. We quantify the temporal variation of the plasmoid numbers and
the density filling factor of the plasmoids for different physical
conditions. We also find that the maximum plasmoid numbers scale as
$S_L^{0.223}$. Within the nonlinearly coalescing plasmoid chains,
localized cool condensations gather, realizing density and temperature
contrasts similar to coronal rain or prominences.

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Title: Two-fluid implementation in MPI-AMRVAC with applications to
the solar chromosphere
Authors: Braileanu, B. Popescu; Keppens, R.
2022A&A...664A..55B Altcode:
Context. The chromosphere is a partially ionized layer of the solar
atmosphere, which acts as the transition between the photosphere
where the gas is almost neutral and the fully ionized corona. As the
collisional coupling between neutral and charged particles decreases
in the upper part of the chromosphere, the hydrodynamical timescales
may become comparable to the collisional timescale, thus calling for
the application of a two-fluid model. Aims: In this paper, we
describe the implementation and validation of a two-fluid model that
simultaneously evolves charges and neutrals, coupled by collisions. Methods: The two-fluid equations are implemented in the fully
open-source MPI-AMRVAC code. In the photosphere and the lower part of
the solar atmosphere, where collisions between charged and neutral
particles are very frequent, an explicit time-marching would be too
restrictive, since, to maintain stability, the time step needs to be
proportional to the inverse of the collision frequency. This caveat
can be overcome by evaluating the collisional terms implicitly, using
an explicit-implicit (IMEX) scheme. Out of the various IMEX variants
implemented, we focused on the IMEX-ARS3 scheme and we used it for
all simulations presented in this paper. The modular structure of
the code allows us to directly apply all other code functionality
- in particular, its automated grid adaptivity - to the two-fluid
model. Results: Our implementation recovers and significantly
extends the available (analytic or numerical) test results for two-fluid
chargeneutral evolutions. We demonstrate wave damping, propagation,
and interactions in stratified settings, as well as Riemann problems
for coupled plasma-neutral mixtures. We generalized a shock-dominated
evolution from single to two-fluid regimes and made contact with recent
findings on typical plasma-neutral instabilities. Conclusions:
The cases presented here cover very different collisional regimes
and our results are fully consistent with related findings from the
literature. If collisional time and length scales are smaller than the
hydrodynamical scales usually considered in the solar chromosphere,
the density structures seen in the neutral and charged fluids will
be similar, with the effect of elastic collisions between charges and
neutrals shown to be similar to the effects of diffusivity. Otherwise,
density structures are different and the decoupling in velocity
between the two species increases, and neutrals may, for instance,
show Kelvin-Helmholtz roll-up while the charges do not. The use of IMEX
schemes efficiently avoids the small time step constraints of fully
explicit implementations in strongly collisional regimes. Implementing
an adaptive mesh refinement (AMR) greatly decreases the computational
cost, as compared to uniform grid runs at the same effective resolution.

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Title: BxC: a swift generator for 3D magnetohydrodynamic turbulence
Authors: Durrive, J. -B.; Changmai, M.; Keppens, R.; Lesaffre, P.;
Maci, D.; Momferatos, G.
2022arXiv220703373D Altcode:
Magnetohydrodynamic turbulence is central to laboratory and
astrophysical plasmas, and is invoked for interpreting many observed
scalings. Verifying predicted scaling law behaviour requires
extreme-resolution direct numerical simulations (DNS), with needed
computing resources excluding systematic parameter surveys. We here
present an analytic generator of realistically looking turbulent
magnetic fields, that computes 3D ${\cal{O}}(1000^3)$ solenoidal
vector fields in minutes to hours on desktops. Our model is inspired
by recent developments in 3D incompressible fluid turbulence theory,
where a Gaussian white noise vector subjected to a non-linear
transformation results in an intermittent, multifractal random
field. Our $B\times C$ model has only few parameters that have
clear geometric interpretations. We directly compare a (costly) DNS
with a swiftly $B\times C$-generated realization, in terms of its
(i) characteristic sheet-like structures of current density, (ii)
volume-filling aspects across current intensity, (iii) power-spectral
behaviour, (iv) probability distribution functions of increments for
magnetic field and current density, structure functions, spectra of
exponents, and (v) partial variance of increments. The model even allows
to mimic time-evolving magnetic and current density distributions and
can be used for synthetic observations on 3D turbulent data cubes.

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Title: 2.5D turbulent magnetic reconnection behaviour in the solar
prominence due to Rayleigh-Taylor instability
Authors: Changmai, Madhurjya; Keppens, Rony
2022cosp...44.1099C Altcode:
The internal dynamic of solar prominences has been observed to be highly
complex for many decades, many of which also indicate the possibility
of turbulence. Prominences represent large-scale, dense condensations
suspended against gravity at great heights within the solar
atmosphere. Therefore, it is no surprise that the fundamental process of
the Rayleigh-Taylor (RT) instability has been suggested as the potential
mechanism for driving the dynamics and turbulence remarked upon within
observations. We begin with the 2.5D ideal magnetohydrodynamic (MHD)
high-resolution simulations with the open-source {\tt MPI-AMRVAC}
code and follow the far nonlinear evolution of an RT instability that
starts at the prominence-corona interface. We use statistical analysis
to investigate the evolution of turbulent regimes, which corresponds
to the observational counterpart. Furthermore, the strength of the
mean magnetic field directed into the 2D plane, and its orientation
with the plane itself, creates a system with varying turbulent
behaviour. The intermittent heating and energy dissipation events are
caused by magnetic reconnection, which we investigate in detail by the
2.5D fully-resistive MHD model. Based on the evolution of plasma beta
($\beta$) along the prominence's height, the stratified numerical model
generates different fluctuation statistics. As a result, we observe
that the turbulent dynamics and prominence reconnection events are
fairly distinct from those occurring elsewhere in the solar corona.

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Title: Resistive tearing growth rate modification by equilibrium flow
Authors: de Jonghe, Jordi; Keppens, Rony
2022cosp...44.1510D Altcode:
Ever since its discovery by Furth, Killeen, and Rosenbluth the resistive
tearing instability has been a well-studied phenomenon related to
magnetic reconnection and a conversion of magnetic energy into thermal
and/or kinetic energy. This conversion of energy may result in solar
flares. Since magnetic reconnection can be triggered by the resistive
tearing instability, effects that may increase the tearing growth rate
become of interest. Whilst the literature regularly claims that flow
has a stabilising effect on the resistive tearing mode, a parametric
study with the modern linear 1D magnetohydrodynamic (MHD) spectral code
Legolas (https://legolas.science) has revealed that equilibrium flow
can both increase and damp the tearing growth rate, depending on the
parameter regime. Relevant parameters include, but are not limited to,
the flow speed, the plasma-$\beta$, and the angle between the flow and
the wavevector, which contribute to the relation of flow shear versus
magnetic shear.

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Title: Estimating uncertainties in the back-mapping of the fast
solar wind
Authors: Koukras, Alexandros; Dolla, Laurent; Keppens, Rony
2022cosp...44.1546K Altcode:
Despite the fact that the sources of the fast solar wind are known to
be the coronal holes, the exact acceleration mechanism that drives
the fast solar wind is still not fully understood. An important
approach that can improve our understanding is the combination of
remote sensing and in situ measurements, which is often referred to as
linkage analysis. In order to combine these observables it is necessary
to accurately identify the source location of the in situ solar wind
with a process called back-mapping. Typically, back-mapping consists of
two main parts, the ballistic mapping from in situ to a point in the
outer corona, where the solar wind radially draws the magnetic field
into the Parker Spiral and the magnetic mapping, where the solar wind
follows the magnetic field line topology down to the solar surface. By
examining the different sources that can effect the derived back-mapped
position of the solar wind, we aim to provide a more precise estimate of
the source location. This can then be used to improve the connection of
remote sensing with in situ measurements. For the ballistic mapping we
created custom velocity profiles based on the Parker approximations for
small and large distances from the Sun. These profiles are constrained
by remote observations of the fast solar wind close to the Sun and are
used to examine the uncertainty in the ballistic mapping. The magnetic
topology is derived with a potential field source surface extrapolation
(PFSS), which takes as input a photospheric synoptic magnetogram. The
sensitivity of the extrapolated field in the initial conditions is
examined by adding noise to the input magnetogram and performing
a Monte Carlo simulation, where for multiple noise realizations we
calculate the source position of the solar wind. Next, the effect
of free parameters of the framework (like the height of the source
surface) is examined and statistical estimates are derived. Lastly,
we use a Gaussian Mixture clustering to provide the optimal grouping
of the back-mapped points and an estimate of the uncertainty in the
source location. Our uncertainty estimation is compared with other
similar frameworks, like the Magnetic Connectivity-Tool of IRAP.

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Title: Multi-threaded prominence oscillations
Authors: Jerčić, Veronika; Keppens, Rony; Zhou, Yuhao
2022cosp...44.2497J Altcode:
Solar prominences are plasma structures that are two orders of magnitude
colder and denser than the background corona in which they reside. They
are highly dynamic and highly structured but nonetheless can persist
for several days or even weeks. The building blocks of prominences are
relatively small thread-like structures, also known as fibrils. The
fibrils have lifetimes on a time scale of minutes during which they
exhibit multiple flows (so-called counterstreamings). One of the ways
the fibrils can be formed is a result of multiple heating events
at the footpoints of the coronal loop. Those heating events cause
evaporation of the chromospheric plasma that in the corona experiences
'catastrophic cooling'. Condensation happens and prominence fibrils
are formed. Embedded in the magnetically dominated, dynamic corona,
prominences (i.e. their fibrils) are also often seen oscillating
[1]. One of the possible (and very frequent) drivers of those
oscillations are coronal shock waves. Understanding the interplay
of the mechanisms that cause prominence formation and oscillation
provides important insight into the solar corona. Multiple numerical
simulations of oscillations have been conducted [2, 3, 4] in order to
analyze the exact mechanisms governing prominence behaviour. Most
of the studies on prominence oscillation to date ignored their
finer structure. Further on, most studies simply imposed velocity
perturbations directly on the prominence. That did not allow possible
effects induced by the presence of the driver. We simulate a 2D
adiabatic prominence where we focus on its thread-like structure and
its oscillations resulting from a realistic source we impose. We
notice longitudinal oscillations, for which our results show that
the pendulum model failed to estimate the period of the prominence
oscillation. Besides the global, longitudinal motion, transverse
oscillations are also evident. They represent small scale oscillations
that have an important influence on the total motion of the individual
fibrils. We extend this study by including non-adiabatic effects in
the system (thermal conduction, radiative cooling, steady background
and random localized heating). Using the localized heating events
we are able to realistically form prominence fibrils. We explore how
different parameters of the formation process affect the properties
of the formed fibrils. What makes the counterstreaming flows in the
domain and how does the different energy input change the resulting
fibrils? The magnetohydrodynamic (MHD) equations were solved using an
open-source MHD code, MPI-AMRVAC ([5], http://amrvac.org/). References
[1] M. Luna, J. Karpen, J. L. Ballester, K. Muglach, J. Terradas,
T. Kucera, and H. Gilbert, Astrophys. J. Suppl.Ser., 236 (2018) [2]
Y. Zhou, C. Xia, R. Keppens, C. Fang, and P. F. Chen, Astrophys. J.,
856 (2018) [3] V. Liakh, M. Luna, and E. Khomenko, Astron. Astrophys.,
637 (2020) [4] Y. H. Zhou, P. F. Chen, J. Hong and C. Fang, Nature
Astronomy, 994 (2020) [5] C. Xia, J. Teunissen, I. El Mellah, E. Chané,
and R. Keppens, Astrophys. J. Suppl. Ser, 234 (2018)

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Title: Three-Dimensional MHD Wave Propagation Near a Coronal Null
Point: a New Wave Mode Decomposition Approach
Authors: Yadav, Nitin; Keppens, Rony; Popescu Braileanu, Beatrice
2022cosp...44.2546Y Altcode:
Ubiquitous vortex flows at the solar surface excite magnetohydrodynamic
(MHD) waves that propagate to higher layers of the solar atmosphere. In
the solar corona, these waves frequently encounter magnetic null
points. The interaction of MHD waves with a coronal magnetic null
in realistic 3D setups requires an appropriate wave identification
method. We present a new MHD wave decomposition method that overcomes
the limitations of existing wave identification methods. Our method
allows to investigate the energy fluxes in different MHD modes at
different locations of the solar atmosphere as waves generated by
vortex flows travel through the solar atmosphere and pass near the
magnetic null. We use the open-source MPI-AMRVAC code to simulate wave
dynamics through a coronal null configuration. We apply a rotational
wave driver at our bottom photospheric boundary to mimic vortex flows
at the solar surface. To identify the wave energy fluxes associated
with different MHD wave modes, we employ a wave-decomposition method
that is able to uniquely distinguish different MHD modes. Our proposed
method utilizes the geometry of an individual magnetic field-line in 3D
space to separate out velocity perturbations associated with the three
fundamental MHD waves. We compare our method with an existing wave
decomposition method that uses magnetic flux surfaces instead. Over
selected flux surfaces, we calculate and analyze temporally averaged
wave energy fluxes, as well as acoustic and magnetic energy fluxes. Our
wave decomposition method allows us to estimate the relative strengths
of individual MHD wave energy fluxes. Our method for wave identification
is consistent with previous flux-surface-based methods and gives
expected results in terms of wave energy fluxes at various locations of
the null configuration. We show that ubiquitous vortex flows excite MHD
waves that contribute significantly to the Poynting flux in the solar
corona. Alfvén wave energy flux accumulates on the fan surface and fast
wave energy flux accumulates near the null point. There is a strong
current density buildup at the spine and fan surface. The proposed
method has advantages over previously utilized wave decomposition
methods, since it may be employed in realistic simulations or magnetic
extrapolations, as well as in real solar observations, whenever the 3D
field line shape is known. The essential characteristics of MHD wave
propagation near a null, such as wave energy flux accumulation and
current buildup at specific locations, translate to the more realistic
setup presented here. The enhancement in energy flux associated with
magneto-acoustic waves near nulls may have important implications in
the formation of jets and impulsive plasma flows.

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Title: Solar tornadoes: Thermal instability in helical magnetic
field configurations with flow
Authors: Hermans, Joris; Keppens, Rony
2022cosp...44.2539H Altcode:
Condensations are observed in many astrophysical environments. In solar
physics, common phenomena are coronal rain and prominences. Coronal
rain consists of transient dense blobs that form in magnetic loops and
rain down along the magnetic field lines \cite{Antolin}. Prominences
are cold, dense structures suspended in the hot, tenuous corona by the
magnetic field \cite{Parenti}. Solar tornadoes are a class of solar
prominences \cite{Pettit}, based on their apparent rotating shape,
and are in some cases viewed as feet of large, horizontal prominences
by which they are connected into the chromosphere. Whether or not
they rotate is a topic of debate with two main opposing views: actual
rotating magnetic structures \cite{Yang2018}, versus counterstreaming
flows \cite{Barczynski2021}. A process to form condensations without
self-gravity is thermal instability, where structures are formed
due to energy loss by radiative emission. Thermal instability is
a very likely mechanism to form solar prominences or coronal rain
in the solar corona, as shown by multidimensional MHD simulations
\cite{Jack} and spectral analysis \cite{Claesatmos}, alike. We used
the newly developed spectral eigenvalue code Legolas \cite{Legolas}
to study the instabilities of 1D equilibria, which may represent the
tornado-like feet of prominences.. We compare three helical magnetic
configurations, under influence of rotational flow, optically thin
radiative cooling and anisotropic thermal conduction. This gives
us insight into the arising magnetothermal instabilities, which are
responsible for the formation of condensations and the dynamics. A
follow-up 2.5D numerical simulation was set up using the open-source
software MPI-AMRVAC \cite{amrvac}. Modern MHD simulations allow us to
study the nonlinear and dynamic evolution of the condensations formed by
thermal instability in these rotating magnetic structures at ultra-high
resolution. \begin{thebibliography}{99} \bibitem{Antolin}
P. Antolin, Plasma Physics and Controlled Fusion, 62, 014016 (2020)
\bibitem{Parenti} S. Parenti, Living Reviews in Solar Physics, 11,
1 (2014) \bibitem{Pettit} E. Pettit, Astrophysical Journal, 76,
9 (1932) \bibitem{Yang2018} Z. Yang, et al., ApJ, 852, 79 (2018)
\bibitem{Barczynski2021} K. Barczynski, et al., Astronomy &amp;
Astrophysics, 653, A94 (2021) \bibitem{Jack} J.M. Jenkins, et al.,
Astronomy &amp; Astrophysics, 646, A134 (2021) \bibitem{Claesatmos}
N. Claes, et al., Solar Physics, 296, 143, (2021) \bibitem{Legolas}
N. Claes, et al., ApJS, 251, 25 (2020) \bibitem{amrvac} C. Xia,
J. Teunissen, I. El Mellah, E. Chané, &amp; R. Keppens ApJS, 234, 30
(2018) \end{thebibliography}

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Title: Non-adiabatic effects of the evolution of tearing mode in
a guiding magnetic field: Explosive reconnection and formation
of plasmoids
Authors: Sen, Samrat; Keppens, Rony
2022cosp...44.1501S Altcode:
Nonlinear evolution of the tearing mode is investigated
in a guiding magnetic field within the framework of
resistive magnetohydrodynamic simulation using MPI-AMRVAC
(\url{http://amrvac.org}). Earlier analytic studies by Ledentsov
(2021\href{https://ui.adsabs.harvard.edu/abs/2021SoPh..296...74L/abstract}{a},\href{https://ui.adsabs.harvard.edu/abs/2021SoPh..296...93L/abstract}{b},\href{https://ui.adsabs.harvard.edu/abs/2021SoPh..296..117L/abstract}{c})
address the thermal instability growth rate and spatial periodicity of
the current layers in the linear regime taking into account viscosity,
electrical and thermal conductivity, and radiative cooling. In this
work, we incorporate the non-adiabatic effects: radiative cooling,
and background heating relevant to the solar corona. The parametric
survey of different resistivities and plasma-$\beta$ for estimating
the instability growth rate in the linear and nonlinear regimes is
explored. We notice that at relatively high values of resistivity ($\sim
10^{-3}$ in dimensionless unit), above the threshold usually quoted
for explosive secondary tearing events (which is $\sim 10^{-5}$),
we get explosive behavior where thermal instability and tearing
behavior reinforce each other. We calculate the mean nonlinear growth
rates for different resistivities and find it to follow a power-law
distribution. The fragmentation of the current sheet and the formation
of the plasmoids in the nonlinear phase of the evolution due to the
thermal and tearing instabilities are noticed. We also estimate the
temporal variation of the plasmoid numbers and the density filling
factor of the plasmoids.

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Title: Resolving the solar prominence/filament paradox using the
magnetic Rayleigh-Taylor instability
Authors: Jenkins, Jack M.; Keppens, Rony
2022NatAs...6..942J Altcode: 2022NatAs.tmp..153J
Prominences and filaments are manifestations of magnetized, levitated
plasma within the solar coronal atmosphere. Their structure is assumed
to be driven by the ambient magnetic field, but various open questions
pertaining to their formation and evolution persist. In particular,
the discrepancy between their appearance if projected against
the solar disk or at the limb remain unexplained. State-of-the-art
magnetohydrodynamic simulations yield a fully three-dimensional model
that successfully unites the extreme ultraviolet and hydrogen Hα
views of quiescent prominences that contain radial striations with the
equivalent on-disk filaments comprised of finite width threads. We
analyse all hydromagnetic sources of the vorticity evolution and
find it consistent with the nonlinear development of the magnetic
Rayleigh-Taylor instability. We show that this universal gravitational
interchange process can explain the apparent dichotomy of the quiescent
prominence/filament appearances. Our simulation could also be used
to predict what the instruments associated with the Solar Orbiter and
the Inouye Solar Telescope (DKIST) will observe.

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Title: Implementation of the Soloviev equilibrium as a new CME model
in EUHFORIA
Authors: Linan, Luis; Keppens, Rony; Maharana, Anwesha; Poedts,
Stefaan; Schmieder, Brigitte
2022cosp...44.2431L Altcode:
The EUropean Heliosphere FORecasting Information Asset (EUHFORIA) is
designed to model the evolution of solar eruptions in the heliosphere
and to accurately forecast their geo-effectiveness. In EUHFORIA,
Coronal Mass Ejections (CMEs) are superposed on a steady background
solar wind and injected at $r=0.1\;AU$ into a 3D time-dependent ideal
magnetohydrodynamics heliospheric domain. Our study focuses on the
implementation of a new CME model to improve and extend the CME models
that are currently implemented, for instance by providing a more
realistic geometry or a faster execution time. The novel CME model
is based on an analytical solution of the Grad-Shafranov equation,
called the Soloviev solution, which describes a plasma equilibrium in
a toroidal geometry (Soloviev, Reviews of Plasma Physics, 1975). One of
the main advantages is that magnetic field and other physical quantities
like pressure and density can be determined in terms of an analytic
magnetic flux formula. This flux being a polynomial function of the
local coordinates, we can directly control the interior properties
(in terms of shape and topology) within the cross-section of the toroid
with the spherical inner boundary at $r=0.1\;AU$. Hence, in practice,
the numerical computation of this model is less time consuming than the
FRi3D CME model that requires the numerical solution of differential
equations in each time step (Isavnin, Astrophys. J., 2016). Furthermore,
our implementation offers a wide range of free parameters, including the
shape of the model (aspect ratio, shape of the poloidal cross-section)
to the distribution and strength of the magnetic field lines in the
torus. This suffices to approach the geometry and characteristics
of observed CMEs. Some parameters are limited well-defined ranges,
to ensure basic physical aspects like positivity of thermodynamic
quantities. After the Soloviev CME is injected into the heliospheric
domain of EUHFORIA as a time-dependent boundary condition, it is
self-consistently evolved by the magnetohydrodynamics equations to
Earth. Finally, we present a test case CME modelled with Soloviev
and compare the plasma and magnetic field predictions with the
observations. This research has received funding from the European
Union's Horizon 2020 research and innovation programme under grant
agreement No 870405 (EUHFORIA 2.0)

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Title: Legolas: magnetohydrodynamic spectroscopy with viscosity and
Hall current
Authors: De Jonghe, J.; Claes, N.; Keppens, R.
2022JPlPh..88c9021D Altcode: 2022arXiv220607377D
Many linear stability aspects in plasmas are heavily influenced
by non-ideal effects beyond the basic ideal magnetohydrodynamics
(MHD) description. Here, the extension of the modern open-source
MHD spectroscopy code Legolas with viscosity and the Hall current is
highlighted and benchmarked on a stringent set of historic and recent
findings. The viscosity extension is demonstrated in a cylindrical
set-up featuring Taylor-Couette flow and in a viscoresistive plasma
slab with a tearing mode. For the Hall extension, we show how the full
eigenmode spectrum relates to the analytic dispersion relation in an
infinite homogeneous medium. We quantify the Hall term influence on the
resistive tearing mode in a Harris current sheet, including the effect
of compressibility, which is absent in earlier studies. Furthermore,
we illustrate how Legolas mimics the incompressible limit easily to
compare with literature results. Going beyond published findings, we
emphasise the importance of computing the full eigenmode spectrum, and
how elements of the spectrum are modified by compressibility. These
extensions allow for future stability studies with Legolas that
are relevant to ongoing dynamo experiments, protoplanetary disks or
magnetic reconnection.

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Title: Two-fluid implementation in MPI-AMRVAC, with applications in
the solar chromosphere
Authors: Popescu Braileanu, B.; Keppens, R.
2022arXiv220505049P Altcode:
The chromosphere is a partially ionized layer of the solar atmosphere,
the transition between the photosphere where the gas is almost neutral
and the fully ionized corona. As the collisional coupling between
neutral and charged particles decreases in the upper part of the
chromosphere, the hydrodynamical timescales may become comparable to
the collisional timescale, and a two-fluid model is needed. In this
paper we describe the implementation and validation of a two-fluid
model which simultaneously evolves charges and neutrals, coupled
by collisions. The two-fluid equations are implemented in the fully
open-source MPI-AMRVAC code. In the photosphere and the lower part of
the solar atmosphere, where collisions between charged and neutral
particles are very frequent, an explicit time-marching would be too
restrictive, since for stability the timestep needs to be proportional
to the inverse of the collision frequency. This is overcome by
evaluating the collisional terms implicitly using an explicit-implicit
(IMEX) scheme. The cases presented cover very different collisional
regimes and our results are fully consistent with related literature
findings. If collisional time and length scales are smaller than the
hydrodynamical scales usually considered in the solar chromosphere,
density structures seen in the neutral and charged fluids are similar,
with the effect of elastic collisions between charges and neutrals being
similar to diffusivity. Otherwise, density structures are different and
the decoupling in velocity between the two species increases. The use of
IMEX schemes efficiently avoids the small timestep constraints of fully
explicit implementations in strongly collisional regimes. Adaptive
Mesh Refinement (AMR) greatly decreases the computational cost,
compared to uniform grid runs at the same effective resolution.

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Title: The Super-Alfvénic Rotational Instability in Accretion Disks
about Black Holes
Authors: Goedbloed, Hans; Keppens, Rony
2022ApJS..259...65G Altcode: 2022arXiv220111551G
The theory of instability of accretion disks about black holes, neutron
stars, or protoplanets is revisited by means of the recent method of
the Spectral Web. The cylindrical accretion disk differential equation
is shown to be governed by the forward and backward Doppler-shifted
continuous Alfvén spectra ${{\rm{\Omega }}}_{{\rm{A}}}^{\pm }\equiv
m{\rm{\Omega }}\pm {\omega }_{{\rm{A}}}$ , where ω A is
the static Alfvén frequency. It is crucial to take nonaxisymmetry (m
≠ 0) and super-Alfvénic rotation of the Doppler frames (∣mΩ∣
≫ ∣ω A∣) into account. The continua ${{\rm{\Omega
}}}_{{\rm{A}}}^{+}$ and ${{\rm{\Omega }}}_{{\rm{A}}}^{-}$ then overlap,
ejecting a plethora of super-Alfvénic rotational instabilities
(SARIs). In-depth analysis for small inhomogeneity shows that the
two Alfvén singularities reduce the extent of the modes to sizes
much smaller than the width of the accretion disk. Generalization
for large inhomogeneity leads to the completely unprecedented result
that, for mode numbers ∣k∣ ≫ ∣m∣, any complex ω in a wide
neighborhood of the real axis is an approximate "eigenvalue." The
difference with genuine eigenmodes is that the amount of complementary
energy to excite the modes is tiny, ∣W com∣ ≤ c, with
c the machine accuracy of the computation. This yields a multitude
of two-dimensional continua of quasi-discrete modes: quasi-continuum
SARIs. We conjecture that the onset of 3D turbulence in magnetized
accretion disks is governed not by the excitation of discrete
axisymmetric magnetorotational instabilities but by the excitation
of modes from these two-dimensional continua of quasi-discrete
nonaxisymmetric SARIs.

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Title: 3D MHD wave propagation near a coronal null point: New wave
mode decomposition approach
Authors: Yadav, N.; Keppens, R.; Popescu Braileanu, B.
2022A&A...660A..21Y Altcode: 2022arXiv220109704Y
Context. Ubiquitous vortex flows at the solar surface excite
magnetohydrodynamic (MHD) waves that propagate to higher layers of
the solar atmosphere. In the solar corona, these waves frequently
encounter magnetic null points. The interaction of MHD waves with a
coronal magnetic null in realistic 3D setups requires an appropriate
wave identification method. Aims: We present a new MHD wave
decomposition method that overcomes the limitations of existing wave
identification methods. Our method allows for an investigation of the
energy fluxes in different MHD modes at different locations of the
solar atmosphere as waves generated by vortex flows travel through
the solar atmosphere and pass near the magnetic null. Methods:
We used the open-source MPI-AMRVAC code to simulate wave dynamics
through a coronal null configuration. We applied a rotational wave
driver at our bottom photospheric boundary to mimic vortex flows at
the solar surface. To identify the wave energy fluxes associated with
different MHD wave modes, we employed a wave decomposition method that
is able to uniquely distinguish different MHD modes. Our proposed
method utilizes the geometry of an individual magnetic field-line
in the 3D space to separate the velocity perturbations associated
with the three fundamental MHD waves. We compared our method with an
existing wave decomposition method that uses magnetic flux surfaces
instead. Over the selected flux surfaces, we calculated and analyzed
the temporally averaged wave energy fluxes, as well as the acoustic
and magnetic energy fluxes. Our wave decomposition method allowed
us to estimate the relative strengths of individual MHD wave energy
fluxes. Results: Our method for wave identification is consistent
with previous flux-surface-based methods and provides the expected
results in terms of the wave energy fluxes at various locations of
the null configuration. We show that ubiquitous vortex flows excite
MHD waves that contribute significantly to the Poynting flux in the
solar corona. Alfvén wave energy flux accumulates on the fan surface
and fast wave energy flux accumulates near the null point. There is
a strong current density buildup at the spine and fan surface. 
Conclusions: The proposed method has advantages over previously utilized
wave decomposition methods, since it may be employed in realistic
simulations or magnetic extrapolations, as well as in real solar
observations whenever the 3D fieldline shape is known. The essential
characteristics of MHD wave propagation near a null - such as wave
energy flux accumulation and current buildup at specific locations -
translate to the more realistic setup presented here. The enhancement
in energy flux associated with magneto-acoustic waves near nulls may
have important implications in the formation of jets and impulsive
plasma flows.

---------------------------------------------------------
Title: Plasmoid-fed Prominence Formation (PF2) During
Flux Rope Eruption
Authors: Zhao, Xiaozhou; Keppens, Rony
2022ApJ...928...45Z Altcode: 2022arXiv220208367Z
We report a new, plasmoid-fed scenario for the formation of an
eruptive prominence (PF2), involving reconnection and
condensation. We use grid-adaptive resistive two-and-a-half-dimensional
magnetohydrodynamic simulations in a chromosphere-to-corona setup to
resolve this plasmoid-fed scenario. We study a preexisting flux rope
(FR) in the low corona that suddenly erupts due to catastrophe, which
also drives a fast shock above the erupting FR. A current sheet (CS)
forms underneath the erupting FR, with chromospheric matter squeezed
into it. The plasmoid instability occurs and multiple magnetic
islands appear in the CS once the Lundquist number reaches ~3.5 ×
104. The remnant chromospheric matter in the CS is then
transferred to the FR by these newly formed magnetic islands. The dense
and cool mass transported by the islands accumulates in the bottom of
the FR, thereby forming a prominence during the eruption phase. More
coronal plasma continuously condenses into the prominence due to the
thermal instability as the FR rises. Due to the fine structure brought
in by the PF2 process, the model naturally forms filament
threads, aligned above the polarity inversion line. Synthetic views
at our resolution of 15 km show many details that may be verified in
future high-resolution observations.

---------------------------------------------------------
Title: Coronal Rain in Randomly Heated Arcades
Authors: Li, Xiaohong; Keppens, Rony; Zhou, Yuhao
2022ApJ...926..216L Altcode: 2021arXiv211202702L
Adopting the MPI-AMRVAC code, we present a 2.5-dimensional
magnetohydrodynamic simulation, which includes thermal conduction
and radiative cooling, to investigate the formation and evolution
of the coronal rain phenomenon. We perform the simulation in
initially linear force-free magnetic fields that host chromospheric,
transition-region, and coronal plasma, with turbulent heating localized
on their footpoints. Due to thermal instability, condensations start
to occur at the loop top, and rebound shocks are generated by the
siphon inflows. Condensations fragment into smaller blobs moving
downwards, and as they hit the lower atmosphere, concurrent upflows
are triggered. Larger clumps show us clear coronal rain showers
as dark structures in synthetic EUV hot channels and as bright
blobs with cool cores in the 304 Å channel, well resembling real
observations. Following coronal rain dynamics for more than 10 hr, we
carry out a statistical study of all coronal rain blobs to quantify
their widths, lengths, areas, velocity distributions, and other
properties. The coronal rain shows us continuous heating-condensation
cycles, as well as cycles in EUV emissions. Compared to the previous
studies adopting steady heating, the rain happens faster and in more
erratic cycles. Although most blobs are falling downward, upward-moving
blobs exist at basically every moment. We also track the movement of
individual blobs to study their dynamics and the forces driving their
movements. The blobs have a prominence-corona transition-region-like
structure surrounding them, and their movements are dominated by the
pressure evolution in the very dynamic loop system.

---------------------------------------------------------
Title: Multi-threaded prominence oscillations triggered by a coronal
shock wave
Authors: Jerčić, V.; Keppens, R.; Zhou, Y.
2022A&A...658A..58J Altcode: 2021arXiv211109019J
Context. Understanding the interplay between ubiquitous coronal
shock waves and the resulting prominence oscillations is a key
factor in improving our knowledge of prominences and the solar
corona overall. In particular, prominences are a key element of
the solar corona and represent a window into an as yet unexplained
processes in the Sun's atmosphere. Aims: To date, most studies
on oscillations of prominences have ignored their finer structure
and analyzed them strictly as monolithic bodies. In this work, we
study the causal relations between a localised energy release and a
remote prominence oscillation, where the prominence has a realistic
thread-like structure. Methods: In our work, we used an
open source magnetohydrodynamic code known as MPI-AMRVAC to create
a multi-threaded prominence body. In this domain, we introduced an
additional energy source from which a shock wave originates, thereby
inducing prominence oscillation. We studied two cases with different
source amplitudes to analyze its effect on the oscillations. 
Results: Our results show that the frequently used pendulum model
does not suffice to fully estimate the period of the prominence
oscillation, in addition to showing that the influence of the source
and the thread-like prominence structure needs to be taken into
account. Repeated reflections and transmissions of the initial shock
wave occur at the specific locations of multiple high-temperature and
high-density gradients in the domain. This includes the left and right
transition region located at the footpoints of the magnetic arcade,
as well as the various transition regions between the prominence and
the corona. This results in numerous interferences of compressional
waves propagating within and surrounding the prominence plasma. They
contribute to the restoring forces of the oscillation, causing the
period to deviate from the expected pendulum model, in addition to
leading to differences in attributed damping or even growth in amplitude
between the various threads. Along with the global longitudinal motion
that result from the shock impact, small-scale transverse oscillations
are also evident. Multiple high-frequency oscillations represent the
propagation of magnetoacoustic waves. The damping we see is linked
to the conversion of energy and its exchange with the surrounding
corona. Our simulations demonstrate the exchange of energy between
different threads and their different modes of oscillation.

---------------------------------------------------------
Title: Radiation-hydrodynamics with MPI-AMRVAC . Flux-limited
diffusion
Authors: Moens, N.; Sundqvist, J. O.; El Mellah, I.; Poniatowski,
L.; Teunissen, J.; Keppens, R.
2022A&A...657A..81M Altcode: 2021arXiv210403968M
Context. Radiation controls the dynamics and energetics of many
astrophysical environments. To capture the coupling between the
radiation and matter, however, is often a physically complex and
computationally expensive endeavor. Aims: We sought to develop
a numerical tool to perform radiation-hydrodynamics simulations in
various configurations at an affordable cost. Methods: We
built upon the finite volume code MPI-AMRVAC to solve the equations
of hydrodynamics on multi-dimensional adaptive meshes and introduce a
new module to handle the coupling with radiation. A non-equilibrium,
flux-limiting diffusion approximation was used to close the radiation
momentum and energy equations. The time-dependent radiation energy
equation was then solved within a flexible framework, fully accounting
for radiation forces and work terms and further allowing the user to
adopt a variety of descriptions for the radiation-matter interaction
terms ("opacities"). Results: We validated the radiation module
on a set of standard test cases for which different terms of the
radiative energy equation predominate. As a preliminary application to
a scientific case, we calculated spherically symmetric models of the
radiation-driven and optically thick supersonic outflows from massive
Wolf-Rayet stars. This also demonstrates our code's flexibility, as
the illustrated simulation combines opacities typically used in static
stellar structure models with a parametrized form for the enhanced
line-opacity expected in supersonic flows. Conclusions: This
new module provides a convenient and versatile tool for performing
multi-dimensional and high-resolution radiative-hydrodynamics
simulations in optically thick environments with the MPI-AMRVAC
code. The code is ready to be used for a variety of astrophysical
applications, where our first target is set to be multi-dimensional
simulations of stellar outflows from Wolf-Rayet stars.

---------------------------------------------------------
Title: Magnetohydrodynamic Spectroscopy of a Non-Adiabatic Solar
Atmosphere
Authors: Claes, Niels; Keppens, R.
2021AGUFMSH42B..08C Altcode:
In this work we present a detailed, high-resolution eigenspectrum
analysis of the solar atmosphere using our recently developed
Legolas code to calculate full spectra and eigenfunctions of
various equilibrium configurations, based on fully realistic solar
atmospheric models including gravity, optically thin radiative losses,
and anisotropic thermal conduction. Our models treat a stratified,
magnetized atmosphere with density and temperature profiles based
on a widely used semi-empirical model. We mainly focus on thermal
instabilities, together with a new outlook on the slow and thermal
continua and their behavior in different chromospheric and coronal
regions. We show that thermal instabilities are ubiquitously present
in our solar atmospheric models, which implies a great variety of
linear pathways to form condensations. Since these instabilities lie
at the very basis of prominence formation and coronal rain, knowing
all MHD modes of possibly coupled mode types in the magnetized solar
atmosphere and how they modify as a result of including non-adiabatic
effects, is thus a clear necessity. We demonstrate for the first time
the intricate structure of the thermal, slow and Alfven continua, and
the way the many discrete modes organize in (coupled) thermal, slow,
Alfven and fast wave sequences. This essentially gives us a linear
preview of how nonlinear simulations should develop as a result of
(interacting) instabilities. We also encounter regions where the slow,
thermal, and fast modes all have unstable wave mode solutions, along
with situations where the thermal and slow continua become purely
imaginary and merge on the imaginary spectral axis. Since many linear
waves and instabilities can be at play in realistic solar atmospheric
evolutions, modern nonlinear simulations can benefit greatly from the
full knowledge of all linear instabilities and eigenoscillations of
a given configuration, while the MHD magnetothermal subspectrum is
interesting in its own right for quantifying thermal instability. The
spectra discussed illustrate clearly that thermal instabilities
(both discrete and continuum modes) and magneto-thermal overstable
propagating modes are of great importance in the solar atmosphere and
may very well be responsible for much of the observed fine-structuring
and multi-thermal dynamics.

---------------------------------------------------------
Title: The 2.5D and 3D Structure and Evolution of Solar Prominence
Plasma Condensations
Authors: Jenkins, Jack; Keppens, R.
2021AGUFMSH43A..05J Altcode:
Solar prominences exist as a delicate balance between both magnetic
and gravitational forces, and thermal and mechanical energies,
within the solar corona. Despite extensive research on the topic,
many questions remain outstanding most notably of which are as to
their general internal magnetic structure, condensation formation
mechanism, and subsequent evolution. Beginning with the pioneering
work of Kaneko &amp; Yokoyama (2015) we have revisited the so-called
levitation-condensation mechanism for the ab-initio formation of solar
prominences. Our initial 2.5D, 5.6 km resolution study (Jenkins &amp;
Keppens, 2021) demonstrated that the thermal runaway condensation can
happen at any location, not solely in the bottom part of the flux rope
where the majority of stable material is believed to reside. In the
presence of gravity, intricate thermodynamic evolutions and shearing
flows developed spontaneously, themselves inducing further fine-scale
(magneto)hydrodynamic instabilities. Of particular note, the condensing
prominence plasma in our baroclitic atmosphere evolved according to the
internal pressure and density gradients but specifically misalignments
therein i.e., baroclinicity, hinting relevance to the Rayleigh-Taylor
instability or RTI process in 3D. Our recent extension to 3D achieves
a resolution still far in excess of all modern solar observatories
at ~20 km. As anticipated, we find that the additional dimension
permits those dynamics that were previously suppressed within the
2.5D implementation. Namely, we relate the interchange mode of the
Rayleigh-Taylor instability, now explicitly linked to additional
(baroclitic) solenoidal source terms within the evolving vorticity
formalism, to the evolution of those falling fingers and rising plumes
characteristic of solar prominences. Fundamentally, we will show that
our visualisations of the condensations within the flux rope topology
are able to reconcile the longstanding discrepancy between the filament
and prominence projections.

---------------------------------------------------------
Title: Effect of optically thin cooling curves on condensation
formation: Case study using thermal instability
Authors: Hermans, J.; Keppens, R.
2021A&A...655A..36H Altcode: 2021arXiv210707569H
Context. Non-gravitationally induced condensations are observed in
many astrophysical environments. In solar physics, common phenomena
are coronal rain and prominences. These structures are formed due to
energy loss by optically thin radiative emission. Instead of solving
the full radiative transfer equations, precomputed cooling curves
are typically used in numerical simulations. In the literature,
a wide variety of cooling curves exist, and they are quite often
used as unquestionable ingredients. Aims: We here determine
the effect of the optically thin cooling curves on the formation and
evolution of condensations. We also investigate the effect of numerical
settings. This includes the resolution and the low-temperature treatment
of the cooling curves, for which the optically thin approximation
is not valid. Methods: We performed a case study using
thermal instability as a mechanism to form in situ condensations. We
compared 2D numerical simulations with different cooling curves using
interacting slow magnetohydrodynamic (MHD) waves as trigger for the
thermal instability. Furthermore, we discuss a bootstrap measure to
investigate the far non-linear regime of thermal instability. In the
appendix, we include the details of all cooling curves implemented in
MPI-AMRVAC and briefly discuss a hydrodynamic variant of the slow MHD
waves setup for thermal instability. Results: For all tested
cooling curves, condensations are formed. The differences due to the
change in cooling curve are twofold. First, the growth rate of the
thermal instability is different, leading to condensations that form
at different times. Second, the morphology of the formed condensation
varies widely. After the condensation forms, we find fragmentation
that is affected by the low-temperature treatment of the cooling
curves. Condensations formed using cooling curves that vanish for
temperatures lower than 20 000 K appear to be more stable against
dynamical instabilities. We also show the need for high-resolution
simulations. The bootstrap procedure allows us to continue the
simulation into the far non-linear regime, where the condensation
fragments dynamically align with the background magnetic field. The
non-linear regime and fragmentation in the hydrodynamic case differ
greatly from the low-beta MHD case. Conclusions: We advocate the
use of modern cooling curves, based on accurate computations and current
atomic parameters and solar abundances. Our bootstrap procedure can be
used in future multi-dimensional simulations to study fine-structure
dynamics in solar prominences. Movies are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202140665/olm">https://www.aanda.org</A>

---------------------------------------------------------
Title: When Hot Meets Cold: Post-flare Coronal Rain
Authors: Ruan, Wenzhi; Zhou, Yuhao; Keppens, Rony
2021ApJ...920L..15R Altcode: 2021arXiv210911873R
Most solar flares demonstrate a prolonged, hour-long post-flare (or
gradual) phase, characterized by arcade-like, post-flare loops (PFLs)
visible in many extreme ultraviolet (EUV) passbands. These coronal
loops are filled with hot (~30 MK) and dense plasma that evaporated
from the chromosphere during the impulsive phase of the flare, and
they very gradually recover to normal coronal density and temperature
conditions. During this gradual cooling down to ~1 MK regimes, much
cooler (~0.01 MK) and denser coronal rain is frequently observed inside
PFLs. Understanding PFL dynamics in this long-duration, gradual phase is
crucial to the entire corona-chromosphere mass and energy cycle. Here
we report a simulation in which a solar flare evolves from pre-flare,
over the impulsive phase all the way into its gradual phase, which
successfully reproduces post-flare coronal rain. This rain results from
catastrophic cooling caused by thermal instability, and we analyze the
entire mass and energy budget evolution driving this sudden condensation
phenomenon. We find that the runaway cooling and rain formation also
induces the appearance of dark post-flare loop systems, as observed in
EUV channels. We confirm and augment earlier observational findings,
suggesting that thermal conduction and radiative losses alternately
dominate the cooling of PFLs.

---------------------------------------------------------
Title: Data-constrained Magnetohydrodynamic Simulation of a
Long-duration Eruptive Flare
Authors: Guo, Yang; Zhong, Ze; Ding, M. D.; Chen, P. F.; Xia, Chun;
Keppens, Rony
2021ApJ...919...39G Altcode: 2021arXiv210615080G
We perform a zero-β magnetohydrodynamic simulation for the C7.7
class flare initiated at 01:18 UT on 2011 June 21 using the Message
Passing Interface Adaptive Mesh Refinement Versatile Advection Code
(MPI-AMRVAC). The initial condition for the simulation involves a
flux rope, which we realize through the regularized Biot-Savart laws,
whose parameters are constrained by observations from the Atmospheric
Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO) and
the Extreme Ultraviolet Imager (EUVI) on the twin Solar Terrestrial
Relations Observatory (STEREO). This data-constrained initial state
is then relaxed to a force-free state by the magnetofrictional module
in MPI-AMRVAC. The further time-evolving simulation results reproduce
the eruption characteristics obtained by SDO/AIA 94 Å, 304 Å, and
STEREO/EUVI 304 Å observations fairly well. The simulated flux rope
possesses similar eruption direction, height range, and velocity to
the observations. In particular, the two phases of slow evolution
and fast eruption are reproduced by varying the density distribution
in the light of the draining process of the filament material. Our
data-constrained simulations also show other advantages, such as a
large field of view (about 0.76 R⊙). We study the twist
of the magnetic flux rope and the decay index of the overlying field,
and find that in this event, both the magnetic strapping force and the
magnetic tension force are sufficiently weaker than the magnetic hoop
force, thus allowing the successful eruption of the flux rope. We also
find that the anomalous resistivity is necessary to keep the correct
morphology of the erupting flux rope.

---------------------------------------------------------
Title: Effects of ambipolar diffusion on waves in the solar
chromosphere
Authors: Popescu Braileanu, B.; Keppens, R.
2021A&A...653A.131P Altcode: 2021arXiv210510285P
Context. The chromosphere is a partially ionized layer of the solar
atmosphere that mediates the transition between the photosphere where
the gas motion is determined by the gas pressure and the corona
dominated by the magnetic field. Aims: We study the effect
of partial ionization for 2D wave propagation in a gravitationally
stratified, magnetized atmosphere characterized by properties that are
similar to those of the solar chromosphere. Methods: We adopted
an oblique uniform magnetic field in the plane of propagation with
a strength that is suitable for a quiet sun region. The theoretical
model we used is a single fluid magnetohydrodynamic approximation,
where ion-neutral interaction is modeled by the ambipolar diffusion
term. Magnetic energy can be converted into internal energy through the
dissipation of the electric current produced by the drift between ions
and neutrals. We used numerical simulations in which we continuously
drove fast waves at the bottom of the atmosphere. The collisional
coupling between ions and neutrals decreases with the decrease in
the density and the ambipolar effect thus becomes important. 
Results: Fast waves excited at the base of the atmosphere reach
the equipartition layer and are reflected or transmitted as slow
waves. While the waves propagate through the atmosphere and the
density drops, the waves steepen into shocks. Conclusions: The
main effect of ambipolar diffusion is damping of the waves. We find
that for the parameters chosen in this work, the ambipolar diffusion
affects the fast wave before it is reflected, with damping being more
pronounced for waves which are launched in a direction perpendicular
to the magnetic field. Slow waves are less affected by ambipolar
effects. The damping increases for shorter periods and greater magnetic
field strengths. Small scales produced by the nonlinear effects and the
superposition of different types of waves created at the equipartition
height are efficiently damped by ambipolar diffusion.

---------------------------------------------------------
Title: Magnetic island merging: Two-dimensional MHD simulation and
test-particle modeling
Authors: Zhao, Xiaozhou; Bacchini, Fabio; Keppens, Rony
2021PhPl...28i2113Z Altcode: 2021arXiv210813508Z
In an idealized system where four current channels interact in a
two-dimensional periodic setting, we follow the detailed evolution of
current sheets (CSs) forming in between the channels as a result of
a large-scale merging. A central X-point collapses, and a gradually
extending CS marks the site of continuous magnetic reconnection. Using
grid-adaptive, non-relativistic, resistive magnetohydrodynamic
(MHD) simulations, we establish that slow, near-steady Sweet-Parker
reconnection transits to a chaotic, multi-plasmoid fragmented state
when the Lundquist number exceeds about 3 × 10 4 , well
in the range of previous studies on plasmoid instability. The extreme
resolution employed in the MHD study shows significant magnetic island
substructures. With relativistic test-particle simulations, we explore
how charged particles can be accelerated in the vicinity of an O-point,
either at embedded tiny-islands within larger "monster"-islands or
near the centers of monster-islands. While the planar MHD setting
artificially causes strong acceleration in the ignored third direction,
it also allows for the full analytic study of all aspects leading to
the acceleration and the in-plane-projected trapping of particles in
the vicinities of O-points. Our analytic approach uses a decomposition
of the particle velocity in slow- and fast-changing components, akin
to the Reynolds decomposition in turbulence studies. Our analytic
description is validated with several representative test-particle
simulations. We find that after an initial non-relativistic motion
throughout a monster island, particles can experience acceleration in
the vicinity of an O-point beyond √{ 2 } c / 2 ≈ 0.7 c , at which
speed the acceleration is at its highest efficiency.

