arXiv subjects

Rony Keppens

Publications and source records attributed to Rony Keppens.

At least 19 recordsLinked to original sources

arXiv2026

Astrophysics on GPUs: introducing AGILE 1.0

We present AGILE, a GPU-enabled adaptive mesh refinement (AMR) framework for the solution of (near-) conservation laws which occur in astro- and solar-physical applications. AGILE is written in modern fortran 2003, inherits a part of its modules and mesh handling from MPI-AMRVAC, and achieves excellent GPU performance via OpenACC offloading. We here discuss the design decisions which enable AGILE to perform cost-efficient and scalable deeply nested AMR simulations with moderate block sizes of e.g. $16^3$ cells. AGILE currently implements several physics modules, ie. hydrodynamics, frozen-field hydrodynamics, magnetohydrodynamics and special-relativistic hydrodynamics and can easily be extended further through its modular design. Besides strong scaling tests to up to 2048 GPUs and standard benchmarks which show consistent performance across a large range of devices and problem sizes, we demonstrate AGILE's capabilities by means of state-of-the art science applications with all currently available physics modules.

astro-ph.IM↗

arXiv2026

Once more: Leaky MHD waves in coronal magnetic flux tubes

By a detailed comparison of leaky magnetohydrodynamic waves in coronal magnetic flux tubes with leaky electromagnetic waves in dielectric media it is shown that the latter kind may be called quasi-normal modes, since they can be regularised by a normalisation which systematically cuts off the contribution of the external homogeneous region, whereas such a possibility is forbidden for the former kind by the conservation of magnetic flux. Consequently, leaky magnetohydrodynamic waves cannot be systematically applied to coronal seismology, i.e. to the inverse spectral problem of determining the different equilibrium distributions of the fields by comparing the spectra they produce with the observed ones.

astro-ph.SR↗

arXiv2026

High-energy Particle Transport in Three-dimensional Anisotropic Turbulent Magnetic Fields

The understanding and modeling of high-energy particles transport in turbulent magnetic fields is an important open question in space- and astrophysics. The multiscale, nonlinear nature of turbulence, and the high variability of turbulence properties across different environments, make it particularly challenging to reach a full understanding of the interactions between particles and turbulent fluctuations. Using synthetic, realistically looking turbulent magnetic field realizations generated by the BxC toolkit, we investigate how the scattering of particles is affected by anisotropic fluctuations in strongly turbulent fields. We find evidence that, in the absence of a uniform background or guide magnetic field, the scattering process is not governed by the turbulence correlation length. We then further verify this hypothesis by studying particle transport in the presence of a guide field. We find evidence of a different scattering mechanism than the usual pitch-angle diffusion used to describe scattering in strong-guide-field settings.

physics.space-ph↗

arXiv2026

foap4: Adaptive mesh refinement with OpenACC, MPI, and p4est

GPUs and other accelerators are increasingly used for scientific computing. In the future, we want to add GPU support to parallel adaptive mesh refinement (AMR) codes written in Fortran. To understand which changes are necessary to obtain good performance we have developed foap4, an AMR framework implemented in Fortran that uses OpenACC, MPI, and the p4est library. We discuss the design and implementation of the framework. Several benchmark problems are considered, in which Euler's equations of gas dynamics are solved using explicit time integration. These benchmarks are performed in both 2D and 3D, using static and adaptive meshes, for varying problem sizes on different hardware. Our results show that AMR simulations can be carried out efficiently on GPUs with OpenACC and MPI, even when using relatively small grid blocks of $8^3$ or $16^3$ cells.

physics.comp-ph↗

arXiv2026

Thermal instability in coronal loops: linking eigenvalue spectra to time-dependent evolution

