Author name code: keppens ADS astronomy entries on 2022-09-14 author:"Keppens, Rony" ------------------------------------------------------------------------ Title: Thermally enhanced tearing in solar current sheets: explosive reconnection with plasmoid-trapped condensations Authors: Sen, Samrat; Keppens, Rony Bibcode: 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. Title: Two-fluid implementation in MPI-AMRVAC with applications to the solar chromosphere Authors: Braileanu, B. Popescu; Keppens, R. Bibcode: 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. Title: BxC: a swift generator for 3D magnetohydrodynamic turbulence Authors: Durrive, J. -B.; Changmai, M.; Keppens, R.; Lesaffre, P.; Maci, D.; Momferatos, G. Bibcode: 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. Title: 2.5D turbulent magnetic reconnection behaviour in the solar prominence due to Rayleigh-Taylor instability Authors: Changmai, Madhurjya; Keppens, Rony Bibcode: 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. Title: Resistive tearing growth rate modification by equilibrium flow Authors: de Jonghe, Jordi; Keppens, Rony Bibcode: 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. Title: Estimating uncertainties in the back-mapping of the fast solar wind Authors: Koukras, Alexandros; Dolla, Laurent; Keppens, Rony Bibcode: 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. Title: Multi-threaded prominence oscillations Authors: Jerčić, Veronika; Keppens, Rony; Zhou, Yuhao Bibcode: 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) 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 Bibcode: 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. Title: Solar tornadoes: Thermal instability in helical magnetic field configurations with flow Authors: Hermans, Joris; Keppens, Rony Bibcode: 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 & Astrophysics, 653, A94 (2021) \bibitem{Jack} J.M. Jenkins, et al., Astronomy & 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, & R. Keppens ApJS, 234, 30 (2018) \end{thebibliography} 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 Bibcode: 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. Title: Resolving the solar prominence/filament paradox using the magnetic Rayleigh-Taylor instability Authors: Jenkins, Jack M.; Keppens, Rony Bibcode: 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. Title: Implementation of the Soloviev equilibrium as a new CME model in EUHFORIA Authors: Linan, Luis; Keppens, Rony; Maharana, Anwesha; Poedts, Stefaan; Schmieder, Brigitte Bibcode: 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) Title: Legolas: magnetohydrodynamic spectroscopy with viscosity and Hall current Authors: De Jonghe, J.; Claes, N.; Keppens, R. Bibcode: 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. Title: Two-fluid implementation in MPI-AMRVAC, with applications in the solar chromosphere Authors: Popescu Braileanu, B.; Keppens, R. Bibcode: 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. Title: The Super-Alfvénic Rotational Instability in Accretion Disks about Black Holes Authors: Goedbloed, Hans; Keppens, Rony Bibcode: 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. Title: 3D MHD wave propagation near a coronal null point: New wave mode decomposition approach Authors: Yadav, N.; Keppens, R.; Popescu Braileanu, B. Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 & 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 & 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. Bibcode: 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 https://www.aanda.org Title: When Hot Meets Cold: Post-flare Coronal Rain Authors: Ruan, Wenzhi; Zhou, Yuhao; Keppens, Rony Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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 https://www.aanda.org 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 Bibcode: 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 Bibcode: 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. \& Keppens, R. 2019, Astronomy \& Astrophysics, 624, A96. [b] Claes, N., Keppens, R. \& Xia, C. 2020, Astronomy \& Astrophysics, submitted. Title: Prominence formation by levitation-condensation at extreme resolutions Authors: Jenkins, Jack; Keppens, Rony Bibcode: 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 \& 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 \& R. Keppens, 2018, ApJ Suppl. 234, 30

$[3]$ `Toward detailed prominence seismology. II. Charting the continuous magnetohydrodynamic spectrum', J.W.S. Blokland \& R. Keppens, 2011, A \& A 532, A94

$[4]$ `Thermal instabilities: Fragmentation and field misalignment of filament fine structure', N. Claes, R. Keppens \& C. Xia, 2020, A \& A 636, A112