---------------------------------------------------------
Title: Magnetohydrodynamic Spectroscopy of a Non-adiabatic Solar
Atmosphere
Authors: Claes, Niels; Keppens, Rony
2021SoPh..296..143C Altcode: 2021arXiv210809467C
The quantification of all possible waves and instabilities in
any given system is of paramount importance, and knowledge of the
full magnetohydrodynamic (MHD) spectrum allows one to predict the
(in)stability of a given equilibrium state. This is highly relevant
in many (astro)physical disciplines, and when applied to the solar
atmosphere it may yield various new insights in processes such as
prominence formation and coronal-loop oscillations. In this work we
present a detailed, high-resolution spectroscopic study of the solar
atmosphere, where we use our newly developed Legolas code to calculate
the full spectrum with corresponding eigenfunctions of equilibrium
configurations that are based on fully realistic solar atmospheric
models, including gravity, optically thin radiative losses, and thermal
conduction. Special attention is given to thermal instabilities, known
to be responsible for the formation of prominences, together with
a new outlook on the thermal and slow continua and how they behave
in different chromospheric and coronal regions. We show that thermal
instabilities are unavoidable in our solar atmospheric models and that
there exist certain regions where the thermal, slow, and fast modes
all have unstable wave-mode solutions. We also encounter regions where
the slow and thermal continua become purely imaginary and merge on the
imaginary axis. The spectra discussed in this work illustrate clearly
that thermal instabilities (both discrete and continuum modes) and
magneto-thermal overstable propagating modes are ubiquitous throughout
the solar atmosphere, and they may well be responsible for much of
the observed fine-structuring and multi-thermal dynamics.

---------------------------------------------------------
Title: An MHD spectral theory approach to Jeans' magnetized
gravitational instability
Authors: Durrive, Jean-Baptiste; Keppens, Rony; Langer, Mathieu
2021MNRAS.506.2336D Altcode: 2021MNRAS.tmp.1506D; 2021arXiv210607681D
In this paper, we revisit the governing equations for linear
magnetohydrodynamic (MHD) waves and instabilities existing within a
magnetized, plane-parallel, self-gravitating slab. Our approach allows
for fully non-uniformly magnetized slabs, which deviate from isothermal
conditions, such that the well-known Alfvén and slow continuous spectra
enter the description. We generalize modern MHD textbook treatments,
by showing how self-gravity enters the MHD wave equation, beyond the
frequently adopted Cowling approximation. This clarifies how Jeans'
instability generalizes from hydro to MHD conditions without assuming
the usual Jeans' swindle approach. Our main contribution lies in
reformulating the completely general governing wave equations in a
number of mathematically equivalent forms, ranging from a coupled
Sturm-Liouville formulation, to a Hamiltonian formulation linked to
coupled harmonic oscillators, up to a convenient matrix differential
form. The latter allows us to derive analytically the eigenfunctions
of a magnetized, self-gravitating thin slab. In addition, as an
example, we give the exact closed form dispersion relations for
the hydrodynamical p- and Jeans-unstable modes, with the latter
demonstrating how the Cowling approximation modifies due to a proper
treatment of self-gravity. The various reformulations of the MHD
wave equation open up new avenues for future MHD spectral studies of
instabilities as relevant for cosmic filament formation, which can
e.g. use modern formal solution strategies tailored to solve coupled
Sturm-Liouville or harmonic oscillator problems.

---------------------------------------------------------
Title: Erratum: Legolas: A Modern Tool for Magnetohydrodynamic
Spectroscopy (2020, ApJS, 251, 25)
Authors: Claes, Niels; De Jonghe, Jordi; Keppens, Rony
2021ApJS..254...45C Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Two Fluid Treatment of Whistling Behavior and the Warm Appleton
Hartree Extension
Authors: De Jonghe, J.; Keppens, R.
2021JGRA..12628953D Altcode: 2021arXiv210405275D
As an application of the completely general, ideal two fluid analysis
of waves in a warm ion electron plasma, where six unique wave pair
labels (S, A, F, M, O, and X) were identified, we here connect to the
vast body of literature on whistler waves. We show that all six mode
pairs can demonstrate whistling behavior, when we allow for whistling
of both descending and ascending frequency types, and when we study
the more general case of oblique propagation to the background
magnetic field. We show how the general theory recovers all known
approximate group speed expressions for both classical whistlers
and ion cyclotron whistlers, which we here extend to include ion
contributions and deviations from parallel propagation. At oblique
angles and at perpendicular propagation, whistlers are investigated
using exact numerical evaluations of the two fluid dispersion relation
and their group speeds under Earth's magnetosphere conditions. This
approach allows for a complete overview of all whistling behavior and we
quantify the typical frequency ranges where they must be observable. We
use the generality of the theory to show that pair plasmas in pulsar
magnetospheres also feature whistling behavior, although not of the
classical type at parallel propagation. Whistling of the high frequency
modes is discussed as well, and we give the extension of the Appleton
Hartree relation for cold plasmas, to include the effect of a nonzero
thermal electron velocity. We use it to quantify the Faraday rotation
effect at all angles, and compare its predictions between the cold
and warm Appleton Hartree equation.

---------------------------------------------------------
Title: Turbulence characteristics of Solar Prominences due to Rayleigh
Taylor Instabilities
Authors: Changmai, Madhurjya; Keppens, Rony
2021EGUGA..2311979C Altcode:
The purpose of our study is to deepen our understanding on the
turbulence that arises from Rayleigh Taylor Instabilities in quiescent
solar prominences. Quiescent prominences in the solar corona are
cool and dense condensates that show internal dynamics over a wide
range of spatial and temporal scales. These dynamics are dominated
by vertical flows in the prominence body where the mean magnetic
field is predominantly in the horizontal direction and the magnetic
pressure suspends the dense prominence material. We perform numerical
simulations using MPI-AMRVAC (http://amrvac.org) to study the Rayleigh
Taylor Instabilitiy at the prominence-corona transition region using the
Ideal-magentohydrodyamics approach. High resolution simulations achieve
a resolution of ∼23 km for ∼21 min transitioning from a multi-mode
perturbation instability to the non-linear regime and finally a fully
turbulent prominence. We use statistical methods to quantify the rich
dynamics in quiescent prominence as being indicative of turbulence.

---------------------------------------------------------
Title: Prominence Formation by Levitation-Condensation at Extreme
Resolutions
Authors: Jenkins, Jack; Keppens, Rony
2021EGUGA..23.2445J Altcode:
We revisit the so-called levitation-condensation mechanism for the
ab-inito formation of solar prominences: cool and dense clouds in
the million-degree solar atmosphere. Levitation-condensation occurs
following the formation of a flux rope in response to the deformation of
a force-free coronal arcade by controlled magnetic footpoint motions and
subsequent reconnection. Existing coronal plasma gets lifted within the
forming rope, therein isolating a collection of matter now more dense
than its immediate surroundings. This denser region ultimately suffers
a thermal instability driven by radiative losses, and a prominence
forms. We improve on various aspects that were left unanswered in the
early work, by revisiting this model with our modern open-source grid-
adaptive simulation code [amrvac.org]. Most notably, this tool enables
a resolution of 5.6 km within a 24 Mm x 25 Mm domain size; the full
global flux rope dynamics and local plasma dynamics are captured in
unprecedented detail. Our 2.5D simulation (where the flux rope has
realistic helical magnetic field lines) demonstrates that the thermal
runaway condensation can happen at any location, not solely in the
bottom part of the flux rope where the majority of prominence material
is assumed to reside. Intricate thermodynamic evolution and shearing
flows develop spontaneously, themselves inducing further fine-scale
(magneto)hydrodynamic instabilities. Our analysis touches base with
advanced linear magnetohydrodynamic stability theory, e.g. with the
Convective Continuum Instability or CCI process as well as with in-situ
thermal instability studies. We find that condensing prominence plasma
evolves according to the internal pressure and density gradients as
found previously for coronal rain condensations, but also misalignments
therein suggesting the relevance of the Rayleigh-Taylor instability
or RTI process in 3D. We also find evidence for resistively-driven
dynamics in the prominence body, in close analogy with analytical
predictions. These findings are relevant for modern studies of full
3D prominence formation and structuring. Most notably, we anticipate
obtaining similar resolutions within a fully 3D setup. Such an
achievement will afford us the exciting opportunity to offer crucial
explanations as to the persistent discrepancy in prominence appearance
when projected off- or on-disk.

---------------------------------------------------------
Title: Turbulence driven by chromospheric evaporations in solar flares
Authors: Ruan, Wenzhi; Xia, Chun; Keppens, Rony
2021EGUGA..2311065R Altcode:
Chromospheric evaporations are frequently observed at the footpoints
of flare loops in flare events. The evaporations flows driven by
thermal conduction or fast electron deposition often have high speed
of hundreds km/s. Since the speed of the observed evaporation flows
is comparable to the local Alfven speed, it is reasonable to consider
the triggering of Kelvin-Helmholtz instabilities. Here we revisit
a scenario which stresses the importance of the Kelvin-Helmholtz
instability (KHI) proposed by Fang et al. (2016). This scenario
suggests that evaporations flows from two footpoints of a flare loop
can meet each other at the looptop and produce turbulence there via
KHI. The produced KHI turbulence can play important roles in particle
accelerations and generation of strong looptop hard X-ray sources. We
investigate whether evaporation flows can produce turbulence inside
the flare loop with the help of numerical simulation. KHI turbulence
is successfully produced in our simulation. The synthesized soft X-ray
curve demonstrating a clear quasi-periodic pulsation (QPP) with period
of 26 s. The QPP is caused by a locally trapped, fast standing wave
that resonates in between KHI vortices.

---------------------------------------------------------
Title: Transition region adaptive conduction (TRAC) in
multidimensional magnetohydrodynamic simulations
Authors: Zhou, Yu-Hao; Ruan, Wen-Zhi; Xia, Chun; Keppens, Rony
2021A&A...648A..29Z Altcode: 2021arXiv210207549Z
Context. In solar physics, a severe numerical challenge for modern
simulations is properly representing a transition region between
the million-degree hot corona and a much cooler plasma of about 10
000 K (e.g., the upper chromosphere or a prominence). In previous 1D
hydrodynamic simulations, the transition region adaptive conduction
(TRAC) method has been proven to capture aspects better that are
related to mass evaporation and energy exchange. Aims: We aim
to extend this method to fully multidimensional magnetohydrodynamic
(MHD) settings, as required for any realistic application in the solar
atmosphere. Because modern MHD simulation tools efficiently exploit
parallel supercomputers and can handle automated grid refinement,
we design strategies for any-dimensional block grid-adaptive MHD
simulations. Methods: We propose two different strategies and
demonstrate their working with our open-source MPI-AMRVAC code. We
benchmark both strategies on 2D prominence formation based on the
evaporation-condensation scenario, where chromospheric plasma is
evaporated through the transition region and then is collected and
ultimately condenses in the corona. Results: A field-line-based
TRACL method and a block-based TRACB method are introduced and compared
in block grid-adaptive 2D MHD simulations. Both methods yield similar
results and are shown to satisfactorily correct the underestimated
chromospheric evaporation, which comes from a poor spatial resolution
in the transition region. Conclusions: Because fully resolving
the transition region in multidimensional MHD settings is virtually
impossible, TRACB or TRACL methods will be needed in any 2D or 3D
simulations involving transition region physics.

---------------------------------------------------------
Title: Prominence formation by levitation-condensation at extreme
resolutions
Authors: Jenkins, J. M.; Keppens, R.
2021A&A...646A.134J Altcode: 2020arXiv201113428J
Context. Prominences in the solar atmosphere represent an intriguing
and delicate balance of forces and thermodynamics in an evolving
magnetic topology. How this relatively cool material comes to reside at
coronal heights, and what drives its evolution prior to, during, and
after its appearance, remains an area full of open questions. 
Aims: We here set forth to identify the physical processes driving the
formation and evolution of prominence condensations within 2.5D magnetic
flux ropes. We deliberately focus on the levitation-condensation
scenario, where a coronal flux rope forms and eventually demonstrates
in situ condensations, revisiting it at extreme resolutions down
to order 6 km in scale. Methods: We perform grid-adaptive
numerical simulations in a 2.5D translationally invariant setup,
where we can study the distribution of all metrics involved in
advanced magnetohydrodynamic stability theory for nested flux rope
equilibria. We quantify in particular convective continuum instability
(CCI), thermal instability (TI), baroclinicity, and mass-slipping
metrics within a series of numerical simulations of prominences formed
via levitation-condensation. Results: Overall, we find that
the formation and evolution of prominence condensations happens in a
clearly defined sequence regardless of resolution, with background field
strength between 3 and 10 Gauss. The CCI governs the slow evolution
of plasma prior to the formation of distinct condensations found to be
driven by the TI. Evolution of the condensations towards the topological
dips of the magnetic flux rope is a consequence of these condensations
initially forming out of pressure balance with their surroundings. From
the baroclinicity distributions, smaller-scale rotational motions
are inferred within forming and evolving condensations. Upon the
complete condensation of a prominence `monolith', the slow descent
of this plasma towards lower heights appears consistent with the
mass-slippage mechanism driven by episodes of both local current
diffusion and magnetic reconnection. Movies are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202039630/olm">https://www.aanda.org</A>

---------------------------------------------------------
Title: From Nonlinear Force-free Field Models to Data-driven
Magnetohydrodynamic Simulations
Authors: Guo, Yang; Chen, P. F.; Keppens, Rony; Xia, Chun; Ding,
Mingde; Yang, Kai; Zhong, Ze
2021cosp...43E1777G Altcode:
To study the origin, structures, and dynamics of various solar
activities, such as flares, prominences/filaments, and coronal
mass ejections, we have to know the 3D magnetic field in the solar
corona. Since many static phenomena in the corona live in a low
beta environment, they can be modelled as a force-free state. We
have implemented a new nonlinear force-free field (NLFFF) algorithm
in the Message Passing Interface Adaptive Mesh Refinement Versatile
Advection Code (MPI-AMRVAC), which could construct an NLFFF model in
both Cartesian and spherical coordinate systems, and in all uniform,
adaptive mesh refinement, and stretched grids. The NLFFF models
have been applied to observations to study the magnetic structures
of flux ropes, the coronal emission in extreme ultraviolet lines
and the morphology of flare ribbons. To further study the dynamic
eruption of a magnetic flux rope, we have developed a data-driven
magnetohydrodynamic (MHD) model using the zero-beta MHD equations. The
NLFFF model is served as the initial condition, and the time series
of observed magnetic field and velocity field provide the boundary
conditions. This model can reproduce the evolution of a magnetic flux
rope in its dynamic eruptive phase. We also find that a data-constrained
boundary condition, where the bottom boundary is fixed to the initial
values, reproduces a similar simulation result as the data-driven
simulation. The data-driven MHD model has also been applied to study
a failed eruption, where the torus instability, kink instability,
and additional components of Lorentz forces are studied in detail.

---------------------------------------------------------
Title: Thermal instabilities: fragmentation and field misalignment
of filament fine structure
Authors: Claes, Niels; Keppens, Rony; Xia, Chun
2021cosp...43E.966C Altcode:
Prominences show a surprising amount of fine structure, and it is
widely believed that their threads as seen in H-alpha observations
provide indirect information on the magnetic field topology. Both
prominence and coronal rain condensations most likely originate
from thermal instabilities in the solar corona, and how the nonlinear
instability evolution shapes their observed fine structure is still not
understood. We investigate the spontaneous emergence and evolution of
fine structure in high-density condensations formed through the process
of thermal instability, under typical solar coronal condensations. Our
study reveals intricate multi-dimensional processes that happen
through in situ condensations in a representative coronal volume, in
a low plasma beta regime. Using MPI-AMRVAC (amrvac.org), we performed
multiple 2D and 3D numerical simulations of interacting slow MHD wave
modes when all relevant non-adiabatic effects are included, extending
our previous work [a]. Multiple levels of adaptive mesh refinement
ensure that any emerging fine structure is automatically resolved. We
show that the interaction of multiple slow modes in a regime unstable
to the thermal mode leads to thermal instability. Initially this forms
pancake-like structures almost orthogonal to the magnetic field, while
low-pressure induced inflows of matter generate rebound shocks. This
is succeeded by the rapid disruption of these pancake-sheets through
thin-shell instabilities evolving naturally from minute ram pressure
imbalances. This eventually creates high-density blobs accompanied by
thread-like features due to shear flow effects. The further evolution
of these blobs follows the magnetic field lines such that a dynamical
realignment with the magnetic field appears. However, the emerging
thread-like features are not at all field-aligned, implying only
a weak link between fine structure orientation and magnetic field
topology [b]. This would imply that threads formed by nonlinear thermal
instability evolution do not strictly outline magnetic field structure,
which has far-reaching implications for field topology interpretations
based on H-alpha observations. [a] Claes, N. \&amp; Keppens, R. 2019,
Astronomy \&amp; Astrophysics, 624, A96. [b] Claes, N., Keppens,
R. \&amp; Xia, C. 2020, Astronomy \&amp; Astrophysics, submitted.

---------------------------------------------------------
Title: Prominence formation by levitation-condensation at extreme
resolutions
Authors: Jenkins, Jack; Keppens, Rony
2021cosp...43E.971J Altcode:
Following up on pioneering work presented in [1], we revisit the
so-called levitation-condensation mechanism for the ab-inito formation
of solar prominences: cool and dense clouds in the million-degree solar
atmosphere. Levitation-condensation occurs following the formation of a
flux rope in response to the deformation of a force-free coronal arcade
by controlled magnetic footpoint motions. Existing coronal plasma gets
lifted within the forming rope, therein isolating a collection of matter
now more dense than its immediate surroundings. This denser region
ultimately suffers a thermal instability driven by radiative losses,
and a prominence forms. We improve on various aspects that were left
unanswered in the original work, by revisiting this model with our
modern open-source grid-adaptive simulation code [amrvac.org, see
[2]]. Most notably, this tool enables a resolution of 5.6 km within
a 24 Mm x 25 Mm domain size; the full global flux rope dynamics and
local plasma dynamics are captured in unprecedented detail. Our 2.5D
simulation (where the flux rope has realistic helical magnetic field
lines) demonstrates that the thermal runaway condensation can happen
at any location, not solely in the bottom part of the flux rope where
the majority of material is believed to reside. Intricate thermodynamic
evolutions and shearing flows develop spontaneously, themselves inducing
further fine-scale (magneto)hydrodynamic instabilities. Our analysis
makes explicit links with advanced linear magnetohydrodynamic stability
theory, e.g. with the Convective Continuum Instability or CCI process
[3] as well as with in-situ thermal instability studies [4]. We find
that condensing prominence plasma evolves according to the internal
pressure and density gradients as found for coronal rain condensations
[e.g., 5], but also misalignments therein hinting relevance to the
Rayleigh-Taylor instability or RTI process in 3D [6]. We also find
evidence for resistively-driven dynamics in the prominence body,
in close analogy with analytical predictions [7]. These findings
are relevant for modern studies of full 3D prominence formation
and structuring [e.g., 8]. Most crucially, we anticipate obtaining
similar resolutions within a fully 3D setup will afford us the exciting
opportunity to offer explanations as to the persistent discrepancy in
prominence appearance when projected against the solar disk vs. above
the limb. References $[1]$ `Numerical study on in-situ
prominence formation by radiative condensation in the solar corona',
T. Kaneko \&amp; T. Yokoyama, 2015, ApJ 806, 115 $[2]$ `MPI-AMRVAC
2.0 for solar and astrophysical applications', C. Xia, J. Teunissen,
I. El Mellah, E. Chane \&amp; R. Keppens, 2018, ApJ Suppl. 234,
30 $[3]$ `Toward detailed prominence seismology. II. Charting
the continuous magnetohydrodynamic spectrum', J.W.S. Blokland \&amp;
R. Keppens, 2011, A \&amp; A 532, A94 $[4]$ `Thermal instabilities:
Fragmentation and field misalignment of filament fine structure',
N. Claes, R. Keppens \&amp; C. Xia, 2020, A \&amp; A 636, A112 $[5]$ `Simulating coronal condensation dynamics in 3D', S. P. Moschou,
R. Keppens, C. Xia \&amp; X. Fang, 2015, Adv. Space Res. 56, 2738 $[6]$ `The magnetic Rayleigh-Taylor instability in solar prominences',
A. Hillier, 2018, RvMPP, 2, 1 $[7]$ `The Hydromagnetic Interior
of a Solar Quiescent Prominence. II. Magnetic Discontinuities and
Cross-field Mass Transport', Low B.C. et al, 2012, ApJ 757, 21 $[8]$ `Formation and plasma circulation of solar prominences',
C. Xia \&amp; R. Keppens, 2016, ApJ 823, 22

---------------------------------------------------------
Title: The magnetic flux rope structure of a triangulated solar
filament
Authors: Guo, Yang; Chen, P. F.; Keppens, Rony; Xia, Chun; Ding,
Mingde; Xu, Yu
2021cosp...43E1734G Altcode:
We construct a magnetic flux rope model for a prominence observed at
01:11 UT on 2011 June 21 in AR 11236 using the following methods,
triangulation from multi perspective observations, the flux
rope embedding method, the regularized Biot-Savart laws, and the
magnetofrictional method. First, the prominence path is reconstructed
with the triangulation with 304 Å images observed by the Atmospheric
Imaging Assembly on board Solar Dynamics Observatory (SDO) and by the
Extreme Ultraviolet Imager on board the twin Solar Terrestrial Relations
Observatory. Then, a flux rope is constructed with the regularized
Biot-Savart laws using the information of its axis. Next, it is embedded
into a potential magnetic field computed from the photospheric radial
magnetic field observed by the Helioseismic and Magnetic Imager on
board SDO. The combined magnetic field is finally relaxed by the
magnetofrictional method to reach a nonlinear force-free state. It
is found that both models constructed by the regularized Biot-Savart
laws and after the magnetofrictional relaxation coincide with the
304 Å images. The distribution of magnetic dips coincides with part
of the prominence material, and the quasi-separatrix layers wrap the
magnetic flux ropes, displaying hyperbolic flux tube structures. These
models have the advantages of constructing magnetic flux ropes in the
higher atmosphere and weak magnetic field regions, which could be used
as initial conditions for magnetohydrodynamic simulations of coronal
mass ejections.

---------------------------------------------------------
Title: Legolas: A Modern Tool for Magnetohydrodynamic Spectroscopy
Authors: Claes, Niels; De Jonghe, Jordi; Keppens, Rony
2020ApJS..251...25C Altcode: 2020arXiv201014148C
Magnetohydrodynamic (MHD) spectroscopy is central to many astrophysical
disciplines, ranging from helio- to asteroseismology, over solar
coronal (loop) seismology, to the study of waves and instabilities in
jets, accretion disks, or solar/stellar atmospheres. MHD spectroscopy
quantifies all linear (standing or traveling) wave modes, including
overstable (i.e., growing) or damped modes, for a given configuration
that achieves force and thermodynamic balance. Here, we present Legolas,
a novel, open-source numerical code to calculate the full MHD spectrum
of one-dimensional equilibria with flow, balancing pressure gradients,
Lorentz forces, centrifugal effects, and gravity, and enriched with
nonadiabatic aspects like radiative losses, thermal conduction, and
resistivity. The governing equations use Fourier representations in
the ignorable coordinates, and the set of linearized equations is
discretized using finite elements in the important height or radial
variation, handling Cartesian and cylindrical geometries using the same
implementation. A weak Galerkin formulation results in a generalized
(non-Hermitian) matrix eigenvalue problem, and linear algebraic
algorithms calculate all eigenvalues and corresponding eigenvectors. We
showcase a plethora of well-established results, ranging from p and
g modes in magnetized, stratified atmospheres, over modes relevant
for coronal loop seismology, thermal instabilities, and discrete
overstable Alfvén modes related to solar prominences, to stability
studies for astrophysical jet flows. We encounter (quasi-)Parker,
(quasi-)interchange, current-driven, and Kelvin-Helmholtz instabilities,
as well as nonideal quasi-modes, resistive tearing modes, up to
magnetothermal instabilities. The use of high resolution sheds new
light on previously calculated spectra, revealing interesting spectral
regions that have yet to be investigated.

---------------------------------------------------------
Title: A two-fluid analysis of waves in a warm ion-electron plasma
Authors: De Jonghe, J.; Keppens, R.
2020PhPl...27l2107D Altcode: 2020arXiv201106282D
Following recent work, we discuss waves in a warm ideal two-fluid plasma
consisting of electrons and ions starting from a completely general,
ideal two-fluid dispersion relation. The plasma is characterized
by five variables: the electron and ion magnetizations, the squared
electron and ion sound speeds, and a parameter describing the angle
between the propagation vector and the magnetic field. The dispersion
relation describes six pairs of waves which we label S, A, F, M, O,
and X. Varying the angle, it is argued that parallel and perpendicular
propagation (with respect to the magnetic field) exhibit unique
behavior. This behavior is characterized by the crossing of wave modes
which is prohibited at oblique angles. We identify up to six different
parameter regimes where a varying number of exact mode crossings in the
special parallel or perpendicular orientations can occur. We point out
how any ion-electron plasma has a critical magnetization (or electron
cyclotron frequency) at which the cutoff ordering changes, leading
to different crossing behaviors. These are relevant for exotic plasma
conditions found in pulsar and magnetar environments. Our discussion
is fully consistent with ideal relativistic MHD and contains light
waves. Additionally, by exploiting the general nature of the dispersion
relation, phase and group speed diagrams can be computed at arbitrary
wavelengths for any parameter regime. Finally, we recover earlier
approximate dispersion relations that focus on low-frequency limits and
make direct correspondences with some selected kinetic theory results.

---------------------------------------------------------
Title: Legolas: Large Eigensystem Generator for One-dimensional
pLASmas
Authors: Claes, Niels; De Jonghe, Jordi; Keppens, Rony
2020ascl.soft10013C Altcode:
Legolas (Large Eigensystem Generator for One-dimensional pLASmas) is a
finite element code for MHD spectroscopy of 1D Cartesian/cylindrical
equilibria with flow that balance pressure gradients, enriched with
various non-adiabatic effects. The code's capabilities range from
full spectrum calculations to eigenfunctions of specific modes to
full-on parametric studies of various equilibrium configurations in
different geometries.

---------------------------------------------------------
Title: Relativistic AGN jets - III. Synthesis of synchrotron emission
from double-double radio galaxies
Authors: Walg, S.; Achterberg, A.; Markoff, S.; Keppens, R.; Porth, O.
2020MNRAS.497.3638W Altcode: 2020arXiv200714815W; 2020MNRAS.tmp.2338W
The class of double-double radio galaxies (DDRGs) relates to episodic
jet outbursts. How various regions and components add to the total
intensity in radio images is less well known. In this paper, we
synthesize synchrotron images for DDRGs based on special relativistic
hydrodynamic simulations, making advanced approximations for the
magnetic fields. We study the synchrotron images for three different
radial jet profiles; ordered, entangled, or mixed magnetic fields;
spectral ageing from synchrotron cooling; the contribution from
different jet components; the viewing angle and Doppler (de-)boosting;
and the various epochs of the evolution of the DDRG. To link our results
to observational data, we adopt to J1835+6204 as a reference source. In
all cases, the synthesized synchrotron images show two clear pairs
of hotspots, in the inner and outer lobes. The best resemblance is
obtained for the piecewise isochoric jet model, for a viewing angle of
approximately θ ∼ -71°, i.e. inclined with the lower jet towards
the observer, with predominantly entangled (≳70 per cent of the
magnetic pressure) in turbulent, rather than ordered fields. The effects
of spectral ageing become significant when the ratio of observation
frequencies and cut-off frequency νobs/ν∞, 0
≳ 10-3, corresponding to ∼3 × 102 MHz. For
viewing angles θ ≲ |-30°|, a DDRG morphology can no longer be
recognized. The second jets must be injected within ≲ 4 per cent
of the lifetime of the first jets for a DDRG structure to emerge,
which is relevant for active galactic nuclei feedback constraints.

---------------------------------------------------------
Title: Mesoscale Phenomena during a Macroscopic Solar Eruption
Authors: Zhao, Xiaozhou; Keppens, Rony
2020ApJ...898...90Z Altcode:
Our previous magnetohydrodynamic simulation of a macroscopic solar
eruption discussed in Zhao et al. (2019) showed that the current
sheet (CS) evolution during eruption went through four stages: the
CS growth stage, the dynamic growth stage, the hot CS stage, and the
dynamic hot CS stage. We now focus on various mesoscale phenomena
associated with the ongoing reconnection. In the dynamic growth stage,
the remnant chromospheric matter in the CS is quasi-periodically pushed
into the prominence, inducing fast shocks propagating at a speed of
210 km s-1. In the hot CS phase, various shock features
relevant for particle acceleration are identified throughout the flare
loop. Finally, during both dynamic stages, we quantify the properties
of magnetic islands. A typical island is accelerated to Alfvénic
speed by the Lorentz force and cools down by radiative cooling and
thermal conduction. It also tends to expand in size before colliding
with another island, with the FR or with the flare arcade. Islands in
the dynamic growth stage have a higher density and lower temperature,
and vice versa in the dynamic hot CS stage. Islands tend to move
upward in the dynamic growth stage, while almost equal fractions
of downward-moving and upward-moving islands in the dynamic hot CS
stage. Translating the island trajectories to phase space, we find
that the function $\dot{y}=({{ay}}^{2}+{by}+c)\exp (\lambda y)$ fits
the trajectory well, and its two fixed points represent the creation
and the annihilation of the island.

---------------------------------------------------------
Title: The Triple-layered Leading Edge of Solar Coronal Mass Ejections
Authors: Mei, Z. X.; Keppens, R.; Cai, Q. W.; Ye, J.; Li, Y.; Xie,
X. Y.; Lin, J.
2020ApJ...898L..21M Altcode:
In a high-resolution, 3D resistive magnetohydrodynamic simulation of an
eruptive magnetic flux rope (MFR), we revisit the detailed 3D magnetic
structure of a coronal mass ejection (CME). Our results highlight
that there exists a helical current ribbon/boundary (HCB) that wraps
around the CME bubble. This HCB results from the interaction between
the CME bubble and the ambient magnetic field, where it represents a
tangential discontinuity in the magnetic topology. Its helical shape
is ultimately caused by the kinking of the MFR that resides within
the CME bubble. In synthetic Solar Dynamics Observatory/Atmospheric
Imaging Assembly images, processed to logarithmic scale to enhance
otherwise unobservable features, we show a clear triple-layered leading
edge: a bright fast shock front, followed by a bright HCB, and within
it a bright MFR. These are arranged in sequence and expand outward
continuously. For kink unstable eruptions, we suggest that the HCB
is a possible explanation for the bright leading edges seen near CME
bubbles and also for the non-wave component of global EUV disturbances.

---------------------------------------------------------
Title: A Fully Self-consistent Model for Solar Flares
Authors: Ruan, Wenzhi; Xia, Chun; Keppens, Rony
2020ApJ...896...97R Altcode: 2020arXiv200508578R
The "standard solar flare model" collects all physical ingredients
identified by multiwavelength observations of our Sun: magnetic
reconnection, fast particle acceleration, and the resulting emission
at various wavelengths, especially in soft to hard X-ray channels. Its
cartoon representation is found throughout textbooks on solar and plasma
astrophysics and guides interpretations of unresolved energetic flaring
events on other stars, accretion disks, and jets. To date, a fully
self-consistent model that reproduces the standard scenario in all its
facets is lacking, since this requires the combination of a large-scale,
multidimensional magnetohydrodynamic (MHD) plasma description with a
realistic fast electron treatment. Here we demonstrate such a novel
combination, where MHD combines with an analytic fast electron model,
adjusted to handle time-evolving, reconnecting magnetic fields and
particle trapping. This allows us to study (1) the role of fast electron
deposition in the triggering of chromospheric evaporation flows,
(2) the physical mechanisms that generate various hard X-ray sources
at chromospheric footpoints or looptops, and (3) the relationship
between soft X-ray and hard X-ray fluxes throughout the entire flare
loop evolution. For the first time, this self-consistent solar flare
model demonstrates the observationally suggested relationship between
flux swept out by the hard X-ray footpoint regions and the actual
reconnection rate at the X-point, which is a major unknown in flaring
scenarios. We also demonstrate that a looptop hard X-ray source can
result from fast electron trapping.

---------------------------------------------------------
Title: MHD simulation of solar flare by applying analytical energetic
fast electron model
Authors: Ruan, Wenzhi; Keppens, Rony
2020EGUGA..22.4982R Altcode:
In order to study the evaporation of chromospheric plasma and the
formation of hard X-ray (HXR) sources in solar flare events, we coupled
an analytic energetic electron model with the multi-dimensional MHD
simulation code MPI-AMRVAC. The transport of fast electrons accelerated
in the flare looptop is governed by the test particle beam approach
reported in Emslie et al. (1978), now used along individual field
lines. Anomalous resistivity, thermal conduction, radiative losses
and gravity are included in the MHD model. The reconnection process
self-consistently leads to formation of a flare loop system and
the evaporation of chromospheric plasma is naturally recovered. The
non-thermal HXR emission is synthesized from the local fast electron
spectra and local plasma density, and thermal bremsstrahlung soft
X-ray (SXR) emission is synthesized based on local plasma density
and temperature. We found that thermal conduction is an efficient way
to trigger evaporation flows. We also found that the generation of a
looptop HXR source is a result of fast electron trapping, as evidenced
by the pitch angle evolution. By comparing the SXR flux and HXR flux,
we found that a possible reason for the "Neupert effect" is that the
increase of non-thermal and thermal energy follows the same tendency.

---------------------------------------------------------
Title: Wind morphology around cool evolved stars in binaries. The
case of slowly accelerating oxygen-rich outflows
Authors: El Mellah, I.; Bolte, J.; Decin, L.; Homan, W.; Keppens, R.
2020A&A...637A..91E Altcode: 2020arXiv200104482E
Context. The late evolutionary phase of low- and intermediate-mass
stars is strongly constrained by their mass-loss rate, which is orders
of magnitude higher than during the main sequence. The wind surrounding
these cool expanded stars frequently shows nonspherical symmetry, which
is thought to be due to an unseen companion orbiting the donor star. The
imprints left in the outflow carry information about the companion
and also the launching mechanism of these dust-driven winds. Aims: We study the morphology of the circumbinary envelope and
identify the conditions of formation of a wind-captured disk around
the companion. Long-term orbital changes induced by mass loss and mass
transfer to the secondary are also investigated. We pay particular
attention to oxygen-rich, that is slowly accelerating, outflows in
order to look for systematic differences between the dynamics of the
wind around carbon and oxygen-rich asymptotic giant branch (AGB)
stars. Methods: We present a model based on a parametrized
wind acceleration and a reduced number of dimensionless parameters
to connect the wind morphology to the properties of the underlying
binary system. Thanks to the high performance code MPI-AMRVAC, we ran
an extensive set of 72 three-dimensional hydrodynamics simulations of a
progressively accelerating wind propagating in the Roche potential of a
mass-losing evolved star in orbit with a main sequence companion. The
highly adaptive mesh refinement that we used, enabled us to resolve
the flow structure both in the immediate vicinity of the secondary,
where bow shocks, outflows, and wind-captured disks form, and up
to 40 orbital separations, where spiral arms, arcs, and equatorial
density enhancements develop. Results: When the companion
is deeply engulfed in the wind, the lower terminal wind speeds and
more progressive wind acceleration around oxygen-rich AGB stars make
them more prone than carbon-rich AGB stars to display more disturbed
outflows, a disk-like structure around the companion, and a wind
concentrated in the orbital plane. In these configurations, a large
fraction of the wind is captured by the companion, which leads to
a significant shrinking of the orbit over the mass-loss timescale,
if the donor star is at least a few times more massive than its
companion. In the other cases, an increase of the orbital separation is
to be expected, though at a rate lower than the mass-loss rate of the
donor star. Provided the companion has a mass of at least a tenth of the
mass of the donor star, it can compress the wind in the orbital plane
up to large distances. Conclusions: The grid of models that we
computed covers a wide scope of configurations: We vary the terminal
wind speed relative to the orbital speed, the extension of the dust
condensation region around the cool evolved star relative to the orbital
separation, and the mass ratio, and we consider a carbon-rich and an
oxygen-rich donor star. It provides a convenient frame of reference
to interpret high-resolution maps of the outflows surrounding cool
evolved stars. Movie associated to Fig. 7 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/202037492/olm">https://www.aanda.org</A>

---------------------------------------------------------
Title: Thermal instabilities: Fragmentation and field misalignment
of filament fine structure
Authors: Claes, N.; Keppens, R.; Xia, C.
2020A&A...636A.112C Altcode: 2020arXiv200310947C
Context. Prominences show a surprising amount of fine structure and
it is widely believed that their threads, as seen in Hα observations,
provide indirect information concerning magnetic field topology. Both
prominence and coronal rain condensations most likely originate
from thermal instabilities in the solar corona. It is still not
understood how non-linear instability evolution shapes the observed fine
structure of prominences. Investigating this requires multidimensional,
high-resolution simulations to resolve all emerging substructure in
great detail. Aims: We investigate the spontaneous emergence
and evolution of fine structure in high-density condensations formed
through the process of thermal instability under typical solar coronal
conditions. Our study reveals intricate multidimensional processes that
occur through in situ condensations in a representative coronal volume
in a low plasma beta regime. Methods: We quantified slow wave
eigenfunctions used as perturbations and discuss under which conditions
the thermal mode is unstable when anisotropic thermal conduction
effects are included. We performed 2D and 3D numerical simulations of
interacting slow magnetohydrodynamic (MHD) wave modes when all relevant
non-adiabatic effects are included. Multiple levels of adaptive mesh
refinement achieve a high resolution near regions with high density,
thereby resolving any emerging fine structure automatically. Our study
employs a local periodic coronal region traversed by damped slow waves
inspired by the presence of such waves observed in actual coronal
magnetic structures. Results: We show that the interaction
of multiple slow MHD wave modes in a regime unstable to the thermal
mode leads to thermal instability. This initially forms pancake-like
structures almost orthogonal to the local magnetic field, while
low-pressure induced inflows of matter generate rebound shocks. This
is succeeded by the rapid disruption of these pancake sheets through
thin-shell instabilities evolving naturally from minute ram pressure
imbalances. This eventually creates high-density blobs accompanied by
thread-like features from shear flow effects. The further evolution
of the blobs follows the magnetic field lines, such that a dynamical
realignment with the background magnetic field appears. However,
the emerging thread-like features are not at all field-aligned,
implying only a very weak link between fine structure orientation and
magnetic field topology. Conclusions: As seen in our synthetic
Hα views, threads formed by non-linear thermal instability evolution
do not strictly outline magnetic field structure and this finding has
far-reaching implications for field topology interpretations based on
Hα observations. The movie attached to Fig. 12 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/202037616/olm">https://www.aanda.org</A>

---------------------------------------------------------
Title: 3D numerical experiment for EUV waves caused by flux rope
eruption
Authors: Mei, Z. X.; Keppens, R.; Cai, Q. W.; Ye, J.; Xie, X. Y.;
Li, Y.
2020MNRAS.493.4816M Altcode:
We present a 3D magnetohydrodynamic numerical experiment of an eruptive
magnetic flux rope (MFR) and the various types of disturbances it
creates, and employ forward modelling of extreme ultraviolet (EUV)
observables to directly compare numerical results and observations. In
the beginning, the MFR erupts and a fast shock appears as an expanding
3D dome. Under the MFR, a current sheet grows, in which magnetic
field lines reconnect to form closed field lines, which become the
outermost part of an expanding coronal mass ejection (CME) bubble. In
our synthetic SDO/AIA images, we can observe the bright fast shock dome
and the hot MFR in the early stages. Between the MFR and the fast shock,
a dimming region appears. Later, the MFR expands so its brightness
decays and it becomes difficult to identify the boundary of the CME
bubble and distinguish it from the bright MFR in synthetic images. Our
synthetic images for EUV disturbances observed at the limb support the
bimodality interpretation for coronal disturbances. However, images for
disturbances propagating on-disc do not support this interpretation
because the morphology of the bright MFR does not lead to circular
features in the EUV disturbances. At the flanks of the CME bubble, slow
shocks, velocity vortices and shock echoes can also be recognized in
the velocity distribution. The slow shocks at the flanks of the bubble
are associated with a 3D velocity separatrix. These features are found
in our high-resolution simulation, but may be hard to observe as shown
in the synthetic images.

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Title: MPI-AMRVAC: a parallel, grid-adaptive PDE toolkit
Authors: Keppens, Rony; Teunissen, Jannis; Xia, Chun; Porth, Oliver
2020arXiv200403275K Altcode:
We report on the latest additions to our open-source, block-grid
adaptive framework MPI-AMRVAC, which is a general toolkit for
especially hyperbolic/parabolic partial differential equations
(PDEs). Applications traditionally focused on shock-dominated,
magnetized plasma dynamics described by either Newtonian or special
relativistic (magneto)hydrodynamics, but its versatile design easily
extends to different PDE systems. Here, we demonstrate applications
covering any-dimensional scalar to system PDEs, with e.g. Korteweg-de
Vries solutions generalizing early findings on soliton behaviour,
shallow water applications in round or square pools, hydrodynamic
convergence tests as well as challenging computational fluid and plasma
dynamics applications. The recent addition of a parallel multigrid
solver opens up new avenues where also elliptic constraints or stiff
source terms play a central role. This is illustrated here by solving
several multi-dimensional reaction-diffusion-type equations. We document
the minimal requirements for adding a new physics module governed by any
nonlinear PDE system, such that it can directly benefit from the code
flexibility in combining various temporal and spatial discretisation
schemes. Distributed through GitHub, MPI-AMRVAC can be used to perform
1D, 1.5D, 2D, 2.5D or 3D simulations in Cartesian, cylindrical or
spherical coordinate systems, using parallel domain-decomposition,
or exploiting fully dynamic block quadtree-octree grids.

---------------------------------------------------------
Title: Magnetohydrodynamic Nonlinearities in Sunspot Atmospheres:
Chromospheric Detections of Intermediate Shocks
Authors: Houston, S. J.; Jess, D. B.; Keppens, R.; Stangalini, M.;
Keys, P. H.; Grant, S. D. T.; Jafarzadeh, S.; McFetridge, L. M.;
Murabito, M.; Ermolli, I.; Giorgi, F.
2020ApJ...892...49H Altcode: 2020arXiv200212368H
The formation of shocks within the solar atmosphere remains one of
the few observable signatures of energy dissipation arising from the
plethora of magnetohydrodynamic waves generated close to the solar
surface. Active region observations offer exceptional views of wave
behavior and its impact on the surrounding atmosphere. The stratified
plasma gradients present in the lower solar atmosphere allow for the
potential formation of many theorized shock phenomena. In this study,
using chromospheric Ca II λ8542 line spectropolarimetric data of a
large sunspot, we examine fluctuations in the plasma parameters in
the aftermath of powerful shock events that demonstrate polarimetric
reversals during their evolution. Modern inversion techniques are
employed to uncover perturbations in the temperatures, line-of-sight
velocities, and vector magnetic fields occurring across a range of
optical depths synonymous with the shock formation. Classification
of these nonlinear signatures is carried out by comparing the
observationally derived slow, fast, and Alfvén shock solutions with
the theoretical Rankine-Hugoniot relations. Employing over 200,000
independent measurements, we reveal that the Alfvén (intermediate)
shock solution provides the closest match between theory and
observations at optical depths of log10τ =-4, consistent
with a geometric height at the boundary between the upper photosphere
and lower chromosphere. This work uncovers first-time evidence of the
manifestation of chromospheric intermediate shocks in sunspot umbrae,
providing a new method for the potential thermalization of wave energy
in a range of magnetic structures, including pores, magnetic flux ropes,
and magnetic bright points.

---------------------------------------------------------
Title: Simulations of the W50-SS433 system
Authors: Millas, Dimitrios; Porth, Oliver; Keppens, Rony
2020IAUS..342..257M Altcode:
Supernovae and astrophysical jets are two of the most energetic
and intriguing objects in the universe. We examine an interesting
scenario that involves the interaction of these two extreme phenomena,
motivated by observations of the W50-SS433 system: a jet launched from
the microquasar SS433 (an X-ray binary) located inside a supernova
remnant, W50. These observations revealed a unique morphology of the
remnant, attributed to the presence of the jet. We performed full 3D
relativistic hydrodynamic simulations to better capture the interaction
between the remnant and the jet and post-processed the data with a
radiative transfer code to create emission maps.

---------------------------------------------------------
Title: Clumpy wind accretion in Supergiant X-ray Binaries
Authors: Mellah, Ileyk El; Sander, Andreas A. C.; Sundqvist, Jon O.;
Keppens, Rony
2019IAUS..346...34M Altcode:
Supergiant X-ray Binaries host a compact object, generally a neutron
star, orbiting an evolved O/B star. Mass transfer proceeds through the
intense radiatively-driven wind of the stellar donor, a fraction of
which is captured by the gravitational field of the neutron star. The
subsequent accretion process onto the neutron star is responsible
for the abundant X-ray emission from those systems. They also display
variations in time of the X-ray flux by a factor of a few 10, along
with changes in the hardness ratios believed to be due to varying
absorption along the line-of-sight. We used the most recent results on
the inhomogeneities (aka clumps) in the non-stationary wind of massive
hot stars to evaluate their impact on the time-variable accretion
process. We ran three-dimensional simulations of the wind in the
vicinity of the accretor to witness the formation of the bow shock and
follow the inhomogeneous flow over several spatial orders of magnitude,
down to the neutron star magnetosphere. In particular, we show that
the impact of the clumps on the time-variability of the intrinsic
mass accretion rate is severely damped by the crossing of the shock,
compared to the purely ballistic Bondi-Hoyle-Lyttleton estimation. We
also account for the variable absorption due to clumps passing by the
line-of-sight and estimate the final effective variability of the mass
accretion rate for different orbital separations. These results are
confronted to recent analysis of Vela X-1 observations with Chandra
by Grinberg et al. (2017). It shows that clumps account well for
time-variability at low luminosity but can not generate, per se,
the high luminosity activity observed.

---------------------------------------------------------
Title: Ideal MHD instabilities for coronal mass ejections: interacting
current channels and particle acceleration
Authors: Keppens, Rony; Guo, Yang; Makwana, Kirit; Mei, Zhixing;
Ripperda, Bart; Xia, Chun; Zhao, Xiaozhou
2019RvMPP...3...14K Altcode: 2019arXiv191012659K
We review and discuss insights on ideal magnetohydrodynamic (MHD)
instabilities that can play a role in destabilizing solar coronal flux
rope structures. For single flux ropes, failed or actual eruptions may
result from internal or external kink evolutions, or from torus unstable
configurations. We highlight recent findings from 3D magnetic field
reconstructions and simulations where kink and torus instabilities play
a prominent role. For interacting current systems, we critically discuss
different routes to coronal dynamics and global eruptions, due to
current channel coalescence or to tilt-kink scenarios. These scenarios
involve the presence of two nearby current channels and are clearly
distinct from the popular kink or torus instability. Since the solar
corona is pervaded with myriads of magnetic loops—creating interacting
flux ropes typified by parallel or antiparallel current channels as
exemplified in various recent observational studies—coalescence or
tilt-kink evolutions must be very common for destabilizing adjacent
flux rope systems. Since they also evolve on ideal MHD timescales,
they may well drive many sympathetic eruptions witnessed in the solar
corona. Moreover, they necessarily lead to thin current sheets that
are liable to reconnection. We review findings from 2D and 3D MHD
simulations for tilt and coalescence evolutions, as well as on particle
acceleration aspects derived from computed charged particle motions
in tilt-kink disruptions and coalescing flux ropes. The latter were
recently studied in two-way coupled kinetic-fluid models, where the
complete phase-space information of reconnection is incorporated.

---------------------------------------------------------
Title: The Magnetic Flux Rope Structure of a Triangulated Solar
Filament
Authors: Guo, Yang; Xu, Yu; Ding, M. D.; Chen, P. F.; Xia, Chun;
Keppens, Rony
2019ApJ...884L...1G Altcode:
Solar magnetic flux ropes are core structures driving solar
activities. We construct a magnetic flux rope for a filament/prominence
observed at 01:11 UT on 2011 June 21 in AR 11236 with a combination of
state-of-the-art methods, including triangulation from multiperspective
observations, the flux rope embedding method, the regularized
Biot-Savart laws, and the magnetofrictional method. First, the path
of the filament is reconstructed via the triangulation with 304 Å
images observed by the Atmospheric Imaging Assembly on board Solar
Dynamics Observatory (SDO) and by the Extreme Ultraviolet Imager on
board the twin Solar Terrestrial Relations Observatory. Then, a flux
rope is constructed with the regularized Biot-Savart laws using the
information of its axis. Next, it is embedded into a potential magnetic
field computed from the photospheric radial magnetic field observed by
the Helioseismic and Magnetic Imager on board SDO. The combined magnetic
field is finally relaxed by the magnetofrictional method to reach a
nonlinear force-free state. It is found that both models constructed
by the regularized Biot-Savart laws and after the magnetofrictional
relaxation coincide with the 304 Å images. The distribution of magnetic
dips coincides with part of the filament/prominence material, and
the quasi-separatrix layers wrap the magnetic flux ropes, displaying
hyperbolic flux tube structures. These models have the advantages of
constructing magnetic flux ropes in the higher atmosphere and weak
magnetic field regions, which could be used as initial conditions for
magnetohydrodynamic simulations of coronal mass ejections.

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Title: Multilayered Kelvin-Helmholtz Instability in the Solar Corona
Authors: Yuan, Ding; Shen, Yuandeng; Liu, Yu; Li, Hongbo; Feng,
Xueshang; Keppens, Rony
2019ApJ...884L..51Y Altcode: 2019arXiv191005710Y
The Kelvin-Helmholtz (KH) instability is commonly found in many
astrophysical, laboratory, and space plasmas. It could mix plasma
components of different properties and convert dynamic fluid energy from
large-scale structure to smaller ones. In this study, we combined the
ground-based New Vacuum Solar Telescope (NVST) and the Solar Dynamic
Observatories/Atmospheric Imaging Assembly (AIA) to observe the plasma
dynamics associated with active region 12673 on 2017 September 9. In
this multitemperature view, we identified three adjacent layers of
plasma flowing at different speeds, and detected KH instabilities at
their interfaces. We could unambiguously track a typical KH vortex and
measure its motion. We found that the speed of this vortex suddenly
tripled at a certain stage. This acceleration was synchronized with
the enhancements in emission measure and average intensity of the 193
Å data. We interpret this as evidence that KH instability triggers
plasma heating. The intriguing feature in this event is that the KH
instability observed in the NVST channel was nearly complementary to
that in the AIA 193 Å. Such a multithermal energy exchange process is
easily overlooked in previous studies, as the cold plasma component is
usually not visible in the extreme-ultraviolet channels that are only
sensitive to high-temperature plasma emissions. Our finding indicates
that embedded cold layers could interact with hot plasma as invisible
matters. We speculate that this process could occur at a variety of
length scales and could contribute to plasma heating.