Cool, dense condensations such as coronal rain and prominences suggest that coronal plasma can undergo runaway radiative cooling. Connecting this behaviour to linear thermal modes requires us to fully understand the deeper connection between eigenvalue spectra and actual time-dependent evolution. We aim to clarify this intricate link for a simplified, coronal-only model of a stratified coronal loop by combining spectral, linear initial-value, and nonlinear simulations of the same loop setup. We study waves and instabilities, as well as temporal evolutions for a 1D hydrostatic, thermally balanced loop with optically thin radiation and prescribed heating. The non-adiabatic spectrum is computed with our open-source Legolas code. We demonstrate our newly developed boundary value-initial value solver Legolas-IVP, where linear evolutions are performed for controlled perturbations, and fully equivalent nonlinear runs are carried out with MPI-AMRVAC. The spectrum contains discrete acoustic modes and a thermally unstable branch including a thermal continuum. Linear initial-value experiments with isochoric, isobaric, and isentropic pulses highlight how the polarisation of the eigenmodes demonstrates physically consistent behaviour expected from the eigenspectrum. Even in the linear stage, thermal imbalance drives siphon-like flows toward the cooling region. Growth rates from Legolas-IVP agree with spectral predictions and are reproduced in MPI-AMRVAC, which follows the condensation through runaway cooling to chromospheric temperatures, with the cool dense blob sliding under gravity toward the loop footpoint. The spectral-linear-nonlinear investigation demonstrates a direct link between thermal eigenmodes and condensation dynamics, providing a basis for extending to fully 3D MHD models.

astro-ph.SR↗

arXiv2026

On the dynamics, thermodynamics and fine structure of virtual erupting filaments

It is not fully understood why some solar filaments erupt while others do not. Those that do typically undergo a slow rise followed by an acceleration phase, though this transition requires further investigation. Erupting prominences have been observed to heat up during the acceleration phase, but the origin of this heating remains unclear. Moreover, some coronal mass ejections possess additional fine structure in white-light observations beyond the classical three-part morphology. We aim to elaborate on the dynamics of erupting prominences, investigate the heating during the acceleration phase, and correlate our findings with observations. We employ the open-source MPI-AMRVAC code to solve the 2.5D MHD equations on a coronal domain extending to 300 Mm, using adaptive mesh refinement to attain high resolution. Controlled combinations of footpoint shearing and converging motions applied to an initial magnetic arcade produce erupting flux ropes with self-consistent prominence and coronal rain formation due to thermal instability. We find both non-erupting and erupting cases related to the system energization. Comparison with observations from the AIA Filament Eruption Catalog shows that the slow-rise and impulsive phases are modulated by magnetic reconnection. The transition to acceleration corresponds to an increase in the inflow Alfvén Mach number. Thermal conduction and compressional heating can lead to prominence evaporation. We obtain nested circular fine structure in EUV images of the ejected flux ropes, partly resulting from plasmoid interactions. We conclude that internal heating processes and magnetic reconnection play key roles in the early evolution of CMEs.

astro-ph.SR↗

arXiv2026

Nonthermal electron acceleration in turbulent post-flare coronal loops

The generation of nonthermal electrons during solar flares plays a critical role in energy transport from the corona to the chromosphere, producing regions of observed intense X-ray emission. Turbulence in post-flare loops, particularly from Kelvin-Helmholtz instabilities (KHI), has been suggested and investigated as a mechanism for trapping and accelerating electrons in such scenarios. Starting from past results, we aim to characterize the energization process of electrons trapped in a turbulent post-flare looptop, quantifying the contributions of different acceleration mechanisms, and establishing a coherent numerical framework for describing particle energetics. We perform test-particle simulations with the guiding-centre approximation on top of a 2.5D magnetohydrodynamic model of a time-evolving post-flare coronal looptop. We implement an improved formulation of the guiding-centre equations that explicitly conserves energy, enabling a consistent analysis of electron acceleration in the turbulent plasma. We find that, in the plasma turbulence inside the looptop, electrons develop suprathermal energy distributions with tails compatible with hard X-ray emission. The dominant energization channel arises from perpendicular gradient effects in the form of second-order Fermi-like stochastic acceleration, while curvature effects are dominant for particles on long trajectories. Statistical correlations with the measured particle pitch angle confirm that the strongest acceleration occurs for electrons trapped in bouncing motions within turbulent magnetic structures. Our results provide an understanding of how KHI-induced turbulence in coronal looptops produces and sustains populations of trapped nonthermal electrons. We dissect and clarify the relative roles of different magnetic effects and the emergence of stochastic Fermi-like energization.