$[5]$ `Simulating coronal condensation dynamics in 3D', S. P. Moschou, R. Keppens, C. Xia \& 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 \& 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 Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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 https://www.aanda.org Title: Thermal instabilities: Fragmentation and field misalignment of filament fine structure Authors: Claes, N.; Keppens, R.; Xia, C. Bibcode: 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 https://www.aanda.org 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. Bibcode: 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. Title: MPI-AMRVAC: a parallel, grid-adaptive PDE toolkit Authors: Keppens, Rony; Teunissen, Jannis; Xia, Chun; Porth, Oliver Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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. Title: Multilayered Kelvin-Helmholtz Instability in the Solar Corona Authors: Yuan, Ding; Shen, Yuandeng; Liu, Yu; Li, Hongbo; Feng, Xueshang; Keppens, Rony Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 (< 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 https://www.aanda.org Title: W 50 and SS 433 Authors: Bowler, Michael G.; Keppens, Rony Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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 http://www.aanda.org Title: Coronal rain in magnetic bipolar weak fields Authors: Xia, C.; Keppens, R.; Fang, X. Bibcode: 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 http://www.aanda.org 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 Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Title: Internal Dynamics of a Twin-layer Solar Prominence Authors: Xia, C.; Keppens, R. Bibcode: 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. Bibcode: 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. Bibcode: 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. Title: Simulating coronal condensation dynamics in 3D Authors: Moschou, S. P.; Keppens, R.; Xia, C.; Fang, X. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 (>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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 & 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 < Prot < 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 2012rjag.book..341K Altcode: No abstract at ADS Title: Two-component Jets and the Fanaroff-Riley Dichotomy Authors: Meliani, Z.; Keppens, R. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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 & 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. Bibcode: 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 <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 Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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 & 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: Jet Stability: A Computational Survey Authors: Keppens, Rony; Meliani, Zakaria; Baty, Hubert; van der Holst, Bart Bibcode: 2010LNP...791..179K Altcode: No abstract at ADS Title: Spectra and energies of cosmic rays in young supernova remnants Authors: Schure, Klara; Achterberg, Bram; Keppens, Rony; Vink, Jacco Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 2009iac..talk...79K Altcode: 2009iac..talk..109K No abstract at ADS Title: Decelerating Relativistic Two-Component Jets Authors: Meliani, Z.; Keppens, R. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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&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. Bibcode: 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. Bibcode: 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. Bibcode: 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<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. Bibcode: 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. Bibcode: 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: Magnetohydrostatic Solar Prominences in Near-Potential Coronal Magnetic Fields Authors: Petrie, G. J. D.; Blokland, J. W. S.; Keppens, R. Bibcode: 2008ASPC..383..413P Altcode: We present numerical magnetohydrostatic (MHS) 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 magnetohydrodynamics equilibrium solver and describe magnetic fields in and around prominences and the cool prominence plasma that these fields support. The many examples computed here with temperature and entropy prescribed as a free functions of the magnetic flux function include one which reproduces precisely the three-part structure often encountered in observations: a cool dense prominence surrounded by a cavity, within a flux rope embedded in a hot corona. Title: Accretion funnels onto weakly magnetized young stars Authors: Bessolaz, N.; Zanni, C.; Ferreira, J.; Keppens, R.; Bouvier, J. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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&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 Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 2004ESASP.575..337A Altcode: 2004soho...15..337A No abstract at ADS Title: Transonic instabilities in accretion disks Authors: Goedbloed, Hans; Keppens, Rony Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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<~MA<~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. Bibcode: 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>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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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, & 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. Bibcode: 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 & 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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<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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1999JPlPh..61....1K Altcode: 1999astro.ph..1166K Available from http://journals.cambridge.org/bin/bladerunner?REQUNIQ=1105385252&REQSESS=958582&118000REQEVENT=&REQINT1=18471&REQAUTH=0 Title: Numerical Simulations of Stellar Winds Authors: Keppens, R.; Goedbloed, J. P. Bibcode: 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. Title: 3D Nonlinear MHD Wave Heating of Coronal LoopsCD Authors: Poedts, S.; Keppens, R.; Beliën, A. J. C. Bibcode: 1999ASSL..240..319P Altcode: 1999numa.conf..319P No abstract at ADS Title: Implicit and semi-implicit schemes in the Versatile Advection Code: numerical tests Authors: Toth, G.; Keppens, R.; Botchev, M. A. Bibcode: 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. Title: Eigenfrequencies and optimal driving frequencies of 1D non-uniform magnetic flux tubes Authors: Stenuit, H.; Keppens, R.; Goossens, M. Bibcode: 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. Bibcode: 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&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. Bibcode: 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&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. Bibcode: 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 & 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). Title: The magnetic structure of pores and sunspots derived from Advanced Stokes Polarimeter data. Authors: Keppens, R.; Martinez Pillet, V. Bibcode: 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. Bibcode: 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 & 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. Bibcode: 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. Bibcode: 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 & 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<=M_*_/Msun_<=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 <=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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1994ASPC...64..193K Altcode: 1994csss....8..193K No abstract at ADS