---------------------------------------------------------
Title: General-relativistic Resistive Magnetohydrodynamics with
Robust Primitive-variable Recovery for Accretion Disk Simulations
Authors: Ripperda, B.; Bacchini, F.; Porth, O.; Most, E. R.; Olivares,
H.; Nathanail, A.; Rezzolla, L.; Teunissen, J.; Keppens, R.
2019ApJS..244...10R Altcode: 2019arXiv190707197R
Recent advances in black hole astrophysics, particularly the first
visual evidence of a supermassive black hole at the center of the galaxy
M87 by the Event Horizon Telescope, and the detection of an orbiting
“hot spot” nearby the event horizon of Sgr A* in the Galactic center
by the Gravity Collaboration, require the development of novel numerical
methods to understand the underlying plasma microphysics. Non-thermal
emission related to such hot spots is conjectured to originate from
plasmoids that form due to magnetic reconnection in thin current layers
in the innermost accretion zone. Resistivity plays a crucial role in
current sheet formation, magnetic reconnection, and plasmoid growth in
black hole accretion disks and jets. We included resistivity in the
three-dimensional general-relativistic magnetohydrodynamics (GRMHD)
code BHAC and present the implementation of an implicit-explicit
scheme to treat the stiff resistive source terms of the GRMHD
equations. The algorithm is tested in combination with adaptive
mesh refinement to resolve the resistive scales and a constrained
transport method to keep the magnetic field solenoidal. Several novel
methods for primitive-variable recovery, a key part in relativistic
magnetohydrodynamics codes, are presented and compared for accuracy,
robustness, and efficiency. We propose a new inversion strategy
that allows for resistive-GRMHD simulations of low gas-to-magnetic
pressure ratio and highly magnetized regimes as applicable for black
hole accretion disks, jets, and neutron-star magnetospheres. We apply
the new scheme to study the effect of resistivity on accreting black
holes, accounting for dissipative effects as reconnection.

---------------------------------------------------------
Title: Waves in a warm pair plasma: a relativistically complete
two-fluid analysis
Authors: Keppens, Rony; Goedbloed, Hans; Durrive, Jean-Baptiste
2019JPlPh..85d9008K Altcode:
We present an ideal two-fluid wave mode analysis for a pair plasma,
extending an earlier study for cold conditions to the warm pair plasma
case. Starting from the completely symmetrized means for writing the
governing linearized equations in the pair fluid rest frame, we discuss
the governing dispersion relation containing all six pairs of forward
and backward propagating modes, which are conveniently labelled as S, A,
F, M, O and X. These relate to the slow (S), Alfvén (A) and fast (F)
magnetohydrodynamic waves, include a modified (M) electrostatic mode,
as well as the electromagnetic O and X branches. In the dispersion
relation, only two parameters appear, which define the pair plasma
magnetization E2\in [0,\infty ] and the squared pair plasma
sound speed v2 , measured in units of the light speed c
. The description is valid also in the highly relativistic regime,
where either a high magnetization and/or a relativistic temperature
(hence sound speed) is reached. We recover the exact relativistic
single-fluid magnetohydrodynamic expressions for the S, A and F
families in the low wavenumber-frequency regime, which can be obtained
for any choice of the equation of state. We argue that, as in a cold
pair plasma, purely parallel or purely perpendicular propagation with
respect to the magnetic field vector \boldsymbol{B} is special, and
near-parallel or near-perpendicular orientations demonstrate avoided
crossings of branches at computable wavenumbers and frequencies. The
complete six-mode phase and group diagram views are provided as well,
visually demonstrating the intricate anisotropies in all wave modes,
as well as their transformations. Analytic expressions for all six
wave group speeds at both small and large wavenumbers complement
the analysis.

---------------------------------------------------------
Title: Particle Orbits at the Magnetopause: Kelvin-Helmholtz Induced
Trapping
Authors: Leroy, M. H. J.; Ripperda, B.; Keppens, R.
2019JGRA..124.6715L Altcode: 2018arXiv181004324L
The Kelvin-Helmholtz instability is a known mechanism for penetration
of solar wind matter into the magnetosphere. Using three-dimensional,
resistive magnetohydrodynamic simulations, the double midlatitude
reconnection (DMLR) process was shown to efficiently exchange solar wind
matter into the magnetosphere, through mixing and reconnection. Here
we compute test particle orbits through DMLR configurations. In the
instantaneous electromagnetic fields, charged particle trajectories
are integrated using the guiding center approximation. The mechanisms
involved in the electron particle orbits and their kinetic energy
evolutions are studied in detail, to identify specific signatures of the
DMLR through particle characteristics. The charged particle orbits are
influenced mainly by magnetic curvature drifts. We identify complex,
temporarily trapped trajectories where the combined electric field
and (reconnected) magnetic field variations realize local cavities
where particles gain energy before escaping. By comparing the orbits
in strongly deformed fields due to the Kelvin-Helmholtz instability
development, with the textbook mirror-drift orbits resulting from our
initial configuration, we identify effects due to current sheets formed
in the DMLR process. We do this in various representative stages during
the DMLR development.

---------------------------------------------------------
Title: Test particles in relativistic resistive magnetohydrodynamics
Authors: Ripperda, Bart; Porth, Oliver; Keppens, Rony
2019JPhCS1225a2018R Altcode: 2018arXiv181004323R
The Black Hole Accretion Code (BHAC) has recently been extended with the
ability to evolve charged test particles according to the Lorentz force
within resistive relativistic magnetohydrodynamics simulations. We apply
this method to evolve particles in a reconnecting current sheet that
forms due to the coalescence of two magnetic flux tubes in 2D Minkowski
spacetime. This is the first analysis of charged test particle evolution
in resistive relativistic magnetohydrodynamics simulations. The energy
distributions of an ensemble of 100.000 electrons are analyzed, as
well as the acceleration of particles in the plasmoids that form in the
reconnection layer. The effect of the Lundquist number, magnetization,
and plasma-β on the particle energy distribution is explored for a
range of astrophysically relevant parameters. We find that electrons
accelerate to non-thermal energies in the thin current sheets in
all cases. We find two separate acceleration regimes: An exponential
increase of the Lorentz factor during the island coalescence where
the acceleration depends linearly on the resistivity and a nonlinear
phase with high variability. These results are relevant for determining
energy distributions and acceleration sites obtaining radiation maps
in large-scale magnetohydrodynamics simulations of black hole accretion
disks and jets.

---------------------------------------------------------
Title: Extreme-ultraviolet and X-Ray Emission of Turbulent Solar
Flare Loops
Authors: Ruan, Wenzhi; Xia, Chun; Keppens, Rony
2019ApJ...877L..11R Altcode:
Turbulence has been observed in flare loops and is believed to be
crucial for the acceleration of particles and in the emission of X-ray
photons in flares, but how the turbulence is produced is still an
open question. A scenario proposed by Fang et al. suggests that fast
evaporation flows from flare loop footpoints can produce turbulence in
the looptop via the Kelvin-Helmholtz instability (KHI). We revisit and
improve on this scenario and study how the KHI turbulence influences
extreme-ultraviolet (EUV) and X-ray emission. A 2.5D numerical
simulation is performed in which we incorporate the penetration
of high-energy electrons as a spatio-temporal dependent trigger for
chromospheric evaporation flows. EUV, soft X-ray (SXR), and hard X-ray
(HXR) emission are synthesized based on the evolving plasma parameters
and given energetic electron spectra. KHI turbulence leads to clear
brightness fluctuations in the EUV, SXR, and HXR emission, with the
SXR light curve demonstrating a clear quasi-periodic pulsation (QPP)
with period of 26 s. This QPP derives from a locally trapped, fast
standing wave that resonates in between KHI vortices. The spectral
profile of the Fe XXI 1354 line is also synthesized and found to be
broadened due to the turbulent motion of plasma. HXR tends to mimic
the variation of SXR flux and the footpoint HXR spectrum is flatter
than the looptop HXR spectrum.

---------------------------------------------------------
Title: Relativistic resistive magnetohydrodynamic reconnection and
plasmoid formation in merging flux tubes
Authors: Ripperda, B.; Porth, O.; Sironi, L.; Keppens, R.
2019MNRAS.485..299R Altcode: 2018arXiv181010116R; 2019MNRAS.tmp..383R
We apply the general relativistic resistive magnetohydrodynamics
code BHAC to perform a 2D study of the formation and evolution of
a reconnection layer in between two merging magnetic flux tubes
in Minkowski space-time. Small-scale effects in the regime of low
resistivity most relevant for dilute astrophysical plasmas are resolved
with very high accuracy due to the extreme resolutions obtained with
adaptive mesh refinement. Numerical convergence in the highly non-linear
plasmoid-dominated regime is confirmed for a sweep of resolutions. We
employ both uniform resistivity and non-uniform resistivity based on
the local, instantaneous current density. For uniform resistivity we
find Sweet-Parker reconnection, from η = 10-2 down to η =
10-4, for a reference case of magnetization σ = 3.33 and
plasma-β = 0.1. For uniform resistivity η = 5 × 10-5
the tearing mode is recovered, resulting in the formation of secondary
plasmoids. The plasmoid instability enhances the reconnection rate to
vrec ∼ 0.03c compared to vrec ∼ 0.01c for
η = 10-4. For non-uniform resistivity with a base level
η0 = 10-4 and an enhanced current-dependent
resistivity in the current sheet, we find an increased reconnection
rate of vrec ∼ 0.1c. The influence of the magnetization σ
and the plasma-β is analysed for cases with uniform resistivity η =
5 × 10-5 and η = 10-4 in a range 0.5 ≤ σ
≤ 10 and 0.01 ≤ β ≤ 1 in regimes that are applicable for black
hole accretion discs and jets. The plasmoid instability is triggered
for Lundquist numbers larger than a critical value of Sc
≈ 8000.

---------------------------------------------------------
Title: Thermal stability of magnetohydrodynamic modes in homogeneous
plasmas
Authors: Claes, N.; Keppens, R.
2019A&A...624A..96C Altcode:
Context. Thermal instabilities give rise to condensations in the
solar corona, and are the most probable scenario for coronal rain and
prominence formation. We revisit the original theoretical treatment
done by Field (1965, ApJ, 142, 531) in a homogeneous plasma with
heat-loss effects and combine this with state-of-the-art numerical
simulations to verify growth-rate predictions and address the long-term
non-linear regime. We especially investigate interaction between
multiple magnetohydrodynamic (MHD) wave modes and how they in turn
trigger thermal mode development. Aims: We assess how well
the numerical MHD simulations retrieve the analytically predicted
growth rates. We complete the original theory with quantifications
of the eigenfunctions, calculated to consistently excite each wave
mode. Thermal growth rates are fitted also in the non-linear regime
of multiple wave-wave interaction setups, at the onset of thermal
instability formation. Methods: We performed 2D numerical MHD
simulations, including an additional (radiative) heat-loss term and
a constant heating term to the energy equation. We mainly focus on
the thermal (i.e. entropy) and slow MHD wave modes and use the wave
amplitude as a function of time to make a comparison to predicted growth
rates. Results: It is shown that the numerical MHD simulations
retrieve analytically predicted growth rates for all modes, of thermal
and slow or fast MHD type. In typical coronal conditions, the latter are
damped due to radiative losses, but their interaction can cause slowly
changing equilibrium conditions which ultimately trigger thermal mode
development. Even in these non-linear wave-wave interaction setups,
the growth rate of the thermal instability agrees with the exponential
profile predicted by linear theory. The non-linear evolutions show
systematic field-guided motions of condensations with fairly complex
morphologies, resulting from thermal modes excited through damped slow
MHD waves. These results are of direct interest to the study of solar
coronal rain and prominence fine structure. Our wave-wave interaction
setups are relevant for coronal loop sections which are known to host
slow wave modes, and hence provide a new route to explain the sudden
onset of coronal condensation.

---------------------------------------------------------
Title: A fresh look at waves in ion-electron plasmas
Authors: Keppens, Rony; Goedbloed, Hans
2019FrASS...6...11K Altcode:
Exploiting the general dispersion relation describing all waves
in an ideal ion-electron fluid, we revisit established treatments
on wave families in a cold ion-electron plasma. These contain the
magnetohydrodynamic Alfvén and fast waves at low frequencies, long
wavelengths, but are enriched by short wavelength resonance behaviours,
electrostatic and electromagnetic mode types, and cut-off frequencies
distinguishing propagating from evanescent waves. Our theoretical
treatment exploits purely polynomial expressions, which for the cold
ion-electron case only depend on 2 parameters: the ratio of masses
over charges μ and the ratio E of the electron gyro frequency to
the combined ion-electron plasma frequency. We provide a complete
description of all waves, which stresses the intricate variation of
all five branches of eigenfrequencies ω(k,θ) depending on wavenumber
k and angle θ between wavevector and magnetic field B. Corresponding
5-mode phase and group diagrams provide insight on wave transformations
and energy transport. Special cases, like the high frequency modes
in magneto-ionic theory following from Appleton-Hartree dispersion
relations, are naturally recovered and critically discussed. Faraday
rotation for electromagnetic waves is extended to all propagation
angles θ. The discussion covers all cold ion-electron plasma waves,
up into the relativistic regime.

---------------------------------------------------------
Title: Wind Roche lobe overflow in high-mass X-ray binaries. A
possible mass-transfer mechanism for ultraluminous X-ray sources
Authors: El Mellah, I.; Sundqvist, J. O.; Keppens, R.
2019A&A...622L...3E Altcode: 2018arXiv181012937E
Ultraluminous X-ray sources (ULXs) have such high X-ray luminosities
that they were long thought to be accreting intermediate-mass black
holes. Yet, some ULXs have been shown to display periodic modulations
and coherent pulsations suggestive of a neutron star in orbit around
a stellar companion and accreting at super-Eddington rates. In
this Letter, we propose that the mass transfer in ULXs could be
qualitatively the same as in supergiant X-ray binaries (SgXBs),
with a wind from the donor star highly beamed towards the compact
object. Since the star does not fill its Roche lobe, this mass
transfer mechanism known as "wind Roche lobe overflow" can remain
stable even for large donor-star-to-accretor mass ratios. Based on
realistic acceleration profiles derived from spectral observations
and modeling of the stellar wind, we compute the bulk motion of the
wind to evaluate the fraction of the stellar mass outflow entering
the region of gravitational predominance of the compact object. The
density enhancement towards the accretor leads to mass-transfer rates
systematically much larger than the mass-accretion rates derived by
the Bondi-Hoyle-Lyttleton formula. We identify orbital and stellar
conditions for a SgXBs to transfer mass at rates necessary to reach the
ULX luminosity level. These results indicate that Roche-lobe overflow is
not the only way to funnel large quantities of material into the Roche
lobe of the accretor. With the stellar mass-loss rates and parameters
of M101 ULX-1 and NGC 7793 P13, wind Roche-lobe overflow can reproduce
mass-transfer rates that qualify an object as an ULX.

---------------------------------------------------------
Title: Wave modes in a cold pair plasma: the complete phase and
group diagram point of view
Authors: Keppens, Rony; Goedbloed, Hans
2019JPlPh..85a1701K Altcode:
We present a complete analysis of all wave modes in a cold pair
plasma, significantly extending standard textbook treatments. Instead
of identifying the maximal number of two propagating waves at fixed
frequency ω, we introduce a unique labelling of all 5 mode pairs
described by the general dispersion relation ω(k), starting from
their natural ordering at small wavenumber k. There, the 5 pairs start
off as Alfvén (A), fast magnetosonic (F), modified electrostatic (M)
and electromagnetic O and X branches, and each ω(k) branch smoothly
connects to large wavenumber resonances or limits. For cold pair
plasmas, these 5 branches show avoided crossings, which become true
crossings at exactly parallel or perpendicular orientation. Only for
those orientations, we find a changed connectivity between small and
large wavenumber behaviour. Analysing phase and group diagrams for
all 5 wave modes, distinctly different from the Clemmow-Mullaly-Allis
representation, reveals the true anisotropy of the A, M and O branches.

---------------------------------------------------------
Title: Formation of wind-captured disks in supergiant X-ray
binaries. Consequences for Vela X-1 and Cygnus X-1
Authors: El Mellah, I.; Sander, A. A. C.; Sundqvist, J. O.; Keppens, R.
2019A&A...622A.189E Altcode: 2018arXiv181012933E
Context. In supergiant X-ray binaries (SgXB), a compact object captures
a fraction of the wind of an O/B supergiant on a close orbit. Proxies
exist to evaluate the efficiency of mass and angular momentum accretion,
but they depend so dramatically on the wind speed that given the
current uncertainties, they only set loose constraints. Furthermore,
these proxies often bypass the impact of orbital and shock effects on
the flow structure. Aims: We study the wind dynamics and angular
momentum gained as the flow is accreted. We identify the conditions
for the formation of a disk-like structure around the accretor and the
observational consequences for SgXB. Methods: We used recent
results on the wind launching mechanism to compute 3D streamlines,
accounting for the gravitational and X-ray ionizing influence of the
compact companion on the wind. Once the flow enters the Roche lobe of
the accretor, we solved the hydrodynamics equations with cooling. Results: A shocked region forms around the accretor as the flow is
beamed. For wind speeds on the order of the orbital speed, the shock
is highly asymmetric compared to the axisymmetric bow shock obtained
for a purely planar homogeneous flow. With net radiative cooling,
the flow always circularizes for sufficiently low wind speeds. 
Conclusions: Although the donor star does not fill its Roche lobe, the
wind can be significantly beamed and bent by the orbital effects. The
net angular momentum of the accreted flow is then sufficient to form
a persistent disk-like structure. This mechanism could explain the
proposed limited outer extension of the accretion disk in Cygnus X-1
and suggests the presence of a disk at the outer rim of the neutron
star magnetosphere in Vela X-1 and has dramatic consequences on the
spinning up of the accretor.

---------------------------------------------------------
Title: Forward Modeling of SDO/AIA and X-Ray Emission from a Simulated
Flux Rope Ejection
Authors: Zhao, Xiaozhou; Xia, Chun; Van Doorsselaere, Tom; Keppens,
Rony; Gan, Weiqun
2019ApJ...872..190Z Altcode: 2019arXiv190409965Z
We conduct forward-modeling analysis based on our 2.5 dimensional
magnetohydrodynamics (MHD) simulation of magnetic flux rope (MFR)
formation and eruption driven by photospheric converging motion. The
current sheet (CS) evolution during the MFR formation and eruption
process in our MHD simulation can be divided into four stages. The
first stage shows the CS forming and gradually lengthening. Resistive
instabilities that disrupt the CS mark the beginning of the second
stage. Magnetic islands disappear in the third stage and reappear
in the fourth stage. Synthetic images and light curves of the seven
Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA)
channels, i.e., 94 Å, 131 Å, 171 Å, 193 Å, 211 Å, 304 Å, and 335
Å, and the 3-25 keV thermal X-ray are obtained with forward-modeling
analysis. The loop-top source and the coronal sources of the soft
X-ray are reproduced in forward modeling. The light curves of the
seven SDO/AIA channels start to rise once resistive instabilities
develop. The light curve of the 3-25 keV thermal X-ray starts to go
up when the reconnection rate reaches one of its peaks. Quasiperiodic
pulsations (QPPs) appear twice in the SDO/AIA 171 Å, 211 Å, and 304
Å channels, corresponding to the period of chaotic (re)appearance and
CS-guided displacements of the magnetic islands. QPPs appear once in
the SDO/AIA 94 Å and 335 Å channels after the disruption of the CS
by resistive instabilities and in the 193 Å channel when the chaotic
motion of the magnetic islands reappears.

---------------------------------------------------------
Title: Relativistic 3D Hydrodynamic Simulations of the W50-SS433
System
Authors: Millas, Dimitrios; Porth, Oliver; Keppens, Rony
2019ASSP...55...71M Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Enhanced accretion and wind-captured discs in high mass
X-ray binaries
Authors: El Mellah, I.; Sundqvist, J. O.; Keppens, R.
2019MmSAI..90..185E Altcode:
The historical gravitational wave detections of last years ushered in
a new era for the study of massive binaries evolution. In high mass
X-ray binaries, a transient albeit decisive phase preceding compact
binaries, a compact accretor orbits a massive star and captures part
of its intense stellar wind. From the stellar photosphere down to
the vicinity of the compact object, the flow undergoes successive
phases. Our numerical simulations offer a comprehensive picture of the
accretion process along this journey. We report new results on the
impact of the wind micro-structure on the X-ray time variability and
how the revised downwards wind speed implies a significantly different
flow geometry than the one previously considered. For wind speeds of
the order of the orbital speed or lower, accretion is significantly
enhanced and provided cooling is accounted for, transient disc-like
structures form beyond the neutron star magnetosphere, with dramatic
consequences on the torques applied to the compact object. The
recent observational reports on the limited extent of the accretion
disc in Cygnus X-1 suggest that the disc is produced by this mechanism
rather than a Roche lobe overflow of the companion star. In Vela X-1,
such a structure remains to be observed but its indirect signatures
through jets or the torques it applies on the neutron star could well
be within our observational grasp. This accretion regime could
also account for large mass transfer rates, up to levels suitable for
ultra-luminous X-ray sources, without Roche lobe overflow of the donor
star, a situation observed in M101 ULX-1.

---------------------------------------------------------
Title: Solar Magnetic Flux Rope Eruption Simulated by a Data-driven
Magnetohydrodynamic Model
Authors: Guo, Yang; Xia, Chun; Keppens, Rony; Ding, M. D.; Chen, P. F.
2019ApJ...870L..21G Altcode: 2018arXiv181210030G
The combination of magnetohydrodynamic (MHD) simulation and
multi-wavelength observations is an effective way to study the
mechanisms of magnetic flux rope eruption. We develop a data-driven MHD
model using the zero-β approximation. The initial condition is provided
by a nonlinear force-free field derived from the magneto-frictional
method based on vector magnetic field observed by the Helioseismic and
Magnetic Imager on board the Solar Dynamics Observatory. The bottom
boundary uses observed time series of the vector magnetic field and the
vector velocity derived by the Differential Affine Velocity Estimator
for Vector Magnetograms. We apply the data-driven model to active
region 11123 observed from 06:00 UT on 2010 November 11 to about 2 hr
later. The evolution of the magnetic field topology coincides with
the flare ribbons observed in the 304 and 1600 Å wavebands by the
Atmospheric Imaging Assembly. The morphology, propagation path, and
propagation range of the flux rope are comparable with the observations
in 304 Å. We also find that a data-constrained boundary condition,
where the bottom boundary is fixed to the initial values, reproduces
a similar simulation result. This model can reproduce the evolution
of a magnetic flux rope in its dynamic eruptive phase.

---------------------------------------------------------
Title: Solar flares and Kelvin-Helmholtz instabilities: A parameter
survey
Authors: Ruan, W.; Xia, C.; Keppens, R.
2018A&A...618A.135R Altcode: 2018arXiv180902410R
Context. Hard X-ray (HXR) sources are frequently observed near the
top of solar flare loops, which are also bright in soft X-ray (SXR)
and extreme ultraviolet (EUV) wavebands. We revisit a recent scenario
proposed by Fang et al. (2016) to trigger loop-top turbulence in
flaring loops, which can help explain variations seen in SXR and EUV
brightenings and potentially impact and induce HXR emission. It is
conjectured that evaporation flows from flare-impacted chromospheric
footpoints interact with each other near the loop top and produce
turbulence via the Kelvin-Helmholtz instability (KHI). Aims: By
performing a rigorous parameter survey, in which we vary the duration,
total amount, and asymmetry of the energy deposition at both footpoints,
we assess the relevance of the KHI in triggering and sustaining
loop-top turbulence. We synthesize SXR and EUV emission and discuss
the possibility of HXR emission through bremsstrahlung or inverse
Compton processes, which scatter SXR photons to HXR photons via the
inverse Compton mechanism. Methods: We performed 2.5D numerical
simulations in which the magnetohydrodynamic model incorporates a
realistic photosphere to coronal stratification, parametrized heating,
radiative losses, and field-aligned anisotropic thermal conduction. We
focus on the trigger of the KHI and the resulting turbulence, as well as
identify various oscillatory patterns that appear in the evolutions. Results: We find that a M2.2-class related amount of energy should be
deposited in less than four minutes to trigger a KHI interaction. Slower
deposition, or lesser energy (&lt; 0.33 × 1029 ergs) rather
leads to mere loop-top compression sites bounded by shocks, without KHI
development. Asymmetry in the footpoint deposition determines whether
the KHI turbulent zone gets produced away from the apex, and asymmetric
cases can show a slow-mode mediated, periodic displacement of the
turbulent zone. Our reference simulation further demonstrates a clear
25 s periodicity in the declining phase of the SXR light curve, wherein
compressional effects dominate. Conclusions: When turbulence is
produced in the loop apex, an index of -5/3 can be found in the spectra
of velocity and magnetic field fluctuations. Typical values for M-class
flares routinely show KHI development. The synthesized SXR light curve
shows a clear periodic signal related to the sloshing motion of the
vortex pattern created by the KHI. The movies are available at <A
href="https://www.aanda.org/10.1051/0004-6361/201833362/olm">https://www.aanda.org</A>

---------------------------------------------------------
Title: W 50 and SS 433
Authors: Bowler, Michael G.; Keppens, Rony
2018A&A...617A..29B Altcode: 2018arXiv180510094B
Context. The Galactic microquasar SS 433 launches oppositely directed
jets at speeds approximately a quarter of the speed of light. These
appear to have punched through and beyond the supposed supernova
remnant shell W 50. The problems with this interpretation are: (i)
the precessing jets have somehow been collimated before reaching the
shell; (ii) without deceleration, only recently launched jets would
have reached no further; and (iii) certain features in the lobes are
moving slowly or are stationary. Aims: Hydrodynamic computations
have demonstrated that for at least one set of parameters describing
the ambient medium, jets that diverge and precess are both decelerated
and collimated; the conformation of W 50 could then have been sculpted
by the jets of SS 433. However, the parameters adopted for density and
pressure in these computations are not consistent with observations
of jets at a few years old; nor do they represent conditions within a
supernova remnant. Our aim is to investigate whether the computations
already performed can be scaled to a realistic W 50. Methods:
We find simple and physically based scaling relations. The distance to
collimation varies inversely with the square root of the pressure of
the ambient medium and the speed with which the head of a collimated
jet propagates scales with the square root of the temperature. We
extrapolate the results of the hydrodynamic computations to lower
densities and pressures. Results: The jets of SS 433, launched
into an ambient medium of pressure 10-9 erg cm-3
and temperature 108 K, within a supernova remnant, could
be responsible for the characteristics of W 50. The precessing jets
are collimated within 10 pc and the head of the resulting cylindrical
jet propagates slowly. Conclusions: The problems of relating
W 50 to SS 433 may now be solved.

---------------------------------------------------------
Title: Diverse Stratosphere Circulation in tidally locked Exo-Earths
Authors: Carone, Ludmila; Keppens, Rony; Decin, Leen; Henning, Thomas
2018EPSC...12..903C Altcode:
We show that the circulation in a transient stratosphere on habitable
exoplanets can be very diverse, ranging from a scenario with efficient
equator-to-polewards circulation (like on Earth) to the exact opposite,
'Anti-Brewer-Dobson'-circulation that confines air masses to the
stratospheric equatorial region

---------------------------------------------------------
Title: Clumpy wind accretion in Supergiant X-ray binaries
Authors: El Mellah, Ileyk; Keppens, Rony; Sundqvist, Jon
2018cosp...42E.973E Altcode:
Supergiant X-ray Binaries (SgXB) host a neutron star (NS)
accreting a fraction of the intense wind from an evolved O/B
Supergiant companion. The X-ray emission associated to accretion
displays photometric and spectroscopic variability in time which has
partly been attributed to overdensities (aka clumps) in the stellar
wind. Recently, the micro-structure of the wind mass and dimension of
these clumps. To evaluate the impact of the serendipitous a has been
computed by Sundqvist et al (2017), shedding new light the on the
mass and dimension of these clumps. To evaluate the impact of their
serendipitous accretion on the time variability of the mass accretion
rates, we plunge the NS into the wind and performed 3D simulations of
the accretion process. We follow the inhomogeneous flow over several
orders of magnitude, from the hydrodynamical bow shock down to the NS
magnetosphere, and identify the conditions favorable to the formation
of a transient disc-like structure within the shocked region. We also
account for the variable absorption due to unaccreted clumps passing by
the line-of-sight and estimate the final effective variability of the
mass accretion rate for different orbital separations. By confronting
our results to observations of Vela X-1 by Grinberg et al (2017), we
conclude that, if the variability at low luminosity is essentially due
to clumps, they can not explain, per se, the flaring activity which
must find its origin within the NS magnetosphere.

---------------------------------------------------------
Title: Scooping up a prominence: embedding a filament in a CME
Authors: Keppens, Rony; Gan, Weiqun; Xia, Chun; Zhao, Xiaozhou
2018cosp...42E1737K Altcode:
We report on thermodynamically consistent magnetohydrodynamic
simulations from chromosphere to corona, where an erupting flux rope
gets formed by photospheric converging motions. Our model shows how
chromospheric material can get levitated into the flux rope and form a
filament. The flux rope-filament system transits from quasi-equilibrium
stages to an accelerated eruption, and this transition coincides
with a changeover in the reconnection occurring in the current sheet
underneath the flux rope. A slow Sweet-Parker stage transforms to an
unsteady bursty reconnection regime with multiple islands. The largest
of these islands are seen to merge with the chromospheric fields below
to produce flare arcade loops. Our simulation unifies a variety of
processes, from small-scale reconnection structures up to full-scale
embedded filaments in a coronal mass ejection.

---------------------------------------------------------
Title: Understanding formation and structure of solar prominences
via multidimensional simulations
Authors: Xia, Chun; Keppens, Rony
2018cosp...42E3708X Altcode:
Solar prominences are one of the most common activities in the
corona. The formation of magnetic and plasma structures of prominences
is far from fully understood. Observationsand theoretical studies
suggest that typical prominences are hosted in helical magnetic flux
ropes. With the aid of multidimensional magnetohydrodynamic (MHD)
simulations, we provide numerical models to explain the formation of
flux ropes driven by photospheric motion and magnetic reconnection at
footpoints of sheared magnetic loops. The physical mechanism responsible
for prominence plasma formation is believed to be thermal instability,
which may be triggered by thermal nonequilibrium process with strong
heating and chromospheric evaporation near footpoints of magnetic
loops or by compression resulted from the topological change of
magnetic field via coronal reconnection. Both scenarios are presented
by multidimensional MHD simulations. The observed ubiquitous dense
downflows and light upflowsin quiescent prominences are difficult
to interpret as plasma with high conductivity seems to move across
horizontal magnetic field lines. Multidimensional MHD simulations on a
local portion of prominence with parallel field lines, suggest magnetic
Rayleigh-Taylor instability is responsible for the phenomenon. Our
full prominence model, as a result of in-situ plasma condensations
in a magnetic flux rope driven by continuous plasma evaporation from
chromosphere, reproduced a fragmented, highly dynamic state with
continuous reappearance of multiple blobs and thread structures that
move mainly downward dragging along mass-loaded field lines, which
may explain the dense downflows of quiescent prominences. With steady
footpoint heating, the modeled prominence established a dynamic balance
between the drainage of the prominence plasma back to the chromosphere
and the formation of prominence plasma via continuous condensation.

---------------------------------------------------------
Title: Multidimensional simulations on evaporation-condensation in
complex coronal magnetic field
Authors: Xia, Chun; Keppens, Rony
2018cosp...42E3709X Altcode:
We present multidimensional simulations to illuminate that
evaporation-condensation process can produce both prominence and
coronal rain depending on the complexity of magnetic topology. On
a two-dimensional (2D) magnetic arcade, we demonstrate how
evaporation-condensation can induce in-situ cross-field condensations
which fall along arcade loops as coronal rain. The virtual coronal
rain displays the deformation of blobs into V-shaped features and
streaming lines. The transition region between cool dense material
and hot coronal plasma is revealed with high-resolution simulation. We
found shear flows along neighboring loops are siphon flows set up by
multiple blob dynamics and they affect the deformation of the falling
blobs. On a three-dimensional (3D) weak magnetic bipolar arcade, we
found fast in-situ condensation across loop top due to symmetry. The
first large-scale condensation on the loop top suffers Rayleigh-Taylor
instability and becomes fragmented into smaller blobs. The blobs fall
vertically dragging magnetic loops until they enter the lower region
with stronger magnetic field and start to fall along the loops from loop
top to loop footpoints as coronal rain. On a 2D quadrupolar magnetic
system with a coronal null point and four groups of magnetic loops,
we present how evaporation-condensation produce a prominence in the
dipped magnetic region. And subsequent descending of the prominence
triggers magnetic reconnection near null point, which leads to
redistribution of prominence material to underlying loops to be coronal
rain. In a 3D helical magnetic flux rope, we reproduced a prominence
formation as a result of in-situ plasma condensations collected in
the dipped magnetic regions of the flux rope. The prominence is born
and maintained in a fragmented, highly dynamic state with a continuous
reappearance of multiple blobs and thread structures that move mainly
downward dragging along mass-loaded field lines. A plasma circulation
is found by self-organized dynamic balance between the drainage
of prominence plasma back to the chromosphere and the formation of
prominence plasma via continuous condensation. Common features, such
as, rebound shocks generated by the siphon inflows during condensation
and counter-streaming shearing flows around dynamic condensed blobs,
are discussed.

---------------------------------------------------------
Title: Kelvin-Helmholtz instabilities: a novel ingredient to the
standard flare model
Authors: Keppens, Rony; Xia, Chun; Ruan, Wenzhi
2018cosp...42E1735K Altcode:
In the standard flare model, energy deposition near the chromosphere
from downwards accelerated particles leads to evaporation flows that
invade the flaring loop. We modeled this process in isolation of the
overarching reconnection site, and found that one can frequently
encounter situations where these upflows from both loop legs meet
up in a loop-top localized, turbulent fashion. At the loop apex,
Kelvin-Helmholtz instability (KHI) of the interacting flows sets in and
thermal soft X-ray photons are abound in the interaction zone. This,
together with the intrinsically fragmented magnetic field topology
due to the vortical disruption can explain hard X-ray sources in
loop apexes: electrons trapped and accelerated in the turbulent
region can upscatter soft X-ray photons. A parametric survey in a
magnetohydrodynamic setting finds that the trigger of KHI and the
generation of turbulence are determined by the amount of energy
deposited in the chromospheric foot-points and the time scale of
energy deposition.

---------------------------------------------------------
Title: Magnetic reconnection during eruptive magnetic flux ropes
Authors: Keppens, Rony; Lin, Jun; Mei, Zhixing
2018cosp...42E1736K Altcode:
Using highly resolved magnetohydrodynamic simulations, we follow
the eruption of a kink-unstable magnetic flux rope in 3D. Our grid
refinement allows to zoom in on the reconnection sites within the
current sheet. At an estimated Lundquist number of 10000, we retrieve
the 3D generalization of Petschek slow shocks, tubular substructures
indicating tearing disruption, and turbulent interaction of the ejection
flows with the closed magnetic structures above and below the extended
current sheet. The 3D simulations allow synthetic views from varying
line of sight orientations, and show many morphological aspects known
from actual observations.

---------------------------------------------------------
Title: Two-way coupled MHD-PIC simulations of magnetic reconnection
in magnetic island coalescence
Authors: Makwana, Kirit; Keppens, Rony; Lapenta, Giovanni
2018JPhCS1031a2019M Altcode:
We present simulations of magnetic reconnection with a newly developed
coupled MHD-PIC code. In this work a global magnetohydrodynamic (MHD)
simulation receives kinetic feedback within an embedded region that is
modeled by a kinetic particle-in-cell (PIC) code. The PIC code receives
initial and boundary conditions from the MHD simulation, while the
MHD solution is updated with the PIC state. We briefly describe this
coupling mechanism. This method is suitable for simulating magnetic
reconnection problems, as we show with the example of reconnection in
the coalescence of magnetic islands. We compare the MHD, Hall-MHD,
fully PIC and coupled MHD-PIC simulations of the magnetic island
coalescence. We find that the kinetic simulations are very different
from the MHD and Hall-MHD results, while the coupled MHD-PIC simulations
can remedy this discrepancy while saving computing time. The diffusion
region is well resolved in the kinetic simulations, which is also
captured by the coupled MHD-PIC model. The coupled simulation also
reproduces the kinetic Hall magnetic fields correctly. We calculate
the reconnection rates and find differences between the MHD and kinetic
results. We find that the coupled MHD-PIC code can reasonably reproduce
the kinetic reconnection rate when a larger PIC feedback region is used,
while still saving significant computing time.

---------------------------------------------------------
Title: Accretion from a clumpy massive-star wind in supergiant
X-ray binaries
Authors: El Mellah, I.; Sundqvist, J. O.; Keppens, R.
2018MNRAS.475.3240E Altcode: 2017arXiv171108709E
Supergiant X-ray binaries (SgXB) host a compact object, often
a neutron star (NS), orbiting an evolved O/B star. Mass transfer
proceeds through the intense line-driven wind of the stellar donor,
a fraction of which is captured by the gravitational field of the
NS. The subsequent accretion process on to the NS is responsible for
the abundant X-ray emission from SgXB. They also display peak-to-peak
variability of the X-ray flux by a factor of a few 10-100, along with
changes in the hardness ratios possibly due to varying absorption along
the line of sight. We use recent radiation-hydrodynamic simulations of
inhomogeneities (a.k.a. clumps) in the non-stationary wind of massive
hot stars to evaluate their impact on the time-variable accretion
process. For this, we run 3D hydrodynamic simulations of the wind in
the vicinity of the accretor to investigate the formation of the bow
shock and follow the inhomogeneous flow over several spatial orders of
magnitude, down to the NS magnetosphere. In particular, we show that
the impact of the wind clumps on the time variability of the intrinsic
mass accretion rate is severely tempered by the crossing of the shock,
compared to the purely ballistic Bondi-Hoyle-Lyttleton estimation. We
also account for the variable absorption due to clumps passing
by the line of sight and estimate the final effective variability
of the column density and mass accretion rate for different orbital
separations. Finally, we compare our results to the most recent analysis
of the X-ray flux and the hardness ratio in Vela X-1.

---------------------------------------------------------
Title: Three-dimensional MHD Simulations of Solar Prominence
Oscillations in a Magnetic Flux Rope
Authors: Zhou, Yu-Hao; Xia, C.; Keppens, R.; Fang, C.; Chen, P. F.
2018ApJ...856..179Z Altcode: 2018arXiv180303385Z
Solar prominences are subject to all kinds of perturbations during
their lifetime, and frequently demonstrate oscillations. The
study of prominence oscillations provides an alternative way to
investigate their internal magnetic and thermal structures because
the characteristics of the oscillations depend on their interplay
with the solar corona. Prominence oscillations can be classified into
longitudinal and transverse types. We perform three-dimensional ideal
magnetohydrodynamic simulations of prominence oscillations along a
magnetic flux rope, with the aim of comparing the oscillation periods
with those predicted by various simplified models and examining the
restoring force. We find that the longitudinal oscillation has a period
of about 49 minutes, which is in accordance with the pendulum model
where the field-aligned component of gravity serves as the restoring
force. In contrast, the horizontal transverse oscillation has a period
of about 10 minutes and the vertical transverse oscillation has a
period of about 14 minutes, and both of them can be nicely fitted
with a two-dimensional slab model. We also find that the magnetic
tension force dominates most of the time in transverse oscillations,
except for the first minute when magnetic pressure overwhelms it.

---------------------------------------------------------
Title: A Comprehensive Comparison of Relativistic Particle Integrators
Authors: Ripperda, B.; Bacchini, F.; Teunissen, J.; Xia, C.; Porth,
O.; Sironi, L.; Lapenta, G.; Keppens, R.
2018ApJS..235...21R Altcode: 2017arXiv171009164R
We compare relativistic particle integrators commonly used in plasma
physics, showing several test cases relevant for astrophysics. Three
explicit particle pushers are considered, namely, the Boris, Vay,
and Higuera-Cary schemes. We also present a new relativistic fully
implicit particle integrator that is energy conserving. Furthermore,
a method based on the relativistic guiding center approximation is
included. The algorithms are described such that they can be readily
implemented in magnetohydrodynamics codes or Particle-in-Cell codes. Our
comparison focuses on the strengths and key features of the particle
integrators. We test the conservation of invariants of motion and the
accuracy of particle drift dynamics in highly relativistic, mildly
relativistic, and non-relativistic settings. The methods are compared
in idealized test cases, i.e., without considering feedback onto the
electrodynamic fields, collisions, pair creation, or radiation. The test
cases include uniform electric and magnetic fields, {\boldsymbol{E}}×
{\boldsymbol{B}} fields, force-free fields, and setups relevant for
high-energy astrophysics, e.g., a magnetic mirror, a magnetic dipole,
and a magnetic null. These tests have direct relevance for particle
acceleration in shocks and in magnetic reconnection.

---------------------------------------------------------
Title: Stratosphere circulation on tidally locked ExoEarths
Authors: Carone, L.; Keppens, R.; Decin, L.; Henning, Th.
2018MNRAS.473.4672C Altcode: 2017arXiv171111446C
Stratosphere circulation is important to interpret abundances of
photochemically produced compounds like ozone which we aim to observe
to assess habitability of exoplanets. We thus investigate a tidally
locked ExoEarth scenario for TRAPPIST-1b, TRAPPIST-1d, Proxima Centauri
b and GJ 667 C f with a simplified 3D atmosphere model and for different
stratospheric wind breaking assumptions.

---------------------------------------------------------
Title: MPI-AMRVAC 2.0 for Solar and Astrophysical Applications
Authors: Xia, C.; Teunissen, J.; El Mellah, I.; Chané, E.; Keppens, R.
2018ApJS..234...30X Altcode: 2017arXiv171006140X
We report on the development of MPI-AMRVAC version 2.0, which is
an open-source framework for parallel, grid-adaptive simulations
of hydrodynamic and magnetohydrodynamic (MHD) astrophysical
applications. The framework now supports radial grid stretching in
combination with adaptive mesh refinement (AMR). The advantages
of this combined approach are demonstrated with one-dimensional,
two-dimensional, and three-dimensional examples of spherically symmetric
Bondi accretion, steady planar Bondi-Hoyle-Lyttleton flows, and wind
accretion in supergiant X-ray binaries. Another improvement is support
for the generic splitting of any background magnetic field. We present
several tests relevant for solar physics applications to demonstrate
the advantages of field splitting on accuracy and robustness in
extremely low-plasma β environments: a static magnetic flux rope,
a magnetic null-point, and magnetic reconnection in a current sheet
with either uniform or anomalous resistivity. Our implementation
for treating anisotropic thermal conduction in multi-dimensional
MHD applications is also described, which generalizes the original
slope-limited symmetric scheme from two to three dimensions. We perform
ring diffusion tests that demonstrate its accuracy and robustness, and
show that it prevents the unphysical thermal flux present in traditional
schemes. The improved parallel scaling of the code is demonstrated
with three-dimensional AMR simulations of solar coronal rain, which
show satisfactory strong scaling up to 2000 cores. Other framework
improvements are also reported: the modernization and reorganization
into a library, the handling of automatic regression tests, the use
of inline/online Doxygen documentation, and a new future-proof data
format for input/output.

---------------------------------------------------------
Title: Erratum: Reconnection and particle acceleration in interacting
flux ropes - II. 3D effects on test particles in magnetically
dominated plasmas
Authors: Ripperda, B.; Porth, O.; Xia, C.; Keppens, R.
2018MNRAS.473.3128R Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Parametric study on kink instabilities of twisted magnetic
flux ropes in the solar atmosphere
Authors: Mei, Z. X.; Keppens, R.; Roussev, I. I.; Lin, J.
2018A&A...609A...2M Altcode: 2017A&A...609A...2M
 Aims: Twisted magnetic flux ropes (MFRs) in the solar atmosphere
have been researched extensively because of their close connection to
many solar eruptive phenomena, such as flares, filaments, and coronal
mass ejections (CMEs). In this work, we performed a set of 3D isothermal
magnetohydrodynamic (MHD) numerical simulations, which use analytical
twisted MFR models and study dynamical processes parametrically inside
and around current-carrying twisted loops. We aim to generalize
earlier findings by applying finite plasma β conditions. 
Methods: Inside the MFR, approximate internal equilibrium is obtained
by pressure from gas and toroidal magnetic fields to maintain balance
with the poloidal magnetic field. We selected parameter values to
isolate best either internal or external kink instability before
studying complex evolutions with mixed characteristics. We studied
kink instabilities and magnetic reconnection in MFRs with low and
high twists. Results: The curvature of MFRs is responsible for
a tire tube force due to its internal plasma pressure, which tends
to expand the MFR. The curvature effect of toroidal field inside the
MFR leads to a downward movement toward the photosphere. We obtain an
approximate internal equilibrium using the opposing characteristics
of these two forces. A typical external kink instability totally
dominates the evolution of MFR with infinite twist turns. Because of
line-tied conditions and the curvature, the central MFR region loses
its external equilibrium and erupts outward. We emphasize the possible
role of two different kink instabilities during the MFR evolution:
internal and external kink. The external kink is due to the violation
of the Kruskal-Shafranov condition, while the internal kink requires
a safety factor q = 1 surface inside the MFR. We show that in mixed
scenarios, where both instabilities compete, complex evolutions
occur owing to reconnections around and within the MFR. The S-shaped
structures in current distributions appear naturally without invoking
flux emergence. Magnetic reconfigurations common to eruptive MFRs and
flare loop systems are found in our simulations.

---------------------------------------------------------
Title: Clumpy wind accretion in Supergiant X-ray Binaries
Authors: El Mellah, I.; Sundqvist, J. O.; Keppens, R.
2017sf2a.conf..145E Altcode:
Supergiant X-ray binaries (\sgx) contain a neutron star (NS) orbiting a
Supergiant O/B star. The fraction of the dense and fast line-driven wind
from the stellar companion which is accreted by the NS is responsible
for most of the X-ray emission from those system. Classic \sgx display
photometric variability of their hard X-ray emission, typically
from a few 10^{35} to a few 10^{37}erg\cdots^{-1}. Inhomogeneities
(\aka clumps) in the wind from the star are expected to play a role
in this time variability. We run 3D hydrodynamical (HD) finite volume
simulations to follow the accretion of the inhomogeneous stellar wind by
the NS over almost 3 orders of magnitude. To model the unperturbed wind
far upstream the NS, we use recent simulations which managed to resolve
its micro-structure. We observe the formation of a Bondi-Hoyle-Lyttleton
(BHL) like bow shock around the accretor and follow the clumps as they
cross it, down to the NS magnetosphere. Compared to previous estimations
discarding the HD effects, we measure lower time variability due to
both the damping effect of the shock and the necessity to evacuate
angular momentum to enable accretion. We also compute the associated
time-variable column density and compare it to recent observations in
Vela X-1.

---------------------------------------------------------
Title: The SS433 jet from subparsec to parsec scales (Corrigendum)
Authors: Monceau-Baroux, Remi; Porth, Oliver; Meliani, Zakaria;
Keppens, Rony
2017A&A...607C...4M Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Synchrotron Radiation Maps from Relativistic MHD Jet
Simulations
Authors: Millas, Dimitrios; Porth, Oliver; Keppens, Rony
2017Galax...5...79M Altcode:
Relativistic jets from active galactic nuclei (AGN) often display a
non-uniform structure and are, under certain conditions, susceptible to
a number of instabilities. An interesting example is the development
of non-axisymmetric, Rayleigh-Taylor type instabilities in the case
of differentially rotating two-component jets, with the toroidal
component of the magnetic field playing a key role in the development
or suppression of these instabilities. We have shown that higher
magnetization leads to stability against these non-axisymmetric
instabilities. Using ray-casting on data from relativistic MHD
simulations of two-component jets, we now investigate the effect of
these instabilities on the synchrotron emission pattern from the
jets. We recover many well known trends from actual observations,
e.g., regarding the polarization fraction and the distribution of
the position angle of the electric field, in addition to a different
emitting region, depending on the stability of the jet.

---------------------------------------------------------
Title: Reconnection and particle acceleration in interacting flux
ropes - II. 3D effects on test particles in magnetically dominated
plasmas
Authors: Ripperda, B.; Porth, O.; Xia, C.; Keppens, R.
2017MNRAS.471.3465R Altcode: 2017arXiv170708920R
We analyse particle acceleration in explosive reconnection events in
magnetically dominated proton-electron plasmas. Reconnection is driven
by large-scale magnetic stresses in interacting current-carrying flux
tubes. Our model relies on development of current-driven instabilities
on macroscopic scales. These tilt-kink instabilities develop in an
initially force-free equilibrium of repelling current channels. Using
magnetohydrodynamics (MHD) methods we study a 3D model of repelling
and interacting flux tubes in which we simultaneously evolve test
particles, guided by electromagnetic fields obtained from MHD. We
identify two stages of particle acceleration; initially particles
accelerate in the current channels, after which the flux ropes start
tilting and kinking and particles accelerate due to reconnection
processes in the plasma. The explosive stage of reconnection produces
non-thermal energy distributions with slopes that depend on plasma
resistivity and the initial particle velocity. We also discuss the
influence of the length of the flux ropes on particle acceleration and
energy distributions. This study extends previous 2.5D results to 3D
setups, providing all ingredients needed to model realistic scenarios
like solar flares, black hole flares and particle acceleration in
pulsar wind nebulae: formation of strong resistive electric fields,
explosive reconnection and non-thermal particle distributions. By
assuming initial energy equipartition between electrons and protons,
applying low resistivity in accordance with solar corona conditions
and limiting the flux rope length to a fraction of a solar radius, we
obtain realistic energy distributions for solar flares with non-thermal
power-law tails and maximum electron energies up to 11 MeV and maximum
proton energies up to 1 GeV.