astro-ph.SR↗

arXiv2025

Data-constrained magnetohydrodynamic simulation of global solar corona including solar wind effects within 2.5 $R_\odot$

Total solar eclipses (TSEs) provide a unique opportunity to observe the large-scale solar corona. The solar wind plays an important role in forming the large-scale coronal structure and magnetohydrodynamic (MHD) simulations are used to reproduce it for further studying coronal mass ejections (CMEs). We conduct a data-constrained MHD simulation of the global solar corona including solar wind effects of the 2024 April 8 TSE with observed magnetograms using the Message Passing Interface Adaptive Mesh Refinement Versatile Advection Code (MPI-AMRVAC) within 2.5 $R_\odot$. This TSE happened within the solar maximum, hence the global corona was highly structured. Our MHD simulation includes the energy equation with a reduced polytropic index $γ=1.05$. We compare the global magnetic field for multiple magnetograms and use synchronic frames from the Solar Dynamics Observatory/Helioseismic and Magnetic Imager to initialize the magnetic field configuration from a magneto-frictionally equilibrium solution, called the Outflow field. We detail the initial and boundary conditions employed to time-advance the full set of ideal MHD equations such that the global corona is relaxed to a steady state. The magnetic field, the velocity field, and distributions of the density and thermal pressure are successfully reproduced. We demonstrate direct comparisons with TSE images in white-light and Fe XIV emission augmented with quasi-separatrix layers, the integrated current density, and the synthetic white-light radiation, and find a good agreement between simulations and observations. This provides a fundamental background for future simulations to study the triggering and acceleration mechanisms of CMEs under solar wind effects.

astro-ph.SR↗

arXiv2025

Secondary small-scale dynamics of a Rayleigh-Taylor unstable solar prominence

Quiescent solar prominences show distinct small-scale dynamics in observations. Their internal density contrasts with the surrounding corona make them susceptible to Rayleigh-Taylor (RT) instabilities, leading to vertically structured prominence morphologies when observed at the solar limb. As a result, prominences develop bubbles and plumes, along with secondary Kelvin-Helmholtz (KH) roll-ups along their edges. Recent observations also suggest magnetic reconnection events within the RT-driven turbulent flows. We perform high-resolution 2.5D resistive magnetohydrodynamic simulations using the open-source MPI-AMRVAC code, reaching a spatial resolution of $\sim 11.7$ km in a 2D domain of size 30 Mm$\times$30 Mm and evolving the system for approximately 10 minutes of solar time. A dense, magnetic pressure supported prominence serves as the initial state, which becomes unstable at the prominence-corona interface. The resulting interaction between RT and KH instabilities leads to the formation of current sheets and localized reconnection events. The reconnection-driven outflows form energetic jets that enhance energy transport and dissipation across the prominence. We analyze our high-resolution prominence simulation using synthetic images of the broadband SDO/AIA 094, 171, and 193 Å and narrowband H$α$ filters, to compare the developing fine-scale structures with their observational counterparts. Most secondary instabilities emerge in the hotter coronal regions surrounding the cooler prominence core. While our simulated features match observed scales, speeds, and duration, the simulated activity remains concentrated in hot, surrounding coronal plasma rather then the cool prominence material, implying that key physical ingredients may be missing. Future 3D studies in more realistic magnetic configurations are required to address these limitations.

astro-ph.SR↗

arXiv2025

Modeling multiphase plasma in the corona: prominences and rain

We review major achievements in our understanding of multiphase coronal plasma, where cool-dense and hot-tenuous matter coexists, brought about by advances in modeling and theory, inspired by observations. We give an overview of models that self-consistently form solar (or stellar) prominences and filaments, or (postflare) coronal rain, and clarify how these different phenomena share a common physical origin, relating radiative losses and coronal heating. While we do not fully understand the coronal heating, multi-dimensional models of solar prominence and rain formation demonstrate how thermal instability triggers condensations, and how their morphology may reveal aspects of the applied heating at play. We emphasize how the many pathways to linear instability due to combined ingredients of heat-loss, gravity, flows, and magnetic topologies are all involved in the resulting nonlinear magnetohydrodynamics. We provide some challenges to future model efforts, especially concerning prominence fine structure, internal dynamics, and their overall lifecycle.