---------------------------------------------------------
Title: Interfacing MHD Single Fluid and Kinetic Exospheric Solar
Wind Models and Comparing Their Energetics
Authors: Moschou, Sofia-Paraskevi; Pierrard, Viviane; Keppens, Rony;
Pomoell, Jens
2017SoPh..292..139M Altcode: 2017arXiv170901605M
An exospheric kinetic solar wind model is interfaced with
an observation-driven single-fluid magnetohydrodynamic (MHD)
model. Initially, a photospheric magnetogram serves as observational
input in the fluid approach to extrapolate the heliospheric magnetic
field. Then semi-empirical coronal models are used for estimating the
plasma characteristics up to a heliocentric distance of 0.1 AU. From
there on, a full MHD model that computes the three-dimensional
time-dependent evolution of the solar wind macroscopic variables
up to the orbit of Earth is used. After interfacing the density and
velocity at the inner MHD boundary, we compare our results with those
of a kinetic exospheric solar wind model based on the assumption of
Maxwell and Kappa velocity distribution functions for protons and
electrons, respectively, as well as with in situ observations at 1
AU. This provides insight into more physically detailed processes,
such as coronal heating and solar wind acceleration, which naturally
arise from including suprathermal electrons in the model. We are
interested in the profile of the solar wind speed and density at 1 AU,
in characterizing the slow and fast source regions of the wind, and in
comparing MHD with exospheric models in similar conditions. We calculate
the energetics of both models from low to high heliocentric distances.

---------------------------------------------------------
Title: Magnetic reconnection during eruptive magnetic flux ropes
Authors: Mei, Z. X.; Keppens, R.; Roussev, I. I.; Lin, J.
2017A&A...604L...7M Altcode:
 Aims: We perform a three-dimensional (3D) high resolution
numerical simulation in isothermal magnetohydrodynamics to study
the magnetic reconnection process in a current sheet (CS) formed
during an eruption of a twisted magnetic flux rope (MFR). Because
the twist distribution violates the Kruskal-Shafranov condition, the
kink instability occurs, and the MFR is distorted. The centre part of
the MFR loses its equilibrium and erupts upward, which leads to the
formation of a 3D CS underneath it. Methods: In order to study
the magnetic reconnection inside the CS in detail, mesh refinement has
been used to reduce the numerical diffusion and we estimate a Lundquist
number S = 104 in the vicinity of the CS. Results:
The refined mesh allows us to resolve fine structures inside the 3D
CS: a bifurcating sheet structure signaling the 3D generalization of
Petschek slow shocks, some distorted-cylindrical substructures due to
the tearing mode instabilities, and two turbulence regions near the
upper and the lower tips of the CS. The topological characteristics
of the MFR depend sensitively on the observer's viewing angle: it
presents as a sigmoid structure, an outwardly expanding MFR with
helical distortion, or a flare-CS-coronal mass ejection symbiosis
as in 2D flux-rope models when observed from the top, the front,
or the side. The movie associated to Fig. 2 is available at <A
href="http://www.aanda.org/10.1051/0004-6361/201731146/olm">http://www.aanda.org</A>

---------------------------------------------------------
Title: Coronal rain in magnetic bipolar weak fields
Authors: Xia, C.; Keppens, R.; Fang, X.
2017A&A...603A..42X Altcode: 2017arXiv170601804X
 Aims: We intend to investigate the underlying physics for
the coronal rain phenomenon in a representative bipolar magnetic
field, including the formation and the dynamics of coronal rain
blobs. Methods: With the MPI-AMRVAC code, we performed three
dimensional radiative magnetohydrodynamic (MHD) simulation with
strong heating localized on footpoints of magnetic loops after a
relaxation to quiet solar atmosphere. Results: Progressive
cooling and in-situ condensation starts at the loop top due to
radiative thermal instability. The first large-scale condensation
on the loop top suffers Rayleigh-Taylor instability and becomes
fragmented into smaller blobs. The blobs fall vertically dragging
magnetic loops until they reach low-β regions and start to fall along
the loops from loop top to loop footpoints. A statistic study of the
coronal rain blobs finds that small blobs with masses of less than
1010 g dominate the population. When blobs fall to lower
regions along the magnetic loops, they are stretched and develop a
non-uniform velocity pattern with an anti-parallel shearing pattern
seen to develop along the central axis of the blobs. Synthetic images
of simulated coronal rain with Solar Dynamics Observatory Atmospheric
Imaging Assembly well resemble real observations presenting dark
falling clumps in hot channels and bright rain blobs in a cool
channel. We also find density inhomogeneities during a coronal rain
"shower", which reflects the observed multi-stranded nature of coronal
rain. Movies associated to Figs. 3 and 7 are available at <A
href="http://www.aanda.org/10.1051/0004-6361/201730660/olm">http://www.aanda.org</A>

---------------------------------------------------------
Title: Formation and Initiation of Erupting Flux Rope and Embedded
Filament Driven by Photospheric Converging Motion
Authors: Zhao, Xiaozhou; Xia, Chun; Keppens, Rony; Gan, Weiqun
2017ApJ...841..106Z Altcode:
In this paper, we study how a flux rope (FR) is formed and evolves
into the corresponding structure of a coronal mass ejection
(CME) numerically driven by photospheric converging motion. A
two-and-a-half-dimensional magnetohydrodynamics simulation is conducted
in a chromosphere-transition-corona setup. The initial arcade-like
linear force-free magnetic field is driven by an imposed slow
motion converging toward the magnetic inversion line at the bottom
boundary. The convergence brings opposite-polarity magnetic flux
to the polarity inversion, giving rise to the formation of an FR by
magnetic reconnection and eventually to the eruption of a CME. During
the FR formation, an embedded prominence gets formed by the levitation
of chromospheric material. We confirm that the converging flow is a
potential mechanism for the formation of FRs and a possible triggering
mechanism for CMEs. We investigate the thermal, dynamical, and magnetic
properties of the FR and its embedded prominence by tracking their
thermal evolution, analyzing their force balance, and measuring their
kinematic quantities. The phase transition from the initiation phase to
the acceleration phase of the kinematic evolution of the FR was observed
in our simulation. The FR undergoes a series of quasi-static equilibrium
states in the initiation phase; while in the acceleration phase the FR
is driven by Lorentz force and the impulsive acceleration occurs. The
underlying physical reason for the phase transition is the change of
the reconnection mechanism from the Sweet-Parker to the unsteady bursty
regime of reconnection in the evolving current sheet underneath the FR.

---------------------------------------------------------
Title: Reconnection and particle acceleration in interacting flux
ropes - I. Magnetohydrodynamics and test particles in 2.5D
Authors: Ripperda, B.; Porth, O.; Xia, C.; Keppens, R.
2017MNRAS.467.3279R Altcode: 2016arXiv161109966R
Magnetic reconnection and non-thermal particle distributions
associated with current-driven instabilities are investigated by
means of resistive magnetohydrodynamics (MHD) simulations combined
with relativistic test particle methods. We propose a system with
two parallel, repelling current channels in an initially force-free
equilibrium, as a simplified representation of flux ropes in a stellar
magnetosphere. The current channels undergo a rotation and separation
on Alfvénic time-scales, forming secondary islands and (up to tearing
unstable) current sheets in which non-thermal energy distributions
are expected to develop. Using the recently developed particle module
of our open-source grid-adaptive mpi-amrvac software, we simulate MHD
evolution combined with test particle treatments in MHD snapshots. We
explore under which plasma-β conditions the fastest reconnection
occurs in 2.5D scenarios, and in these settings, test particles are
evolved. We quantify energy distributions, acceleration mechanisms,
relativistic corrections to the particle equations of motion and
effects of resistivity in magnetically dominated proton-electron
plasmas. Due to large resistive electric fields and indefinite
acceleration of particles in the infinitely long current channels,
hard energy spectra are found in 2.5D configurations. Solutions to
these numerical artefacts are proposed for both 2.5D setups and future
3D work. We discuss the MHD of an additional kink instability in 3D
setups and the expected effects on energy distributions. The obtained
results hold as a proof-of-principle for test particle approaches in
MHD simulations, relevant to explore less idealized scenarios like
solar flares and more exotic astrophysical phenomena, like black hole
flares, magnetar magnetospheres and pulsar wind nebulae.

---------------------------------------------------------
Title: Rotation and toroidal magnetic field effects on the stability
of two-component jets
Authors: Millas, Dimitrios; Keppens, Rony; Meliani, Zakaria
2017MNRAS.470..592M Altcode: 2017arXiv170606912M
Several observations of astrophysical jets show evidence of a
structure in the direction perpendicular to the jet axis, leading to
the development of 'spine and sheath' models of jets. Most studies
focus on a two-component jet consisting of a highly relativistic inner
jet and a slower - but still relativistic - outer jet surrounded by an
unmagnetized environment. These jets are believed to be susceptible
to a relativistic Rayleigh-Taylor-type instability, depending on the
effective inertia ratio of the two components. We extend previous
studies by taking into account the presence of a non-zero toroidal
magnetic field. Different values of magnetization are examined to detect
possible differences in the evolution and stability of the jet. We find
that the toroidal field, above a certain level of magnetization σ,
roughly equal to 0.01, can stabilize the jet against the previously
mentioned instabilities and that there is a clear trend in the behaviour
of the average Lorentz factor and the effective radius of the jet
when we continuously increase the magnetization. The simulations are
performed using the relativistic MHD module from the open source,
parallel, grid adaptive, mpi-amrvac code.

---------------------------------------------------------
Title: Influence of environmental parameters on the Kelvin-Helmholtz
instability at the magnetopause
Authors: Leroy, Matthieu; Keppens, Rony
2017EGUGA..19.1582L Altcode:
Influence of environmental parameters on the Kelvin-Helmholtz
instability at the magnetopause M. Leroy1, R. Keppens1 1 Centre for
mathematical Plasma-Astrophysics, Department Wiskunde, KU Leuven,
Celestijnenlaan 200B, bus 2400, B-3001 Leuven, Belgium The process
dominating the development of a large boundary layer at the interface
between the solar wind (SW) and the magnetosphere (MS) during northward
interplanetary magnetic field is still not fully understood. However
the Kelvin-Helmholtz instability (KHI), which can induce magnetic
reconnection events through its non-linear phase vortices, being the
major actor is in good agreement with the observations around the
magnetopause so far. Numerous numerical studies have investigated the
topic with many interesting results but most of these were considering
two-dimensional situations with simplified magnetic configuration and
often neglecting the inhomogeneities for the sake of clarity. Given
the typical parameters at the SW/MS interface, the situation must
be considered in the frame of Hall-MHD, due to the fact that the
current layers widths and the gradient lengths can be in the order
of the ion inertial length. As a consequence of Hall-MHD creating a
third vector component from two planar ones, and also because flow and
magnetic field variations in the equatorial plane can affect the field
configuration at a distance in all directions and not only locally,
the simulations must also be performed away from the equatorial plane
and a three-dimensional treatment is necessary. In this work, different
configurations than can occur in the KHI scenario are studied in a
three-dimensional (3D) Hall-MHD setting, where the double mid-latitude
reconnection (DMLR) process exposed by Faganello, Califano et al. is
triggered by the equatorial roll-ups. Their previous work is extended
here with in particular a larger simulation box and the addition of
a density contrast and variations of the interface configuration. The
influence of various parameters on the growth rate of the KHI and thus
the efficiency of the DMLR is assessed. In the scope of assessing the
effect of the Hall term on the physical processes, the simulations
are also performed in the MHD frame. These different configurations
may have discernible signatures that can be identified by spacecrafts
diagnostics, therefore fields and particles data that would be recorded
by spacecrafts during such an event are simulated and compared to real
in-situ data.

---------------------------------------------------------
Title: Shock-cloud interaction and gas-dust spatial separation
Authors: Monceau-Baroux, Rémi; Keppens, Rony
2017A&A...600A.134M Altcode:
Context. We revisit the study of shocks interacting with molecular
clouds, incorporating coupled gas-dust dynamics. Aims: We study
the effect of different parameters on the shock-cloud interaction, such
as the dust-to-gas ratio or the Mach number of the impinging shock. By
solving self-consistently for drag-coupled gas and dust evolutions,
we can assess the frequently made assumption that the dust is locked
to the dynamics of the gas so that dust observations would result in
direct information on the gas distribution. Methods: We used a
multi-fluid model where the dust is represented by grain-size specific
pressureless fluids. The dust and gas interact through a drag force,
and we used four dust species with weighted representative sizes between
1 and 500 nm. We use the open source code MPI-AMRVAC for a parametric
study of the effect of the gas-dust ratio and the Mach number of the
shock. By using the radiative transfer code SKIRT, we create synthetic
millimeter wavelength maps to connect to observations. Results:
We find that the presence of dust does not significantly affect the
dynamics of the gas for realistic dust-gas ratios, and this is the case
throughout the range of Mach numbers explored (1.5-10). For high Mach
numbers, we find a significant discrepancy between the distribution
of the dust and gas after the cloud-shock interaction with the larger
dust species clearly lagging the heavily mixed and accelerated gas
(re)distribution. Conclusions: We conclude that observational
studies of dusty environments may need to account for clearly separated
spatial distributions of dust and gas, especially those studies that
are representative of molecular clouds that have been interacting with
high Mach number shock fronts.

---------------------------------------------------------
Title: How is the Jovian main auroral emission affected by the
solar wind?
Authors: Chané, E.; Saur, J.; Keppens, R.; Poedts, S.
2017JGRA..122.1960C Altcode:
The influence of the solar wind on Jupiter's magnetosphere is
studied via three-dimensional global MHD simulations. We especially
examine how solar wind density variations affect the main auroral
emission. Our simulations show that a density increase in the solar
wind has strong effects on the Jovian magnetosphere: the size of
the magnetosphere decreases, the field lines are compressed on the
dayside and elongated on the nightside (this effect can be seen
even deep inside the magnetosphere), and dawn-dusk asymmetries are
enhanced. Our results also show that the main oval becomes brighter
when the solar wind is denser. But the precise response of the main
oval to such a density enhancement in the solar wind depends on the
local time: on the nightside the main oval becomes brighter, while on
the dayside it first turns slightly darker for a few hours and then
also becomes brighter. Once the density increase in the solar wind
reaches the magnetosphere, the magnetopause moves inward, and in less
than 5 h, a new approximate equilibrium position is obtained. But
the magnetosphere as a whole needs much longer to adapt to the new
solar wind conditions. For instance, the total electrical current
closing in the ionosphere slowly increases during the simulation and
it takes about 60 h to reach a new equilibrium. By then the currents
have increased by as much as 45%.

---------------------------------------------------------
Title: Erratum: Connecting the dots - III. Nightside cooling and
surface friction affect climates of tidally locked terrestrial planets
Authors: Carone, L.; Keppens, R.; Decin, L.
2016MNRAS.463.3114C Altcode: 2016MNRAS.tmp.1323C
No abstract at ADS

---------------------------------------------------------
Title: The Role of Kelvin-Helmholtz Instability for Producing Loop-top
Hard X-Ray Sources in Solar Flares
Authors: Fang, Xia; Yuan, Ding; Xia, Chun; Van Doorsselaere, Tom;
Keppens, Rony
2016ApJ...833...36F Altcode:
We propose a model for the formation of loop-top hard X-ray (HXR)
sources in solar flares through the inverse Compton mechanism,
scattering the surrounding soft X-ray (SXR) photons to higher energy
HXR photons. We simulate the consequences of a flare-driven energy
deposit in the upper chromosphere in the impulsive phase of single
loop flares. The consequent chromosphere evaporation flows from both
footpoints reach speeds up to hundreds of kilometers per second, and
we demonstrate how this triggers Kelvin-Helmholtz instability (KHI)
in the loop top, under mildly asymmetric conditions, or more toward
the loop flank for strongly asymmetric cases. The KHI vortices further
fragment the magnetic topology into multiple magnetic islands and
current sheets, and the hot plasma within leads to a bright loop-top
SXR source region. We argue that the magnetohydrodynamic turbulence
that appears at the loop apex could be an efficient accelerator of
non-thermal particles, which the island structures can trap at the
loop-top. These accelerated non-thermal particles can upscatter the
surrounding thermal SXR photons emitted by the extremely hot evaporated
plasma to HXR photons.

---------------------------------------------------------
Title: Hall-MHD simulations of the Kelvin-Helmholtz instability at
the solar wind/magnetosphere interface
Authors: Leroy, M. H. J.; Keppens, R.
2016sf2a.conf..107L Altcode:
The process feeding the development of the boundary layer at the
interface between the solar wind (SW) and the magnetosphere (MS)
during northward interplanetary magnetic field is still not fully
understood, though the Kelvin-Helmholtz instability (KHI) being the
major actor is in good agreement with the observations so far. In
this work, we study different configurations than can occur in the
KHI scenario in a three-dimensional (3D) Hall-MHD setting, where the
double mid-latitude reconnection (DMLR) process exposed by Faganello,
Califano et al. is triggered by the equatorial roll-ups. Their previous
work is extended here with a larger simulation box and the addition
of a density contrast. The influence of the parameters on the growth
rate of the KHI and thus the efficiency of the DMLR is assessed. The
effect of the Hall term on the physical processes is also investigated.

---------------------------------------------------------
Title: Magneto-frictional Modeling of Coronal Nonlinear Force-free
Fields. II. Application to Observations
Authors: Guo, Y.; Xia, C.; Keppens, R.
2016ApJ...828...83G Altcode:
A magneto-frictional module has been implemented and tested in
the Message Passing Interface Adaptive Mesh Refinement Versatile
Advection Code (MPI-AMRVAC) in the first paper of this series. Here,
we apply the magneto-frictional method to observations to demonstrate
its applicability in both Cartesian and spherical coordinates, and
in uniform and block-adaptive octree grids. We first reconstruct a
nonlinear force-free field (NLFFF) on a uniform grid of 1803
cells in Cartesian coordinates, with boundary conditions provided by
the vector magnetic field observed by the Helioseismic and Magnetic
Imager (HMI) on board the Solar Dynamics Observatory (SDO) at 06:00
UT on 2010 November 11 in active region NOAA 11123. The reconstructed
NLFFF successfully reproduces the sheared and twisted field lines and
magnetic null points. Next, we adopt a three-level block-adaptive grid
to model the same active region with a higher spatial resolution on
the bottom boundary and a coarser treatment of regions higher up. The
force-free and divergence-free metrics obtained are comparable to
the run with a uniform grid, and the reconstructed field topology
is also very similar. Finally, a group of active regions, including
NOAA 11401, 11402, 11405, and 11407, observed at 03:00 UT on 2012
January 23 by SDO/HMI is modeled with a five-level block-adaptive
grid in spherical coordinates, where we reach a local resolution of
0\buildrel{\circ}\over{.} 06 pixel-1 in an area of 790 Mm
× 604 Mm. Local high spatial resolution and a large field of view
in NLFFF modeling can be achieved simultaneously in parallel and
block-adaptive magneto-frictional relaxations.

---------------------------------------------------------
Title: Magneto-frictional Modeling of Coronal Nonlinear Force-free
Fields. I. Testing with Analytic Solutions
Authors: Guo, Y.; Xia, C.; Keppens, R.; Valori, G.
2016ApJ...828...82G Altcode:
We report our implementation of the magneto-frictional method in the
Message Passing Interface Adaptive Mesh Refinement Versatile Advection
Code (MPI-AMRVAC). The method aims at applications where local adaptive
mesh refinement (AMR) is essential to make follow-up dynamical modeling
affordable. We quantify its performance in both domain-decomposed
uniform grids and block-adaptive AMR computations, using all frequently
employed force-free, divergence-free, and other vector comparison
metrics. As test cases, we revisit the semi-analytic solution of Low
and Lou in both Cartesian and spherical geometries, along with the
topologically challenging Titov-Démoulin model. We compare different
combinations of spatial and temporal discretizations, and find that the
fourth-order central difference with a local Lax-Friedrichs dissipation
term in a single-step marching scheme is an optimal combination. The
initial condition is provided by the potential field, which is the
potential field source surface model in spherical geometry. Various
boundary conditions are adopted, ranging from fully prescribed cases
where all boundaries are assigned with the semi-analytic models,
to solar-like cases where only the magnetic field at the bottom is
known. Our results demonstrate that all the metrics compare favorably
to previous works in both Cartesian and spherical coordinates. Cases
with several AMR levels perform in accordance with their effective
resolutions. The magneto-frictional method in MPI-AMRVAC allows us
to model a region of interest with high spatial resolution and large
field of view simultaneously, as required by observation-constrained
extrapolations using vector data provided with modern instruments. The
applications of the magneto-frictional method to observations are
shown in an accompanying paper.

---------------------------------------------------------
Title: Connecting the dots - III. Nightside cooling and surface
friction affect climates of tidally locked terrestrial planets
Authors: Carone, L.; Keppens, R.; Decin, L.
2016MNRAS.461.1981C Altcode: 2016arXiv160509545C; 2016MNRAS.tmp..931C
We investigate how nightside cooling and surface friction affect
surface temperatures and large-scale circulation for tidally locked
Earth-like planets. For each scenario, we vary the orbital period
between Prot = 1 and 100 d and capture changes in climate
states. We find drastic changes in climate states for different
surface friction scenarios. For very efficient surface friction
(ts,fric = 0.1 d), the simulations for short rotation
periods (Prot ≤ 10 d) show predominantly standing
extratropical Rossby waves. These waves lead to climate states
with two high-latitude westerly jets and unperturbed meridional
direct circulation. In most other scenarios, simulations with short
rotation periods exhibit instead dominance by standing tropical Rossby
waves. Such climate states have a single equatorial westerly jet,
which disrupts direct circulation. Experiments with weak surface
friction (ts,fric = 10-100 d) show decoupling between
surface temperatures and circulation, which leads to strong cooling of
the nightside. The experiment with ts,fric = 100 d assumes
climate states with easterly flow (retrograde rotation) for medium and
slow planetary rotations Prot = 12-100 d. We show that an
increase of nightside cooling efficiency by one order of magnitude
compared to the nominal model leads to a cooling of the nightside
surface temperatures by 80-100 K. The dayside surface temperatures
only drop by 25 K at the same time. The increase in thermal forcing
suppresses the formation of extratropical Rossby waves on small planets
(RP = 1REarth) in the short rotation period regime
(Prot ≤ 10 d).

---------------------------------------------------------
Title: Pinwheels in the sky, with dust: 3D modelling of the Wolf-Rayet
98a environment
Authors: Hendrix, Tom; Keppens, Rony; van Marle, Allard Jan; Camps,
Peter; Baes, Maarten; Meliani, Zakaria
2016MNRAS.460.3975H Altcode: 2016MNRAS.tmp..943H; 2016arXiv160509239H
The Wolf-Rayet 98a (WR 98a) system is a prime target for interferometric
surveys, since its identification as a `rotating pinwheel nebulae',
where infrared images display a spiral dust lane revolving with a 1.4
yr periodicity. WR 98a hosts a WC9+OB star, and the presence of dust
is puzzling given the extreme luminosities of Wolf-Rayet stars. We
present 3D hydrodynamic models for WR 98a, where dust creation and
redistribution are self-consistently incorporated. Our grid-adaptive
simulations resolve details in the wind collision region at scales below
one percent of the orbital separation (∼4 au), while simulating up
to 1300 au. We cover several orbital periods under conditions where
the gas component alone behaves adiabatic, or is subject to effective
radiative cooling. In the adiabatic case, mixing between stellar winds
is effective in a well-defined spiral pattern, where optimal conditions
for dust creation are met. When radiative cooling is incorporated,
the interaction gets dominated by thermal instabilities along the
wind collision region, and dust concentrates in clumps and filaments
in a volume-filling fashion, so WR 98a must obey close to adiabatic
evolutions to demonstrate the rotating pinwheel structure. We mimic
Keck, ALMA or future E-ELT observations and confront photometric
long-term monitoring. We predict an asymmetry in the dust distribution
between leading and trailing edge of the spiral, show that ALMA and
E-ELT would be able to detect fine-structure in the spiral indicative of
Kelvin-Helmholtz development, and confirm the variation in photometry
due to the orientation. Historic Keck images are reproduced, but their
resolution is insufficient to detect the details we predict.

---------------------------------------------------------
Title: Dust grain coagulation modelling : From discrete to continuous
Authors: Paruta, P.; Hendrix, T.; Keppens, R.
2016A&C....16..155P Altcode:
In molecular clouds, stars are formed from a mixture of gas, plasma
and dust particles. The dynamics of this formation is still actively
investigated and a study of dust coagulation can help to shed light
on this process. Starting from a pre-existing discrete coagulation
model, this work aims to mathematically explore its properties and
its suitability for numerical validation. The crucial step is in our
reinterpretation from its original discrete to a well-defined continuous
form, which results in the well-known Smoluchowski coagulation
equation. This opens up the possibility of exploiting previous results
in order to prove the existence and uniqueness of a mass conserving
solution for the evolution of dust grain size distribution. Ultimately,
to allow for a more flexible numerical implementation, the problem is
rewritten as a non-linear hyperbolic integro-differential equation
and solved using a finite volume discretisation. It is demonstrated
that there is an exact numerical agreement with the initial discrete
model, with improved accuracy. This is of interest for further work
on dynamically coupled gas with dust simulations.

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Title: Internal Dynamics of a Twin-layer Solar Prominence
Authors: Xia, C.; Keppens, R.
2016ApJ...825L..29X Altcode:
Modern observations revealed rich dynamics within solar prominences. The
globally stable quiescent prominences, characterized by the presence
of thin vertical threads and falling knobs, are frequently invaded
by small rising dark plumes. These dynamic phenomena are related to
magnetic Rayleigh-Taylor instability, since prominence matter, 100
times denser than surrounding coronal plasma, is lifted against gravity
by weak magnetic field. To get a deeper understanding of the physics
behind these phenomena, we use three-dimensional magnetohydrodynamic
simulations to investigate the nonlinear magnetoconvective motions in a
twin-layer prominence in a macroscopic model from chromospheric layers
up to 30 Mm height. The properties of simulated falling “fingers”
and uprising bubbles are consistent with those in observed vertical
threads and rising plumes in quiescent prominences. Both sheets of
the twin-layer prominence show a strongly coherent evolution due
to their magnetic connectivity, and demonstrate collective kink
deformation. Our model suggests that the vertical threads of the
prominence as seen in an edge-on view, and the apparent horizontal
threads of the filament when seen top-down are different appearances
of the same structures. Synthetic images of the modeled twin-layer
prominence reflect the strong degree of mixing established over the
entire prominence structure, in agreement with the observations.

---------------------------------------------------------
Title: Formation and Plasma Circulation of Solar Prominences
Authors: Xia, C.; Keppens, R.
2016ApJ...823...22X Altcode: 2016arXiv160305397X
Solar prominences are long-lived cool and dense plasma curtains in
the hot and rarefied outer solar atmosphere or corona. The physical
mechanism responsible for their formation and especially for their
internal plasma circulation has been uncertain for decades. The observed
ubiquitous downflows in quiescent prominences are difficult to interpret
because plasma with high conductivity seems to move across horizontal
magnetic field lines. Here we present three-dimensional numerical
simulations of prominence formation and evolution in an elongated
magnetic flux rope as a result of in situ plasma condensations fueled
by continuous plasma evaporation from the solar chromosphere. The
prominence is born and maintained in a fragmented, highly dynamic state
with continuous reappearance of multiple blobs and thread structures
that move mainly downward, dragging along mass-loaded field lines. The
circulation of prominence plasma is characterized by the dynamic balance
between the drainage of prominence plasma back to the chromosphere and
the formation of prominence plasma via continuous condensation. Plasma
evaporates from the chromosphere, condenses into the prominence in the
corona, and drains back to the chromosphere, establishing a stable
chromosphere-corona plasma cycle. Synthetic images of the modeled
prominence with the Solar Dynamics Observatory Atmospheric Imaging
Assembly closely resemble actual observations, with many dynamical
threads underlying an elliptical coronal cavity.

---------------------------------------------------------
Title: Synthetic Radio Views of Simulated Solar Flux Ropes
Authors: Kuznetsov, A. A.; Keppens, R.; Xia, C.
2016SoPh..291..823K Altcode: 2016SoPh..tmp...32K; 2016arXiv160102370K
We produce synthetic radio views of simulated flux ropes in the
solar corona, where finite-β magnetohydrodynamic (MHD) simulations
serve to mimic the flux-rope formation stages, as well as their stable
endstates. These endstates represent twisted flux ropes where balancing
Lorentz forces, gravity, and pressure gradients determine the full
thermodynamic variation throughout the flux rope. The models obtained
are needed to quantify radiative transfer in radio bands, and they
allow us to contrast weak with strong magnetic-field conditions. Field
strengths of up to 100 G in the flux rope yield radio views dominated
by optically thin free-free emission. The forming flux rope shows
clear morphological changes in its emission structure as it deforms
from an arcade to a flux rope, both on disk and at the limb. For an
active-region filament channel with a field strength of up to 680 G
in the flux rope, gyroresonance emission (from the third and fourth
gyrolayers) can be detected, and it even dominates free-free emission
at frequencies of up to 7 GHz. Finally, we also show synthetic views
of a simulated filament embedded within a (weak-field) flux rope,
resulting from an energetically consistent MHD simulation. For this
filament, synthetic views at the limb show clear similarities with
actual observations, and the transition from optically thick (below
10 GHz) to optically thin emission can be reproduced. On the disk,
its dimension and temperature conditions are as yet not realistic
enough to yield the observed radio-brightness depressions.

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Title: Simulating coronal condensation dynamics in 3D
Authors: Moschou, S. P.; Keppens, R.; Xia, C.; Fang, X.
2015AdSpR..56.2738M Altcode: 2015arXiv150505333M
We present numerical simulations in 3D settings where coronal rain
phenomena take place in a magnetic configuration of a quadrupolar
arcade system. Our simulation is a magnetohydrodynamic simulation
including anisotropic thermal conduction, optically thin radiative
losses, and parametrised heating as main thermodynamical features to
construct a realistic arcade configuration from chromospheric to coronal
heights. The plasma evaporation from chromospheric and transition region
heights eventually causes localised runaway condensation events and
we witness the formation of plasma blobs due to thermal instability,
that evolve dynamically in the heated arcade part and move gradually
downwards due to interchange type dynamics. Unlike earlier 2.5D
simulations, in this case there is no large scale prominence formation
observed, but a continuous coronal rain develops which shows clear
indications of Rayleigh-Taylor or interchange instability, that causes
the denser plasma located above the transition region to fall down,
as the system moves towards a more stable state. Linear stability
analysis is used in the non-linear regime for gaining insight and
giving a prediction of the system's evolution. After the plasma blobs
descend through interchange, they follow the magnetic field topology
more closely in the lower coronal regions, where they are guided by
the magnetic dips.

---------------------------------------------------------
Title: Simulating the Formation and Evolution of Solar Prominences
in Coronal Cavities
Authors: Xia, C.; Keppens, R.
2015AGUFMSH53B2492X Altcode:
The physical mechanism responsible for the formation and the mass
cycling of solar prominences has been uncertain for decades, because
of the difficulty of knowing the three-dimensional (3D) magnetic
field hosting prominences and the mass supply from chromosphere
to prominences. Here we report comprehensive 3D simulations which
demonstrate that the chromospheric evaporation and the coronal
condensation in a magnetic flux rope lead to the formation of a
quiescent prominence with complex internal fluid dynamics. First, we
simulate the formation of a stable magnetic flux rope in the corona
starting from a sheared magnetic bipolar arcade driven by shearing and
converging flows at the bottom, using isothermal magnetohydrodynamics
(MHD) modeling including gravity. Second, we fill the magnetic
flux rope with hydrostatic plasma from chromosphere to corona and
simulate a quiet sun in an equilibrium using full thermodynamic MHD
with anisotropic thermal conduction, optically thin radiative losses,
and parameterized heating. Then, we add extra strong heating localized
in two circular regions covering chromospheric foot points of the flux
rope. As the plasma is evaporated into corona, the lower part of the
flux rope evolve into thermally unstable situation due to dominative
radiative losses, where multiple blobs and threads of condensations
form and move continuously mainly along local magnetic field. Some
of the condensations fall down to chromosphere without support of
magnetic dips near the foot region of the flux rope. Others linger in
magnetic dips and descend slowly. Synthetic images of Solar Dynamics
Observatory views with the Atmospheric Imaging Assembly shows many
properties of quiescent prominences from real observations, such as,
dynamics dark threads under elliptical coronal cavity.

---------------------------------------------------------
Title: Solar Wind Modelling: MHD And Kinetic Treatments with Kappa
Distributions for the Electrons
Authors: Moschou, S. P.; Pierrard, V.; Keppens, R.; Pomoell, J.
2015AGUFMSH31A2389M Altcode:
We want to constrain a kinetic solar wind model with Kappa-distributed
electrons using observation-driven magnetohydrodynamics (MHD) modelling
and in-situ data.Solar wind modelling efforts are presented using MHD -
based modelling as well as a kinetic approach. In the fluid approach,
photospheric magnetograms serve as observational input in semi-empirical
coronal models that are used for estimating the plasma characteristics
up to a heliocentric distance of 0.1AU. From there on a full MHD model
which computes the three-dimensional time-dependent evolution of the
macroscopic variables of the solar wind up to the orbit of the Earth
is recruited. In the kinetic approach, an exospheric kinetic solar
wind model based on the assumption of Maxwell and Kappa velocity
distributions functions for protons and electrons respectively is
used to determine appropriate boundary conditions to obtain the best
comparison with available observations at the Earth's orbit. This
will provide insight on more physically detailed processes, such as
coronal heating and solar wind acceleration, that naturally arise by
inclusion of suprathermal electrons in the model. We are interested
in the profile of the solar wind speed and density at 1 AU, in
characterising the slow and fast source regions of the wind and in
comparing the features of that with results of exospheric models in
similar conditions. We start from similar boundary conditions at the
exobase and propagate the solution up to 1AU to compare MHD and kinetic
treatments with observations.

---------------------------------------------------------
Title: Connecting the dots - II. Phase changes in the climate dynamics
of tidally locked terrestrial exoplanets
Authors: Carone, L.; Keppens, R.; Decin, L.
2015MNRAS.453.2412C Altcode: 2015arXiv150800419C
We investigate 3D atmosphere dynamics for tidally locked terrestrial
planets with an Earth-like atmosphere and irradiation for different
rotation periods (Prot = 1-100 d) and planet sizes
(RP = 1-2REarth) with unprecedented fine
detail. We could precisely identify three climate state transition
regions that are associated with phase transitions in standing
tropical and extratropical Rossby waves. We confirm that the climate on
fast-rotating planets may assume multiple states (Prot ≤ 12
d for RP = 2REarth). Our study is, however, the
first to identify the type of planetary wave associated with different
climate states: the first state is dominated by standing tropical Rossby
waves with fast equatorial superrotation. The second state is dominated
by standing extratropical Rossby waves with high-latitude westerly jets
with slower wind speeds. For very fast rotations (Prot ≤
5 d for RP = 2REarth), we find another climate
state transition, where the standing tropical and extratropical
Rossby wave can both fit on the planet. Thus, a third state with a
mixture of the two planetary waves becomes possible that exhibits
three jets. Different climate states may be observable, because the
upper atmosphere's hotspot is eastward shifted with respect to the
substellar point in the first state, westward shifted in the second
state and the third state shows a longitudinal `smearing' of the spot
across the substellar point. We show, furthermore, that the largest
fast-rotating planet in our study exhibits atmosphere features known
from hot Jupiters like fast equatorial superrotation and a temperature
chevron in the upper atmosphere.

---------------------------------------------------------
Title: Modeling of Reflective Propagating Slow-mode Wave in a
Flaring Loop
Authors: Fang, X.; Yuan, D.; Van Doorsselaere, T.; Keppens, R.; Xia, C.
2015ApJ...813...33F Altcode: 2015arXiv150904536F
Quasi-periodic propagating intensity disturbances have been observed in
large coronal loops in extreme ultraviolet images over a decade, and are
widely accepted to be slow magnetosonic waves. However, spectroscopic
observations from Hinode/EIS revealed their association with persistent
coronal upflows, making this interpretation debatable. We perform
a 2.5D magnetohydrodynamic simulation to imitate the chromospheric
evaporation and the following reflected patterns in a flare loop. Our
model encompasses the corona, transition region, and chromosphere. We
demonstrate that the quasi periodic propagating intensity variations
captured by the synthesized Solar Dynamics Observatory/Atmospheric
Imaging Assembly 131, 94 Å emission images match the previous
observations well. With particle tracers in the simulation, we confirm
that these quasi periodic propagating intensity variations consist
of reflected slow mode waves and mass flows with an average speed
of 310 km s-1 in an 80 Mm length loop with an average
temperature of 9 MK. With the synthesized Doppler shift velocity
and intensity maps of the Solar and Heliospheric Observatory/Solar
Ultraviolet Measurement of Emitted Radiation Fe xix line emission,
we confirm that these reflected slow mode waves are propagating waves.

---------------------------------------------------------
Title: Coronal Rain in Magnetic Arcades: Rebound Shocks, Limit Cycles,
and Shear Flows
Authors: Fang, X.; Xia, C.; Keppens, R.; Van Doorsselaere, T.
2015ApJ...807..142F Altcode: 2015arXiv150700882F
We extend our earlier multidimensional, magnetohydrodynamic simulations
of coronal rain occurring in magnetic arcades with higher resolution,
grid-adaptive computations covering a much longer (&gt;6 hr) time
span. We quantify how blob-like condensations forming in situ grow
along and across field lines and show that rain showers can occur in
limit cycles, here demonstrated for the first time in 2.5D setups. We
discuss dynamical, multi-dimensional aspects of the rebound shocks
generated by the siphon inflows and quantify the thermodynamics of
a prominence-corona transition-region-like structure surrounding the
blobs. We point out the correlation between condensation rates and the
cross-sectional size of loop systems where catastrophic cooling takes
place. We also study the variations of the typical number density,
kinetic energy, and temperature while blobs descend, impact, and sink
into the transition region. In addition, we explain the mechanisms
leading to concurrent upflows while the blobs descend. As a result,
there are plenty of shear flows generated with relative velocity
difference around 80 km s-1 in our simulations. These shear
flows are siphon flows set up by multiple blob dynamics and they in
turn affect the deformation of the falling blobs. In particular, we
show how shear flows can break apart blobs into smaller fragments,
within minutes.

---------------------------------------------------------
Title: Solar Prominences: "Double, Double... Boil and Bubble"
Authors: Keppens, R.; Xia, C.; Porth, O.
2015ApJ...806L..13K Altcode: 2015arXiv150505268K
Observations revealed rich dynamics within prominences, the cool
(104 K), macroscopic (sizes of order 100 Mm) “clouds”
in the million degree solar corona. Even quiescent prominences are
continuously perturbed by hot, rising bubbles. Since prominence matter
is hundredfold denser than coronal plasma, this bubbling is related
to Rayleigh-Taylor instabilities. Here we report on true macroscopic
simulations well into this bubbling phase, adopting an MHD description
from chromospheric layers up to 30 Mm height. Our virtual prominences
rapidly establish fully nonlinear (magneto)convective motions where
hot bubbles interplay with falling pillars, with dynamical details
including upwelling pillars forming within bubbles. Our simulations
show impacting Rayleigh-Taylor fingers reflecting on transition region
plasma, ensuring that cool, dense chromospheric material gets mixed with
prominence matter up to very large heights. This offers an explanation
for the return mass cycle mystery for prominence material. Synthetic
views at extreme ultraviolet wavelengths show remarkable agreement
with observations, with clear indications of shear-flow induced
fragmentations.

---------------------------------------------------------
Title: Modelling ripples in Orion with coupled dust dynamics and
radiative transfer
Authors: Hendrix, T.; Keppens, R.; Camps, P.
2015A&A...575A.110H Altcode: 2015arXiv150204011H
 Aims: In light of the recent detection of direct evidence for
the formation of Kelvin-Helmholtz instabilities in the Orion nebula,
we expand upon previous modelling efforts by numerically simulating
the shear-flow driven gas and dust dynamics in locations where the Hii
region and the molecular cloud interact. We aim to directly confront
the simulation results with the infrared observations. Methods:
To numerically model the onset and full nonlinear development of the
Kelvin-Helmholtz instability we take the setup proposed to interpret the
observations, and adjust it to a full 3D hydrodynamical simulation that
includes the dynamics of gas as well as dust. A dust grain distribution
with sizes between 5-250 nm is used, exploiting the gas+dust module
of the MPI-AMRVAC code, in which the dust species are represented
by several pressureless dust fluids. The evolution of the model is
followed well into the nonlinear phase. The output of these simulations
is then used as input for the SKIRT dust radiative transfer code to
obtain infrared images at several stages of the evolution, which can
be compared to the observations. Results: We confirm that a
3D Kelvin-Helmholtz instability is able to develop in the proposed
setup, and that the formation of the instability is not inhibited by
the addition of dust. Kelvin-Helmholtz billows form at the end of the
linear phase, and synthetic observations of the billows show striking
similarities to the infrared observations. It is pointed out that the
high density dust regions preferentially collect on the flanks of the
billows. To get agreement with the observed Kelvin-Helmholtz ripples,
the assumed geometry between the background radiation, the billows
and the observer is seen to be of critical importance.

---------------------------------------------------------
Title: Evolution of Fast Magnetoacoustic Pulses in Randomly Structured
Coronal Plasmas
Authors: Yuan, D.; Pascoe, D. J.; Nakariakov, V. M.; Li, B.;
Keppens, R.
2015ApJ...799..221Y Altcode: 2014arXiv1411.4152Y
We investigate the evolution of fast magnetoacoustic pulses in randomly
structured plasmas, in the context of large-scale propagating waves in
the solar atmosphere. We perform one-dimensional numerical simulations
of fast wave pulses propagating perpendicular to a constant magnetic
field in a low-β plasma with a random density profile across the
field. Both linear and nonlinear regimes are considered. We study
how the evolution of the pulse amplitude and width depends on their
initial values and the parameters of the random structuring. Acting
as a dispersive medium, a randomly structured plasma causes amplitude
attenuation and width broadening of the fast wave pulses. After the
passage of the main pulse, secondary propagating and standing fast
waves appear. Width evolution of both linear and nonlinear pulses can
be well approximated by linear functions; however, narrow pulses may
have zero or negative broadening. This arises because narrow pulses are
prone to splitting, while broad pulses usually deviate less from their
initial Gaussian shape and form ripple structures on top of the main
pulse. Linear pulses decay at an almost constant rate, while nonlinear
pulses decay exponentially. A pulse interacts most efficiently with a
random medium with a correlation length of about half of the initial
pulse width. This detailed model of fast wave pulses propagating in
highly structured media substantiates the interpretation of EIT waves
as fast magnetoacoustic waves. Evolution of a fast pulse provides us
with a novel method to diagnose the sub-resolution filamentation of
the solar atmosphere.

---------------------------------------------------------
Title: The SS433 jet from subparsec to parsec scales
Authors: Monceau-Baroux, Rémi; Porth, Oliver; Meliani, Zakaria;
Keppens, Rony
2015A&A...574A.143M Altcode:
Context. Relativistic jets associated with compact objects, as in the
X-ray binary SS433, are known to be multiscale because they spawn over
many orders of magnitude in distance. Here we model the precessing
SS433 jet and study its dynamics from \vartheta (0.01) to \vartheta(1)
parsec scales. Aims: We aim to solve the discrepancy between
the observations on a 0.1 pc scale of SS433, where the jet is clearly
precessing with an angle of 20°, and the larger scale observations
where the jet of SS433 interacts with the associated supernova remnant
W50, requiring a precessing angle of 10°. Methods: We use
3D special relativistic hydrodynamical simulations on a domain of a
scale of 1 pc. We use the finite volume code MPI-AMRVAC, solving the
relativistic variant of the Euler equations. To cover lengthscale
variations from \vartheta(0.001) pc as the jet beam width up to
the domain size, we take full advantage of code parallelization and
its adaptive mesh refinement scheme. Results: We found that
by means of a simple hydrodynamical process, the jet of SS433 can
transit from a precessing jet with an angle of 20°, to a continuous
hollow non-precessing jet with a smaller opening angle of about
10°. Successive windings of the precessing jet helix undergo gradual
deceleration by ISM interaction, to ultimately merge in a hollow
straight jet at distances where the ram pressure of individual jet
elements match the ISM pressure at about 0.068 pc from the source. Conclusions: We solve the discrepancy with an elegant and simple
model that does not require the jet of SS433 to undergo any temporal
changes in jet injection dynamics, but does so as a consequence of a
hydrodynamically enforced spatial recollimation. Our simulation thus
serves to validate simpler model prescriptions for SS433 on large
scales, where a continuous jet profile suffices.

---------------------------------------------------------
Title: Modelling colliding wind binaries in 2D
Authors: Hendrix, T.; Keppens, R.
2015wrs..conf..279H Altcode:
We look at how the dynamics of colliding wind binaries (CWB) can be
investigated in 2D, and how several parameters influence the dynamics
of the small scale structures inside the colliding wind and the
shocked regions, as well as in how the dynamics influence the shape
of the collision region at large distances. The parameters we adopt
are based on the binary system WR98a, one of the few Wolf-Rayet (WR)
dusty pinwheels known.

---------------------------------------------------------
Title: Interacting Tilt and Kink Instabilities in Repelling Current
Channels
Authors: Keppens, R.; Porth, O.; Xia, C.
2014ApJ...795...77K Altcode: 2014arXiv1409.4543K
We present a numerical study in resistive magnetohydrodynamics (MHD)
where the initial equilibrium configuration contains adjacent,
oppositely directed, parallel current channels. Since oppositely
directed current channels repel, the equilibrium is liable to an ideal
magnetohydrodynamic tilt instability. This tilt evolution, previously
studied in planar settings, involves two magnetic islands or flux
ropes, which on Alfvénic timescales undergo a combined rotation
and separation. This in turn leads to the creation of (near) singular
current layers, posing severe challenges to numerical approaches. Using
our open-source grid-adaptive MPI-AMRVAC software, we revisit the
planar evolution case in compressible MHD, as well as its extension
to two-and-a-half-dimensional (2.5D) and full three-dimensional (3D)
scenarios. As long as the third dimension can be ignored, pure tilt
evolutions result that are hardly affected by out of plane magnetic
field components. In all 2.5D runs, our simulations do show secondary
tearing type disruptions throughout the near singular current sheets
in the far nonlinear saturation regime. In full 3D runs, both current
channels can be liable to additional ideal kink deformations. We
discuss the effects of having both tilt and kink instabilities acting
simultaneously in the violent, reconnection-dominated evolution. In
3D, both the tilt and the kink instabilities can be stabilized by
tension forces. As a concrete space plasma application, we argue that
interacting tilt-kink instabilities in repelling current channels
provide a novel route to initiate solar coronal mass ejections,
distinctly different from the currently favored pure kink or torus
instability routes.

---------------------------------------------------------
Title: Connecting the dots: a versatile model for the atmospheres
of tidally locked Super-Earths
Authors: Carone, L.; Keppens, R.; Decin, L.
2014MNRAS.445..930C Altcode: 2014arXiv1405.6109C
Radiative equilibrium temperatures are calculated for the troposphere
of a tidally locked Super-Earth based on a simple greenhouse model,
using Solar system data as a guideline. These temperatures provide in
combination with a Newtonian relaxation scheme thermal forcing for
a 3D atmosphere model using the dynamical core of the Massachusetts
Institute of Technology global circulation model. Our model is of the
same conceptional simplicity than the model of Held &amp; Suarez and
is thus computationally fast. Furthermore, because of the coherent,
general derivation of radiative equilibrium temperatures, our model is
easily adaptable for different planets and atmospheric scenarios. As
a case study relevant for Super-Earths, we investigate a Gl581g-like
planet with Earth-like atmosphere and irradiation and present results
for two representative rotation periods of Prot = 10 d and
Prot = 36.5 d. Our results provide proof of concept and
highlight interesting dynamical features for the rotating regime 3 &lt;
Prot &lt; 100 d, which was shown by Edson et al. to be an
intermediate regime between equatorial superrotation and divergence. We
confirm that the Prot = 10 d case is more dominated by
equatorial superrotation dynamics than the Prot = 36.5 d
case, which shows diminishing influence of standing Rossby-Kelvin waves
and increasing influence of divergence at the top of the atmosphere. We
argue that this dynamical regime change relates to the increase in
Rossby deformation radius, in agreement with previous studies. However,
we also pay attention to other features that are not or only in partial
agreement with other studies, like, e.g. the number of circulation
cells and their strength, the role and extent of thermal inversion
layers, and the details of heat transport.

---------------------------------------------------------
Title: Gas Acceleration by Fast Dust Particles and the Dusty
Rayleigh-Taylor Instability
Authors: Hendrix, T.; Keppens, R.
2014ASPC..488...77H Altcode:
We use the gas+dust module of the MPI-AMRVAC code, and demonstrate
our ability to reproduce a Sod shock tube test with the addition of
dust. As a more concrete application, we discuss a more detailed setup
in which fast dust particles flow into a stationary gas, leading to
its fast acceleration. We introduce a density discontinuity in the
domain, and investigate how the interaction of the inflowing dust
and the accelerated gas with a density discontinuity alters the
formation of the Rayleigh-Taylor instability (RTI). This setup is
of interest in stellar surroundings where radiative acceleration can
cause fast acceleration of dust particles, while the gas only feels
this acceleration through interaction with the dust particles.

---------------------------------------------------------
Title: MPI-AMRVAC for Solar and Astrophysics
Authors: Porth, O.; Xia, C.; Hendrix, T.; Moschou, S. P.; Keppens, R.
2014ApJS..214....4P Altcode: 2014arXiv1407.2052P
In this paper, we present an update to the open source MPI-AMRVAC
simulation toolkit where we focus on solar and non-relativistic
astrophysical magnetofluid dynamics. We highlight recent developments
in terms of physics modules, such as hydrodynamics with dust coupling
and the conservative implementation of Hall magnetohydrodynamics. A
simple conservative high-order finite difference scheme that works
in combination with all available physics modules is introduced and
demonstrated with the example of monotonicity-preserving fifth-order
reconstruction. Strong stability-preserving high-order Runge-Kutta time
steppers are used to obtain stable evolutions in multi-dimensional
applications, realizing up to fourth-order accuracy in space and
time. With the new distinction between active and passive grid cells,
MPI-AMRVAC is ideally suited to simulate evolutions where parts
of the solution are controlled analytically or have a tendency to
progress into or out of a stationary state. Typical test problems
and representative applications are discussed with an outlook toward
follow-up research. Finally, we discuss the parallel scaling of the
code and demonstrate excellent weak scaling up to 30, 000 processors,
allowing us to exploit modern peta-scale infrastructure.

---------------------------------------------------------
Title: Simulating the in Situ Condensation Process of Solar
Prominences
Authors: Xia, C.; Keppens, R.; Antolin, P.; Porth, O.
2014ApJ...792L..38X Altcode: 2014arXiv1408.4249X
Prominences in the solar corona are a hundredfold cooler and denser
than their surroundings, with a total mass of 1013 up
to 1015 g. Here, we report on the first comprehensive
simulations of three-dimensional, thermally and gravitationally
stratified magnetic flux ropes where in situ condensation to a
prominence occurs due to radiative losses. After a gradual thermodynamic
adjustment, we witness a phase where runaway cooling occurs while
counter-streaming shearing flows drain off mass along helical field
lines. After this drainage, a prominence-like condensation resides
in concave upward field regions, and this prominence retains its
overall characteristics for more than two hours. While condensing,
the prominence establishes a prominence-corona transition region where
magnetic field-aligned thermal conduction is operative during the
runaway cooling. The prominence structure represents a force-balanced
state in a helical flux rope. The simulated condensation demonstrates a
right-bearing barb, as a remnant of the drainage. Synthetic images at
extreme ultraviolet wavelengths follow the onset of the condensation,
and confirm the appearance of horns and a three-part structure for the
stable prominence state, as often seen in erupting prominences. This
naturally explains recent Solar Dynamics Observatory views with
the Atmospheric Imaging Assembly on prominences in coronal cavities
demonstrating horns.

---------------------------------------------------------
Title: Rayleigh-Taylor instability in magnetohydrodynamic simulations
of the Crab nebula
Authors: Porth, Oliver; Komissarov, Serguei S.; Keppens, Rony
2014MNRAS.443..547P Altcode: 2014arXiv1405.4029P
In this paper, we discuss the development of Rayleigh-Taylor
(RT) filaments in axisymmetric simulations of pulsar wind nebulae
(PWN). High-resolution adaptive mesh refinement magnetohydrodynamic
simulations are used to resolve the non-linear evolution of the
instability. The typical separation of filaments is mediated by
the turbulent flow in the nebula and hierarchical growth of the
filaments. The strong magnetic dissipation and field randomization
found in recent global three-dimensional simulations of PWN suggest
that magnetic tension is not strong enough to suppress the growth of RT
filaments, in agreement with the observations of prominent filaments
in the Crab nebula. The long-term axisymmetric results presented here
confirm this finding.

---------------------------------------------------------
Title: The Dynamics of Funnel Prominences
Authors: Keppens, R.; Xia, C.
2014ApJ...789...22K Altcode: 2014arXiv1405.3419K
We present numerical simulations in 2.5D settings where large-scale
prominences form in situ out of coronal condensation in magnetic dips,
in close agreement with early as well as recent reporting of funnel
prominences. Our simulation uses full thermodynamic magnetohydrodynamics
with anisotropic thermal conduction, optically thin radiative losses,
and parameterized heating as main ingredients to establish a realistic
arcade configuration from chromosphere to corona. The chromospheric
evaporation, especially from transition region heights, ultimately
causes thermal instability, and we witness the growth of a prominence
suspended well above the transition region, continuously gaining mass
and cross-sectional area. Several hours later, the condensation has
grown into a structure connecting the prominence-corona transition
region with the underlying transition region, and a continuous downward
motion from the accumulated mass represents a drainage that matches
observational findings. A more dynamic phase is found as well, with
coronal rain, induced wave trains, and even a reconnection event when
the core prominence plasma weighs down the field lines until a flux
rope is formed. The upper part of the prominence is then trapped in
a flux-rope structure, and we argue for its violent kink-unstable
eruption as soon as the (ignored) length dimension would allow for
ideal kink deformations.

---------------------------------------------------------
Title: Multi-scale virtual view on the precessing jet SS433
Authors: Monceau-Baroux, R.; Porth, O.; Meliani, Z.; Keppens, R.
2014xru..confE.147M Altcode:
Observations of SS433 infer how an X-ray binary gives rise to a
corkscrew patterned relativistic jet. XRB SS433 is well known on a
large range of scales for wich we realize 3D simulation and radio
mappings. For our study we use relativistic hydrodynamic in special
relativity using a relativistic effective polytropic index. We use
parameters extracted from observations to impose thermodynamical
conditions of the ISM and jet. We follow the kinetic and thermal
energy content, of the various ISM and jet regions. Our simulation
follows simultaneously the evolution of the population of electrons
which are accelerated by the jet. The evolving spectrum of these
electrons, together with an assumed equipartition between dynamic and
magnetic pressure, gives input for estimating the radio emission from
our simulation. Ray tracing according to a direction of sight then
realizes radio mappings of our data. Single snapshots are realised
to compare with VLA observation as in Roberts et al. 2008. A radio
movie is realised to compare with the 41 days movie made with the VLBA
instrument. Finaly a larger scale simulation explore the discrepancy
of opening angle between 10 and 20 degree between the large scale
observation of SS433 and its close in observation.

---------------------------------------------------------
Title: Relativistic AGN jets - II. Jet properties and mixing effects
for episodic jet activity
Authors: Walg, S.; Achterberg, A.; Markoff, S.; Keppens, R.; Porth, O.
2014MNRAS.439.3969W Altcode: 2014MNRAS.tmp..428W; 2013arXiv1311.4234W
Various radio galaxies show signs of having gone through episodic
jet outbursts in the past. An example is the class of double-double
radio galaxies (DDRGs). However, to follow the evolution of an
individual source in real-time is impossible due to the large
time-scales involved. Numerical studies provide a powerful tool to
investigate the temporal behaviour of episodic jet outbursts in a
(magneto)hydrodynamical setting. We simulate the injection of two jets
from active galactic nuclei (AGNs), separated by a short interruption
time. Three different jet models are compared. We find that an AGN
jet outburst cycle can be divided into four phases. The most prominent
phase occurs when the restarted jet is propagating completely inside the
hot and inflated cocoon left behind by the initial jet. In that case,
the jet-head advance speed of the restarted jet is significantly higher
than the initial jet-head. While the head of the initial jet interacts
strongly with the ambient medium, the restarted jet propagates almost
unimpeded. As a result, the restarted jet maintains a strong radial
integrity. Just a very small fraction of the amount of shocked jet
material flows back through the cocoon compared to that of the initial
jet and much weaker shocks are found at the head of the restarted
jet. We find that the features of the restarted jet in this phase
closely resemble the observed properties of a typical DDRG.

---------------------------------------------------------
Title: Solution to the Sigma Problem of Pulsar Wind Nebulae
Authors: Porth, Oliver; Komissarov, Serguei S.; Keppens, Rony
2014IJMPS..2860168P Altcode:
Pulsar wind nebulae (PWN) provide a unique test-bed for the
study of highly relativistic processes right at our astronomical
doorstep. In this contribution we will show results from the first
3D RMHD simulations of PWN. Of key interest to our study is the long
standing "sigma-problem" that challenges MHD models of Pulsars and their
nebulae now for 3 decades. Earlier 2D MHD models were very successful
in reproducing the morphology of the inner Crab nebula showing a jet,
torus, concentric wisps and a variable knot. However, these models
are limited to a purely toroidal field geometry which leads to an
exaggerated compression of the termination shock and polar jet —
in contrast to the observations. In three dimensions, the toroidal
field structure is susceptible to current driven instabilities; hence
kink instability and magnetic dissipation govern the dynamics of the
nebula flow. This leads to a resolution of the sigma-problem once
also the pulsar's obliqueness (striped wind) is taken into account. In
addition, we present polarized synchrotron maps constructed from the
3D simulations, showing the wealth of morphological features reproduced
in 2D is preserved in the 3D case.

---------------------------------------------------------
Title: Effect of dust on Kelvin-Helmholtz instabilities
Authors: Hendrix, T.; Keppens, R.
2014A&A...562A.114H Altcode: 2014arXiv1401.6774H
Context. Dust is present in a large variety of astrophysical fluids,
ranging from tori around supermassive black holes to molecular clouds,
protoplanetary discs, and cometary outflows. In many such fluids,
shearing flows are present, which can lead to the formation of
Kelvin-Helmholtz instabilities (KHI) and may change the properties
and structures of the fluid through processes such as mixing and
clumping of dust. Aims: We study the effects of dust on the
KHI by performing numerical hydrodynamical dust+gas simulations. We
investigate how the presence of dust changes the growth rates of the
KHI in 2D and 3D and how the KHI redistributes and clumps dust. We
investigate if similarities can be found between the structures in 3D
KHI and those seen in observations of molecular clouds. Methods:
We perform numerical multifluid hydrodynamical simulations in addition
to the gas a number of dust fluids. Each dust fluid represents a portion
of the particle size-distribution. We study how dust-to-gas mass density
ratios between 0.01 and 1 alter the growth rate in the linear phase
of the KHI. We do this for a wide range of perturbation wavelengths,
and compare these values to the analytical gas-only growth rates. As
the formation of high-density dust structures is of interest in many
astrophysical environments, we scale our simulations with physical
quantities that are similar to values in molecular clouds. 
Results: Large differences in dynamics are seen for different grain
sizes. We demonstrate that high dust-to-gas ratios significantly reduce
the growth rate of the KHI, especially for short wavelengths. We compare
the dynamics in 2D and 3D simulations, where the latter demonstrates
additional full 3D instabilities during the non-linear phase, leading
to increased dust densities. We compare the structures formed by the
KHI in 3D simulations with those in molecular clouds and see how the
column density distribution of the simulation shares similarities
with log-normal distributions with power-law tails sometimes seen in
observations of molecular clouds.

---------------------------------------------------------
Title: Three-dimensional magnetohydrodynamic simulations of the
Crab nebula
Authors: Porth, Oliver; Komissarov, Serguei S.; Keppens, Rony
2014MNRAS.438..278P Altcode: 2013arXiv1310.2531P; 2013MNRAS.tmp.2943P
In this paper, we give a detailed account of the first three-dimensional
(3D) relativistic magnetohydrodynamic simulations of pulsar wind
nebulae, with parameters most suitable for the Crab nebula. In contrast
to the previous 2D simulations, we also consider pulsar winds with
much stronger magnetization, up to σ ≃ few. The 3D models preserve
the separation of the post-termination shock flow into the equatorial
and polar components, but the polar jets are disrupted by the kink
mode of the current driven instability and `dissolve' into the main
body of the nebula after propagation of several shock radii. With the
exception of the region near the termination shock, the 3D models do
not exhibit the strong z-pinch configuration characteristic of the 1D
and 2D models. Contrary to the expectations based on 1D analytical and
semi-analytical models, the 3D solutions with highly magnetized pulsar
winds still produce termination shocks with radii comparable to those
deduced from the observations. The reason for this is not only the
randomization of magnetic field observed in the 3D solutions, but also
the magnetic dissipation inside the nebula. Assuming that the particle
acceleration occurs only at the termination shock, we produced synthetic
maps of the Crab nebula synchrotron emission. These maps retain most
of the features revealed in the previous 2D simulations, including
thin wisps and the inner knot. The polarization and variability of the
inner knot is in a particularly good agreement with the observations
of the Crab nebula and the overall polarization of the inner nebula is
also reproduced quite well. However, the polar jet is not as bright
as observed, suggesting that an additional particle acceleration,
presumably related to the magnetic dissipation, has to be invoked.

---------------------------------------------------------
Title: Prominence Formation and Destruction
Authors: Xia, Chun; Antolin, Patrick; Keppens, Rony
2014IAUS..300..468X Altcode:
In earlier work, we demonstrated the in-situ formation of a quiescent
prominence in a sheared magnetic arcade by chromospheric evaporation
and thermal instability in a multi-dimensional MHD model. Here,
we improve our setup and reproduce the formation of a curtain-like
prominence from first principles, while showing the coexistence of the
growing, large-scale prominence with short-lived dynamic coronal rain
in overlying loops. When the localized heating is gradually switched
off, the central prominence expands laterally beyond the range of its
self-created magnetic dips and falls down along the arched loops. The
dipped loops recover their initially arched shape and the prominence
plasma drains to the chromosphere completely.

---------------------------------------------------------
Title: Modeling Magnetic Flux Ropes
Authors: Xia, Chun; Keppens, Rony
2014IAUS..300..121X Altcode:
The magnetic configuration hosting prominences can be a large-scale
helical magnetic flux rope. As a necessary step towards future
prominence formation studies, we report on a stepwise approach
to study flux rope formation. We start with summarizing our recent
three-dimensional (3D) isothermal magnetohydrodynamic (MHD) simulation
where a flux rope is formed, including gas pressure and gravity. This
starts from a static corona with a linear force-free bipolar magnetic
field, altered by lower boundary vortex flows around the main polarities
and converging flows towards the polarity inversion. The latter flows
induce magnetic reconnection and this forms successive new helical
loops so that a complete flux rope grows and ascends. After stopping
the driving flows, the system relaxes to a stable helical magnetic
flux rope configuration embedded in an overlying arcade. Starting from
this relaxed isothermal endstate, we next perform a thermodynamic MHD
simulation with a chromospheric layer inserted at the bottom. As a
result of a properly parametrized coronal heating, and due to radiative
cooling and anisotropic thermal conduction, the system further relaxes
to an equilibrium where the flux rope and the arcade develop a fully
realistic thermal structure. This paves the way to future simulations
for 3D prominence formation.

---------------------------------------------------------
Title: Atmospheric dynamics on tidally locked Earth-like planets in
the habitable zone of an M dwarf star
Authors: Carone, Ludmila; Keppens, Rony; Decin, Leen
2014IAUS..299..376C Altcode:
We investigated the large scale atmospheric circulation of Gl581g, a
potentially habitable planet around an M dwarf star, using an idealized
dry global circulation model (GCM) with simplified thermal forcing as a
first step towards a systematic extended parameter study. The results
are compared with the work of Joshi et al. (1997) who investigated a
tidally-locked habitable Earth analogue with less than half the rotation
period of Gl581g. The extent, form and strength of the atmospheric
circulation in each model generally agree with each other, even
though the models differ in key parameters such as planetary radius,
surface gravity, forcing scheme and rotation period. The substellar
point is associated with an uprising direct circulation-branch of a
Hadley-like cell with return flow over the poles. It is compelling
to assume that the substellar point of a tidally locked terrestrial
exoplanet behaves dynamically like the Earth's tropic associated with
clouds and precipitation, making it an ideal target for habitability.

---------------------------------------------------------
Title: Three-dimensional Prominence-hosting Magnetic Configurations:
Creating a Helical Magnetic Flux Rope
Authors: Xia, C.; Keppens, R.; Guo, Y.
2014ApJ...780..130X Altcode: 2013arXiv1311.5478X
The magnetic configuration hosting prominences and their surrounding
coronal structure is a key research topic in solar physics. Recent
theoretical and observational studies strongly suggest that a helical
magnetic flux rope is an essential ingredient to fulfill most of the
theoretical and observational requirements for hosting prominences. To
understand flux rope formation details and obtain magnetic
configurations suitable for future prominence formation studies,
we here report on three-dimensional isothermal magnetohydrodynamic
simulations including finite gas pressure and gravity. Starting from
a magnetohydrostatic corona with a linear force-free bipolar magnetic
field, we follow its evolution when introducing vortex flows around
the main polarities and converging flows toward the polarity inversion
line near the bottom of the corona. The converging flows bring the
feet of different loops together at the polarity inversion line, where
magnetic reconnection and flux cancellation happen. Inflow and outflow
signatures of the magnetic reconnection process are identified, and
thereby the newly formed helical loops wind around preexisting ones so
that a complete flux rope grows and ascends. When a macroscopic flux
rope is formed, we switch off the driving flows and find that the
system relaxes to a stable state containing a helical magnetic flux
rope embedded in an overlying arcade structure. A major part of the
formed flux rope is threaded by dipped field lines that can stably
support prominence matter, while the total mass of the flux rope is
in the order of 4-5× 1014 g.

---------------------------------------------------------
Title: Coronal rain: multi-dimensional aspects from numerical surveys
Authors: Keppens, Rony; Xia, Chun; Fang, Xia
2014cosp...40E1455K Altcode:
The enigmatic coronal rain phenomenon has frequently been studied
using essentially one-dimensional, thermodynamically driven evolutions
along rigid magnetic field lines. The onset of thermal instability and
follow-up runaway catastrophic cooling then allows for almost cyclic
behavior, with individual blobs forming and `raining' down along the
loop legs. Using our recent progression to 2.5 dimensional computations
for thermodynamically stratified, full magnetohydrodynamic evolutions
of heated arcades, we can identify several multi-dimensional aspects
in coronal rain showers. These include the force field variations
influencing the overall lateral sizes for coronal rain blobs, the fact
that shear flow patterns in adjacent loops modify local thermodynamic
instability development, wave trains seen to trail descending blobs,
etc. Our grid-adaptive simulations follow multiple cycles of raining
arcade setups, and allow to draw statistics that favorably compare to
modern high resolution views.

---------------------------------------------------------
Title: Relativistic 3D precessing jet simulations for the X-ray
binary SS433
Authors: Monceau-Baroux, Rémi; Porth, Oliver; Meliani, Zakaria;
Keppens, Rony
2014A&A...561A..30M Altcode: 2013arXiv1311.7593M
Context. Modern high-resolution radio observations allow us a closer
look into the objects that power relativistic jets. This is especially
the case for SS433, an X-ray binary that emits a precessing jet that
is observed down to the subparsec scale. Aims: We aim to study
full 3D dynamics of relativistic jets associated with active galactic
nuclei or X-ray binaries (XRB). In particular, we incorporate the
precessing motion of a jet into a model for the jet associated with
the XRB SS433. Our study of the jet dynamics in this system focuses
on the subparsec scales. We investigate the impact of jet precession
and the variation of the Lorentz factor of the injected matter on the
general 3D jet dynamics and its energy transfer to the surrounding
medium. After visualizing and quantifying jet dynamics, we aim to
realize synthetic radio mapping of the data, to compare our results
with observations. Methods: For our study we used a block-tree
adaptive mesh refinement scheme and an inner time-dependent boundary
prescription to inject precessing bipolar supersonic jets. Parameters
extracted from observations were used. Different 3D jet realizations
that match the kinetic flux of the SS433 jet were intercompared,
which vary in density contrast and jet beam velocity. We tracked
the energy content deposited in different regions of the domain
affected by the jet. Our code allows us to follow the adiabatic
cooling of a population of relativistic particles injected by the
jet. This evolving energy spectrum of accelerated electrons, using
a pressure-based proxy for the magnetic field, allowed us to obtain
the radio emission from our simulation. Results: We find a
higher energy transfer for a precessing jet than for standing jets
with otherwise identical parameters as a result of the effectively
increased interaction area. We obtain synthetic radio maps for all jets,
from which one can see that dynamical flow features are clearly linked
with enhanced emission sites. Conclusions: The synthetic radio
map best matches a jet model with the canonical propagation speed
of 0.26c and a precession angle of 20°. Overdense precessing jets
experience significant deceleration in their propagation through the
interstellar medium, while the overall jet is of helical shape. Our
results show that the kinematic model for SS433 has to be corrected
for deceleration assuming ballistic propagation on a subparsec scale.

---------------------------------------------------------
Title: Modeling Prominence Formation in 2.5D
Authors: Fang, X.; Xia, C.; Keppens, R.
2014IAUS..300..410F Altcode:
We use a 2.5-dimensional, fully thermodynamically and
magnetohydrodynamically compatible model to imitate the formation
process of normal polarity prominences on top of initially linear
force-free arcades above photospheric neutral lines. In magnetic
arcades hosting chromospheric, transition region, and coronal plasma,
we perform a series of numerical simulations to do a parameter survey
for multi-dimensional evaporation-condensation prominence models. The
investigated parameters include the fixed angle of the magnetic arcade,
the strength and spatial range of the localized chromospheric heating.

---------------------------------------------------------
Title: Relativistic modeling for precessing jets: the SS433 X-ray
binary environment
Authors: Keppens, Rony; Porth, Oliver; Monceau-Baroux, Remi
2014cosp...40E1454K Altcode:
We present numerical simulations to complement modern radio
observations of the helical jets seen in association with X-ray binary
SS433. Adopting a 3D relativistic hydrodynamic model, we go beyond the
pure kinematic model frequently used to interpret the radio VLA views,
pointing out that the gradual build-up of the full helical jet path
naturally results in a somewhat decelerated propagation. Synthetic
radio maps of the simulated, precessing jets confirm the basic scenario
of an overdense jet injected at 0.26c, prevailing at the sub-parsec
scale distances. Recent extensions to either larger simulated domains,
or to closer in regions including time-variable ejection patterns,
will be presented.

---------------------------------------------------------
Title: 3D simulation of prominence magnetic structure: a helical
magnetic flux rope
Authors: Xia, Chun; Guo, Yang; Keppens, Rony
2014cosp...40E3658X Altcode:
The magnetic configuration hosting prominences and their surrounding
coronal structure is a key research topic in solar physics. Recent
theoretical and observational studies strongly suggest that a helical
magnetic flux rope is an essential ingredient to fulfill most of the
theoretical and observational requirements for hosting prominences. To
understand flux rope formation details and obtain magnetic
configurations suitable for future prominence formation studies,
we here report on three-dimensional isothermal magnetohydrodynamic
simulations including finite gas pressure and gravity. Starting
from a magnetohydrostatic corona with a linear force-free bipolar
magnetic field, we follow its evolution when introducing vortex flows
around the main polarities and converging flows towards the polarity
inversion line near the bottom of the corona. The converging flows
bring feet of different loops together at the polarity inversion line
and magnetic reconnection and flux cancellation happens. Inflow and
outflow signatures of the magnetic reconnection process are identified,
and the thereby newly formed helical loops wind around pre-existing
ones so that a complete flux rope grows and ascends. When a macroscopic
flux rope is formed, we switch off the driving flows and find that the
system relaxes to a stable state containing a helical magnetic flux
rope embedded in an overlying arcade structure. A major part of the
formed flux rope is threaded by dipped field lines which can stably
support prominence matter.

---------------------------------------------------------
Title: MHD waves and instabilities for gravitating, magnetized
configurations in motion
Authors: Keppens, Rony; Goedbloed, Hans J. P.
2014cosp...40E1452K Altcode:
Seismic probing of equilibrium configurations is of course well-known
from geophysics, but has also been succesfully used to determine the
internal structure of the Sun to an amazing accuracy. The results of
helioseismology are quite impressive, although they only exploit an
equilibrium structure where inward gravity is balanced by a pressure
gradient in a 1D radial fashion. In principle, one can do the
same for stationary, gravitating, magnetized plasma equilibria, as
needed to perform MHD seismology in astrophysical jets or accretion
disks. The introduction of (sheared) differential rotation does
require the important switch from diagnosing static to stationary
equilibrium configurations. The theory to describe all linear
waves and instabilities in ideal MHD, given an exact stationary,
gravitating, magnetized plasma equilibrium, in any dimensionality
(1D, 2D, 3D) has been known since 1960, and is governed by the
Frieman-Rotenberg equation. The full (mathematical) power of spectral
theory governing physical eigenmode determination comes into play
when using the Frieman-Rotenberg equation for moving equilibria, as
applicable to astrophysical jets, accretion disks, but also solar
flux ropes with stationary flow patterns. I will review exemplary
seismic studies of flowing equilibrium configurations, covering
solar to astrophysical configurations in motion. In that case, even
essentially 1D configurations require quantification of the spectral
web of eigenmodes, organizing the complex eigenfrequency plane.

---------------------------------------------------------
Title: 3D simulations of pulsar wind nebulae
Authors: Porth, Oliver; Komissarov, Serguei; Keppens, Rony
2014cosp...40E2606P Altcode:
In this presentation I will show results from global 3D RMHD
simulations of PWN. Of key interest to our study is the long standing
"sigma-problem" that challenges MHD models of Pulsars and their nebulae
now for over 3 decades. In contrast to previous 2D simulations,
we also consider pulsar winds with much stronger magnetization,
up to sigma_0≃3. Our 3D models preserve the separation of
the post-termination shock flow into the equatorial and polar
components. However, the polar jets (excessively strong in 2D)
are disrupted by the kink mode of the current driven instability and
'dissolve' into the main body of the nebula after propagation of several
shock radii. With the exception of the region near the termination
shock, the 3D models do not exhibit the strong z-pinch configuration
characteristic of 1D and 2D models. This leads to a resolution of the
sigma-problem once also the pulsar's obliqueness (striped wind) is taken
into account, since contrary to expectations based on 1D analytical
and semi-analytical models, the 3D solutions with highly magnetised
pulsar winds still produce termination shocks with radii comparable
to those deduced from observations. In addition, I will present
synchrotron maps and animations constructed from the 3D simulations,
showing a remarkable resemblance with the available observations of
Crab nebula. The polarisation and variability of the inner knot is in
particularly good agreement and the overall polarisation of the inner
nebula is reproduced well. However, the polar jet is not as bright as
observed, suggesting that additional particle acceleration, presumably
related to magnetic dissipation, has to be invoked.

---------------------------------------------------------
Title: Interacting tilt and kink instabilities in repelling current
channels
Authors: Keppens, Rony; Porth, Oliver
2014cosp...40E1453K Altcode:
I present a numerical study in resistive magnetohydrodynamics where
the initial equilibrium configuration contains adjacent, oppositely
directed current channels. Since oppositely directed current channels
repel, the equilibrium is liable to an ideal magnetohydrodynamic
tilt instability. This tilt evolution in planar settings involves
two magnetic islands, which on Alfvenic timescales undergo a combined
rotation and displacement. This deformation leads to the creation of
(near) singular current layers, posing severe challenges to numerical
approaches. Using our open-source grid-adaptive MPI-AMRVAC software,
we revisit the planar evolution case in compressible MHD, as well
as its extension to 2.5D and full 3D scenarios. As long as the third
dimension is ignorable, pure tilt evolutions result which are hardly
affected by an added perpendicular magnetic field component. In all
2.5D runs, our simulations show evidence for secondary tearing type
disruptions throughout the near singular current sheets. In full
3D, both current channels can be liable to additional ideal kink
deformations. We discuss the effects of having both kink and tilt
instabilities acting simultaneously in the violent, reconnection
dominated evolution. Possible laboratory as well as space plasma
applications will briefly be discussed.

---------------------------------------------------------
Title: 3D simulation on solar prominence and coronal cavity
Authors: Xia, Chun; Keppens, Rony; Porth, Oliver
2014cosp...40E3659X Altcode:
The formation of solar prominences within a coronal cavity has been
detected by recent SDO/AIA observations. A bright emission cloud
shifts the peak brightness progressively from hot channels to cool
channels, which shows evidence of plasma cooling and condensation during
prominence formation. In order to study prominence plasma formation
in realistic magnetic configurations, we perform three-dimensional
magnetohydrodynamic simulations considering thermodynamics in the solar
corona including radiative cooling, anisotropic thermal conduction, and
parameterized coronal heating, based on a numerical isothermal magnetic
flux rope from our previous work. Due to excess density inside the flux
rope, runaway radiative cooling causes a dramatic drop of temperature
leading to plasma condensation in the middle dipped region of the
flux rope. The cool dense condensation forms a slab-shape prominence
stably supported by dipped field lines while the density depletion in
the rest part of the flux rope creates a coronal cavity.

---------------------------------------------------------
Title: Nonlinear evolution of the magnetized Kelvin-Helmholtz
instability: From fluid to kinetic modeling
Authors: Henri, P.; Cerri, S. S.; Califano, F.; Pegoraro, F.; Rossi,
C.; Faganello, M.; Šebek, O.; Trávníček, P. M.; Hellinger,
P.; Frederiksen, J. T.; Nordlund, A.; Markidis, S.; Keppens, R.;
Lapenta, G.
2013PhPl...20j2118H Altcode: 2013arXiv1310.7707H
The nonlinear evolution of collisionless plasmas is typically a
multi-scale process, where the energy is injected at large, fluid
scales and dissipated at small, kinetic scales. Accurately modelling
the global evolution requires to take into account the main micro-scale
physical processes of interest. This is why comparison of different
plasma models is today an imperative task aiming at understanding
cross-scale processes in plasmas. We report here the first comparative
study of the evolution of a magnetized shear flow, through a variety of
different plasma models by using magnetohydrodynamic (MHD), Hall-MHD,
two-fluid, hybrid kinetic, and full kinetic codes. Kinetic relaxation
effects are discussed to emphasize the need for kinetic equilibriums
to study the dynamics of collisionless plasmas in non trivial
configurations. Discrepancies between models are studied both in the
linear and in the nonlinear regime of the magnetized Kelvin-Helmholtz
instability, to highlight the effects of small scale processes on
the nonlinear evolution of collisionless plasmas. We illustrate how
the evolution of a magnetized shear flow depends on the relative
orientation of the fluid vorticity with respect to the magnetic field
direction during the linear evolution when kinetic effects are taken
into account. Even if we found that small scale processes differ
between the different models, we show that the feedback from small,
kinetic scales to large, fluid scales is negligible in the nonlinear
regime. This study shows that the kinetic modeling validates the use
of a fluid approach at large scales, which encourages the development
and use of fluid codes to study the nonlinear evolution of magnetized
fluid flows, even in the collisionless regime.

---------------------------------------------------------
Title: Non-resonant magnetohydrodynamics streaming instability near
magnetized relativistic shocks
Authors: Casse, F.; Marcowith, A.; Keppens, R.
2013MNRAS.433..940C Altcode: 2013arXiv1305.0847C; 2013MNRAS.tmp.1433C
We present in this paper both a linear study and numerical relativistic
magnetohydrodynamic (MHD) simulations of the non-resonant streaming
instability occurring in the precursor of relativistic shocks. In the
shock front rest frame, we perform a linear analysis of this instability
in a likely configuration for ultra-relativistic shock precursors. This
considers magneto-acoustic waves having a wave vector perpendicular
to the shock front and the large-scale magnetic field. Our linear
analysis is achieved without any assumption on the shock velocity
and is thus valid for all velocity regimes. In order to check our
calculation, we also perform relativistic MHD simulations describing
the propagation of the aforementioned magneto-acoustic waves through
the shock precursor. The numerical calculations confirm our linear
analysis, which predicts that the growth rate of the instability
is maximal for ultra-relativistic shocks and exhibits a wavenumber
dependence ∝ kx1/2. Our numerical simulations
also depict the saturation regime of the instability where we show that
the magnetic amplification is moderate but nevertheless significant
(δB/B ≤ 10). This latter fact may explain the presence of strong
turbulence in the vicinity of relativistic magnetized shocks. Our
numerical approach also introduces a convenient means to handle
isothermal (ultra-)relativistic MHD conditions.

---------------------------------------------------------
Title: Relativistic AGN jets I. The delicate interplay between jet
structure, cocoon morphology and jet-head propagation
Authors: Walg, S.; Achterberg, A.; Markoff, S.; Keppens, R.;
Meliani, Z.
2013MNRAS.433.1453W Altcode: 2013MNRAS.tmp.1526W; 2013arXiv1305.2157W
Astrophysical jets reveal strong signs of radial structure. They
suggest that the inner region of the jet, the jet spine, consists of a
low-density, fast-moving gas, while the outer region of the jet consists
of a more dense and slower moving gas, called the jet sheath. Moreover,
if jets carry angular momentum, the resultant centrifugal forces lead
to a radial stratification. Current observations are not able to fully
resolve the radial structure, so little is known about its actual
profile. We present three active galactic nuclei jet models in 2.5D
of which two have been given a radial structure. The first model is a
homogeneous jet, the only model that does not carry angular momentum;
the second model is a spine-sheath jet with an isothermal equation of
state; and the third jet model is a (piecewise) isochoric spine-sheath
jet, with constant but different densities for jet spine and jet
sheath. In this paper, we look at the effects of radial stratification
on jet integrity, mixing between the different jet components and global
morphology of the jet-head and surrounding cocoon. We consider steady
jets that have been active for 23 Myr. All jets have developed the same
number of strong internal shocks along their jet axis at the final
time of simulation. These shocks arise when vortices are being shed
by the jet-head. We find that all three jets maintain their stability
all the way up to the jet-head. The isothermal jet maintains part
of its structural integrity at the jet-head where the distinction
between jet spine and jet sheath material can still be made. In
this case, mixing between jet spine and jet sheath within the jet is
fairly inefficient. The isochoric jet, on the other hand, loses its
structural jet integrity fairly quickly after the jet is injected. At
its jet-head, little structure is maintained and the central part of
the jet predominantly consists of jet sheath material. In this case,
jet spine and jet sheath material mix efficiently within the jet. We
find that the propagation speed for all three models is less than
expected from simple theoretical predictions. We propose this is due
to an enlarged cross-section of the jet which impacts with the ambient
medium. We show that in these models, the effective surface area is 16
times as large in the case of the homogeneous jet, 30 times as large
in the case of the isochoric jet and can be up to 40 times as large
in the case of the isothermal jet.

---------------------------------------------------------
Title: Multidimensional Modeling of Coronal Rain Dynamics
Authors: Fang, X.; Xia, C.; Keppens, R.
2013ApJ...771L..29F Altcode: 2013arXiv1306.4759F
We present the first multidimensional, magnetohydrodynamic simulations
that capture the initial formation and long-term sustainment of
the enigmatic coronal rain phenomenon. We demonstrate how thermal
instability can induce a spectacular display of in situ forming
blob-like condensations which then start their intimate ballet on top
of initially linear force-free arcades. Our magnetic arcades host a
chromospheric, transition region, and coronal plasma. Following coronal
rain dynamics for over 80 minutes of physical time, we collect enough
statistics to quantify blob widths, lengths, velocity distributions,
and other characteristics which directly match modern observational
knowledge. Our virtual coronal rain displays the deformation of
blobs into V-shaped features, interactions of blobs due to mostly
pressure-mediated levitations, and gives the first views of blobs
that evaporate in situ or are siphoned over the apex of the background
arcade. Our simulations pave the way for systematic surveys of coronal
rain showers in true multidimensional settings to connect parameterized
heating prescriptions with rain statistics, ultimately allowing us to
quantify the coronal heating input.

---------------------------------------------------------
Title: Parametric survey of longitudinal prominence oscillation
simulations
Authors: Zhang, Q. M.; Chen, P. F.; Xia, C.; Keppens, R.; Ji, H. S.
2013A&A...554A.124Z Altcode: 2013arXiv1304.3798Z
Context. Longitudinal filament oscillations recently attracted
increasing attention, while the restoring force and the damping
mechanisms are still elusive. Aims: We intend to investigate
the underlying physics for coherent longitudinal oscillations of the
entire filament body, including their triggering mechanism, dominant
restoring force, and damping mechanisms. Methods: With the
MPI-AMRVAC code, we carried out radiative hydrodynamic numerical
simulations of the longitudinal prominence oscillations. We modeled
two types of perturbations of the prominence, impulsive heating at one
leg of the loop and an impulsive momentum deposition, which cause the
prominence to oscillate. We studied the resulting oscillations for a
large parameter scan, including the chromospheric heating duration,
initial velocity of the prominence, and field line geometry. 
Results: We found that both microflare-sized impulsive heating at
one leg of the loop and a suddenly imposed velocity perturbation
can propel the prominence to oscillate along the magnetic dip. Our
extensive parameter survey resulted in a scaling law that shows that
the period of the oscillation, which weakly depends on the length and
height of the prominence and on the amplitude of the perturbations,
scales with √R/g⊙, where R represents the curvature
radius of the dip, and g⊙ is the gravitational acceleration
of the Sun. This is consistent with the linear theory of a pendulum,
which implies that the field-aligned component of gravity is the
main restoring force for the prominence longitudinal oscillations, as
confirmed by the force analysis. However, the gas pressure gradient
becomes significant for short prominences. The oscillation damps
with time in the presence of non-adiabatic processes. Radiative
cooling is the dominant factor leading to damping. A scaling law
for the damping timescale is derived, i.e., τ~ l1.63
D0.66w-1.21v0-0.30,
showing strong dependence on the prominence length l, the geometry
of the magnetic dip (characterized by the depth D and the width w),
and the velocity perturbation amplitude v0. The larger
the amplitude, the faster the oscillation damps. We also found that
mass drainage significantly reduces the damping timescale when the
perturbation is too strong.

---------------------------------------------------------
Title: Solution to the sigma problem of pulsar wind nebulae.
Authors: Porth, O.; Komissarov, S. S.; Keppens, R.
2013MNRAS.431L..48P Altcode: 2012arXiv1212.1382P
We present first results of 3D relativistic magnetohydrodynamical
simulations of pulsar wind nebulae. They show that the kink instability
and magnetic dissipation inside these nebulae may be the key processes
allowing them to reconcile their observations with the theory of
pulsar winds. In particular, the size of the termination shock,
obtained in the simulations, agrees very well with the observations
even for Poynting-dominated pulsar winds. Due to magnetic dissipation
the total pressure in the simulated nebulae is particle-dominated and
more or less uniform. While in the main body of the simulated nebulae
the magnetic field becomes rather randomized, close to the termination
shock, it is dominated by the regular toroidal field freshly injected
by the pulsar wind. This field is responsible for driving polar outflows
and may explain the high polarization observed in pulsar wind nebulae.

---------------------------------------------------------
Title: Dust Dynamics in Kelvin-Helmholtz Instabilities
Authors: Hendrix, Tom; Keppens, Rony
2013EPJWC..4606003H Altcode:
The Kelvin-Helmholtz instability (KHI) is a fluid instability
which arises when two contacting flows have different tangential
velocities. As shearing flows are very common in all sorts of
(astro)physical fluid setups, the KHI is frequently encountered. In
many astrophysical fluids the gas fluid in loaded with additional
dust particles. Here we study the influence of these dust particles
on the initiation of the KHI, as well as the effect the KHI has on the
density distribution of dust species in a range of different particle
sizes. This redistribution by the instability is of importance in the
formation of dust structures in astrophysical fluids. To study the
effect of dust on the linear and nonlinear phase of the KHI, we use the
multi-fluid dust + gas module of the MPI-AMRVAC [1] code to perform 2D
and 3D simulations of KHI in setups with physical quantities relevant
to astrophysical fluids. A clear dependency on dust sizes is seen,
with larger dust particles displaying significantly more clumping than
smaller ones.

---------------------------------------------------------
Title: SWIFF: Space weather integrated forecasting framework
Authors: Lapenta, Giovanni; Pierrard, Viviane; Keppens, Rony; Markidis,
Stefano; Poedts, Stefaan; Šebek, Ondřej; Trávníček, Pavel M.;
Henri, Pierre; Califano, Francesco; Pegoraro, Francesco; Faganello,
Matteo; Olshevsky, Vyacheslav; Restante, Anna Lisa; Nordlund, Åke;
Trier Frederiksen, Jacob; Mackay, Duncan H.; Parnell, Clare E.;
Bemporad, Alessandro; Susino, Roberto; Borremans, Kris
2013JSWSC...3A..05L Altcode:
SWIFF is a project funded by the Seventh Framework Programme of the
European Commission to study the mathematical-physics models that
form the basis for space weather forecasting. The phenomena of space
weather span a tremendous scale of densities and temperature with
scales ranging 10 orders of magnitude in space and time. Additionally
even in local regions there are concurrent processes developing at
the electron, ion and global scales strongly interacting with each
other. The fundamental challenge in modelling space weather is the
need to address multiple physics and multiple scales. Here we present
our approach to take existing expertise in fluid and kinetic models to
produce an integrated mathematical approach and software infrastructure
that allows fluid and kinetic processes to be modelled together. SWIFF
aims also at using this new infrastructure to model specific coupled
processes at the Solar Corona, in the interplanetary space and in the
interaction at the Earth magnetosphere.

---------------------------------------------------------
Title: Solar prominences: formation, force balance, internal dynamics
Authors: Keppens, R.; Xia, C.; Chen, P.; Blokland, J. W. S.
2013ASPC..470...37K Altcode:
Prominences represent fascinating large-scale, cool and dense
structures, suspended in the hot and tenuous solar corona above
magnetic neutral lines. Starting from magnetohydrostatic force
balance arguments, their differing magnetic topology distinguishes
Kippenhahn-Schlüter (1957) versus Kuperus-Raadu (1974) types. In both,
the concave-upward parts of magnetic field lines or ‘dips’ host
and support prominence material via the magnetic tension force against
gravity. We highlight recent insights into prominence physics, where we
start from modern magnetohydrodynamic equilibrium computations, allowing
to mimic flux-rope embedded multi-layer prominence configurations of
Kuperus-Raadu type. These can be analysed for linear stability, and
by quantifying the eigenfrequencies of flux-surface localized modes,
charting out the continuous parts of the MHD spectrum, we pave the way
for more detailed prominence seismology. Perhaps the most elusive aspect
of prominence physics is their sudden formation, and we demonstrate
recent achievements in both rigid field, and fully multi-dimensional
simulation efforts. The link with the thermal instability of
optically thin radiative plasmas is clarified, and we show the first
evaporation-condensation model study where we can demonstrate how the
formed prominence stays in a force balanced state, which can be compared
to the original Kippenhahn-Schlüter type magnetohydrostatic model.

---------------------------------------------------------
Title: Multi-dimensional models of circumstellar shells around
evolved massive stars
Authors: van Marle, A. J.; Keppens, R.
2012A&A...547A...3V Altcode: 2012arXiv1209.4496V
Context. Massive stars shape their surrounding medium through the
force of their stellar winds, which collide with the circumstellar
medium. Because the characteristics of these stellar winds vary over the
course of the evolution of the star, the circumstellar matter becomes
a reflection of the stellar evolution and can be used to determine
the characteristics of the progenitor star. In particular, whenever
a fast wind phase follows a slow wind phase, the fast wind sweeps
up its predecessor in a shell, which is observed as a circumstellar
nebula. Aims: We make 2D and 3D numerical simulations of fast
stellar winds sweeping up their slow predecessors to investigate
whether numerical models of these shells have to be 3D, or whether
2D models are sufficient to reproduce the shells correctly. Methods: We use the MPI-AMRVAC code, using hydrodynamics with
optically thin radiative losses included, to make numerical models of
circumstellar shells around massive stars in 2D and 3D and compare
the results. We focus on those situations where a fast Wolf-Rayet
star wind sweeps up the slower wind emitted by its predecessor,
being either a red supergiant or a luminous blue variable. 
Results: As the fast Wolf-Rayet wind expands, it creates a dense
shell of swept up material that expands outward, driven by the high
pressure of the shocked Wolf-Rayet wind. These shells are subject
to a fair variety of hydrodynamic-radiative instabilities. If the
Wolf-Rayet wind is expanding into the wind of a luminous blue variable
phase, the instabilities will tend to form a fairly small-scale,
regular filamentary lattice with thin filaments connecting knotty
features. If the Wolf-Rayet wind is sweeping up a red supergiant wind,
the instabilities will form larger interconnected structures with less
regularity. The numerical resolution must be high enough to resolve
the compressed, swept-up shell and the evolving instabilities, which
otherwise may not even form. Conclusions: Our results show that
3D models, when translated to observed morphologies, give realistic
results that can be compared directly to observations. The 3D structure
of the nebula will help to distinguish different progenitor scenarios.

---------------------------------------------------------
Title: The effect of angular opening on the dynamics of relativistic
hydro jets
Authors: Monceau-Baroux, R.; Keppens, R.; Meliani, Z.
2012A&A...545A..62M Altcode: 2012arXiv1211.1590M
Context. Relativistic jets emerging from active galactic nuclei (AGN)
cores transfer energy from the core of the AGN to their surrounding
interstellar/intergalactic medium through shock-related and hydrodynamic
instability mechanisms. Because jets are observed to have finite
opening angles, one needs to quantify the role of conical versus
cylindrical jet propagation in this energy transfer. Aims:
We adopt parameters representative for Faranoff-Riley class II AGN
jets with finite opening angles. We study how such an opening angle
affects the overall dynamics of the jet and its interaction with
its surrounding medium and therefore how it influences the energy
transfer between the AGN and the external medium. We also point
out how the characteristics of this external medium, such as its
density profile, play a role in the dynamics. Methods: This
study exploits our parallel adaptive mesh refinement code MPI-AMRVAC
with its special relativistic hydrodynamic model, incorporating an
equation of state with varying effective polytropic index. We initially
studied mildly underdense jets up to opening angles of 10 degrees,
at Lorentz factors of about 10, inspired by input parameters derived
from observations. Instantaneous quantifications of the various
interstellar medium (ISM) volumes affected by jet injection and
their energy content allows one to quantify the role of mixing versus
shock-heated cocoon regions over the simulated time intervals. 
Results: We show that a wider opening angle jet results in a faster
deceleration of the jet and leads to a wider radial expansion zone
dominated by Kelvin-Helmholtz and Rayleigh-Taylor instabilities. The
energy transfer mainly occurs in the shocked ISM region by both the
frontal bow shock and cocoon-traversing shock waves, in a roughly 3
to 1 ratio to the energy transfer of the mixing zone, for a 5 degree
opening angle jet. The formation of knots along the jet may be related
to X-ray emission blobs known from observations. A rarefaction wave
induces a dynamically formed layered structure of the jet beam. Conclusions: Finite opening angle jets can efficiently transfer
significant fractions (25% up to 70%) of their injected energy over a
growing region of shocked ISM matter. The role of the ISM stratification
is prominent for determining the overall volume that is affected by
relativistic jet injection. While our current 2D simulations give us
clear insights into the propagation characteristics of finite opening
angle, hydrodynamic relativistic jets, we need to expand this work
to 3D.

---------------------------------------------------------
Title: MPI-AMRVAC: MPI-Adaptive Mesh Refinement-Versatile Advection
Code
Authors: van der Holst, Bar; Keppens, Rony; Meliani, Zakaria; Porth,
Oliver; van Marle, Allard Jan; Delmont, Peter; Xia, Chun
2012ascl.soft08014V Altcode:
MPI-AMRVAC is an MPI-parallelized Adaptive Mesh Refinement code, with
some heritage (in the solver part) to the Versatile Advection Code or
VAC, initiated by Gábor Tóth at the Astronomical Institute at Utrecht
in November 1994, with help from Rony Keppens since 1996. Previous
incarnations of the Adaptive Mesh Refinement version of VAC were of
restricted use only, and have been used for basic research in AMR
strategies, or for well-targeted applications. This MPI version uses
a full octree block-based approach, and allows for general orthogonal
coordinate systems. MPI-AMRVAC aims to advance any system of (primarily
hyperbolic) partial differential equations by a number of different
numerical schemes. The emphasis is on (near) conservation laws, with
shock-dominated problems as a main research target. The actual equations
are stored in separate modules, can be added if needed, and they can
be selected by a simple configuration of the VACPP preprocessor. The
dimensionality of the problem is also set through VACPP. The numerical
schemes are able to handle discontinuities and smooth flows as well.

---------------------------------------------------------
Title: Dust distribution in circumstellar shells
Authors: van Marle, Allard-Jan; Meliani, Zakaria; Keppens, Rony;
Decin, Leen
2012IAUS..283..516V Altcode: 2011arXiv1110.3144V
We present numerical simulations of the hydrodynamical interactions
that produce circumstellar shells. These simulations include several
scenarios, such as wind-wind interaction and wind-ISM collisions. In
our calculations we have taken into account the presence of dust in
the stellar wind. Our results show that, while small dust grains tend
to be strongly coupled to the gas, large dust grains are only weakly
coupled. As a result, the distribution of the large dust grains is
not representative of the gas distribution. Combining these results
with observations may give us a new way of validating hydrodynamical
models of the circumstellar medium.

---------------------------------------------------------
Title: Numerical Simulation of Flares in GRB Afterglow Phase
Authors: Meliani, Z.; Vlasis, A.; Keppens, R.
2012ASPC..459..118M Altcode:
We investigate numerically the various evolutionary phases in
the interaction of relativistic shells with its surrounding cold
interstellar medium (ISM) and shell-shell interaction. We do this for
1D. This is relevant for gamma-ray bursts (GRBs) and the observed
flares, and we demonstrate that, thanks to the AMR strategy, we
resolve the internal structure of the shocked shell and ISM matter
and shell-shell matter, which will leave its imprint on the GRB
afterglow. Also, we perform high resolution numerical simulations
of late collisions between two ultra-relativistic shells in order to
explore the flares in the afterglow phase of GRB. We examine the case
where a cold uniform shell collides with a self-similar Blandford and
McKee shell in a constant density environment and consider cases with
different Lorentz factor and energy for the uniform shell. We produce
the corresponding on-axis light curves and emission images for the
afterglow phase and examine the occurrence of optical and radio flares
assuming a spherical explosion and a hard-edged jet scenario. For our
simulations we use the Adaptive Mesh Refinement version of the Versatile
Advection Code (AMRVAC) coupled to a linear radiative transfer code
to calculate synchrotron emission. We find steeply rising flare like
behavior for small jet opening angles and more gradual rebrightenings
for large opening angles. Synchrotron self-absorption is found to
strongly influence the onset and shape of the radio flare.