astro-ph.SR↗

arXiv2025

Particle Acceleration and Transport in the Large-scale Current Sheet under an Erupting Magnetic Flux Rope

We investigate the acceleration and transport of electrons in the highly fine-structured current sheet that develops during magnetic flux rope (MFR) eruptions. Our work combines ultra-resolved MHD simulations of MFR eruption, with test-particle studies performed using the guiding center approximation. Our grid-adaptive, fully three-dimensional, high-resolution magnetohydrodynamic simulations model MFR eruptions that form complex current sheet topologies, serving as background electromagnetic fields for particle acceleration. Within the current sheet, tearing-mode instabilities give rise to mini flux ropes. Electrons become temporarily trapped within these elongated structures, undergoing acceleration and transport processes that significantly differ from those observed in two-dimensional or two-and-a-half-dimensional simulations. Our findings reveal that these fine-scale structures act as efficient particle accelerators, surpassing the acceleration efficiency of single X-line reconnection events, and are capable of energizing electrons to energies exceeding 100 keV. High-energy electrons accelerated in different mini flux ropes follow distinct trajectories due to spatially varying magnetic field connectivity, ultimately precipitating onto opposite sides of flare ribbons. Remarkably, double electron sources at the flare ribbons originate from different small flux rope acceleration regions, rather than from the same reconnecting field line as previously suggested. Distinct small flux ropes possess opposite magnetic helicity to accelerate electrons to source regions with different magnetic polarities, establishing a novel conjugate double source configuration. Furthermore, electrons escaping from the lower regions exhibit a broken power-law energy spectrum.

astro-ph.SR↗

arXiv2025

Effective resistivity in relativistic collisionless plasmoid-mediated reconnection

Magnetic reconnection can power spectacular high-energy astrophysical phenomena by producing non-thermal energy distributions in highly magnetized regions around compact objects. By means of two-dimensional fully kinetic particle-in-cell (PIC) simulations we investigate relativistic collisionless plasmoid-mediated reconnection in magnetically dominated pair plasmas with and without guide field. In X-points, where diverging flows result in a non-diagonal thermal pressure tensor, a finite residence time for particles gives rise to a localized collisionless effective resistivity. Here, for the first time for relativistic reconnection in a fully developed plasmoid chain we identify the mechanisms driving the non-ideal electric field using a full Ohm's law by means of a statistical analysis based on our PIC simulations. We show that the non-ideal electric field is predominantly driven by gradients of nongyrotropic thermal pressures. We propose a kinetic physics motivated non-uniform effective resistivity model, which is negligible on global scales and becomes significant only locally in X-points, that captures the properties of collisionless reconnection with the aim of mimicking its essentials in non-ideal magnetohydrodynamic descriptions. This effective resistivity model provides a viable opportunity to design physically grounded global models for reconnection-powered high-energy emission.

astro-ph.HE↗

arXiv2025

A visual approach to global accretion disk instabilities

For over 30 years, the Magneto-Rotational Instability has been accepted as the mechanism driving accretion disk turbulence. Its physical basis is well understood, where an interplay between centrifugal forces and magnetic tension transfers angular momentum between oppositely displaced fluid elements. In this work, we revisit this picture in global disk models and various magnetic field topologies and generalise it to non-axisymmetric instabilities like the Super-Alfvénic Rotational Instability (SARI). We use the open-source \texttt{Legolas} software to quantify all complex-valued linear eigenfunctions for the (near-)eigenmodes and visualise the resulting spatio-temporal variations in real space in a manner that can be compared to direct numerical simulations of disks. The field perturbations are fundamentally different between the (axisymmetric) MRI and the novel, ultra-localised SARI modes, which bear some resemblance to spiral modes in galaxies but differ in important ways. We use a combined numerical-analytical approach to study the polarization of the magnetic and velocity field perturbations. Finally, we compare disks of differing magnetic topology where many linear modes are merely superposed to recreate a visual impression of `turbulent' fields and quantify the resulting stresses. We find that even superposed, still linearly growing SARI modes can already provide the needed effective viscosity-related alpha-values invoked for angular momentum transport. 3D views on the magnetic field perturbations show that SARI modes of opposite azimuthal mode number may naturally introduce plasmoid and toroidal flux-tube like field deformations.