---------------------------------------------------------
Title: Dusty Circumstellar Environments: MPI-AMRVAC Simulations
Authors: Keppens, R.; van Marle, A. J.; Meliani, Z.
2012ASPC..459...73K Altcode:
We highlight results obtained on multi-dimensional modeling of
circumstellar environments, where stellar outflows interact mutually
or with the surrounding interstellar medium. We use the MPI-AMRVAC
code, able to handle Newtonian to relativistic gas and plasma dynamic
scenarios. For circumstellar modeling, the code has been extended
by coupling the hydro to (possibly multiple) pressureless dust
species. The dynamics of pressureless dust poses ultimate challenges
to grid-adaptive numerical simulations, as it allows for delta waves in
its purest form. We subsequently illustrate pure gas dynamic scenarios,
where optically thin radiative losses combined with supersonic outflows
cause complex instability dominated circumstellar bubbles. We comment
on single star, as well as on binary system scenarios. In the latter,
we focus on the wind collision front where distinctly different
instabilities manifest themselves in different regions. Finally, we show
coupled gas-dust dynamical treatments, as case scenarios for dust-loaded
stellar outflows, specifically for moving massive stars. This includes
a detailed treatment for dust grains in the stellar wind, accounting
for drag forces between dust and gas.

---------------------------------------------------------
Title: VAC: Versatile Advection Code
Authors: Tóth, Gábor; Keppens, Rony
2012ascl.soft07003T Altcode:
The Versatile Advection Code (VAC) is a freely available general
hydrodynamic and magnetohydrodynamic simulation software that works in
1, 2 or 3 dimensions on Cartesian and logically Cartesian grids. VAC
runs on any Unix/Linux system with a Fortran 90 (or 77) compiler and
Perl interpreter. VAC can run on parallel machines using either the
Message Passing Interface (MPI) library or a High Performance Fortran
(HPF) compiler.

---------------------------------------------------------
Title: Formation and long-term evolution of 3D vortices in
protoplanetary discs
Authors: Meheut, H.; Keppens, R.; Casse, F.; Benz, W.
2012A&A...542A...9M Altcode: 2012arXiv1204.4390M
Context. In the context of planet formation, anticyclonic vortices
have recently received much attention for the role they can play in
planetesimal formation. Radial migration of intermediate-size solids
towards the central star may prevent them from growing to larger solid
grains. On the other hand, vortices can trap the dust and accelerate
this growth, counteracting fast radial transport. Several effects
have been shown to affect this scenario, such as vortex migration
or decay. Aims: We aim to study the formation of vortices by
the Rossby wave instability and their long-term evolution in a full
three-dimensional (3D) protoplanetary disc. Methods: We used
a robust numerical scheme combined with adaptive mesh refinement
in cylindrical coordinates, which allowed us to affordably compute
long-term 3D evolutions. We considered a full disc radially and
vertically stratified, in which vortices can be formed by the Rossby
wave instability. Results: We show that the 3D Rossby vortices
grow and survive over hundreds of years without migration. The localised
overdensity that initiated the instability and vortex formation survives
the growth of the Rossby wave instability for very long times. When
the vortices are no longer sustained by the Rossby wave instability,
their shape changes towards more elliptical vortices. This allows them
to survive shear-driven destruction, but they may be prone to elliptical
instability and slow decay. Conclusions: When the conditions for
growing Rossby-wave-related instabilities are maintained in the disc,
large-scale vortices can survive over very long timescales and may be
able to concentrate solids.

---------------------------------------------------------
Title: Observations and simulations of longitudinal oscillations of
an active region prominence
Authors: Zhang, Q. M.; Chen, P. F.; Xia, C.; Keppens, R.
2012A&A...542A..52Z Altcode: 2012arXiv1204.3787Z
Context. Filament longitudinal oscillations have been observed
in Hα observations of the solar disk. Aims: We intend to
find an example of the longitudinal oscillations of a prominence,
where the magnetic dip can be seen directly, and examine the
restoring force of this type of oscillations. Methods:
We carry out a multiwavelength data analysis of the active region
prominence oscillations above the western limb on 2007 February
8. In addition, we perform a one-dimensional hydrodynamic simulation
of the longitudinal oscillations. Results: Our analysis of
high-resolution observations performed by Hinode/SOT indicate that the
prominence, seen as a concave-inward shape in lower-resolution extreme
ultraviolet (EUV) images, consists of many concave-outward threads,
which is indicative of magnetic dips. After being injected into the dip
region, a bulk of prominence material started to oscillate for more
than 3.5 h, with the period of 52 min. The oscillation decayed with
time, on the decay timescale 133 min. Our hydrodynamic simulation
can reproduce the oscillation period, but the damping timescale
in the simulation is 1.5 times as long as the observations. 
Conclusions: The results clearly show the prominence longitudinal
oscillations around the dip of the prominence and our study suggests
that the restoring force of the longitudinal oscillations might be the
gravity. Radiation and heat conduction are insufficient to explain the
decay of the oscillations. Other mechanisms, such as wave leakage and
mass accretion, have to be considered. The possible relation between
the longitudinal oscillations and the later eruption of a prominence
thread, as well as a coronal mass ejection (CME), is also discussed.

---------------------------------------------------------
Title: Simulations of Prominence Formation in the Magnetized Solar
Corona by Chromospheric Heating
Authors: Xia, C.; Chen, P. F.; Keppens, R.
2012ApJ...748L..26X Altcode: 2012arXiv1202.6185X
Starting from a realistically sheared magnetic arcade connecting the
chromospheric, transition region to coronal plasma, we simulate the
in situ formation and sustained growth of a quiescent prominence in
the solar corona. Contrary to previous works, our model captures all
phases of the prominence formation, including the loss of thermal
equilibrium, its successive growth in height and width to macroscopic
dimensions, and the gradual bending of the arched loops into dipped
loops, as a result of the mass accumulation. Our 2.5 dimensional,
fully thermodynamically and magnetohydrodynamically consistent model
mimics the magnetic topology of normal-polarity prominences above a
photospheric neutral line, and results in a curtain-like prominence
above the neutral line through which the ultimately dipped magnetic
field lines protrude at a finite angle. The formation results from
concentrated heating in the chromosphere, followed by plasma evaporation
and later rapid condensation in the corona due to thermal instability,
as verified by linear instability criteria. Concentrated heating
in the lower atmosphere evaporates plasma from below to accumulate
at the top of coronal loops and supply mass to the later prominence
constantly. This is the first evaporation-condensation model study
where we can demonstrate how the formed prominence stays in a force
balanced state, which can be compared to the Kippenhahn-Schlüter type
magnetohydrostatic model, all in a finite low-beta corona.

---------------------------------------------------------
Title: Kinetic structure of collisionless reconnection: hybrid
simulations
Authors: Šebek, O.; Trávníček, P. M.; Hellinger, P.; Lapenta,
G.; Keppens, R.; Olshevsky, V.; Restante, A. L.; Hendrix, T.
2012EGUGA..14.8382S Altcode:
Magnetic reconnection is a fundamental process observed in various
space plasma systems, such as, for example, interface between planetary
magnetosphere and solar wind at the dayside magnetopause. We study
magnetic reconnection by means of two-dimensional hybrid approach
(kinetic ions and fluid electrons). Our initial configuration consists
of Harris equilibrium layer with small amplitude perturbation of
magnetic field. These perturbations are origins of the formation of
magnetic islands. In this study we focus on the role of ionic kinetic
effects during the reconnection process, we examine the temperature
anisotropy and gyrotropy of the ion velocity distribution functions. We
discuss the importance of these kinetic effects by comparing the results
from hybrid simulations with the results from magneto-hydrodynamic
(MHD) simulations results.

---------------------------------------------------------
Title: Late activity in GRB afterglows. A multidimensional approach.
Authors: Vlasis, A.; Meliani, Z.; Keppens, R.
2012MSAIS..21..190V Altcode:
A late activity of the central engine of Gamma-Ray Bursts (GRBs)
followed by energy injection in the external shock has been proposed
in order to explain the strong variability which is often observed
in multiwavelength observations in the afterglow. We perform high
resolution 1D and 2D numerical simulations of late collisions between
two ultra-relativistic shells in order to explore these events. We
examine the case where a cold uniform shell collides with a self-similar
Blandford and McKee shell in a constant density environment and
for the 1D case we produce the corresponding on-axis light curves
for the afterglow phase investigating the occurrence of optical and
radio flares assuming a spherical explosion and a jet scenario with
different opening angles. For our simulations we use the Adaptive
Mesh Refinement version of the Versatile Advection Code (MPI-AMRVAC)
coupled to a linear radiative transfer code to calculate synchrotron
emission. We find steeply rising flare like behavior for small jet
opening angles and more gradual rebrightenings for large opening
angles. Synchrotron self-absorption is found to strongly influence
the onset and shape of the radio flare. Preliminary results of the
dynamics from the 2D simulation are also presented in this paper.

---------------------------------------------------------
Title: On the circumstellar medium of massive stars and how it may
appear in GRB observations .
Authors: van Marle, A. J.; Keppens, R.; Yoon, S. -C.; Langer, N.
2012MSAIS..21...40V Altcode: 2011arXiv1110.3142V
Massive stars lose a large fraction of their original mass over
the course of their evolution. These stellar winds shape the
surrounding medium according to parameters that are the result of the
characteristics of the stars, varying over time as the stars evolve,
leading to both permanent and temporary features that can be used
to constrain the evolution of the progenitor star. Because
long Gamma-Ray Bursts (GRBs) are thought to originate from massive
stars, the characteristics of the circumstellar medium (CSM) should
be observable in the signal of GRBs. This can occur directly, as the
characteristics of the GRB-jet are changed by the medium it collides
with, and indirectly because the GRB can only be observed through the
extended circumstellar bubble that surrounds each massive star. We use computer simulations to describe the circumstellar features
that can be found in the vicinity of massive stars and discuss if,
and how, they may appear in GRB observations. Specifically, we make
hydrodynamical models of the circumstellar environment of a rapidly
rotating, chemically near-homogeneous star, which is represents a
GRB progenitor candidate model. The simulations show that the
star creates a large scale bubble of shocked wind material, which
sweeps up the interstellar medium in an expanding shell. Within this
bubble, temporary circumstellar shells, clumps and voids are created
as a result of changes in the stellar wind. Most of these temporary
features have disappeared by the time the star reaches the end of its
life, leaving a highly turbulent circumstellar bubble behind. Placing
the same star in a high density environment simplifies the evolution
of the CSM as the more confined bubble prohibits the formation of some
of the temporary structures.

---------------------------------------------------------
Title: Jet Structure, Collimation and Stability: Recent Results from
Analytical Models and Simulations
Authors: Keppens, Rony; Meliani, Zakaria
2012rjag.book..341K Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Two-component Jets and the Fanaroff-Riley Dichotomy
Authors: Meliani, Z.; Keppens, R.
2011ASPC..444...75M Altcode:
The two types of Fanaroff-Riley radio loud galaxies, FRI and FRII,
exhibit strong jets but with different properties. These differences may
be associated to the central engine and/or the external medium. The
AGN classification FRI and FRII can be linked to the transverse
stratification of the jet. Indeed, theoretical arguments support
this transverse stratification of jets with two components induced
by intrinsic features of the central engine (accretion disk + black
hole). In fact, according to the observations and theoretical models, a
typical jet has an inner fast low density jet, surrounded by a slower,
denser, extended jet. We elaborate on this model and investigate
for the first time this two-component jet evolution with very high
resolution in 3D. We demonstrate that two-component jets with a high
kinetic energy flux contribution from the inner jet are subject to
the development of a relativistically enhanced, rotation-induced
Rayleigh-Taylor type non-axisymmetric instability. This instability
induces strong mixing between both components, decelerating the inner
jet and leading to overall jet decollimation. This novel scenario of
sudden jet deceleration and decollimation can explain the radio source
Fanaroff-Riley dichotomy as a consequence of the efficiency of the
central engine in launching the inner jet component vs. the outer jet
component. We infer that the FRII/FRI transition, interpreted in our
two-component jet scenario, occurs when the relative kinetic energy
flux of the inner to the outer jet exceeds a critical ratio.

---------------------------------------------------------
Title: Toward detailed prominence seismology. II. Charting the
continuous magnetohydrodynamic spectrum
Authors: Blokland, J. W. S.; Keppens, R.
2011A&A...532A..94B Altcode: 2011arXiv1106.4935B
Context. Starting from accurate magnetohydrodynamic flux rope equilibria
containing prominence condensations, we initiate a systematic survey
of their linear eigenoscillations. This paves the way for more detailed
prominence seismology, which thus far has made dramatic simplifications
about the prevailing magnetic field topologies. Aims: To quantify
the full spectrum of linear MHD eigenmodes, we require knowledge of
all flux-surface localized modes, charting out the continuous parts
of the MHD spectrum. We combine analytical and numerical findings for
the continuous spectrum for realistic prominence configurations, where
a cool prominence is embedded in a hotter cavity, or where the flux
rope contains multiple condensations supported against gravity. 
Methods: The equations governing all eigenmodes for translationally
symmetric, gravitating equilibria containing an axial shear flow,
are analyzed, along with their flux-surface localized limit. The
analysis is valid for general 2.5D equilibria, where either density,
entropy, or temperature vary from one flux surface to another. We
analyze the intricate mode couplings caused by the poloidal variation
in the flux rope equilibria, by performing a small gravity parameter
expansion. We contrast the analytical results with continuous spectra
obtained numerically. Results: For equilibria where the density
is a flux function, we show that continuum modes can be overstable,
and we present the stability criterion for these convective continuum
instabilities. Furthermore, for all equilibria, a four-mode coupling
scheme between an Alfvénic mode of poloidal mode number m and three
neighboring (m - 1,m,m + 1) slow modes is identified, occurring in
the vicinity of rational flux surfaces. For realistically structured
prominence equilibria, this coupling is shown to play an important
role, from weak to stronger gravity parameter g values. The analytic
predictions for small g are compared with numerical spectra,
and progressive deviations for larger g are identified. 
Conclusions: The unstable continuum modes could be relevant for
short-lived prominence configurations. The gaps created by poloidal
mode coupling in the continuous spectrum need further analysis, as
they form preferred frequency ranges for global eigenoscillations.

---------------------------------------------------------
Title: Formation of Solar Filaments by Steady and Nonsteady
Chromospheric Heating
Authors: Xia, C.; Chen, P. F.; Keppens, R.; van Marle, A. J.
2011ApJ...737...27X Altcode: 2011arXiv1106.0094X
It has been established that cold plasma condensations can form in a
magnetic loop subject to localized heating of its footpoints. In this
paper, we use grid-adaptive numerical simulations of the radiative
hydrodynamic equations to investigate the filament formation process
in a pre-shaped loop with both steady and finite-time chromospheric
heating. Compared to previous works, we consider low-lying
loops with shallow dips and use a more realistic description for
radiative losses. We demonstrate for the first time that the onset of
thermal instability satisfies the linear instability criterion. The
onset time of the condensation is roughly ~2 hr or more after the
localized heating at the footpoint is effective, and the growth rate
of the thread length varies from 800 km hr-1 to 4000 km
hr-1, depending on the amplitude and the decay length scale
characterizing this localized chromospheric heating. We show how single
or multiple condensation segments may form in the coronal portion. In
the asymmetric heating case, when two segments form, they approach
and coalesce, and the coalesced condensation later drains down into
the chromosphere. With steady heating, this process repeats with a
periodicity of several hours. While our parametric survey confirms and
augments earlier findings, we also point out that steady heating is not
necessary to sustain the condensation. Once the condensation is formed,
it keeps growing even after the localized heating ceases. In such a
finite-heating case, the condensation instability is maintained by
chromospheric plasma that gets continuously siphoned into the filament
thread due to the reduced gas pressure in the corona. Finally, we
show that the condensation can survive the continuous buffeting of
perturbations from photospheric p-mode waves.

---------------------------------------------------------
Title: Numerical simulations of the circumstellar medium of massive
binaries
Authors: van Marle, Allard Jan; Keppens, Rony
2011IAUS..271..405V Altcode:
We have made 3-D models of the collision of binary star winds and
followed their interaction over multiple orbits. This allows us
to explore how the wind-wind interaction shapes the circumstellar
environment. Specifically, we can model the highly radiative shock that
occurs where the winds collide. We find that the shell that is created
at the collision front between the two winds can be highly unstable,
depending on the characteristics of the stellar winds.

---------------------------------------------------------
Title: Toward detailed prominence seismology. I. Computing accurate
2.5D magnetohydrodynamic equilibria
Authors: Blokland, J. W. S.; Keppens, R.
2011A&A...532A..93B Altcode: 2011arXiv1106.4933B
Context. Prominence seismology exploits our knowledge of the linear
eigenoscillations for representative magnetohydrodynamic models
of filaments. To date, highly idealized models for prominences
have been used, especially with respect to the overall magnetic
configurations. Aims: We initiate a more systematic survey
of filament wave modes, where we consider full multi-dimensional
models with twisted magnetic fields representative of the surrounding
magnetic flux rope. This requires the ability to compute accurate
2.5 dimensional magnetohydrodynamic equilibria that balance Lorentz
forces, gravity, and pressure gradients, while containing density
enhancements (static or in motion). Methods: The governing
extended Grad-Shafranov equation is discussed, along with an analytic
prediction for circular flux ropes for the Shafranov shift of the
central magnetic axis due to gravity. Numerical equilibria are computed
with a finite element-based code, demonstrating fourth order accuracy
on an explicitly known, non-trivial test case. Results: The
code is then used to construct more realistic prominence equilibria,
for all three possible choices of a free flux-function. We quantify the
influence of gravity, and generate cool condensations in hot cavities,
as well as multi-layered prominences. Conclusions: The internal
flux rope equilibria computed here have the prerequisite numerical
accuracy to allow a yet more advanced analysis of the complete spectrum
of linear magnetohydrodynamic perturbations, as will be demonstrated
in the companion paper.

---------------------------------------------------------
Title: Two-shell collisions in the gamma-ray burst afterglow phase
Authors: Vlasis, A.; van Eerten, H. J.; Meliani, Z.; Keppens, R.
2011MNRAS.415..279V Altcode: 2011MNRAS.tmp..859V
Strong optical and radio flares often appear in the afterglow phase
of gamma-ray bursts (GRBs). It has been proposed that colliding
ultrarelativistic shells can produce these flares. Such consecutive
shells can be formed due to the variability in the central source of
a GRB. We perform high-resolution 1D numerical simulations of late
collisions between two ultrarelativistic shells in order to explore
these events. We examine the case where a cold uniform shell collides
with a self-similar Blandford &amp; McKee shell in a constant density
environment and consider cases with different Lorentz factor and
energy for the uniform shell. We produce the corresponding on-axis
light curves and emission images for the afterglow phase and examine
the occurrence of optical and radio flares, assuming a spherical
explosion and a hard-edged jet scenario. For our simulations, we use
the Adaptive Mesh Refinement version of the Versatile Advection Code
coupled to a linear radiative transfer code to calculate synchrotron
emission. We find steeply rising flares like the behaviour of small
jet opening angles and more gradual rebrightenings for large opening
angles. Synchrotron self-absorption is found to strongly influence
the onset and shape of the radio flare.

---------------------------------------------------------
Title: Computing the Dust Distribution in the Bow Shock of a
Fast-moving, Evolved Star
Authors: van Marle, A. J.; Meliani, Z.; Keppens, R.; Decin, L.
2011ApJ...734L..26V Altcode: 2011arXiv1105.2387V
We study the hydrodynamical behavior occurring in the turbulent
interaction zone of a fast-moving red supergiant star, where
the circumstellar and interstellar material collide. In this
wind-interstellar-medium collision, the familiar bow shock, contact
discontinuity, and wind termination shock morphology form, with
localized instability development. Our model includes a detailed
treatment of dust grains in the stellar wind and takes into account the
drag forces between dust and gas. The dust is treated as pressureless
gas components binned per grain size, for which we use 10 representative
grain size bins. Our simulations allow us to deduce how dust grains
of varying sizes become distributed throughout the circumstellar
medium. We show that smaller dust grains (radius &lt;0.045 μm)
tend to be strongly bound to the gas and therefore follow the gas
density distribution closely, with intricate fine structure due to
essentially hydrodynamical instabilities at the wind-related contact
discontinuity. Larger grains which are more resistant to drag forces
are shown to have their own unique dust distribution, with progressive
deviations from the gas morphology. Specifically, small dust grains
stay entirely within the zone bound by shocked wind material. The large
grains are capable of leaving the shocked wind layer and can penetrate
into the shocked or even unshocked interstellar medium. Depending
on how the number of dust grains varies with grain size, this should
leave a clear imprint in infrared observations of bow shocks of red
supergiants and other evolved stars.

---------------------------------------------------------
Title: Shock refraction from classical gas to relativistic plasma
environments
Authors: Keppens, Rony; Delmont, Peter; Meliani, Zakaria
2011IAUS..274..441K Altcode:
The interaction of (strong) shock waves with localized density changes
is of particular relevance to laboratory as well as astrophysical
research. Shock tubes have been intensively studied in the lab for
decades and much has been learned about shocks impinging on sudden
density contrasts. In astrophysics, modern observations vividly
demonstrate how (even relativistic) winds or jets show complex
refraction patterns as they encounter denser interstellar material. In this contribution, we highlight recent insights into shock
refraction patterns, starting from classical up to relativistic hydro
and extended to magnetohydrodynamic scenarios. Combining analytical
predictions for shock refraction patterns exploiting Riemann solver
methodologies, we confront numerical, analytical and (historic)
laboratory insights. Using parallel, grid-adaptive simulations, we
demonstrate the fate of Richtmyer-Meshkov instabilities when going
from gaseous to magnetized plasma scenarios. The simulations invoke
idealized configurations closely resembling lab analogues, while
extending them to relativistic flow regimes.

---------------------------------------------------------
Title: Parameter regimes for slow, intermediate and fast MHD shocks
Authors: Delmont, P.; Keppens, R.
2011JPlPh..77..207D Altcode:
We investigate under which parameter regimes the magnetohydrodynamic
(MHD) Rankine-Hugoniot conditions, which describe discontinuous
solutions to the MHD equations, allow for slow, intermediate and fast
shocks. We derive limiting values for the upstream and downstream
shock parameters for which shocks of a given shock-type can occur. We
revisit this classical topic in nonlinear MHD dynamics, augmenting
the recent time reversal duality finding by in the usual shock frame
parametrization.

---------------------------------------------------------
Title: Radiative cooling in numerical astrophysics: The need for
adaptive mesh refinement
Authors: van Marle, Allard Jan; Keppens, Rony
2011CF.....42...44V Altcode: 2010arXiv1011.2610V
Energy loss through optically thin radiative cooling plays an important
part in the evolution of astrophysical gas dynamics and should therefore
be considered a necessary element in any numerical simulation. Although
the addition of this physical process to the equations of hydrodynamics
is straightforward, it does create numerical challenges that have
to be overcome in order to ensure the physical correctness of the
simulation. First, the cooling has to be treated (semi-)implicitly,
owing to the discrepancies between the cooling timescale and the typical
timesteps of the simulation. Secondly, because of its dependence on
a tabulated cooling curve, the introduction of radiative cooling
creates the necessity for an interpolation scheme. In particular,
we will argue that the addition of radiative cooling to a numerical
simulation creates the need for extremely high resolution, which can
only be fully met through the use of adaptive mesh refinement.

---------------------------------------------------------
Title: Thin shell morphology in the circumstellar medium of massive
binaries
Authors: van Marle, A. J.; Keppens, R.; Meliani, Z.
2011A&A...527A...3V Altcode: 2010arXiv1011.1734V
Context. In massive binaries, the powerful stellar winds of the two
stars collide, leading to the formation of shock-dominated environments
that can be modeled only in 3D. Aims: We investigate the
morphology of the collision-front shell between the stellar winds of
binary components in two long-period binary systems, one consisting of
a hydrogen-rich Wolf-Rayet star (WNL) and an O-star and the other of a
luminous blue variable (LBV) and an O-star. We follow the development
and evolution of instabilities due to both the wind interaction and
the orbital motion, that form in this shell if it is sufficiently
compressed. Methods: We use MPI-AMRVAC to time-integrate the
equations of hydrodynamics, combined with optically thin radiative
cooling, on an adaptive mesh 3D grid. Using parameters for generic
binary systems, we simulate the interaction between the winds of the
two stars. Results: The WNL + O star binary represent a typical
example of an adiabatic wind collision. The resulting shell is thick
and smooth, showing no instabilities. On the other hand, the shell
created by the collision of the O star wind with the LBV wind, as well
as the orbital motion of the binary components, is susceptible to thin
shell instabilities, which create a highly structured morphology. We
identify the instabilities as both linear and non-linear thin-shell
instabilities, there being distinct differences between the leading
and the trailing parts of the collision front. We also find that for
binaries containing a star with a (relatively) slow wind, the global
shape of the shell is determined more by the slow wind velocity and the
orbital motion of the binary, than the ram pressure balance between the
two winds. Conclusions: Additional parametric studies of the
interaction between the massive binary winds are needed to identify
the role and dynamical importance of multiple instabilities at the
collision front, as shown here for an LBV + O star system.

---------------------------------------------------------
Title: Two shell collisions in the GRB afterglow phase
Authors: Vlasis, A.; van Eerten, H. J.; Meliani, Z.; Keppens, R.
2011MmSAI..82..137V Altcode: 2011arXiv1103.2936V
Strong optical and X-Ray flares often appear in the afterglow phase
of Gamma-Ray Bursts (GRBs). We perform high resolution numerical
simulations of late collisions between two ultra-relativistic shells in
order to explore these events. Such consecutive shells can be formed due
to the variability in the central source of a GRB. We examine the case
where a cold uniform shell collides with a self similar relativistic,
shocked shell \citep{BM} in a constant density environment. We produce
the corresponding light curves for the afterglow phase and examine
the occurrence and chromaticity of optical and radio flares assuming
different opening angles. We conclude that occurrence of optical and
radio flares is possible for small opening angles of the jet. For our
simulations we use the Adaptive Mesh Refinement version of the Versatile
Advection Code \citep{Kep03,Mel08} while the synchrotron radiation
has been calculated with the method introduced in \citet{HvE09b}.

---------------------------------------------------------
Title: 3-D simulations of shells around massive stars
Authors: van Marle, Allard Jan; Keppens, Rony; Meliani, Zakaria
2011BSRSL..80..310V Altcode: 2011arXiv1102.0104V
As massive stars evolve, their winds change. This causes a series of
hydrodynamical interactions in the surrounding medium. Whenever a fast
wind follows a slow wind phase, the fast wind sweeps up the slow wind
in a shell, which can be observed as a circumstellar nebula. One of the
most striking examples of such an interaction is when a massive star
changes from a red supergiant into a Wolf-Rayet star. Nebulae resulting
from such a transition have been observed around many Wolf-Rayet stars
and show detailed, complicated structures owing to local instabilities
in the swept-up shells. Shells also form in the case of massive binary
stars, where the winds of two stars collide with one another. Along
the collision front gas piles up, forming a shell that rotates along
with the orbital motion of the binary stars. In this case the shell
follows the surface along which the ram pressure of the two colliding
winds is in balance. Using the MPI-AMRVAC hydrodynamics code we have
made multi-dimensional simulations of these interactions in order to
model the formation and evolution of these circumstellar nebulae and
explore whether full 3D simulations are necessary to obtain accurate
models of such nebulae.

---------------------------------------------------------
Title: Jet simulations and gamma-ray burst afterglow jet breaks
Authors: van Eerten, H. J.; Meliani, Z.; Wijers, R. A. M. J.;
Keppens, R.
2011MNRAS.410.2016V Altcode: 2010MNRAS.tmp.1497V; 2010arXiv1005.3966V
The conventional derivation of the gamma-ray burst afterglow jet break
time uses only the blast wave fluid Lorentz factor and therefore
leads to an achromatic break. We show that in general gamma-ray
burst afterglow jet breaks are chromatic across the self-absorption
break. Depending on circumstances, the radio jet break may be postponed
significantly. Using high-accuracy adaptive mesh fluid simulations
in one dimension, coupled to a detailed synchrotron radiation code,
we demonstrate that this is true even for the standard fireball model
and hard-edged jets. We confirm these effects with a simulation
in two dimensions. The frequency dependence of the jet break is a
result of the angle dependence of the emission, the changing optical
depth in the self-absorbed regime and the shape of the synchrotron
spectrum in general. In the optically thin case the conventional
analysis systematically overestimates the jet break time, leading to
inferred opening angles that are underestimated by a factor of ∼1.3
and explosion energies that are underestimated by a factor of ∼1.7,
for explosions in a homogeneous environment. The methods presented in
this paper can be applied to adaptive mesh simulations of arbitrary
relativistic fluid flows. All analysis presented here makes the usual
assumption of an on-axis observer.

---------------------------------------------------------
Title: Relativistic Hydro and Magnetohydrodynamic Models for AGN
Jet Propagation and Deceleration
Authors: Keppens, R.; Meliani, Z.
2010ASPC..429...91K Altcode:
We present grid-adaptive computational studies of both magnetized and
unmagnetized jet flows, with significantly relativistic bulk speeds,
as appropriate for AGN jets. Our relativistic jet studies shed light
on the observationally established classification of Fanaroff-Riley
galaxies, where the appearance in radio maps distinguishes two
types of jet morphologies. The computational effort involves modern
shock-capturing schemes exploited at very high effective resolutions
due to the dynamic grid adaptivity. Our parallel MPI-AMRVAC code
allows for direct comparisons between TVD Lax-Friedrichs, HLL,
HLLC, and approximate Riemann solver based schemes for hydro and
magnetohydrodynamic applications, in both classical and relativistic
variants. We investigate how density changes in the external medium can
induce one-sided jet decelerations, explaining the existence of hybrid
morphology radio sources. Our simulations explore under which conditions
highly energetic FR II jets may suddenly decelerate and continue with FR
I characteristics. Apart from this externally induced effect, we study
intrinsic jet properties that shed light on FR I/II behavior. For
the latter, we explore the consequences of a radially structured
jet morphology, where interface dynamics can trigger transitions
to highly turbulent flow regimes. Finally, we explore the role of
dynamically important, organized magnetic fields in the collimation of
the relativistic jet flows. We show that the helicity of the magnetic
field is effectively transported down the beam, with compression zones
in between diagonal internal cross-shocks showing stronger toroidal
field regions. The axial flow can reaccelerate downstream to these
internal cross-shocks, as field compression pinches the flow.

---------------------------------------------------------
Title: Relativistic Two-component Jet Evolutions in 2D and 3D
Authors: Meliani, Z.; Keppens, R.
2010ASPC..429..121M Altcode:
Observations of astrophysical jets and theoretical arguments suggest
a transverse stratification with two components induced by intrinsic
features of the central engine (accretion disk + black hole). We
study two-component jet dynamics for an inner fast low density jet,
surrounded by a slower, denser, extended jet. We investigate for the
first time this two-component jet evolution with very high resolution
in 2.5D and 3D. We demonstrate that two-component jets with high
kinetic energy flux contribution from the inner jet are subject to
the development of a relativistically enhanced, rotation-induced
Rayleigh-Taylor type instability. This instability induces strong
mixing between both components, decelerating the inner jet and
leading to overall jet decollimation. The 3D simulation confirms the
dominance of the non-axisymmetric character of this novel explanation
for sudden jet deceleration. We note that it can explain the radio
source dichotomy as a direct consequence of the efficiency of the
central engine in launching the inner jet component. We argue that the
FRII/FRI transition, interpreted in our two-component jet scenario,
occurs when the relative kinetic energy flux of the inner to the outer
jet exceeds a critical ratio.

---------------------------------------------------------
Title: Dynamics and stability of relativistic gamma-ray-bursts
blast waves
Authors: Meliani, Z.; Keppens, R.
2010A&A...520L...3M Altcode: 2010arXiv1009.1224M
 Aims: In gamma-ray-bursts (GRBs), ultra-relativistic blast waves
are ejected into the circumburst medium. We analyse in unprecedented
detail the deceleration of a self-similar Blandford-McKee blast wave
from a Lorentz factor 25 to the nonrelativistic Sedov phase. Our goal
is to determine the stability properties of its frontal shock. 
Methods: We carried out a grid-adaptive relativistic 2D hydro-simulation
at extreme resolving power, following the GRB jet during the entire
afterglow phase. We investigate the effect of the finite initial jet
opening angle on the deceleration of the blast wave, and identify
the growth of various instabilities throughout the coasting shock
front. Results: We find that during the relativistic phase,
the blast wave is subject to pressure-ram pressure instabilities that
ripple and fragment the frontal shock. These instabilities manifest
themselves in the ultra-relativistic phase alone, remain in full
agreement with causality arguments, and decay slowly to finally
disappear in the near-Newtonian phase as the shell Lorentz factor
drops below 3. From then on, the compression rate decreases to levels
predicted to be stable by a linear analysis of the Sedov phase. Our
simulations confirm previous findings that the shell also spreads
laterally because a rarefaction wave slowly propagates to the jet axis,
inducing a clear shell deformation from its initial spherical shape. The
blast front becomes meridionally stratified, with decreasing speed from
axis to jet edge. In the wings of the jetted flow, Kelvin-Helmholtz
instabilities occur, which are of negligible importance from the
energetic viewpoint. Conclusions: Relativistic blast waves are
subject to hydrodynamical instabilities that can significantly affect
their deceleration properties. Future work will quantify their effect
on the afterglow light curves.

---------------------------------------------------------
Title: Time-dependent particle acceleration in supernova remnants
in different environments
Authors: Schure, K. M.; Achterberg, A.; Keppens, R.; Vink, J.
2010MNRAS.406.2633S Altcode: 2010MNRAS.tmp..838S; 2010arXiv1004.2766S
We simulate time-dependent particle acceleration in the blast wave
of a young supernova remnant (SNR), using a Monte Carlo approach
for the diffusion and acceleration of the particles, coupled to a
magnetohydrodynamics code. We calculate the distribution function of the
cosmic rays concurrently with the hydrodynamic evolution of the SNR,
and compare the results with those obtained using simple steady-state
models. The surrounding medium into which the SNR evolves turns out
to be of great influence on the maximum energy to which particles are
accelerated. In particular, a shock going through a ρ ~ r-2
density profile causes acceleration to typically much higher energies
than a shock going through a medium with a homogeneous density
profile. We find systematic differences between steady-state analytical
models and our time-dependent calculation in terms of spectral slope,
maximum energy and the shape of the cut-off of the particle spectrum
at the highest energies. We also find that, provided that the magnetic
field at the reverse shock is sufficiently strong to confine particles,
cosmic rays can be easily re-accelerated at the reverse shock.

---------------------------------------------------------
Title: Advanced Magnetohydrodynamics
Authors: Goedbloed, J. P.; Keppens, Rony; Poedts, Stefaan
2010adma.book.....G Altcode:
Preface; Part III. Flow and Dissipation: 12. Waves and instabilities of
stationary plasmas; 13. Shear flow and rotation; 14. Resistive plasma
dynamics; 15. Computational linear MHD; Part IV. Toroidal Plasmas:
16. Static equilibrium of toroidal plasmas; 17. Linear dynamics of
static toroidal plasmas; 18. Linear dynamics of stationary toroidal
plasmas; Part V. Nonlinear Dynamics: 19. Computational nonlinear MHD;
20. Transonic MHD flows and shocks; 21. Ideal MHD in special relativity;
Appendices; References; Index.

---------------------------------------------------------
Title: Gamma-ray burst afterglows from transrelativistic blast
wave simulations
Authors: van Eerten, H. J.; Leventis, K.; Meliani, Z.; Wijers,
R. A. M. J.; Keppens, R.
2010MNRAS.403..300V Altcode: 2009arXiv0909.2446V; 2010MNRAS.tmp...48V
We present a study of the intermediate regime between ultrarelativistic
and non-relativistic flow for gamma-ray burst afterglows. The
hydrodynamics of spherically symmetric blast waves is numerically
calculated using the AMRVAC adaptive mesh refinement code. Spectra and
light curves are calculated using a separate radiation code that, for
the first time, links a parametrization of the microphysics of shock
acceleration, synchrotron self-absorption and electron cooling to a
high-performance hydrodynamic simulation. For the dynamics, we find that
the transition to the non-relativistic regime generally occurs later
than expected, the Sedov-Taylor solution overpredicts the late-time
blast wave radius and the analytical formula for the blast wave velocity
from Huang, Dai &amp; Lu overpredicts the late-time velocity by a factor
of 4/3. Also, we find that the lab frame density directly behind the
shock front divided by the fluid Lorentz factor squared remains very
close to four times the unshocked density, while the effective adiabatic
index of the shock changes from relativistic to non-relativistic. For
the radiation, we find that the flux may differ up to an order of
magnitude depending on the equation of state that is used for the
fluid and that the counterjet leads to a clear rebrightening at late
times for hard-edged jets. Simulating GRB 030329 using predictions for
its physical parameters from the literature leads to spectra and light
curves that may differ significantly from the actual data, emphasizing
the need for very accurate modelling. Predicted light curves at low
radio frequencies for a hard-edged jet model of GRB 030329 with opening
angle 22° show typically two distinct peaks, due to the combined
effect of jet break, non-relativistic break and counterjet. Spatially
resolved afterglow images show a ring-like structure.

---------------------------------------------------------
Title: Spectra and energies of cosmic rays in young supernova remnants
Authors: Schure, Klara; Achterberg, Bram; Keppens, Rony; Vink, Jacco
2010cosp...38.2733S Altcode: 2010cosp.meet.2733S
Cosmic ray acceleration in supernova remnants (SNRs) is attributed
to the process of diffusive shock acceleration. The maximum energy
to which the cosmic rays are accelerated in SNRs is believed to be
around 1015 eV, close to the break ("knee") in the cosmic ray spectrum
observed on Earth. Many models exist that treat cosmic ray acceleration
in the steady state approximation. We will present our Monte Carlo
method that follows particle acceleration over the life time of the
SNR. This method shows that the maximum-attainable energy depends on
the background into which the supernova explodes. Type Ia supernovae
typically go off in a uniform-density medium, whereas many Type Ib/c-II
explode into a medium with a ρ ∝ r-2 density profile. We show that
in the latter case much higher cosmic ray energies can be attained
for the same explosion energy. Our method also allows us to extract
cosmic ray spectra as a function of time and location in the SNR,
as well as make X-ray synchrotron and pion-decay emissivity maps.

---------------------------------------------------------
Title: Two-Component Jets and the Fanaroff-Riley Dichotomy
Authors: Meliani, Zakaria; Keppens, Rony; Sauty, Christophe
2010IJMPD..19..867M Altcode:
Transversely stratified jets are observed in many classes of
astrophysical objects, ranging from young stellar objects, μ-quasars,
to active galactic nuclei and even in gamma-ray bursts. Theoretical
arguments support this transverse stratification of jets with
two components induced by intrinsic features of the central engine
(accretion disk + black hole). In fact, according to the observations
and theoretical models, a typical jet has an inner fast low density jet,
surrounded by a slower, denser, extended jet. We elaborate on this model
and investigate for the first time this two-component jet evolution with
very high resolution in 3D. We demonstrate that two-component jets with
a high kinetic energy flux contribution from the inner jet are subject
to the development of a relativistically enhanced, rotation-induced
Rayleigh-Taylor type non-axisymmetric instability. This instability
induces-strong mixing between both components, decelerating the inner
jet and leading to overall jet decollimation. This novel scenario of
sudden jet deceleration and decollimation can explain the radio source
Fanaroff-Riley dichotomy as a consequence of the efficiency of the
central engine in launching the inner jet component versus the outer
jet component. We infer that the FRII/FRI transition, interpreted
in our two-component jet scenario, occurs when the relative kinetic
energy flux of the inner to the outer jet exceeds a critical ratio.

---------------------------------------------------------
Title: A new radiative cooling curve based on an up-to-date plasma
emission code
Authors: Schure, K. M.; Kosenko, D.; Kaastra, J. S.; Keppens, R.;
Vink, J.
2009A&A...508..751S Altcode: 2009arXiv0909.5204S
This work presents a new plasma cooling curve that is calculated
using the SPEX package. We compare our cooling rates to those
in previous works, and implement the new cooling function in the
grid-adaptive framework “AMRVAC”. Contributions to the cooling
rate by the individual elements are given, to allow for the creation
of cooling curves tailored to specific abundance requirements. In some
situations, it is important to be able to include radiative losses
in the hydrodynamics. The enhanced compression ratio can trigger
instabilities (such as the Vishniac thin-shell instability) that would
otherwise be absent. For gas with temperatures below 104 K,
the cooling time becomes very long and does not affect the gas on the
timescales that are generally of interest for hydrodynamical simulations
of circumstellar plasmas. However, above this temperature, a significant
fraction of the elements is ionised, and the cooling rate increases
by a factor 1000 relative to lower temperature plasmas. Tables
3 and 4 are only available in electronic form at http://www.aanda.org

---------------------------------------------------------
Title: Relativistic hydro and magnetohydrodynamic models for AGN
jet propagation and deceleration
Authors: Keppens, R.; Meliani, Z.
2009iac..talk...79K Altcode: 2009iac..talk..109K
No abstract at ADS

---------------------------------------------------------
Title: Decelerating Relativistic Two-Component Jets
Authors: Meliani, Z.; Keppens, R.
2009ApJ...705.1594M Altcode: 2009arXiv0910.0332M
Transverse stratification is a common intrinsic feature of astrophysical
jets. There is growing evidence that jets in radio galaxies consist of
a fast low-density outflow at the jet axis, surrounded by a slower,
denser, extended jet. The inner and outer jet components then have
a different origin and launching mechanism, making their effective
inertia, magnetization, associated energy flux, and angular momentum
content different as well. Their interface will develop differential
rotation, where disruptions may occur. Here we investigate the
stability of rotating, two-component relativistic outflows typical for
jets in radio galaxies. For this purpose, we parametrically explore
the long-term evolution of a transverse cross section of radially
stratified jets numerically, extending our previous study where a
single, purely hydrodynamic evolution was considered. We include
cases with poloidally magnetized jet components, covering hydro and
magnetohydrodynamic (MHD) models. With grid-adaptive relativistic MHD
simulations, augmented with approximate linear stability analysis,
we revisit the interaction between the two jet components. We study
the influence of dynamically important poloidal magnetic fields, with
varying contributions of the inner component jet to the total kinetic
energy flux of the jet, on their non-linear azimuthal stability. We
demonstrate that two-component jets with high kinetic energy flux and
inner jet effective inertia which is higher than the outer jet effective
inertia are subject to the development of a relativistically enhanced,
rotation-induced Rayleigh-Taylor-type instability. This instability
plays a major role in decelerating the inner jet and the overall jet
decollimation. This novel deceleration scenario can partly explain
the radio source dichotomy, relating it directly to the efficiency of
the central engine in launching the inner jet component. The FRII/FRI
transition could then occur when the relative kinetic energy flux of
the inner to the outer jet grows beyond a certain threshold.

---------------------------------------------------------
Title: No visible optical variability from a relativistic blast wave
encountering a wind termination shock
Authors: van Eerten, H. J.; Meliani, Z.; Wijers, R. A. M. J.;
Keppens, R.
2009MNRAS.398L..63V Altcode: 2009MNRAS.tmpL.282V; 2009arXiv0906.3629V
Gamma-ray burst afterglow flares and rebrightenings of the optical and
X-ray light curves have been attributed to both late-time inner engine
activity and density changes in the medium surrounding the burster. To
test the latter, we study the encounter between the relativistic
blast wave from a gamma-ray burster and a stellar wind termination
shock. The blast wave is simulated using a high-performance adaptive
mesh relativistic hydrodynamic code, AMRVAC, and the synchrotron
emission is analysed in detail with a separate radiation code. We find
no bump in the resulting light curve, not even for very high density
jumps. Furthermore, by analysing the contributions from the different
shock wave regions we are able to establish that it is essential to
resolve the blast wave structure in order to make qualitatively correct
predictions on the observed output and that the contribution from the
reverse shock region will not stand out, even when the magnetic field
is increased in this region by repeated shocks. This study resolves
a controversy in the recent literature.

---------------------------------------------------------
Title: Evolution of Magnetic Fields in Supernova Remnants
Authors: Schure, K. M.; Vink, J.; Achterberg, A.; Keppens, R.
2009RMxAC..36..350S Altcode: 2008arXiv0810.5150S
Supernova remnants (SNR) are now widely believed to be a source of
cosmic rays (CRs) up to an energy of 1015 eV. The magnetic
fields required to accelerate CRs to sufficiently high energies need
to be much higher than can result from compression of the circumstellar
medium (CSM) by a factor 4, as is the case in strong shocks. Non-thermal
synchrotron maps of these regions indicate that indeed the magnetic
field is much stronger, and for young SNRs has a dominant radial
component while for old SNRs it is mainly toroidal. How these magnetic
fields get enhanced, or why the field orientation is mainly radial for
young remnants, is not yet fully understood. We use an adaptive mesh
refinement MHD code, AMRVAC, to simulate the evolution of supernova
remnants and to see if we can reproduce a mainly radial magnetic field
in early stages of evolution. We follow the evolution of the SNR with
three different configurations of the initial magnetic field in the
CSM: an initially mainly toroidal field, a turbulent magnetic field,
and a field parallel to the symmetry axis. Although for the latter two
topologies a significant radial field component arises at the contact
discontinuity due to the Rayleigh-Taylor instability, no radial
component can be seen out to the forward shock. Ideal MHD appears
not sufficient to explain observations. Possibly a higher compression
ratio and additional turbulence due to dominant presence of CRs can
help us to better reproduce the observations in future studies.

---------------------------------------------------------
Title: Evolution of magnetic fields and cosmic ray acceleration in
supernova remnants
Authors: Schure, K. M.; Vink, J.; Achterberg, A.; Keppens, R.
2009AdSpR..44..433S Altcode: 2009arXiv0905.1134S
Observations show that the magnetic field in young supernova remnants
(SNRs) is significantly stronger than can be expected from the
compression of the circumstellar medium (CSM) by a factor of four
expected for strong blast waves. Additionally, the polarization is
mainly radial, which is also contrary to expectation from compression of
the CSM magnetic field. Cosmic rays (CRs) may help to explain these two
observed features. They can increase the compression ratio to factors
well over those of regular strong shocks by adding a relativistic plasma
component to the pressure, and by draining the shock of energy when CRs
escape from the region. The higher compression ratio will also allow for
the contact discontinuity, which is subject to the Rayleigh-Taylor (R-T)
instability, to reach much further out to the forward shock. This could
create a preferred radial polarization of the magnetic field. With an
Adaptive Mesh Refinement MHD code (AMRVAC), we simulate the evolution
of SNRs with three different configurations of the initial CSM magnetic
field, and look at two different equations of state in order to look
at the possible influence of a CR plasma component. The spectrum of
CRs can be simulated using test particles, of which we also show some
preliminary results that agree well with available analytical solutions.

---------------------------------------------------------
Title: GRADSPH: A parallel smoothed particle hydrodynamics code for
self-gravitating astrophysical fluid dynamics
Authors: Vanaverbeke, S.; Keppens, R.; Poedts, S.; Boffin, H.
2009CoPhC.180.1164V Altcode:
We describe the algorithms implemented in the first version of
GRADSPH, a parallel, tree-based, smoothed particle hydrodynamics
code for simulating self-gravitating astrophysical systems written
in FORTRAN 90. The paper presents details on the implementation
of the Smoothed Particle Hydro (SPH) description, where a gridless
approach is used to model compressible gas dynamics. This is done in
the conventional SPH way by means of ‘particles’ which sample
fluid properties, exploiting interpolating kernels. The equations
of self-gravitating hydrodynamics in the SPH framework are derived
self-consistently from a Lagrangian and account for variable
smoothing lengths (‘GRAD-h’) terms in both the hydrodynamic
and gravitational acceleration equations. A Barnes-Hut tree is
used for treating self-gravity and updating the neighbour list of
the particles. In addition, the code updates particle properties
on their own individual timesteps and uses a basic parallelisation
strategy to speed up calculations on a parallel computer system with
distributed memory architecture. Extensive tests of the code in one
and three dimensions are presented. Finally, we describe the program
organisation of the publicly available 3D version of the code, as well
as details concerning the structure of the input and output files
and the execution of the program. Catalogue identifier: AECX_v1_0
Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AECX_v1_0.html
Program obtainable from: CPC Program Library, Queen's University,
Belfast, N. Ireland Licensing provisions: Standard CPC licence,
http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed
program, including test data, etc.: 11 123 No. of bytes in distributed
program, including test data, etc.: 1 561 909 Distribution
format: tar.gz Programming language: Fortran 90/MPI Computer:
HPC cluster Operating system: Unix Has the code been vectorised or
parallelised?: Yes RAM: 56 Mwords with 1.2 million particles on 1 CPU
Word size: 32 bits Classification: 12 Nature of problem: Evolution of
a self-gravitating fluid. Solution method: Hydrodynamics is described
using SPH, self-gravity using the Barnes-Hut tree method. Running time:
The test case provided with the distribution takes less than 10 minutes
for 500 time steps on 10 processors.

---------------------------------------------------------
Title: Numerical simulations of homologous coronal mass ejections
in the solar wind
Authors: Soenen, A.; Zuccarello, F. P.; Jacobs, C.; Poedts, S.;
Keppens, R.; van der Holst, B.
2009A&A...501.1123S Altcode:
Context: Coronal mass ejections (CMEs) are enormous expulsions of
magnetic flux and plasma from the solar corona. Most scientists agree
that a coronal mass ejection is the sudden release of magnetic free
energy stored in a strongly stressed field. However, the exact reason
for this sudden release is still highly debated. Aims: In an
initial multiflux system in steady state equilibrium, containing
a pre-eruptive region consisting of three arcades with alternating
magnetic flux polarity, we study the initiation and early evolution
properties of a sequence of CMEs by shearing a region slightly
larger than the central arcade. Methods: We solve the ideal
magnetohydrodynamics (MHD) equations in an axisymmetrical domain
from the solar surface up to 30 R_⊙. The ideal MHD equations are
advanced in time over a non uniform grid using a modified version of
the Versatile Advection Code (VAC). Results: By applying shearing
motions on the solar surface, the magnetic field is energised and
multiple eruptions are obtained. Magnetic reconnection first opens the
overlying field and two new reconnections sites set in on either side
of the central arcade. After the disconnection of the large helmet top,
the system starts to restore itself but cannot return to its original
configuration as a new arcade has already started to erupt. This process
then repeats itself as we continue shearing. Conclusions: The
simulations reported in the present paper, demonstrate the ability to
obtain a sequence of CMEs by shearing a large region of the central
arcade or by shearing a region that is only slightly larger than
the central arcade. We show, be it in an axisymmetric configuration,
that the breakout model can not only lead to confined eruptions but
also to actual coronal mass ejections provided the model includes a
realistic solar wind model.

---------------------------------------------------------
Title: Numerical simulations of the solar corona and Coronal Mass
Ejections
Authors: Poedts, Stefaan; Jacobs, Carla; van der Holst, Bart; Chané,
Emmanuel; Keppens, Rony
2009EP&S...61..599P Altcode: 2009EP&S...61L.599P
Numerical simulations of Coronal Mass Ejections (CMEs) can provide a
deeper insight in the structure and propagation of these impressive
solar events. In this work, we present our latest results of numerical
simulations of the initial evolution of a fast CME. For this purpose,
the equations of ideal MagnetoHydroDynamics (MHD) have been solved on
a three-dimensional (3D) mesh by means of an explicit, finite volume
solver, where the simulation domain ranges from the lower solar corona
up to 30 R e. In order to simulate the propagation of a
CME throughout the heliosphere, a magnetic flux rope is superposed on
top of a stationary background solar (MHD) wind with extra density
added to the flux rope. The flux rope is launched by giving it an
extra initial velocity in order to get a fast CME forming a 3D shock
wave. The magnetic field inside the initial flux rope is described in
terms of Bessel functions and possesses a high amount of twist.

---------------------------------------------------------
Title: Relativistic Two-Component Hydrodynamic Jets
Authors: Meliani, Zakaria; Keppens, Rony
2009ASSP...13..581M Altcode: 2009pjc..book..581M
Astrophysical jets from various sources seem to be stratified,
with a fast inner jet and a slower outer jet. As it is likely
that the launching mechanism for each component is different,
their interface will develop differential rotation, while the
outer jet radius represents a second interface where disruptions may
occur. We explore the stability of stratified, rotating, relativistic
two-component jets, in turn embedded in static interstellar medium. In
a grid-adaptive relativistic hydrodynamic simulation with the AMRVAC
(Adaptive Mesh Refinement version of the Versatile Advection code),
the non-linear azimuthal stability of two-component relativistic jets is
investigated. We simulate until multiple inner jet rotations have been
completed. We find evidence for the development of an extended shear
flow layer between the two jet components, resulting from the growth of
a body mode in the inner jet, Kelvin-Helmholtz surface modes at their
original interface, and their nonlinear interaction. Both wave modes
are excited by acoustic waves which are reflected between the symmetry
axis and the interface of the two jet components. Their interaction
induces the growth of near stationary, counterrotating vortices at the
outer edge of the shear flow layer. The presence of a heavy external jet
allows their further development to be slowed down, and maintains of a
collimated flow. At the outer jet boundary, small-scale Rayleigh-Taylor
instabilities develop, without disrupting the jet configuration.We
demonstrate that the cross-section of two-component relativistic jets,
with a heavy, cold outer jet, is non-linearly stable.

---------------------------------------------------------
Title: Extragalactic Jets with Helical Magnetic Fields
Authors: Keppens, Rony; Meliani, Zakaria
2009ASSP...13..555K Altcode: 2009pjc..book..555K
Extragalactic jets harbor dynamically important, organized magnetic
fields. We explore with grid-adaptive, high resolution numerical
simulations the morphology of AGN jets pervaded by helical field
and flow topologies. We concentrate on the long term evolution of
kinetic energy dominated jets, penetrating denser clouds. The jets
have near-equipartition magnetic fields, and radially varying Lorentz
factor profiles maximally reaching Γ ∼ 22. The helicity of the
beam magnetic field is effectively transported down the beam, with
compression zones in between diagonal internal cross-shocks showing
stronger toroidal field. The high speed jets have localized, strong
toroidal field within the backflow vortices and a more poloidal field
layer, compressed between jet beam and backflows. This layer stabilizes
the jet beam. We infer emission intensity, suggesting a clear trend
were highly structured beams are found for toroidal fields, while
inner beam cross-shocks and thin hotspots are detectable for poloidal
topologies. Significant jet deceleration only occurs beyond distances
exceeding {O}(100{R}j), as the axial flow can reaccelerate
downstream to the internal cross shocks. This reacceleration is
magnetically aided by field compression across the internal shocks
that pinch the flow.

---------------------------------------------------------
Title: Jet Stability: A Computational Survey
Authors: Keppens, Rony; Meliani, Zakaria; Baty, Hubert; van der
Holst, Bart
2009LNP...791..179K Altcode:
To investigate stability properties of astrophysical jets,
high-resolution numerical simulations are nowadays used routinely. In
this chapter, we address jet stability issues using two complementary
approaches: one where highly idealized, classical magnetohydrodynamic
(MHD) “jet” configurations are simulated in detail, and one
where the full complexity of relativistic jet flows is mimicked
computationally. In the former, we collect vital insights into
multi-dimensional MHD evolutions, where we start from simple planar,
magnetized shear flows to eventually model full three dimensional,
helically magnetized jet segments. Such a gradual approach allows
an in-depth study of [1] the nonlinear interaction of multiple,
linearly unstable modes; as well as [2] their potential to steepen
into shocks with intricate shock-shock interactions. All these return
to varying degree in the latter approach, where jets are impulsively
injected into the simulation domain, and followed over many dynamical
timescales. In particular, we review selected recent insights gained
from relativistic AGN jet modeling. There, we cover both relativistic
hydro and magnetohydrodynamic simulations. In all these studies, the
use of grid-adaptive codes suited for modern supercomputing facilities
is illustrated.

---------------------------------------------------------
Title: Grid-adaptive Simulations of Relativistic Flows
Authors: Keppens, R.; Meliani, Z.
2009cfd..conf..335K Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Faranoff-Riley type I jet deceleration at density
discontinuities. Relativistic hydrodynamics with a realistic equation
of state
Authors: Meliani, Z.; Keppens, R.; Giacomazzo, B.
2008A&A...491..321M Altcode: 2008arXiv0808.2492M
Aims: We propose a model that could explain the sudden jet
deceleration in active galactic nuclei, thereby invoking density
discontinuities. Motivated particularly by recent indications
from HYbrid MOrphology Radio Sources (HYMORS) that Fanaroff-Riley
classification is induced in some cases by variations in the density
of the external medium. We explore how one-sided jet deceleration and
a transition to FR I type can occur in HYMORS, which start as FR II
(and remain so on the other side). Methods: We implemented the
Synge-type equation of state introduced in the general polytropic
case by Meliani et al. (2004, A&amp;A, 425, 773) into the relativistic
hydrodynamic grid-adaptive AMRVAC code. To demonstrate its accuracy,
we set up various test problems in an appendix, which we compare to
exact solutions that we calculate as well. We use the code to analyse
the deceleration of jets in FR II/FR I radio galaxies, following them
at high resolution across several hundred jet beam radii. Results:
We present results for 10 model computations that vary the inlet Lorentz
factor from 10 to 20, include uniform or decreasing density profiles,
and allow for cylindrical versus conical jet models. As long as the jet
propagates through uniform media, we find that the density contrast
sets most of the propagation characteristics, fully consistent with
previous modelling efforts. When the jet runs into a denser medium,
we find a clear distinction in the decelaration of high-energy jets
depending on the encountered density jump. For fairly high-density
contrast, the jet becomes destabilised and compressed, decelerates
strongly (up to subrelativistic speeds), and can form knots. If the
density contrast is too weak, the high-energy jets continue with FR II
characteristics. The trend is similar for the low-energy jet models,
which start as underdense jets from the outset, and decelerate by
entrainment into the lower region as well. We point out differences that
are found between cylindrical and conical jet models, together with
dynamical details like the Richtmyer-Meshkov instabilities developing
at the original contact interface. Appendices are only available
in electronic form at http://www.aanda.org

---------------------------------------------------------
Title: A multidimensional grid-adaptive relativistic magnetofluid code
Authors: van der Holst, B.; Keppens, R.; Meliani, Z.
2008CoPhC.179..617V Altcode: 2008arXiv0807.0713V
A robust second order, shock-capturing numerical scheme for
multidimensional special relativistic magnetohydrodynamics on
computational domains with adaptive mesh refinement is presented. The
base solver is a total variation diminishing Lax Friedrichs scheme
in a finite volume setting and is combined with a diffusive approach
for controlling magnetic monopole errors. The consistency between
the primitive and conservative variables is ensured at all limited
reconstructions and the spatial part of the four velocity is used as
a primitive variable. Demonstrative relativistic examples are shown
to validate the implementation. We recover known exact solutions to
relativistic MHD Riemann problems, and simulate the shock-dominated
long term evolution of Lorentz factor 7 vortical flows distorting
magnetic island chains.

---------------------------------------------------------
Title: Linear wave propagation in relativistic magnetohydrodynamics
Authors: Keppens, R.; Meliani, Z.
2008PhPl...15j2103K Altcode: 2008arXiv0810.2416K
The properties of linear Alfvén, slow, and fast magnetoacoustic waves
for uniform plasmas in relativistic magnetohydrodynamics (MHD) are
discussed, augmenting the well-known expressions for their phase speeds
with knowledge on the group speed. A 3+1 formalism is purposely adopted
to make direct comparison with the Newtonian MHD limits easier and to
stress the graphical representation of their anisotropic linear wave
properties using the phase and group speed diagrams. By drawing these
for both the fluid rest frame and for a laboratory Lorentzian frame
which sees the plasma move with a three-velocity having an arbitrary
orientation with respect to the magnetic field, a graphical view of
the relativistic aberration effects is obtained for all three MHD
wave families. Moreover, it is confirmed that the classical Huygens
construction relates the phase and group speed diagram in the usual
way, even for the lab frame viewpoint. Since the group speed diagrams
correspond to exact solutions for initial conditions corresponding
to a localized point perturbation, their formulae and geometrical
construction can serve to benchmark current high-resolution algorithms
for numerical relativistic MHD.

---------------------------------------------------------
Title: Extragalactic jets with helical magnetic fields: relativistic
MHD simulations
Authors: Keppens, R.; Meliani, Z.; van der Holst, B.; Casse, F.
2008A&A...486..663K Altcode: 2008arXiv0802.2034K
Context: Extragalactic jets are judged to harbor dynamically important,
organized magnetic fields that presumably aid in the collimation of
the relativistic jet flows. Aims: We here explore the morphology
of AGN jets pervaded by helical field and flow topologies by means of
grid-adaptive, high-resolution numerical simulations. We concentrate
on morphological features of the bow shock and the jet beam behind
the Mach disk, for various jet Lorentz factors and magnetic field
helicities. We investigate the influence of helical magnetic fields on
jet beam propagation in an overdense external medium. We adopt a special
relativistic magnetohydrodynamic (MHD) viewpoint on the shock-dominated
AGN jet evolution. Due to the adaptive mesh refinement (AMR), we can
concentrate on the long-term evolution of kinetic energy-dominated jets,
with beam-averaged Lorentz factor Γ ≃ 7, as they penetrate denser
clouds. These jets have near-equipartition magnetic fields (with the
thermal energy) and radially varying Γ(R) profiles within the jet
radius R&lt;Rj maximally reaching Γ ~ 22. Methods:
We used the AMRVAC code, with a novel hybrid block-based AMR strategy,
to compute ideal plasma dynamics in special relativity. We combined
this with a robust second-order shock-capturing scheme and a diffusive
approach to controlling magnetic monopole errors. Results:
We find that the propagation speed of the bow shock systematically
exceeds the value expected from estimates using beam-average parameters,
in accordance with the centrally-peaked Γ(R) variation. The helicity
of the beam magnetic field is effectively transported down the beam,
with compression zones between the diagonal internal cross-shocks
showing stronger toroidal field regions. In comparison with equivalent
low-relativistic jets (Γ ≃ 1.15), which get surrounded by cocoons
with vortical backflows filled by mainly toroidal field, the high
speed jets only demonstrate localized, strong toroidal field zones
within the backflow vortical structures. These structures are ring-like
due to our axisymmetry assumption and may further cascade to a smaller
scale in 3D. We find evidence of a more poloidal, straight field layer,
compressed between jet beam and backflows. This layer decreases the
destabilizing influence of the backflow on the jet beam. In all cases,
the jet beam contains rich cross-shock patterns, across which part
of the kinetic energy gets transfered. For the high-speed reference
jet considered here, significant jet deceleration only occurs beyond
distances exceeding O(100 R_j), as the axial flow can reaccelerate
downstream to the internal cross shocks. This reacceleration is
magnetically aided by field compression across the internal shocks
that pinch the flow.

---------------------------------------------------------
Title: Relativistic hydrodynamic simulation of jet deceleration in GRB
Authors: Meliani, Z.; Keppens, R.; Casse, F.
2008AIPC.1000..452M Altcode:
Using the novel adaptive mesh refinement code, AMRVAC, we investigate
the interaction between collimated ejecta (jetlike fireball models with
various opening angle) with its surrounding cold Interstellar Medium
(ISM). This is relevant for Gamma Ray Bursts, and we demonstrate that,
thanks to the AMR strategy, we resolve the internal structure of the
shocked shell-ISM matter. We determine the deceleration from an initial
Lorentz factor γ = 100 up to the almost Newtonian γ~O(3) phase of
the flow. We discuss the effect of varying the opening angle on the
deceleration, and pay attention to differences with their 1D isotropic
GRB equivalents. These are due to thermally induced sideways expansions
of both shocked shell and shocked ISM regions. The propagating 2D
ultrarelativistic shell does not accrete all the surrounding medium
located within its initial opening angle. The difference with isotropic
GRB models is quite pronounced for shells with small opening angle. In
the most collimated ejecta (open angle of 1 °), the deceleration
phase (once the reverse shock has traversed the shell structure) shows
distinct modulation, attributed to repeated rarefactions traversing the
shell. These may have a clear impact on the emitted afterglow radiation.

---------------------------------------------------------
Title: On the Properties of Low-β Magnetohydrodynamic Waves in
Curved Coronal Fields
Authors: Terradas, J.; Oliver, R.; Ballester, J. L.; Keppens, R.
2008ApJ...675..875T Altcode:
The solar corona is a complex magnetic environment where several kinds
of waves can propagate. In this work, the properties of fast, Alfvén,
and slow magnetohydrodynamic waves in a simple curved structure are
investigated. We consider the linear regime, i.e., small-amplitude
waves. We study the time evolution of impulsively generated waves in a
coronal arcade by solving the ideal magnetohydrodynamic equations. We
use a numerical code specially designed to solve these equations in
the low-β regime. The results of the simulations are compared with
the eigenmodes of the arcade model. Fast modes propagate nearly
isotropically through the whole arcade and are reflected at the
photosphere, where line-tying conditions are imposed. On the other hand,
Alfvén and slow perturbations are very anisotropic and propagate
along the magnetic field lines. Because of the different physical
properties in different field lines, there is a continuous spectrum
of Alfvén and slow modes. Curvature can have a significant effect
on the properties of the waves. Among other effects, it considerably
changes the frequency of oscillation of the slow modes and enhances
the possible dissipation of the Alfvén modes due to phase mixing.

---------------------------------------------------------
Title: Accretion funnels onto weakly magnetized young stars
Authors: Bessolaz, N.; Zanni, C.; Ferreira, J.; Keppens, R.;
Bouvier, J.
2008A&A...478..155B Altcode: 2007arXiv0712.2921B
Aims: We re-examine the conditions required to steadily deviate
an accretion flow from a circumstellar disc into a magnetospheric
funnel flow onto a slow rotating young forming star. Methods:
New analytical constraints on the formation of accretion funnels flows
due to the presence of a dipolar stellar magnetic field disrupting the
disc are derived. The Versatile Advection Code is used to confirm these
constraints numerically. Axisymmetric MHD simulations are performed,
where a stellar dipole field enters the resistive accretion disc,
whose structure is self-consistently computed. Results: The
analytical criterion derived allows to predict a priori the position
of the truncation radius from a non perturbative accretion disc
model. Accretion funnels are found to be robust features which occur
below the co-rotation radius, where the stellar poloidal magnetic
pressure becomes both at equipartition with the disc thermal pressure
and is comparable to the disc poloidal ram pressure. We confirm the
results of Romanova et al. (2002, ApJ, 578, 420) and find accretion
funnels for stellar dipole fields as low as 140 G in the low accretion
rate limit of 10-9 M_⊙ yr-1. With our present
numerical setup with no disc magnetic field, we found no evidence
of winds, neither disc driven nor X-winds, and the star is only
spun up by its interaction with the disc. Conclusions: Weak
dipole fields, similar in magnitude to those observed, lead to the
development of accretion funnel flows in weakly accreting T Tauri
stars. However, the higher accretion observed for most T Tauri stars
(dot M 10-8 M_⊙ yr-1) requires either larger
stellar field strength and/or different magnetic topologies to allow
for magnetospheric accretion.

---------------------------------------------------------
Title: Evolution of magnetic fields and cosmic ray acceleration in
supernova remnants
Authors: Schure, Klara; Vink, Jacco; Achterberg, Bram; Keppens, Rony
2008cosp...37.2791S Altcode: 2008cosp.meet.2791S
Observations show that the magnetic field in young supernova remnants
(SNRs) is significantly stronger than can be expected from compression
of the circumstellar medium (CSM) by a factor four in strong blast
waves. Additionally, the polarization is mainly radial, which is also
contrary to expected compression of the CSM magnetic field. Cosmic
rays (CRs) may help to explain these two observed features. They can
increase the compression ratio to factors well over those of regular
strong shocks, by adding a relativistic plasma component to the
pressure, and by draining the shock of energy when CRs escape from the
region. The higher compression ratio will also allow for the contact
discontinuity that is subject to the Rayleigh-Taylor (R-T) instability
to reach much further out to the forward shock. This could create a
preferred radial polarization of the magnetic field. With an adaptive
mesh refinement MHD code (AMRVAC), we simulate the evolution of SNRs
with three different configurations of the initial CSM magnetic field,
and look at two different equations of state in order to look at the
possible influence of a CR plasma component. The spectrum of CRs can be
simulated using test particles, of which we also show some preliminary
results that agree well with available analytical solutions.

---------------------------------------------------------
Title: Transverse stability of relativistic two-component jets
Authors: Meliani, Z.; Keppens, R.
2007A&A...475..785M Altcode: 2007arXiv0709.3838M
Context: Astrophysical jets from various sources seem to be stratified,
with a fast inner jet and a slower outer jet. As it is likely that
the launching mechanism for each component is different, their
interface will develop differential rotation, while the outer jet
radius represents a second interface where disruptions may occur. Aims: We explore the stability of stratified, rotating, relativistic
two-component jets, in turn embedded in static interstellar medium. Methods: In a grid-adaptive relativistic hydrodynamic simulation
with the AMRVAC (Adaptive Mesh Refinement version of the Versatile
Advection) code, the non-linear azimuthal stability of two-component
relativistic jets is investigated. We simulate until multiple inner
jet rotations have been completed. Results: We find evidence for
the development of an extended shear flow layer between the two jet
components, resulting from the growth of a body mode in the inner jet,
Kelvin-Helmholtz surface modes at their original interface, and their
nonlinear interaction. Both wave modes are excited by acoustic waves
which are reflected between the symmetry axis and the interface of
the two jet components. Their interaction induces the growth of near
stationary, counterrotating vortices at the outer edge of the shear
flow layer. The presence of a heavy external jet allows their further
development be slowed down, and the maintaince of a collimated flow. At
the outer jet boundary, small-scale Rayleigh-Taylor instabilities
develop, without disrupting the jet configuration. Conclusions:
We demonstrate that the cross-section of two-component relativistic
jets, with a heavy, cold outer jet, is non-linearly stable.

---------------------------------------------------------
Title: Numerical simulations of the initiation and the IP evolution
of coronal mass ejections
Authors: Jacobs, C.; Poedts, S.; van der Holst, B.; Dubey, G.;
Keppens, R.
2007AIPC..934..101J Altcode:
We present recent results from numerical simulations of the initiation
and interplanetary (IP) evolution of Coronal Mass Ejections (CMEs)
in the framework of ideal magnetohydrodynamics (MHD). As a first step,
the magnetic field in the lower corona and the background solar wind
are reconstructed. Both simple, axisymmetric (2.5D) solar wind models
for the quiet sun as more complicated 3D solar wind models taking
into account the actual coronal field through magnetogram data are
reconstructed. In a second step, fast CME events are mimicked by
superposing high-density plasma blobs on the background wind and
launching them in a given direction at a certain speed. In this way,
the evolution of the CME can be modeled and its effects on the coronal
field and background solar wind studied. In addition, more realistic
CME onset models have been developed to investigate the possible role of
magnetic foot point shearing and magnetic flux emergence/disappearence
as triggering mechanisms of the instability. Parameter studies of such
onset models reveal the importance of the background wind model that
is used and of the initiation parameters, such as the amount and the
rate of the magnetic flux emergence or the region and the amount of
foot point shearing.

---------------------------------------------------------
Title: PHOENIX: MHD spectral code for rotating laboratory and
gravitating astrophysical plasmas
Authors: Blokland, J. W. S.; van der Holst, B.; Keppens, R.; Goedbloed,
J. P.
2007JCoPh.226..509B Altcode:
The new PHOENIX code is discussed together with a sample of many
new results that are obtained concerning magnetohydrodynamic
(MHD) spectra of axisymmetric plasmas where flow and gravity are
consistently taken into account. PHOENIX, developed from the CASTOR code
[W. Kerner, J.P. Goedbloed, G.T.A. Huysmans, S. Poedts, E. Schwarz,
J. Comput. Phys. 142 (1998) 271], incorporates purely toroidal, or both
toroidal and poloidal flow and external gravitational fields to compute
the entire ideal or resistive MHD spectrum for general tokamak or
accretion disk configurations. These equilibria are computed by means of
FINESSE [A.J.C. Beliën, M.A. Botchev, J.P. Goedbloed, B. van der Holst,
R. Keppens, J. Comp. Physics 182 (2002) 91], which discriminates between
the different elliptic flow regimes that may occur. PHOENIX makes use
of a finite element method in combination with a spectral method for
the discretization. This leads to a large generalized eigenvalue
problem, which is solved by means of Jacobi-Davidson algorithm
[G.L.G. Sleijpen, H.A. van der Vorst, SIAM J. Matrix Anal. Appl. 17
(1996) 401]. PHOENIX is compared with CASTOR, PEST-1 and ERATO for
an internal mode of Soloviev equilibria. Furthermore, the resistive
internal kink mode has been computed to demonstrate that the code can
accurately handle small values for the resistivity. A new reference
test case for a Soloviev-like equilibrium with toroidal flow shows that,
on a particular unstable mode, the flow has a quantifiable stabilizing
effect regardless of the direction of the flow. PHOENIX reproduces
the Toroidal Flow induced Alfvén Eigenmode (TFAE, [B. van der Holst,
A.J.C. Beliën, J.P. Goedbloed, Phys. Rev. Lett. 84 (2000) 2865]) where
finite resistivity in combination with equilibrium flow effects causes
resonant damping. Localized ideal gap modes are presented for tokamak
plasmas with toroidal and poloidal flow. Finally, we demonstrate the
ability to spectrally diagnose magnetized accretion disk equilibria
where gravity acts together with either purely toroidal flow or both
toroidal and poloidal flow. These cases show that the MHD continua
can be unstable or overstable due to the presence of a gravitational
field together with equilibrium flow-driven dynamics [J.P. Goedbloed,
A.J.C. Beliën, B. van der Holst, R. Keppens, Phys. Plasmas 11
(2004) 28].

---------------------------------------------------------
Title: Magnetohydrostatic Solar Prominences in Near-Potential Coronal
Magnetic Fields
Authors: Petrie, G. J. D.; Blokland, J. W. S.; Keppens, R.
2007ApJ...665..830P Altcode: 2007arXiv0704.3956P
We present numerical magnetohydrostatic solutions describing the
gravitationally stratified, bulk equilibrium of cool, dense prominence
plasma embedded in a near-potential coronal field. These solutions
are calculated using the FINESSE magnetohydrodynamic equilibrium
solver and describe the morphologies of magnetic field distributions
in and around prominences and the cool prominence plasma that these
fields support. The equilibrium condition for this class of problem is
usually different in distinct subdomains separated by free boundaries,
across which solutions are matched by suitable continuity or jump
conditions describing force balance. We employ our precise finite
element elliptic solver to calculate solutions not accessible by
previous analytical techniques with temperature or entropy prescribed
as free functions of the magnetic flux function, including a range of
values of the polytropic index, temperature variations mainly across
magnetic field lines and photospheric field profiles sheared close to
the polarity inversion line. Out of the many examples computed here,
perhaps the most noteworthy is one which reproduces precisely the
three-part structure often encountered in observations: a cool dense
prominence within a cavity/flux rope embedded in a hot corona. The
stability properties of these new equilibria, which may be relevant
to solar eruptions, can be determined in the form of a full resistive
MHD spectrum using a companion hyperbolic stability solver.

---------------------------------------------------------
Title: GRB blastwaves through wind-shaped circumburst media
Authors: Meliani, Z.; Keppens, R.
2007A&A...467L..41M Altcode: 2007arXiv0704.2461M
Context: A significant fraction of progenitors for long gamma-ray bursts
(GRBs) are believed to be massive stars. The investigation of long
GRBs therefore requires modeling the propagation of ultra-relativistic
blastwaves through the circumburst medium surrounding massive stars. We
simulate the expansion of an isotropic, adiabatic relativistic fireball
into the wind-shaped medium around a massive GRB progenitor. The
circumburst medium is composed of a realistically stratified stellar
wind zone up to its termination shock, followed by a region of
shocked wind characterized by a constant density. Aims: We
followed the evolution of the blastwave through all its stages,
including the extremely rapid acceleration up to a Lorentz factor
75 flow, its deceleration by interaction with stellar wind, its
passage of the wind termination shock, until its propagation through
shocked wind. Methods: We used the adaptive mesh refinement
versatile advection code to follow the evolution of the fireball,
from 3.3 s after its initial release up to more than 4.5 days beyond
the burst. Results: We show that the acceleration from purely
thermal to ultra-relativistic kinetic regimes is abrupt and produces
an internally structured blastwave. We resolved the structure of
this ultra-relativistic shell in all stages, thanks to the adaptive
mesh. We comment on the dynamical roles played by forward and reverse
shock pairs in the phase of interaction with the free stellar wind and
clearly identify the complex shock-dominated structure created when
the shell crosses the terminal shock. Conclusions: We show that
in our model where the terminal shock is taken relatively close to the
massive star, the phase of self-similar deceleration of Blandford-McKee
type can only be produced in the constant-density, shocked wind zone.

---------------------------------------------------------
Title: Numerical Simulations of the Initiation and the IP Evolution
of Coronal Mass Ejections
Authors: Poedts, Stefaan; van der Holst, B.; Jacobs, C.; Chane, E.;
Dubey, G.; Keppens, R.
2007AAS...210.2925P Altcode: 2007BAAS...39..141P
We present recent results from numerical simulations of the initiation
and IP evolution of CMEs in the framework of ideal magnetohydrodynamics
(MHD). As a first step, the magnetic field in the lower corona and the
background solar wind are reconstructed. Both simple, axi-symmetric
(2.5D) solar wind models for the quiet sun as more complicated
3D solar wind models taking into account the actual coronal field
through magnetogram data are reconstructed. In a second step,
2.5D fast CME events are mimicked by superposing high-density plasma
blobs on the background wind and launching them in a given direction
at a certain speed. In this way, the evolution of the CME can be
modeled and its effects on the coronal field and background solar
wind studied. In addition, more realistic CME onset models have
been developed to investigate the possible role of magnetic foot
point shearing and magnetic flux emergence/disppearence as triggering
mechanisms of the instability. Parameter studies of such onset models
reveal the importance of the background wind model that is used and
of the initiation parameters, such as the amount and the rate of the
magnetic flux emergence or the region and the amount of foot point
shearing. Last but not least, a simulation of the evolution of
a 3D CME and its magnetic cloud superposed on a 3D solar wind model
is presented and discussed. In this simulation the CME is mimicked
by superposing a magnetic flux rope on top of a stationary background
solar wind with extra density and velocity added to the flux rope. The
magnetic field inside the initial flux rope is described in terms of
Bessel functions and possesses a high amount of twist. Its effect on
the evolution of the CME is studied.

---------------------------------------------------------
Title: Unstable magnetohydrodynamical continuous spectrum of accretion
disks. A new route to magnetohydrodynamical turbulence in accretion
disks
Authors: Blokland, J. W. S.; Keppens, R.; Goedbloed, J. P.
2007A&A...467...21B Altcode: 2007astro.ph..3581B
Context: We present a detailed study of localised magnetohydrodynamical
(MHD) instabilities occurring in two-dimensional magnetized accretion
disks. Aims: We model axisymmetric MHD disk tori, and solve
the equations governing a two-dimensional magnetized accretion disk
equilibrium and linear wave modes about this equilibrium. We show
the existence of novel MHD instabilities in these two-dimensional
equilibria which do not occur in an accretion disk in the cylindrical
limit. Methods: The disk equilibria are numerically computed by
the FINESSE code. The stability of accretion disks is investigated
analytically as well as numerically. We use the PHOENIX code to
compute all the waves and instabilities accessible to the computed
disk equilibrium. Results: We concentrate on strongly magnetized
disks and sub-Keplerian rotation in a large part of the disk. These disk
equilibria show that the thermal pressure of the disk can only decrease
outwards if there is a strong gravitational potential. Our theoretical
stability analysis shows that convective continuum instabilities
can only appear if the density contours coincide with the poloidal
magnetic flux contours. Our numerical results confirm and complement
this theoretical analysis. Furthermore, these results show that the
influence of gravity can either be stabilizing or destabilizing on
this new kind of MHD instability. In the likely case of a non-constant
density, the height of the disk should exceed a threshold before this
type of instability can play a role. Conclusions: This localised
MHD instability provides an ideal, linear route to MHD turbulence in
strongly magnetized accretion disk tori.

---------------------------------------------------------
Title: AMRVAC and relativistic hydrodynamic simulations for gamma-ray
burst afterglow phases
Authors: Meliani, Zakaria; Keppens, Rony; Casse, Fabien; Giannios,
Dimitrios
2007MNRAS.376.1189M Altcode: 2007astro.ph..1434M; 2007MNRAS.tmp..130M
We apply a novel adaptive mesh refinement (AMR) code, AMRVAC (Adaptive
Mesh Refinement version of the Versatile Advection Code), to numerically
investigate the various evolutionary phases in the interaction of
a relativistic shell with its surrounding cold interstellar medium
(ISM). We do this for both 1D isotropic and full 2D jet-like fireball
models. This is relevant for gamma-ray bursts (GRBs), and we demonstrate
that, thanks to the AMR strategy, we resolve the internal structure
of the shocked shell-ISM matter, which will leave its imprint on the
GRB afterglow. We determine the deceleration from an initial Lorentz
factor γ = 100 up to the almost Newtonian phase of the flow. We present
axisymmetric 2D shell evolutions, with the 2D extent characterized by
their initial opening angle. In such jet-like GRB models, we discuss the
differences with the 1D isotropic GRB equivalents. These are mainly due
to thermally induced sideways expansions of both the shocked shell and
shocked ISM regions. We found that the propagating 2D ultrarelativistic
shell does not accrete all the surrounding medium located within
its initial opening angle. Part of this ISM matter gets pushed away
laterally and forms a wide bow-shock configuration with swirling flow
patterns trailing the thin shell. The resulting shell deceleration
is quite different from that found in isotropic GRB models. As long
as the lateral shell expansion is merely due to ballistic spreading
of the shell, isotropic and 2D models agree perfectly. As thermally
induced expansions eventually lead to significantly higher lateral
speeds, the 2D shell interacts with comparably more ISM matter and
decelerates earlier than its isotropic counterpart.

---------------------------------------------------------
Title: MHD simulations of the magnetic coupling between a young star
and its accretion disk
Authors: Bessolaz, N.; Ferreira, J.; Keppens, R.; Bouvier, J.
2006sf2a.conf..447B Altcode:
The magnetic star-disk interaction is important in the context
of the dynamic evolution of low mass protostars. In particular,
Classical T-Tauri Stars (CTTS) have a puzzling low rotation rate
despite accretion. In such a complex star-disk system, we need to take
into account the stellar and disk magnetic fields with a realistic
accretion disk structure. Magnetohydrodynamic (MHD) simulations are
necessary to support and extend analytical work. First, we briefly
review theoretical models and past numerical work. We discuss the
difficulties to set up initial conditions with a realistic accretion
disk structure, as well as the choice of the boundary conditions at
the star surface to correctly handle angular momentum transport. Then,
we present our 2.5D MHD simulations done with the Versatile Advection
Code (VAC), modified here to handle strong dipole stellar fields
by a splitting strategy for the magnetic field. In this paper, we
only consider the stellar magnetic field and its interaction with the
disk. We confirm the process of poloidal magnetic field expansion when
the disk resistivity is negligible, and identify physical conditions
needed for the formation of accretion columns.

---------------------------------------------------------
Title: Relativistic hydro with AMRVAC and simulation of
ultra-relativistic dynamics
Authors: Meliani, Z.; Keppens, R.; Casse, F.
2006sf2a.conf..167M Altcode:
Our aim is to numerically investigate Gamma Ray Burst (GRB) afterglows
in the context of a fireball model. This requires the accurate
computation of relativistic hydrodynamic flows, with a need for Adaptive
Mesh Refinement (AMR) due to the extreme demands for resolving thin
ultra-relativistic `shells' propagating over vast distances. Here, we
concentrate on the precise propagation evolution of such relativistic
shells in spherical symmetric, as well as axisymmetric 2D models. For this purpose, we extended the AMRVAC software ( te{Keppens03})
with a capability to simulate special relativistic hydro scenarios. We
use a robust second order, shock-capturing discretization in a finite
volume treatment in combination with AMR. On the numerical level, we
can ensure physical consistency between the primitive (ρ, vec{v}, p)
and conservative variables at limited linear reconstruction stages, as
well as at all AMR restriction and prolongation stages. Stringent test
cases of special relativistic hydro shock problems benefit optimally
from our AMR strategy.

---------------------------------------------------------
Title: Kelvin-Helmholtz disruptions in extended magnetized jet flows
Authors: Baty, H.; Keppens, R.
2006A&A...447....9B Altcode:
We numerically investigate the long-term temporal evolution of
magnetized jets where the computational domain covers multiple
wavelengths (up to 10) of the fastest growing Kelvin-Helmholtz
unstable mode. The dynamical importance of the magnetic field, which is
initially uniform and flow-aligned, varies over a significant range:
the plasma β in the jets ranges from {\cal O}(1000) (essentially
hydrodynamical) down to {\cal O}(1) (equipartition jets). Our
calculations of two-dimensional, longitudinally periodic, extended
slab configurations identify an inverse cascade process in the overall
disruption to a broadened and heated jet flow. This process occurs for
transonic and supersonic flows as well, with rapid shock-dominated
transients appearing in supersonic cases, and with characteristic
differences depending on the initial jet width. For configurations
with a jet velocity profile having a radius that is much larger than
the vorticity thickness of the flow, the cascade proceeds early
through pairing/merging of individual mode structures on both jet
boundaries. Jets with radii of the order of the vorticity thickness
are strongly unstable to sinuous deformations with boundary layer-layer
interactions between vortex (transonic, weak magnetic field) and shock
(supersonic, strong field) structures in a few sound crossing times. We
back up these findings for planar jets with selected three-dimensional
simulations of extended cylindrical jet configurations. These tend to
have more small-scale fluctuations in their relaxed endstates. The
timescales and overall scenario for the helical disruptions agree
well with the 2D studies. This allows us to discuss the possible
implications of our results in the context of magnetohydrodynamic
stability of astrophysical jets.

---------------------------------------------------------
Title: Magneto-rotational overstability in accretion disks
Authors: Blokland, J. W. S.; van der Swaluw, E.; Keppens, R.;
Goedbloed, J. P.
2005A&A...444..337B Altcode: 2005astro.ph..4381B
We present analytical and numerical studies of magnetorotational
instabilities occuring in magnetized accretion disks. These calculations
are performed for general radially stratified disks in the cylindrical
limit. We elaborate on earlier analytical results and confirm and
expand them with numerical computations of unstable eigenmodes of the
full set of linearised compressible MHD equations. We compare these
solutions with those found from approximate local dispersion equations
from WKB analysis. In particular, we investigate the influence of a
nonvanishing toroidal magnetic field component on the growth rate and
oscillation frequency of magnetorotational instabilities in Keplerian
disks. These calculations are performed for a constant axial magnetic
field strength. We find the persistence of these instabilities in
accretion disks close to equipartition. Our calculations show that
these eigenmodes become overstable (complex eigenvalue), due to the
presence of a toroidal magnetic field component, while their growth
rate reduces slightly. Furthermore, we demonstrate the presence of
magneto-rotational overstabilities in weakly magnetized sub-Keplerian
rotating disks. We show that the growth rate scales with the rotation
frequency of the disk. These eigenmodes also have a nonzero oscillation
frequency, due to the presence of the dominant toroidal magnetic field
component. The overstable character of the MRI increases as the rotation
frequency of the disk decreases.

---------------------------------------------------------
Title: Convective magneto-rotational instabilities in accretion disks
Authors: van der Swaluw, E.; Blokland, J. W. S.; Keppens, R.
2005A&A...444..347V Altcode: 2005astro.ph..4386V
We present a study of instabilities occuring in thick magnetized
accretion disks. We calculate the growth rates of these instabilities
and characterise precisely the contribution of the magneto-rotational
and convective mechanism. All our calculations are performed in
radially stratified disks in the cylindrical limit. The numerical
calculations are performed using the appropriate local dispersion
equation solver discussed in Blokland et al. (2005, A&amp;A, 444,
337). A comparison with recent results by Narayan et al. (2002, ApJ,
577, 295) shows excellent agreement with their approximate growth rates
only if the disks are weakly magnetized. However, for disks close to
equipartition, the dispersion equation from Narayan et al. (2002) loses
its validity. Our calculations allow for quantitative determination of
the increase in growth rate due to the magneto-rotational mechanism. We
find that the increase of the growth rate for long wavelength convective
modes caused by this mechanism is almost neglible. On the other hand,
the growth rate of short wavelength instabilities can be significantly
increased by this mechanism, reaching values up to 60%.

---------------------------------------------------------
Title: Grid-Adaptive Computations of Magnetized Jets
Authors: Keppens, Rony; Baty, Hubert; Casse, Fabien
2005SSRv..121...65K Altcode:
We present grid-adaptive numerical simulations of magnetized plasma
jets, modeled by means of the compressible magnetohydrodynamic
equations. The Adaptive Mesh Refinement strategy makes it possible
to investigate long-term jet dynamics where both large-scale and
small-scale effects are at play. We extend recent findings for uniformly
magnetized, periodic shear layers to planar and fully 3D extended
jet segments. The jet lengths cover multiple, typically 10, axial
wavelengths of the fastest growing Kelvin Helmholtz (KH) like modes. The
dominant linear MHD instabilities of the jet flows are quantified by
means of MHD spectroscopic analysis. In cases characterized by sonic
Mach numbers about unity and large plasma beta values, both single and
double shear layers (planar jets) manifest self-organizing trends to
large scales, e.g. by continuous pairing/merging between co-rotating
vortices, simultaneously with the introduction of small-scale features
by magnetic reconnection events. The vortices form as a result of KH
unstable shear-flow layers, and their coalescence arises from the
growth of subharmonic modes at multiple wavelengths of the fastest
growing KH instability. In extended two-dimensional jet segments,
we investigate how varying jet width alters this coalescence process
occurring at both edges, e.g. by introducing Batchelor-like coupling
between counter-rotating vortices formed at opposing weakly magnetized,
close shear layers. Finally, periodic segments of supersonic magnetized
jets are simulated in two- and three-dimensional cases, which are
characterized by violent shock-dominated transients.

---------------------------------------------------------
Title: Forward modeling of coronal funnels
Authors: Aiouaz, T.; Peter, H.; Keppens, R.
2005A&A...442L..35A Altcode:
We propose a forward modeling approach of coronal funnels to investigate
the outer layers of the solar atmosphere with respect to their
thermodynamical properties and resulting emission line spectra. We
investigate the plasma flow out of funnels with a new 2D MHD time
dependent model including the solar atmosphere all the way from
the chromosphere to the corona. The plasma in the funnel is treated
in the single-fluid MHD approximation including radiative losses,
anisotropic thermal conduction, and two different parameterized heating
functions. We obtain plasma properties (e.g. density, temperature
and flow speed) within the funnel for each heating function. From
the results of the MHD calculation we derive spectral profiles of a
low corona emission line (Ne VIII, 770 Å). This allows us e.g. to
study the Doppler shifts across the funnel. These results indicate a
systematic variation of the Doppler shifts in lines formed in the low
corona depending on the heating function used. The line shift above
the magnetic field concentration in the network is stronger than in the
inter-network in both cases. However, for one of the heating functions,
the maximum blue-shift (outflow) is not to be found in the very center
of the funnel but in the vicinity of the center. This is not the case
of the second heating function where the maximum is well aligned with
the centre of the funnel. This model directly relates for the first
time the form of the heating function to the thermodynamic and spectral
properties of the plasma in a funnel.

---------------------------------------------------------
Title: MHD Spectroscopy of Transonic Flows
Authors: Goedbloed, Hans; Keppens, Rony
2005SSRv..121...55G Altcode:
In previous publications (Keppens et al.: 2002, Astrophys. J. 569, L121;
Goedbloed et al.: 2004a, Phys. Plasmas 11, 28), we have demonstrated
that stationary rotation of magnetized plasma about a compact central
object permits an enormous number of different MHD instabilities,
with the well-known magneto-rotational instability (Velikhov, E. P.:
1959, Soviet Phys. JETP Lett. 36, 995; Chandrasekhar, S.: 1960,
Proc. Natl. Acad. Sci. U.S.A. 46, 253; Balbus, S. A. and Hawley,
J. F.: 1991, Astrophys. J. 376, 214) as just one of them. We here
concentrate on the new instabilities found that are driven by transonic
transitions of the poloidal flow. A particularly promising class of
instabilities, from the point of view of MHD turbulence in accretion
disks, is the class of trans-slow Alfv’en continuum modes, that
occur when the poloidal flow exceeds a critical value of the slow
magnetosonic speed. When this happens, virtually every magnetic/flow
surface of the disk becomes unstable with respect to highly localized
modes of the continuous spectrum. The mode structures rotate, in turn,
about the rotating disk. These structures lock and become explosively
unstable when the mass of the central object is increased beyond a
certain critical value. Their growth rates then become huge, of the
order of the Alfv’en transit time. These instabilities appear to
have all requisite properties to facilitate accretion flows across
magnetic surfaces and jet formation.

---------------------------------------------------------
Title: Relation of the Chromospheric Network to Coronal Funnels and
the Solar Wind
Authors: Aiouaz, T.; Peter, H.; Keppens, R.
2005ESASP.592..135A Altcode: 2005ESASP.592E..20A; 2005soho...16E..20A
No abstract at ADS

---------------------------------------------------------
Title: Transonic instabilities in accretion disks
Authors: Goedbloed, J. P.; Keppens, R.
2005AIPC..784..639G Altcode:
In two previous publications, we have demonstrated that stationary
rotation of magnetized plasma about a compact central object
permits an enormous number of different MHD instabilities, with the
well-known magneto-rotational instability as just one of them. We
here concentrate on the new instabilities found that are driven by
transonic transitions of the poloidal flow. A particularly promising
class of instabilities, from the point of view of MHD turbulence in
accretion disks, is the class of trans-slow Alfvén continuum modes,
that occur when the poloidal flow exceeds a critical value of the slow
magnetosonic speed. When this happens, virtually every magnetic/flow
surface of the disk becomes unstable with respect to highly localized
modes of the continuous spectrum. The mode structures rotate, in turn,
about the rotating disk. These structures lock and become explosively
unstable when the mass of the central object is increased beyond a
certain critical value. Their growth rates then become huge, of the
order of the Alfvén transit time. These instabilities appear to have
all requisite properties to facilitate accretion flows across magnetic
surfaces and jet formation.

---------------------------------------------------------
Title: Extrapolation of a nonlinear force-free field containing a
highly twisted magnetic loop
Authors: Valori, G.; Kliem, B.; Keppens, R.
2005A&A...433..335V Altcode:
The stress-and-relax method for the extrapolation of nonlinear
force-free coronal magnetic fields from photospheric vector
magnetograms is formulated and implemented in a manner analogous to
the evolutionary extrapolation method. The technique is applied to a
numerically constructed force-free equilibrium that has a simple bipolar
structure of the normal field component in the bottom (magnetogram)
plane but contains a highly twisted loop and a shear (current) layer,
with a smooth but strong variation of the force-free parameter α in
the magnetogram. A standard linear force-free extrapolation of this
magnetogram, using the so-called α_best value, is found to fail
in reproducing the twisted loop (or flux rope) and the shear layer;
it yields a loop pair instead and the shear is not concentrated in a
layer. With the nonlinear extrapolation technique, the given equilibrium
is readily reconstructed to a high degree of accuracy if the magnetogram
is sufficiently resolved. A parametric study quantifies the requirements
on the resolution for a successful nonlinear extrapolation. Permitting
magnetic reconnection by a controlled use of resistivity improved the
extrapolation at a resolution comparable to the smallest structures
in the magnetogram.

---------------------------------------------------------
Title: Forward Modelling of Coronal Funnels
Authors: Aiouaz, T.; Peter, H.; Keppens, R.
2004ESASP.575..337A Altcode: 2004soho...15..337A
No abstract at ADS

---------------------------------------------------------
Title: Transonic instabilities in accretion disks
Authors: Goedbloed, Hans; Keppens, Rony
2004physics..11180G Altcode:
In two previous publications$^{1,2}$, we have demonstrated that
stationary rotation of magnetized plasma about a compact central object
permits an enormous number of different MHD instabilities, with the
well-known magneto-rotational instability as just one of them. We here
concentrate on the new instabilities found that are driven by transonic
transitions of the poloidal flow. A particularly promising class of
instabilities, from the point of view of MHD turbulence in accretion
disks, is the class of {\em trans-slow Alfven continuum modes}, that
occur when the poloidal flow exceeds a critical value of the slow
magnetosonic speed. When this happens, virtually every magnetic/flow
surface of the disk becomes unstable with respect to highly localized
modes of the continuous spectrum. The mode structures rotate, in turn,
about the rotating disk. These structure lock and become explosively
unstable when the mass of the central object is increased beyond a
certain critical value. Their growth rates then become huge, of the
order of the Alfven transit time. These instabilities appear to have
all requisite properties to facilitate accretion flows across magnetic
surfaces and jet formation.[1] R. Keppens, F. Casse, J.P. Goedbloed,
"Waves and instabilities in accretion disks: Magnetohydrodynamic
spectroscopic analysis", Astrophys. J. {\bf 569}, L121--L126 (2002).[2]
J.P. Goedbloed, A.J.C. Belien, B. van der Holst, R. Keppens, "Unstable
continuous spectra of transonic axisymmetric plasmas", Phys. Plasmas
{\bf 11}, 28--54 (2004).

---------------------------------------------------------
Title: How Can Jets Survive MHD Instabilities?
Authors: Baty, Hubert; Keppens, Rony; Comte, Pierre
2004Ap&SS.293..131B Altcode:
We present the main findings of two recent studies using high-resolution
MHD simulations of supersonic magnetized shear flow layers. First,
a strong large-scale coalescence effect partially countered by
small-scale reconnection events is shown to dominate the dynamics
in a two-dimensional layer subject to Kelvin-Helmholtz (KH)
instabilities. Second, an interaction mechanism between two different
types of instabilities (KH and current-driven modes) is shown to occur
in a cylindrical jet configuration embedded in an helical magnetic
field. Finally, we discuss the implications of these results for
astrophysical jets survival.

---------------------------------------------------------
Title: Simulating Magnetized Jets
Authors: Keppens, Rony; Baty, Hubert; Bergmans, Jeroen; Casse, Fabien
2004Ap&SS.293..217K Altcode:
A suitable model for the macroscopic behavior of accretion disk-jet
systems is provided by the equations of MagnetoHydroDynamics
(MHD). These equations allow us to perform scale-encompassing numerical
simulations of multidimensional nonlinear magnetized plasma flows. For
that purpose, we continue the development and exploitation of the
Versatile Advection Code (VAC) along with its recent extension which
employs dynamically controlled grid adaptation. In the adaptive mesh
refinement AMRVAC code, modules for simulating any-dimensional special
relativistic hydro- and magnetohydrodynamic problems are currently
operational.