astro-ph.GA↗

arXiv2025

The hydrodynamic thermal continuum, with applications to stratified atmospheres and 1D coronal loop models

Using both analytical and numerical means, we demonstrate that linear stability analysis of a hydrodynamic stratified atmosphere or a 1D coronal loop model in non-adiabatic settings features a thermal continuum corresponding to highly localized eigenfunctions. This thermal continuum can be precomputed, involving the net heat-loss function and its partial derivatives, and is the generalization of the thermal instability introduced by~\citet{Parker1953}. We account for a thermal imbalance, directly affecting thermal instability growthrates. We present completely general equations that govern all eigenmodes, including non-adiabatically affected p- and g-modes of the stratified settings. We intend to clarify how linear thermal instability is relevant for solar loops that show spontaneous in-situ condensations, and eliminate recent confusion on specific isochoric routes to linear instability alongside other thermal instability channels. The thermal continuum, previously identified as a crucial ingredient in magnetohydrodynamic eigenmode spectra for coronal loops and atmospheres, drives multithermal aspects across our universe, such as forming solar coronal rain and prominences, or cold cloud creation in intracluster to interstellar medium environments.

astro-ph.SR↗

arXiv2025

Response of the solar atmosphere to flux emergence: With emergence-driven prominence formation

Flux emergence is crucial for the formation of solar active regions and triggering of various eruptions. However, the detailed mechanisms by which flux emergence drives these eruptions remain unclear and require numerical investigation. Using 2.5-dimensional magnetohydrodynamic simulations, we investigate the interaction between emerging flux and background magnetic fields and dynamics of the induced eruptions. By systematically varying the strength and angle of the emerging magnetic field relative to the background field, we investigate its impact on the initiation and evolution of solar eruptions. The simulations show that magnetic reconnection between the emerging flux and background field drives the formation of current sheets, magnetic islands and multithermal jets. Stronger magnetic fields result in earlier eruptions, more energetic jets, and enhanced heating. The formation and ejection of magnetic islands affect the structure and dynamics of the jet. When the hot and cool components of jets reach the other footpoint of magnetic loops, they will generate spicules near the transition region. Varying the angle between the emerging flux and the background field, we find that larger angles delay filament ascent and eruption timing but facilitate filament formation. Filaments form a hot shell and oscillate with a period of 10 minutes driven by periodic plasma ejections. Repetitive reconnection events inject cold plasma into the self-consistently formed filament channel, introducing a new prominence formation mechanism by flux-emergence-fed injection. Our analysis highlights the dynamic interplay between magnetic reconnection, plasma cooling and heating, and filament dynamics. These findings provide insights into solar eruptions and their observational signatures, emphasizing the role of multi-thermal structures in the corona.

astro-ph.SR↗

arXiv2025

Coronal rain formation in a two-fluid approximation

Coronal rain, observed in 3D spine-fan magnetic configurations, results from thermal instability in the solar corona, where runaway in-situ cooling causes plasma to condense and drain along the magnetic lines. The reconnection of the magnetic field lines around the null point creates jets, seen as denser structures traveling along the field lines. As these dense regions evolve, thermal instability can set in and ultimately form coronal rain. In this paper we study the importance of partial ionization effects in the formation of coronal rain in the late evolution of 3D spine-fan magnetic reconnection in the solar corona. We use a two-fluid model consisting of neutral and charged particles coupled by collisions, where ionization recombination processes are taken into account. To trigger the thermal instability, we here investigate how magnetic reconnection generates flows that lead to the accumulation of higher-density structures along magnetic field lines. The dynamics associated with the spine-fan magnetic reconnection produces current sheets around the null point and flows along the field lines. Blobs similar to coronal rain start to appear after 400 seconds in the simulation domain which follow the field lines from the direction of the perturbed null point. The temperature drop is accompanied by recombination of charged particles. Recombination effects become important in coronal rain evolution when the temperature drops considerably in the condensed structures. The neutrals are slowed down by recombination, producing decoupling in velocity at the size of the blob, but inside the condensing structure, the neutrals can move faster across the field lines, creating small scale structures. This study presents a novel two-fluid approach to coronal rain, showing that incorporating two-fluid effects is essential for accurately capturing its dynamics.

astro-ph.SR↗

arXiv2025

Data-constrained Magnetohydrodynamic Simulation of a Filament Eruption in a Decaying Active Region 13079 on a Global Scale

Filaments are special plasma phenomena embedded in the solar atmosphere, characterized by unique thermodynamic properties and magnetic structures. Magnetohydrodynamic (MHD) simulations are useful to investigate the eruption mechanisms of filaments. We conduct a data-constrained zero-$β$ MHD simulation in spherical coordinates to investigate a C3.5 class flare triggered by an eruptive filament on 2022 August 15 in a decaying weak active region 13079. We reconstruct the three-dimensional coronal magnetic field using vector magnetograms and synoptic maps from the Solar Dynamics Observatory/Helioseismic and Magnetic Imager (SDO/HMI). We transform vector magnetic field into Stonyhurst heliographic spherical coordinates combined with a synoptic map and constructed a potential field source surface (PFSS) model with a magnetic flux rope (MFR) embedded using the Regularized Biot--Savart Laws (RBSL). Subsequently, we conduct a spherical zero-$β$ MHD simulation using the Message Passing Interface Adaptive Mesh Refinement Versatile Advection Code (MPI-AMRVAC) and replicated the entire dynamic process of the filament eruption consistent with observations. With the calculation of time-distance profile, Qusai-Separatrix Layers (QSL), and synthetic radiation from simulated current density, we find a good agreement between our simulation and observations in terms of dynamics and magnetic topology. Technically, we provide a useful method of advanced data-constrained simulation of weak active regions in spherical coordinates. Scientifically, the model allows us to quantitatively describe and diagnose the entire process of filament eruption.

astro-ph.SR↗

arXiv2025

Radiation-magnetohydrodynamics with MPI-AMRVAC using flux-limited diffusion

Context. Radiation plays a significant role in solar and astrophysical environments as it may constitute a sizeable fraction of the energy density, momentum flux, and the total pressure. Modelling the dynamic interaction between radiation and magnetized plasmas in such environments is an intricate and computationally costly task. Aims. The goal of this work is to demonstrate the capabilities of the open-source parallel, block-adaptive computational framework MPI-AMRVAC, in solving equations of radiation-magnetohydrodynamics (RMHD), and to present benchmark test cases relevant for radiation-dominated magnetized plasmas. Methods. The existing magnetohydrodynamics (MHD) and flux-limited diffusion (FLD) radiative-hydrodynamics physics modules are combined to solve the equations of radiation-magnetohydrodynamics (RMHD) on block-adaptive finite volume Cartesian meshes in any dimensionality. Results. We introduce and validate several benchmark test cases such as steady radiative MHD shocks, radiation-damped linear MHD waves, radiation-modified Riemann problems and a multi-dimensional radiative magnetoconvection case. We recall the basic governing Rankine-Hugoniot relations for shocks and the dispersion relation for linear MHD waves in the presence of optically thick radiation fields where the diffusion limit is reached. The RMHD system allows for 8 linear wave types, where the classical 7-wave MHD picture (entropy and three wave pairs for slow, Alfven and fast) is augmented with a radiative diffusion mode. Conclusions. The MPI-AMRVAC code now has the capability to perform multidimensional RMHD simulations with mesh adaptation making it well-suited for larger scientific applications to study magnetized matter-radiation interactions in solar and stellar interiors and atmospheres.

astro-ph.SR↗