---------------------------------------------------------
Title: Transsonic instabilities in tokamaks and astrophysical
accretion flows
Authors: Goedbloed, J. P. (Hans); Beliën, A. J. C.; van der Holst,
B.; Keppens, R.
2004AIPC..703...42G Altcode:
Waves and instabilities of transonically rotating toroidal
plasmas present a very complex problem of interest for the two
unrelated fields of magnetically-dominated laboratory plasmas and
gravitationally-dominated astrophysical plasmas. The complexity
originates from the transonic transitions of the poloidal flow which
causes the character of the rotating equilibrium states to change
dramatically, from elliptic to hyperbolic or vice versa, when the
poloidal velocity surpasses certain critical speeds. Associated with
these transitions the different types of magnetohydrodynamic (MHD)
shocks may appear. Obviously, at such transitions the possible waves
and instabilities of the system also change dramatically. We have
investigated these changes for the two mentioned physical systems,
starting from the point of view that the continuous spectrum of
magnetohydrodynamics presents the best organizing principle for the
structure of the complete spectrum since it is the most robust part of
it. We found a new class of local MHD instabilities, that we called
trans-slow Alfvén continuum modes, which are due to poloidal flows
exceeding the critical slow magnetosonic speed. They operate both in
laboratory plasmas (tokamaks), in the absence of gravitational effects,
and in astrophysical plasmas (accretion tori), when the gravitational
field of a compact object dominates the flow. They become extremely
violent when the mass of the central object is large, providing a new
route to MHD turbulence in plasmas rotating about a massive central
object.

---------------------------------------------------------
Title: Grid-adaptive computations for magnetized astrophysical plasmas
Authors: Keppens, R.; Bergmans, J.; Baty, H.
2004MSAIS...4...61K Altcode:
Magnetized plasma dynamics is of central importance in a great
variety of astrophysical phenomena, and poses particular challenges
to computational studies. We review the development history of the
Versatile Advection Code, a software package designed for simulating
magnetohydrodynamic processes, and discuss its current extension to
grid-adaptive simulations. Adaptive mesh refinement is essential to
capture plasma flow details which play a role in long-term dynamical
evolutions. A specific example is given for magnetized shear flow
layers, where large-scale coalescence effects go hand-in-hand
with small-scale magnetic field reconnections. Grid-adaptivity is
also a prerequisite for accurately handling relativistic hydro- and
magnetohydrodynamic flow problems. Examples of the latter are presented
with an outlook to ongoing astrophysically relevant applications.

---------------------------------------------------------
Title: Dynamics and Properties of Coronal Funnels
Authors: Aiouaz, T.; Peter, H.; Lemaire, P.; Keppens, R.
2004ESASP.547..375A Altcode: 2004soho...13..375A
Coronal funnels are open magnetic structures connecting the chromosphere
with the solar corona [5, 3]. We investigate the stationary plasma
flow out of funnels with a 2D- MHD model. The funnel area function is
derived from a magnetic field model and the funnel is approximately 10
Mm high and 20 Mm wide. The energy balance includes radiative losses,
thermal conduction, and a parametrized heating function. We adjust the
parameters to the quantities measured in the lower solar corona. We
obtained 2D plasma properties (e.g. density, temperature, flow speed,
etc.) within the funnel. From the results of the MHD calculation we
synthesize emision profiles of various lines formed in the transition
region from the chromosphere to the corona. This allows us to study
e.g. the Doppler shifts at various temperatures across the funnel
and thus enables a detailed comparison of the model results with
observations. For this we investigate SUMER data and study Doppler
shifts perpendicular to the chromospheric network for different emission
lines, where a tessalation technique is used to derive the outlines of
the chromospheric network. In this paper typical results are presented
for the Ne VIII(770.4 Å) line. Preliminary results show that these
model caclulations compare well to the observations.

---------------------------------------------------------
Title: Radiatively Inefficient Magnetohydrodynamic Accretion-Ejection
Structures
Authors: Casse, Fabien; Keppens, Rony
2004ApJ...601...90C Altcode: 2003astro.ph.10322C
We present magnetohydrodynamic simulations of a resistive accretion
disk continuously launching transmagnetosonic, collimated jets. We
time-evolve the full set of magnetohydrodynamic equations but neglect
radiative losses in the energetics (radiatively inefficient). Our
calculations demonstrate that a jet is self-consistently produced
by the interaction of an accretion disk with an open, initially
bent large-scale magnetic field. A constant fraction of heated disk
material is launched in the inner equipartition disk regions, leading
to the formation of a hot corona and a bright collimated, superfast
magnetosonic jet. We illustrate the complete dynamics of the “hot”
near-steady state outflow (where thermal pressure~=magnetic pressure)
by showing force balance, energy budget, and current circuits. The
evolution to this near-stationary state is analyzed in terms of the
temporal variation of energy fluxes controlling the energetics of the
accretion disk. We find that unlike advection-dominated accretion flow,
the energy released by accretion is mainly sent into the jet rather
than transformed into disk enthalpy. These magnetized, radiatively
inefficient accretion-ejection structures can account for underluminous
thin disks supporting bright fast collimated jets as seen in many
systems displaying jets (for instance, M87).

---------------------------------------------------------
Title: The two-dimensional magnetohydrodynamic Kelvin-Helmholtz
instability: Compressibility and large-scale coalescence effects
Authors: Baty, H.; Keppens, R.; Comte, P.
2003PhPl...10.4661B Altcode: 2004astro.ph..3125B
The Kelvin-Helmholtz (KH) instability occurring in a single shear flow
configuration that is embedded in a uniform flow-aligned magnetic
field, is revisited by means of high resolution two-dimensional
magnetohydrodynamic simulations. First, the calculations extend
previous studies of magnetized shear flows to a higher compressibility
regime. The nonlinear evolution of an isolated KH billow emerging from
the fastest growing linear mode for a convective sonic Mach number
Mcs=0.7 layer is in many respects similar to its less compressible
counterpart (Mach Mcs=0.5). In particular, the disruptive regime
where locally amplified, initially weak magnetic fields, control
the nonlinear saturation process is found for Alfvén Mach numbers
4&lt;~MA&lt;~30. The most notable difference between
Mcs=0.7 vs Mcs=0.5 layers is that higher density contrasts and fast
magnetosonic shocklet structures are observed. Second, the use of
adaptive mesh refinement allows to parametrically explore much larger
computational domains, including up to 22 wavelengths of the linearly
dominant mode. A strong process of large-scale coalescence is found,
whatever the magnetic field regime. It proceeds through continuous
pairing/merging events between adjacent vortices up to the point
where the final large-scale vortical structure reaches the domain
dimensions. This pairing/merging process is attributed to the growth of
subharmonic modes and is mainly controlled by relative phase differences
between them. These grid-adaptive simulations demonstrate that even in
very weak magnetic field regimes (MA~=30), the large-scale
KH coalescence process can trigger tearing-type reconnection events
previously identified in cospatial current-vortex sheets.

---------------------------------------------------------
Title: Three-dimensional magnetohydrodynamic simulations of in situ
shock formation in the coronal streamer belt
Authors: Zaliznyak, Yu.; Keppens, R.; Goedbloed, J. P.
2003PhPl...10.4478Z Altcode: 2004astro.ph..3122Z
A numerical study of an idealized magnetohydrodynamic (MHD)
configuration consisting of a planar wake flow embedded into a
three-dimensional (3D) sheared magnetic field is presented. The
simulations investigate the possibility for in situ development
of large-scale compressive disturbances at cospatial current
sheet-velocity shear regions in the heliosphere. Using a linear
MHD solver, the systematical investigation of the destabilized
wavenumbers, corresponding growth rates, and physical parameter
ranges for dominant 3D sinuous-type instabilities in an equilibrium
wake-current sheet system was done. Wakes bounded by sufficiently
supersonic (Mach number Ms&gt;2.6) flow streams are found
to support dominant fully 3D sinuous instabilities when the plasma
beta is of order unity. Fully nonlinear, compressible 2.5D and 3D
MHD simulations show the self-consistent formation of shock fronts
of fast magnetosonic type. They carry density perturbations far away
from the wake's center. Shock formation conditions are identified
in sonic and Alfvénic Mach number parameter space. Depending on the
wake velocity contrast and magnetic field magnitude, as well as on the
initial perturbation, the emerging shock patterns can be plane-parallel
as well as fully three-dimensionally structured. Similar large-scale
transients could therefore originate at distances far above coronal
helmet streamers or at the location of the ecliptic current sheet.

---------------------------------------------------------
Title: Simulation of shock waves in the interplanetary medium
Authors: Poedts, S.; van der Holst, B.; Chattopadhyay, I.; Banerjee,
D.; van Lier, T.; Keppens, R.
2003ESASP.535..603P Altcode: 2003iscs.symp..603P
The shocks in the solar corona and interplanetary (IP) space caused
by fast Coronal Mass Ejections (CMEs) are simulated numerically
and their structure and evolution is studied in the framework of
magnetohydrodynamics (MHD). Due to the presence of three characteristic
velocities and the anisotropy induced by the magnetic field, CME
shocks generated in the lower corona can have a complex structure
including secondary shock fronts, over-compressive and compound
shocks, etc. The evolution of these CME shocks is followed during
their propagation through the solar wind and, in particular, through
the critical points in the wind. Particular attention is given to
complex IP events involving two CME shocks colliding to each other,
as often observed. The CME shocks are important for "space weather"
because they can easily be observed in radio wavelengths. This makes
it possible to track the position of the CMEs/magnetic clouds and,
hence, to follow their propagation through the corona.

---------------------------------------------------------
Title: Dynamics and Properties of Coronal Funnels
Authors: Aiouaz, T.; Peter, H.; Lemaire, Philippe; Keppens, Rony
2003ANS...324....7A Altcode: 2003ANS...324..B01A
No abstract at ADS

---------------------------------------------------------
Title: Adaptive Mesh Refinement for conservative systems:
multi-dimensional efficiency evaluation
Authors: Keppens, R.; Nool, M.; Tóth, G.; Goedbloed, J. P.
2003CoPhC.153..317K Altcode: 2004astro.ph..3124K
Obtainable computational efficiency is evaluated when using an Adaptive
Mesh Refinement (AMR) strategy in time accurate simulations governed
by sets of conservation laws. For a variety of 1D, 2D, and 3D hydro-
and magnetohydrodynamic simulations, AMR is used in combination with
several shock-capturing, conservative discretization schemes. Solution
accuracy and execution times are compared with static grid simulations
at the corresponding high resolution and time spent on AMR overhead
is reported. Our examples reach corresponding efficiencies of 5
to 20 in multi-dimensional calculations and only 1.5-8% overhead is
observed. For AMR calculations of multi-dimensional magnetohydrodynamic
problems, several strategies for controlling the ∇.B=0 constraint
are examined. Three source term approaches suitable for cell-centered B
representations are shown to be effective. For 2D and 3D calculations
where a transition to a more globally turbulent state takes place, it
is advocated to use an approximate Riemann solver based discretization
at the highest allowed level(s), in combination with the robust
Total Variation Diminishing Lax-Friedrichs method on the coarser
levels. This level-dependent use of the spatial discretization acts
as a computationally efficient, hybrid scheme.

---------------------------------------------------------
Title: Computer simulations of solar plasmas
Authors: Goedbloed, J. P.; Keppens, R.; Poedts, S.
2003SSRv..107...63G Altcode:
Plasma dynamics has been investigated intensively for toroidal
magnetic confinement in tokamaks with the aim to develop a controlled
thermonuclear energy source. On the other hand, it is known that
more than 90% of visible matter in the universe consists of plasma,
so that the discipline of plasma-astrophysics has an enormous
scope. Magnetohydrodynamics (MHD) provides a common theoretical
description of these two research areas where the hugely different
scales do not play a role. It describes the interaction of electrically
conducting fluids with magnetic fields that are, in turn, produced by
the dynamics of the plasma itself. Since this theory is scale invariant
with respect to lengths, times, and magnetic field strengths, for
the nonlinear dynamics it makes no difference whether tokamaks, solar
coronal magnetic loops, magnetospheres of neutron stars, or galactic
plasmas are described. Important is the magnetic geometry determined
by the magnetic field lines lying on magnetic surfaces where also the
flows are concentrated. Yet, transfer of methods and results obtained
in tokamak research to solar coronal plasma dynamics immediately
runs into severe problems with trans‘sonic’ (surpassing any one
of the three critical MHD speeds) stationary flows. For those flows,
the standard paradigm for the analysis of waves and instabilities,
viz. a split of the dynamics in equilibrium and perturbations, appears
to break down. This problem is resolved by a detailed analysis of the
singularities and discontinuities that appear in the trans‘sonic’
transitions, resulting in a unique characterization of the permissible
flow regimes. It then becomes possible to initiate MHD spectroscopy of
axi-symmetric transonic astrophysical plasmas, like accretion disks or
solar magnetic loops, by computing the complete wave and instability
spectra by means of the same methods (with unprecedented accuracy)
exploited for tokamak plasmas. These large-scale linear programs are
executed in tandem with the non-linear (shock-capturing, massively
parallel) Versatile Advection Code to describe both the linear and
the nonlinear phases of the instabilities.

---------------------------------------------------------
Title: Continuous MHD Jet Launching from Resistive Accretion Disk
Authors: Casse, Fabien L.; Keppens, Rony
2003IAUS..221P.127C Altcode:
We present numerical MHD simulations of a magnetized accretion disk
launching super-fastmagnetosonic jets. These axisymmetric simulations
model a time-dependant resistive accretion disk threaded by an initial
vertical magnetic field. The resistivity is only important inside the
disk and is prescribed as an alpha-type law where the alpha coefficient
αm is smaller than unity. We show that the launching of
a collimated outflow occurs self-consistently and the ejection of
matter is continuous and quasi-stationary. These are the first ever
2.5D simulations of resistive accretion disks launching non-transient
ideal MHD jets. This outflow is safely characterized as a jet since
the flow becomes super-fastmagnetosonic well-collimated and reaches
a quasi-stationary state. We present a complete illustration and
explanation of the `accretion-ejection' mechanism that leads to jet
formation from a magnetized accretion disk. In particular the magnetic
torque inside the disk brakes the matter azimuthally and allows for
accretion while it is responsible for an effective magneto-centrifugal
acceleration in the jet. As such the magnetic field channels the disk
angular momentum and powers the jet acceleration and collimation. The
jet originates from the inner disk region where equipartition between
thermal and magnetic forces is achieved.

---------------------------------------------------------
Title: Interaction of high-velocity pulsars with supernova remnant
shells
Authors: van der Swaluw, E.; Achterberg, A.; Gallant, Y. A.; Downes,
T. P.; Keppens, R.
2003A&A...397..913V Altcode: 2002astro.ph..2232V
Hydrodynamical simulations are presented of a pulsar wind emitted
by a supersonically moving pulsar. The pulsar moves through the
interstellar medium or, in the more interesting case, through the
supernova remnant created at its birth event. In both cases there exists
a three-fold structure consisting of the wind termination shock, contact
discontinuity and a bow shock bounding the pulsar wind nebula. Using
hydrodynamical simulations we study the behaviour of the pulsar wind
nebula inside a supernova remnant, and in particular the interaction
with the outer shell of swept up interstellar matter and the blast
wave surrounding the remnant. This interaction occurs when the pulsar
breaks out of the supernova remnant. We assume the remnant is in the
Sedov stage of its evolution. Just before break-through, the Mach number
associated with the pulsar motion equals Mpsr = 7/sqrt {5},
independent of the supernova explosion energy and pulsar velocity. The
bow shock structure is shown to survive this break-through event.

---------------------------------------------------------
Title: Magnetized Accretion-Ejection Structures: 2.5-dimensional
Magnetohydrodynamic Simulations of Continuous Ideal Jet Launching
from Resistive Accretion Disks
Authors: Casse, Fabien; Keppens, Rony
2002ApJ...581..988C Altcode: 2002astro.ph..8459C
We present numerical magnetohydrodynamic (MHD) simulations of a
magnetized accretion disk launching trans-Alfvénic jets. These
simulations, performed in a 2.5-dimensional time-dependent
polytropic resistive MHD framework, model a resistive accretion
disk threaded by an initial vertical magnetic field. The
resistivity is only important inside the disk and is prescribed as
η=αmVAHexp(- 2Z2/H2),
where VA stands for Alfvén speed, H is the disk
scale height, and the coefficient αm is smaller than
unity. By performing the simulations over several tens of dynamical
disk timescales, we show that the launching of a collimated outflow
occurs self-consistently and the ejection of matter is continuous and
quasi-stationary. These are the first ever simulations of resistive
accretion disks launching nontransient ideal MHD jets. Roughly 15%
of accreted mass is persistently ejected. This outflow is safely
characterized as a jet since the flow becomes superfast magnetosonic,
well collimated, and reaches a quasi-stationary state. We present a
complete illustration and explanation of the “accretion-ejection”
mechanism that leads to jet formation from a magnetized accretion
disk. In particular, the magnetic torque inside the disk brakes the
matter azimuthally and allows for accretion, while it is responsible
for an effective magnetocentrifugal acceleration in the jet. As such,
the magnetic field channels the disk angular momentum and powers the
jet acceleration and collimation. The jet originates from the inner
disk region where equipartition between thermal and magnetic forces
is achieved. A hollow, superfast magnetosonic shell of dense material
is the natural outcome of the inward advection of a primordial field.

---------------------------------------------------------
Title: Interplay between Kelvin-Helmholtz and Current-driven
Instabilities in Jets
Authors: Baty, H.; Keppens, R.
2002ApJ...580..800B Altcode:
We investigate, by means of three-dimensional compressible
magnetohydrodynamic numerical simulations, the interaction of
Kelvin-Helmholtz (KH) and current-driven (CD) instabilities in a
magnetized cylindrical jet configuration. The jet has a supersonic
axial flow, sheared in the radial direction, and is embedded in
a helical magnetic field. The strength of the axial magnetic field
component is chosen to be weak, in accord with the “weak field regime”
previously defined by Ryu, Jones, &amp; Frank for uniformly magnetized
configurations. We follow the time evolution of a periodic section
where the jet surface is perturbed at m=+/-1 azimuthal mode numbers. A
m=-1 KH surface mode linearly develops dominating the m=+1 KH one, in
agreement with results obtained using an independent ideal stability
code. This lifted degeneracy, because of the presence of the helical
field, leads nonlinearly to clear morphological differences in the jet
deformation as compared to uniformly magnetized configurations. As
predicted by stability results, a m=-1 CD instability also develops
linearly inside the jet core for configurations having a small enough
magnetic pitch length. As time proceeds, this magnetic mode interacts
with the KH vortical structures and significantly affects the further
nonlinear evolution. The magnetic field deformation induced by the
CD instability provides a stabilizing effect through its azimuthal
component Bθ. This helps to saturate the KH vortices in
the vicinity of the jet surface. Beyond saturation, the subsequent
disruptive effect on the flow is weaker than in cases having similar
uniform and helical magnetic field configurations without the CD
mode. We discuss the implications of this stabilizing mechanism for
the stability of astrophysical jets.

---------------------------------------------------------
Title: Axisymmetric magnetized winds and stellar spin-down
Authors: van der Holst, B.; Banerjee, D.; Keppens, R.; Poedts, S.
2002ESASP.506...75V Altcode: 2002svco.conf...75V; 2002ESPM...10...75V
We present 2.5D stationary solar/stellar wind numerical simulation
results obtained within the magnetohydrodynamic (MHD) model. This is
an extension of earlier work by Keppens &amp; Goedbloed (1999, 2000),
where spherically symmetric, isothermal, unmagnetized, non-rotating
Parker winds were generalized to axisymmetric, polytropic, magnetized,
rotating models containing both a 'wind' and a 'dead' zone. We study
the influence of stellar rotation and coronal magnetic field strength
on the wind acceleration. Since dynamos in cool stars are thought to
operate more efficiently and to produce a stronger coronal magnetic
field with increasing stellar rotation rate, we assume this increase is
linear. We quantify the stellar angular momentum loss via the magnetized
wind with an equatorial dead zone. The obtained spin-down rates are much
smaller than values obtained from Weber-Davis wind estimates. The need
to invoke a dynamo with magnetic field saturation to lower the spin-down
rates for fast rotators is re-evaluated in view of these results.

---------------------------------------------------------
Title: Waves and Instabilities in Accretion Disks: Magnetohydrodynamic
Spectroscopic Analysis
Authors: Keppens, R.; Casse, F.; Goedbloed, J. P.
2002ApJ...569L.121K Altcode: 2002astro.ph..3237K
A complete analytical and numerical treatment of all magnetohydrodynamic
waves and instabilities for radially stratified, magnetized accretion
disks is presented. The instabilities are a possible source of
anomalous transport. While recovering results on known hydrodynamic
and both weak- and strong-field magnetohydrodynamic perturbations,
the full magnetohydrodynamic spectra for a realistic accretion
disk model demonstrate a much richer variety of instabilities
accessible to the plasma than previously realized. We show that both
weakly and strongly magnetized accretion disks are prone to strong
nonaxisymmetric instabilities. The ability to characterize all waves
arising in accretion disks holds great promise for magnetohydrodynamic
spectroscopic analysis.

---------------------------------------------------------
Title: JOSO report 200-2001 - The Netherlands. Solar Physics in
The Netherlands
Authors: Rutten, R.; Keppens, R.; Fleck, B.
2002joso.book...81R Altcode:
Solar physics research in the Netherlands is carried out at Nijmegen,
Utrecht, Nieuwegein, and Noordwijk.

---------------------------------------------------------
Title: Sunspot Pores
Authors: Keppens, R.
2000eaa..bookE2043K Altcode:
Basic properties of pores...

---------------------------------------------------------
Title: Spin and orbital angular momentum exchange in binary star
systems. II. Ascending the giant branch: a new path to FK Comae stars
Authors: Keppens, R.; Solanki, S. K.; Charbonnel, C.
2000A&A...359..552K Altcode:
Using the model by Keppens (1997), we investigate the angular momentum
(AM) evolution in asymmetric binary star systems from Zero-Age Main
Sequence times until at least one component has ascended the giant
branch. We concentrate on stars ranging in mass from 0.9 Msun
- 1.7 Msun, in almost synchronous, short period systems
(P_orb&lt;9 days). We address synchronization and circularization
by tidal interaction, allowing for structural evolution and
stellar winds. A Weber-Davis prescription is used to quantify the
wind influence, thereby accounting for changes in its acceleration
mechanism from the interplay of the evolving thermal-magneto-centrifugal
effects. We identify a scenario for fast in-spiraling components with d
ln P_orb/dt =~ -{cal O}(10-8) which is primarily driven by
fast structural evolution as the heaviest component ascends the giant
branch. This leads to the formation of contact systems, which ultimately
coalesce and form FK Comae-like objects on relatively short timescales
due to the continuing expansion of the primary. The obtained mass loss
rates and orbital period variations d ln P_orb/dt are confronted with
their observed ranges. The predicted mass loss rates agree with the
solar value on the main sequence and with the Reimers relation in the
giant phase. Observations of period evolution in close, active binaries
suggest, however, that other influences than those considered here must
play an important role. Finally, we point out how the mass asymmetry of
the binary system can be a crucial ingredient in the angular momentum
evolution: while the primary dictates the spin-orbital AM exchange
in the system, the slowly evolving lighter component can develop an
efficient magneto-centrifugally driven wind and thereby drain the AM
from the system.

---------------------------------------------------------
Title: Stellar Winds, Dead Zones, and Coronal Mass Ejections
Authors: Keppens, R.; Goedbloed, J. P.
2000ApJ...530.1036K Altcode: 1999astro.ph.10152K
Axisymmetric stellar wind solutions are presented that were
obtained by numerically solving the ideal magnetohydrodynamic (MHD)
equations. Stationary solutions are critically analyzed using
the knowledge of the flux functions. These flux functions enter
in the general variational principle governing all axisymmetric
stationary ideal MHD equilibria. The magnetized wind solutions for
(differentially) rotating stars contain both a “wind” and a “dead”
zone. We illustrate the influence of the magnetic field topology on the
wind acceleration pattern by varying the coronal field strength and the
extent of the dead zone. This is evident from the resulting variations
in the location and appearance of the critical curves for which the wind
speed equals the slow, Alfvén, and fast speed. Larger dead zones cause
effective, fairly isotropic acceleration to super-Alfvénic velocities
as the polar, open field lines are forced to fan out rapidly with
radial distance. A higher field strength moves the Alfvén transition
outward. In the ecliptic, the wind outflow is clearly modulated by
the extent of the dead zone. The combined effect of a fast stellar
rotation and an equatorial dead zone in a bipolar field configuration
can lead to efficient thermocentrifugal equatorial winds. Such winds
show both a strong poleward collimation and some equatorward streamline
bending due to significant toroidal field pressure at midlatitudes. We
discuss how coronal mass ejections are then simulated on top of the
transonic outflows.

---------------------------------------------------------
Title: Stationary and Time-Dependent MHD Simulations of the Solar Wind
Authors: Keppens, R.; Goedbloed, J. P.
1999ESASP.448.1177K Altcode: 1999ESPM....9.1177K; 1999mfsp.conf.1177K
No abstract at ADS

---------------------------------------------------------
Title: Coronal Heating by Resonant Absorption: The Effects of
Chromospheric Coupling
Authors: Beliën, A. J. C.; Martens, P. C. H.; Keppens, R.
1999ApJ...526..478B Altcode:
We present the first 2.5 dimensional numerical model calculations
of the nonlinear wave dynamics and heating by resonant absorption
in coronal loops with thermal structuring of the transition region
and higher chromosphere. The numerical calculations were done with
the Versatile Advection Code. The transition region can move freely
and is transparent for mass motions from chromosphere to corona. The
loops are excited at the chromospheric level by linearly polarized
monochromatic Alfvén waves. We find that the efficiency of resonant
absorption can be much lower than in equivalent line-tied coronal
loop models. The inefficiency is due to the fast rate at which slow
magnetosonic waves are nonlinearly generated in the chromosphere
and transition region. This leads to considerable transfer of energy
from the Alfvén wave to the magnetosonic waves. Consequently, only a
relatively small fraction of the Poynting flux that is injected into
the loop system at the chromospheric level is available at the coronal
level. Cavity leakage and detuning also have a negative impact on the
efficiency, but less so than the nonlinear energy transfer. Inclusion
of radiative and conductive losses improves the efficiency of resonant
absorption. While the efficiency of resonant absorption heating is low,
our results indicate that heating by compression and dissipation of the
slow magnetosonic waves and shocks can easily lead to a temperature rise
of a few percent, and for larger driver amplitudes even to a rise over
10%. Hence, our results support the idea of indirect coronal heating
through the nonlinear generation of magnetosonic waves that was put
forward more than 20 yr ago. Furthermore, the large transition region
and coronal density oscillations that are associated with the slow
magnetosonic waves provide an explanation for some observed coronal
and transition region loop extreme-ultraviolet intensity variations.

---------------------------------------------------------
Title: The Dynamical Influence Of The Transition Region And
Chromosphere On The Heating Of Coronal Loops By Resonant Absorption
Of Alfvén Waves
Authors: Belien, A. J. C.; Martens, P. C. H.; Keppens, R.
1999ESASP.446..167B Altcode: 1999soho....8..167B
We present a numerical MHD study of coronal heating by resonant
absorption of Alfvén waves using models that include an extended
chromosphere and dynamical transition region. The calculations are
done with the Versatile Advection Code (VAC) and assume axisymmetric
loop configurations. Linear polarized, monochromatic Alfvén waves are
launched at the bottom of our extended chromosphere. The efficiency of
heating by resonant absorption of these waves in the corona is measured
by the ratio of Ohmic dissipation over the incoming Poyting flux at
the bottom of our chromosphere (averaged over a driving period). The
efficiency turns out to be much smaller than in loop models that
do not take the chromospheric and transition region coupling into
account. For our model, the efficiency is typically of the order of 10%
in contrast with values over 90% in models without the coupling taken
into account. The difference can be described in terms of efficient
nonlinear generation of compressive motions in the chromosphere
and transition region, the change of the coronal cavity length as a
consequence of the continuous motion of the transition region (due to
the the Alfvén wave pressure and compressive motions), and coronal
cavity leakage due to a finite Alfvén speed ratio between corona
and chromosphere. The compressive waves and motions lead to density
variations that should be observable. To proove that, our model results
are used to simulate some coronal and transition region CDS EUV line
observations as well as broad band EIT observations. The results are
used to give an explanation of EUV coronal brightenings in terms of
mass motions.

---------------------------------------------------------
Title: Wave Heating and Nonlinear Dynamics of Coronal Loops
Authors: Beliën, A. J. C.; Martens, P. C. H.; Keppens, R.; Tóth, G.
1999ASPC..184..248B Altcode:
We present the first results of 2.5D nonlinear magnetohydrodynamic
wave heating simulations of solar coronal loops with inclusion
of the modeling of the coupling to the transition region and
chromosphere. Magnetic flux tubes with fixed lengths are considered
but the coronal extent of the loops as situated in between the two
transition regions can vary dynamically. The numerical simulations
were carried out with the Versatile Advection Code. The loops are
excited with linearly polarized Alfvén waves at the chromospheric
base. The main finding is that resonant absorption is not efficient
since most of the Poynting flux that enters the loop will be used to
support all the nonlinearly generated magnetoacoustic motions and the
corresponding compression of coronal plasma.

---------------------------------------------------------
Title: Compressible Modelling of Slow Solar Wind Formation
Authors: Dahlburg, R. B.; Einaudi, G.; Keppens, R.
1999AAS...194.3204D Altcode: 1999BAAS...31..870D
Recently we have participated in the development of a theory for the
formation of the slow solar wind (Einaudi et al. 1999). The solar wind
is modelled as a wake embedded in a neutral sheet, which models the
effects of a streamer stalk. Plasmoids are formed as the magnetic field
reconnect. These plasmoids are then accelerated as the wake develops
nonlinearly. Good agreement was obtained with LASCO observations. The
previous theory was limited to the incompressible case. We here present
some results from our more recent atudy of both linear and nonlinear
compressible magnetized shear layers. We find that moving density
enhancements are formed, which accelerate up to a speed comparable
to the slow solar wind speed. At large Mach numbers compressible
disturbances can occur, with large variations in the mass density
and temperature.

---------------------------------------------------------
Title: Nonlinear dynamics of Kelvin-Helmholtz unstable magnetized
jets: Three-dimensional effects
Authors: Keppens, R.; Tóth, G.
1999PhPl....6.1461K Altcode: 1999astro.ph..1383K
A numerical study of the Kelvin-Helmholtz instability in compressible
magnetohydrodynamics is presented. The three-dimensional simulations
consider shear flow in a cylindrical jet configuration, embedded in
a uniform magnetic field directed along the jet axis. The growth of
linear perturbations at specified poloidal and axial mode numbers
demonstrate intricate nonlinear coupling effects. The physical
mechanisms leading to induced secondary Kelvin-Helmholtz instabilities
at higher mode numbers are identified. The initially weak magnetic
field becomes locally dominant in the nonlinear dynamics before
and during saturation. Thereby, it controls the jet deformation
and eventual breakup. The results are obtained using the Versatile
Advection Code [G. Tóth, Astrophys. Lett. Commun. 34, 245 (1996)],
a software package designed to solve general systems of conservation
laws. An independent calculation of the same Kelvin-Helmholtz unstable
jet configuration using a three-dimensional pseudospectral code gives
important insights into the coupling and excitation events of the
various linear mode numbers.

---------------------------------------------------------
Title: Numerical simulations of stellar winds: polytropic models
Authors: Keppens, R.; Goedbloed, J. P.
1999A&A...343..251K Altcode: 1999astro.ph..1380K
We discuss steady-state transonic outflows obtained by direct numerical
solution of the hydrodynamic and magnetohydrodynamic equations. We make
use of the Versatile Advection Code, a software package for solving
systems of (hyperbolic) partial differential equations. We proceed
stepwise from a spherically symmetric, isothermal, unmagnetized,
non-rotating Parker wind to arrive at axisymmetric, polytropic,
magnetized, rotating models. These represent 2D generalisations of
the analytical 1D Weber-Davis wind solution, which we obtain in the
process. Axisymmetric wind solutions containing both a `wind' and a
`dead' zone are presented. Since we are solving for steady-state
solutions, we efficiently exploit fully implicit time stepping. The
method allows us to model thermally and/or magneto-centrifugally driven
stellar outflows. We particularly emphasize the boundary conditions
imposed at the stellar surface. For these axisymmetric, steady-state
solutions, we can use the knowledge of the flux functions to verify
the physical correctness of the numerical solutions.

---------------------------------------------------------
Title: Leaky and resonantly damped flux tube modes reconsidered
Authors: Stenuit, H.; Tirry, W. J.; Keppens, R.; Goossens, M.
1999A&A...342..863S Altcode:
In this research note the results for the eigenfrequencies of the
uniform and non-uniform magnetic flux tubes of Stenuit et al. (1998)
are reconsidered. In that paper it is shown that the eigenfrequencies
may have a damping rate due to two mechanisms causing a loss of
energy. In non-uniform flux tubes the eigenmodes can be damped by
resonant absorption. The other mechanism is leakage of wave energy into
the surroundings, which can occur for both uniform and non-uniform
flux tubes. We point out that the dispersion relations obtained by
Stenuit et al. are correct for leaky and undamped non-leaky modes,
but are not correct for resonantly damped non-leaky modes.

---------------------------------------------------------
Title: Growth and saturation of the Kelvin-Helmholtz instability
with parallel and antiparallel magnetic fields
Authors: Keppens, Rony; Tóth, G.; Westermann, R. H. J.; Goedbloed,
J. P.
1999JPlPh..61....1K Altcode: 1999astro.ph..1166K
Available from <A
href="http://journals.cambridge.org/bin/bladerunner?REQUNIQ=1105385252&amp;REQSESS=958582&amp;118000REQEVENT=&amp;REQINT1=18471&amp;REQAUTH=0">http://journals.cambridge.org/bin/bladerunner?REQUNIQ=1105385252&amp;REQSESS=958582&amp;118000REQEVENT=&amp;REQINT1=18471&amp;REQAUTH=0</A>

---------------------------------------------------------
Title: Numerical Simulations of Stellar Winds
Authors: Keppens, R.; Goedbloed, J. P.
1999SSRv...87..223K Altcode:
We discuss steady-state transonic outflows obtained by direct numerical
solution of the hydrodynamic and magnetohydrodynamic equations. We
make use of the Versatile Advection Code, a software package for
solving systems of (hyperbolic) partial differential equations. We
model thermally and magneto-centrifugally driven stellar outflows
as generalizations of the well-known Parker and Weber-Davis wind
solutions. To obtain steady-state solutions efficiently, we exploit
fully implicit time stepping.

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Title: 3D Nonlinear MHD Wave Heating of Coronal LoopsCD
Authors: Poedts, S.; Keppens, R.; Beliën, A. J. C.
1999ASSL..240..319P Altcode: 1999numa.conf..319P
No abstract at ADS

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Title: Implicit and semi-implicit schemes in the Versatile Advection
Code: numerical tests
Authors: Toth, G.; Keppens, R.; Botchev, M. A.
1998A&A...332.1159T Altcode:
We describe and evaluate various implicit and semi-implicit
time integration schemes applied to the numerical simulation of
hydrodynamical and magnetohydrodynamical problems. The schemes were
implemented recently in the software package Versatile Advection
Code, which uses modern shock capturing methods to solve systems of
conservation laws with optional source terms. The main advantage
of implicit solution strategies over explicit time integration is
that the restrictive constraint on the allowed time step can be
(partially) eliminated, thus the computational cost is reduced. The
test problems cover one and two dimensional, steady state and time
accurate computations, and the solutions contain discontinuities. For
each test, we confront explicit with implicit solution strategies.

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Title: Eigenfrequencies and optimal driving frequencies of 1D
non-uniform magnetic flux tubes
Authors: Stenuit, H.; Keppens, R.; Goossens, M.
1998A&A...331..392S Altcode:
The eigenfrequencies and the optimal driving frequencies for flux tubes
embedded in uniform but wave-carrying surroundings are calculated,
based on matching conditions formulated in terms of the normal acoustic
impedances at the flux tube boundary. The requirement of the equality
of the normal acoustic impedance of the transmitted wave field with
the normal acoustic impedance of the outgoing wave field selects the
eigenmodes, while the equality of the ingoing and the transmitted normal
acoustic impedance selects the optimal driving frequencies (Keppens
1996). Even if the flux tube is uniform, the eigenfrequencies can be
complex due to leakage of wave energy into the surroundings. The case
of uniform flux tubes has been considered previously (e.g. Cally 1986),
and serves as a testcase of our formalism. We extend Cally's results
by taking a radial stratification of the flux tube into account. The
non-uniformity of the flux tube can introduce another cause for energy
loss, namely resonant absorption internal to the flux tube. When
resonant absorption occurs, we must incorporate the appropriate jump
conditions over the dissipative layer(s). This can be done using a
simple numerical scheme as introduced by Stenuit et al. (1995).

---------------------------------------------------------
Title: Polar spots and stellar spindown: is dynamo saturation needed?
Authors: Solanki, S. K.; Motamen, S.; Keppens, R.
1997A&A...325.1039S Altcode:
Dynamo saturation is often invoked when calculating the rotational
evolution of cool stars. At rapid rotation rates a saturated
dynamo reduces the angular momentum carried away by the stellar
wind. This, in turn, may explain the high rotation rates present in
the distribution of rotation periods in young clusters. Here we point
out that concentration of magnetic flux near the poles of rapidly
rotating cool stars provides an alternative to dynamo saturation. A
high-latitude concentration of field on rapid rotators saturates
the angular momentum loss induced by the stellar wind, due to the
reduced torque arm. We show that the inclusion of this effect in
model calculations is able to reproduce the observed high rotation
rates without the need for dynamo saturation. Taken together with the
results of O'Dell et al. (1995A&amp;A...294..715O) this argues against
dynamo saturation at low rotation rates.

---------------------------------------------------------
Title: Polar spots and stellar spindown: is dynamo saturation needed?
Authors: Solanki, S. K.; Motamen, S.; Keppens, R.
1997A&A...324..943S Altcode:
Dynamo saturation is often invoked when calculating the rotational
evolution of cool stars. At rapid rotation rates a saturated
dynamo reduces the angular momentum carried away by the stellar
wind. This, in turn, may explain the high rotation rates present in
the distribution of rotation periods in young clusters. Here we point
out that concentration of magnetic flux near the poles of rapidly
rotating cool stars provides an alternative to dynamo saturation. A
high-latitude concentration of field on rapid rotators saturates
the angular momentum loss induced by the stellar wind, due to the
reduced torque arm. We show that the inclusion of this effect in
model calculations is able to reproduce the observed high rotation
rates without the need for dynamo saturation. Taken together with the
results of O'Dell et al. (1995A&amp;A...294..715O) this argues against
dynamo saturation at low rotation rates.

---------------------------------------------------------
Title: Spin and orbital angular momentum exchange in binary star
systems.
Authors: Keppens, R.
1997A&A...318..275K Altcode:
We present a comprehensive model for studying the angular momentum
(AM) evolution in binary star systems, taking into account:
(i) evolutionary effects of both component stars on the Pre-Main
Sequence (PMS), on the Main Sequence (MS) and during the (initial)
ascent onto the giant branch; (ii) spin-orbital AM exchange through
`tidal' interactions; and (iii) AM loss from one or both component
stars due to stellar winds. This allows us to assess whether, when
and how the synchronization of spin and orbital rotation rates,
and the circularization of eccentric orbits, is achieved within a
composite system of two evolving stars. We develop the formalism for
spin and orbital AM exchange in binary systems such that `standard'
(and sometimes rivaling) theories of tidal interactions and stellar
winds can easily be incorporated and compared, in so far as they lead
to qualitative differences in the overall AM evolution. When using our
model for a binary system of solar-type stars, we use a 2-component
model for each star (as in MacGregor &amp; Brenner 1991), with possibly
differentially rotating core and envelope zones. These two zones are
coupled through visco-magnetic mechanisms. The model calculations
presented illustrate how the combined effects of structural evolution,
tidal interactions, stellar winds, and the visco-magnetic coupling
mechanisms lead to rich scenarios for the AM evolution. We concentrate
in this paper on the model and its potential for gaining new insights
in the physical effects that play a role in the binary AM balance. It
is pointed out how it can be used for a direct interpretation of many
observational results, but this is postponed to a forthcoming paper
(Keppens et al. 1997, in prep).

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Title: The magnetic structure of pores and sunspots derived from
Advanced Stokes Polarimeter data.
Authors: Keppens, R.; Martinez Pillet, V.
1996A&A...316..229K Altcode:
We investigate the radial variation of the magnetic field structure
across sunspots, pores and azimuth centers (ACs). We define ACs as
magnetic structures of about the same size as pores (all structures
studied here are larger than 3 Mm diameter), but without a clear
(at least 5%) continuum decrease associated with them. We start
from the full 3D vector fields as observed with the Advanced Stokes
Polarimeter (ASP), and perform a statistical study of the azimuthally
averaged field components in the local cylindrical reference frame
centered on the structures. Our statistical study comprises a sample
of 16 sunspot observations, a sample of 51 pores, and a sample of
22 ACs. For all structures, we derive mean radial profiles and their
standard deviations. Due to the relatively large sample of pores, we
are able to investigate variations of this mean radial field structure
with the size of the pores. On the basis of our statistics, we identify
systematic changes in the magnetic field structure over a considerable
size range. We suggest how this may be the natural consequence of a
formation scenario for the largest pores, by a lateral clustering of
magnetic elements. Indeed, in this process, an AC may develop into a
dark pore and gradually grow in size through the incremental addition of
magnetic flux. Several observations where ACs turn into pores provide
an estimate of about 4-5x10^19^Mx for the critical magnetic flux at
which such transitions occur. We confirm the existence of a magnetic
canopy for pores of all sizes, as their magnetic extent is virtually
always larger than the associated continuum darkening. We observe
a relatively rapid change in the continuum appearance of a large
pore in the sample. We identify the associated changes in the field
structure, and confront it with the determined mean field variation
across sunspots. It appears that we have witnessed the formation of
a partial penumbra.

---------------------------------------------------------
Title: Hot Magnetic Fibrils: The Slow Continuum Revisited
Authors: Keppens, R.
1996ApJ...468..907K Altcode:
We investigate the importance of the slow continuum (from linear,
ideal magnetohydrodynamics [MHD]) for hot, evacuated, and strongly
magnetic fibrils with nonnegligible radial structure. The radial
structure allows for both slow and Alfvén resonant absorption of
acoustic power (in linear, visco-resistive MHD). When calculating
how efficiently the acoustic power is absorbed by such "hot magnetic
fibrils," embedded in a uniform compressible medium, as a function
of the real driving frequency, it is found that the axisymmetric
component of the acoustic excitation is absorbed quite strongly for
frequencies within the range of the slow continuum. Additionally,
for these one-dimensional hot magnetic fibrils, a sequence of
absorption maxima accumulates in real driving frequency above the
range of the slow continuum, still within the Alfvén continuum. The
maximal absorption coefficients reach 80% and more. We identify the
complex optimal driving frequencies and the associated complex leaky
eigenmodes responsible for these absorption maxima. The leaky
eigenmodes relate to the well-known tube speed modes of a uniform,
hot, and evacuated flux tube. The complex eigenfrequencies of the
leaky eigenmodes of the radially structured fibrils are calculated
from the impedance criterion that these eigenfrequencies satisfy. We define the generally complex optimal driving frequencies to
be those driving frequencies at which total (100%) absorption of the
incoming wave field takes place. They also obey an impedance criterion,
similar to the one that defines the eigenfrequencies. Both impedance
criteria demonstrate clearly the connection between optimal driving
frequencies and leaky eigenmodes. This also calls for a reevaluation
of the results of Goossens &amp; Hollweg, in which optimal and total
resonant absorption for real driving frequencies and the complex
leaky eigenmodes was discussed. For network and plage magnetic
elements in the solar atmosphere, our results may be relevant for wave
interactions within a layer situated at a geometrical height of about
400 km above photospheric τ = 1.

---------------------------------------------------------
Title: Flux Tubes with a Thin Transition Layer: Scattering and
Absorption Properties
Authors: Keppens, R.
1995SoPh..161..251K Altcode:
In this paper, we use the T-matrix formalism to discuss the scattering
and absorption properties of isolated flux tubes. We give a general
expression for the T-matrix of a 1D flux tube in terms of the normal
acoustic impedances for the different components of the acoustic
wavefield. This shows how the (leaky and non-leaky) eigenmodes are
related to those frequencies at which the normal acoustic impedances
for the scattered and the transmitted wavefield are equal.

---------------------------------------------------------
Title: On the evolution of rotational velocity distributions for
solar-type stars.
Authors: Keppens, R.; MacGregor, K. B.; Charbonneau, P.
1995A&A...294..469K Altcode:
We investigate how the distribution of rotational velocities for
late-type stars of a given mass evolves with age, both before
and during residence on the main sequence. Starting from an age
~10^6^years, an assumed pre-main sequence rotational velocity/period
distribution is evolved forward in time using the model described by
MacGregor &amp; Brenner (1991) to trace the rotational histories of
single, constituent stars. This model treats: (i) stellar angular
momentum loss as a result of the torque applied to the convection
zone by a magnetically coupled wind; (ii) angular momentum transport
from the radiative interior to the convective envelope in response
to the rotational deceleration of the stellar surface layers; and
(iii), angular momentum redistribution associated with changes in
internal structure during the process of contraction to the main
sequence. We ascertain how the evolution of a specified, initial
rotational velocity/period distribution is affected by such things as:
(i) the dependence of the coronal magnetic field strength on rotation
rate through a prescribed, phenomenological dynamo relation; (ii) the
magnitude of the timescale τ_c_ characterizing the transfer of angular
momentum from the core to the envelope; (ii) differences in the details
and duration of pre-main sequence structural evolution for stars with
masses in the range 0.8&lt;=M_*_/Msun_&lt;=1.0 and (iv),
the exchange of angular momentum between a star and a surrounding,
magnetized accretion disk during the first few million years of
pre-main sequence evolution following the development of a radiative
core. The results of this extensive parameter study are compared with
the distributions derived from measurements of rotational velocities
of solar-type stars in open clusters with known ages. Starting from an
initial distribution compiled from observations of rotation among T
Tauri stars, we find that reasonable agreement with the distribution
evolution inferred from cluster observations is obtained for: (i)
a dynamo law in which the strength of the coronal field increases
linearly with surface angular velocity for rotation rates &lt;=20
times the present solar rate, and becomes saturated for more rapid
rotation; (ii) a coupling timescale ~10^7^years; (iii) a mix of stellar
masses consisting of roughly equal numbers of 0.8Msun_ and
1.0Msun_ stars; and (iv), disk regulation of the surface
rotation up to an age ~6x10^6^years for stars with initial rotation
periods longer than 5days. A number of discrepancies remain, however:
even with the most favorable choice of model parameters, the present
calculations fail to produce a sufficiently large proportion of slow
(equatorial velocities less than 10km/s) rotators on the Zero-Age
Main Sequence.

---------------------------------------------------------
Title: Multiple Scattering and Resonant Absorption of P modes by
Fibril Sunspots
Authors: Keppens, R.
1995ASPC...76..260K Altcode: 1995gong.conf..260K
No abstract at ADS

---------------------------------------------------------
Title: Multiple Scattering and Resonant Absorption of p-Modes by
Fibril Sunspots
Authors: Keppens, R.; Bogdan, T. J.; Goossens, M.
1994ApJ...436..372K Altcode:
We investigate the scattering and absorption of sound waves by
bundles of magnetic flux tubes. The individual flux tubes within the
bundle have thin nonuniform boundary layers where the thermodynamic
and magnetic properties change continuously to their photospheric
levels. In these nonuniform layers, resonant absorption converts some
of the incident acoustic wave energy into heat and thus the flux-tube
bundle appears as a sink of acoustic power. For a fixed amount of
magnetic flux, we find that composite ('spaghetti') sunspots absorb
much more wave energy than their monolithic counterparts, although
both sunspots scatter comparable amounts of the incident acoustic wave
energy. The extra energy drainage results from the interplay of the
wave scattering back and forth between the tubes and the incremental
loss of acoustic power at each interaction with an individual tube due
to the resonant absorption in its boundary layer. The scattering cross
section is not similarly enhanced because the multiply scattered waves
generally interfere destructively in the far field. Another interesting
consequence of the lack of axisymmetry is that composite sunspots may
show acoustic emission for some multipole components, and absorption
for others. The net absorption cross section is however never negative,
and is nonzero only when the projection of the wave phase speed along
the flux-tube bundle is less than the maximal value of the Alfven
speed. Whereas composite sunspots composed of uniformly magnetized
flux tubes posses narrow scattering resonances, the analogous bundle
of nonuniform fibrils instead exhibits corresponding broad absorption
resonances, resulting from the incremental loss of power on successive
scatters. These broad absorption resonances correspond to leaky (MHD
radiating) eigenmodes of the composite structure. When progressively
more flux tubes are clustered, additional oscillation eigenmodes appear
grouped in a complicated band structure characterized by a (nearly)
common speed of propagation along the bundle.

---------------------------------------------------------
Title: Interaction of acoustic oscillations with magnetic flux tubes
in the solar photosphere
Authors: Keppens, R.
1994STIN...9530199K Altcode:
This thesis touches upon some outstanding puzzles concerning the
Sun and its visible surface layers, the solar photosphere. The solar
photosphere is permeated by all kinds of magnetic features. In chapter
one we summarize their overall surface characteristics and introduce
the equations of magnetohydrodynamics to discuss the magnetic features
in a theoretical perspective. In chapter 2 we introduce the basic
mathematical theory to treat multiple scattering and absorption of
acoustic waves. We start from general solutions to the wave equation,
to develop the T-matrix theory for sound wave interactions. In chapter
3 we linearize the ideal MHD equations, and use these linearized
equations to calculate T-matrices and U-matrices for simple (magnetized)
scatters. Subsequently, we extend the discussion to linear, resistive
MHD, to incorporate resonant absorption. Chapter 4 deals with multiple
scattering and resonant absorption in flux tube bundles.

---------------------------------------------------------
Title: Angular Momentum Loss from the Young Sun: Improved Wind and
Dynamo Models
Authors: Keppens, R.; Charbonneau, P.; MacGregor, K. B.; Brandenburg,
A.
1994ASPC...64..193K Altcode: 1994csss....8..193K
No abstract at ADS