Research Projects
In our group, we develop and use innovative methods for space and plasma physics simulations. Our research is supported by different grants
Third-party Funded Projects
SFB 1491, Project F2: Particle acceleration via magnetic reconnection in different regimes
Magnetic reconnection is a small-scale process in terms of astrophysical spatial and temporal scales, but it is expected to power some of the most energetic processes in the Universe. Magnetic reconnection converts magnetic energy into heating and non-thermal particle acceleration in most astrophysical plasmas. High-energy reconnection events are associated with astrophysical jets, from neutron star- beamed X-ray emission to the extended beam along the axis of rotation in active galactic nuclei (AGN). Powered by super-massive black holes, AGN jets pointing to Earth – called blazars – are the most luminous objects in the known Universe. Particles accelerated or pre-accelerated in magnetic reconnection are expected to produce the characteristic double-humped blazar jet energy spectrum; the fast reconnection scenario associated with tearing/plasmoid instability is compatible with the time scales of the flaring activity observed in these objects. In the last two decades, the understanding of magnetic reconnection in AGN jets has progressed at an astounding pace. Significant progress has been achieved theoretically, with a better understanding of fast reconnection (plasmoid instability) in relativistic regimes, and also computationally. Relativistic MHD simulations have shown plausible scenarios for the formation of reconnecting sites in AGN jets, while relativistic particle-in-cell (PIC) simulations have uncovered scaling laws that relate the reconnection regime (chiefly, magnetization levels) to the spectra of non-thermal particles, at least in certain geometries (e.g. 2D reconnection, starting from a relativistic Harris sheet or force-free configurations, with large simulation boxes).
These two lines of investigation have mostly proceeded in parallel, with limited feedback between studies at the “large, MHD” and “small, kinetic” scales. Still, understanding the coupling between system-scale and kinetic-scale processes is fundamental to understand up to which point magnetic reconnection can accelerate particles in astrophysical jets, and, if further energization is needed to explain observations, which secondary processes are the most likely candidates for it. PIC simulations can self-consistently describe the dynamics of particle acceleration, but fluid simulations are needed to understand how many reconnection sites can develop in a kinking, magnetized plasma column (the jet) and if they suffice to bring a significant fraction of the particles to the energies required to power the emissions, without involving further energization processes. In fact, information on reconnection in the idealized current sheets commonly simulated may fail to account for factors (e.g. development of instabilities, turbulence), that may either hinder or boost particle acceleration. For these reasons, a numerical approach that combines large-scale dynamics with a self-consistent description of particle acceleration processes is needed. The unique combination of skills within the RUB-TP1 chair and the close collaborations with the other projects in the proposal put us in the ideal position to develop this new approach, and quickly exploit the new scientific results we will obtain in the context of multimessenger astrophysics. The improved understanding of AGN reconnection that we will obtain from our innovative modeling approach will allow us to deliver accurate information (spectral index and cutoff energies for an eletron-positron and an electron-proton plasma) on particle energization to a number of other projects in the prosed CRC, where it will be used as initial conditions for cosmic-ray propagation models. This way, we can test the different signatures in the electromagnetic spectrum that arise from purely leptonic scenarios (electron-positron jet) and from (lepto)hadronic scenarios (electron-proton jet).
DFG Project: Investigation of the role of selected collisionless processes in heat flux regulation in the solar wind
Kinetic, collisionless processes control fundamental dynamics in heliospheric plasmas, notwithstanding the extreme separation of scales between system and kinetic scales in the solar wind. Large scale ion and electron properties of the wind are constrained by kinetic instabilities. Observations and simulations, including recent Parker Solar Probe observations few solar radii away from the Sun, show that collisionless processes have a fundamental role in regulating the heliospheric heat flux, which in turn controls the solar wind energy budget and, indirectly, the spatial extension of the heliosphere.Within this project, we will use newly developed numerical methods to contribute to the understanding of collisionless regulation of the heat flux. We will specifically address the role of kinetic processes, such as kinetic instabilities and wave/ particle interaction processes triggered in the presence of large-scale magnetic field fluctuations, in heat flux regulation under different solar wind conditions. We will use our simulations to predict and explain observed Velocity Distribution Functions (VDFs) at different heliocentric distances. We will study the effect on heat flux regulation of solar wind expansion, which alters the evolution of heat flux-regulating instabilities and hence indirectly contributes to heat flux regulation. Our findings will be constantly compared and validated against current and historical observations, chiefly from the Parker Solar Probe and Solar Orbiter missions.
DFG Project: Tearing instability and particle energization in collisionless, suprathermal plasmas
The aptly called tearing instability tears up current sheets (CSs) in a number of plasmoids separated by reconnection X points, and turns a steady-state CS into the locus of violent magnetic- to-kinetic energy conversion and particle heating and acceleration. Understanding under which conditions CSs are unstable to the tearing instability and how efficiently it develops (Objective 1) is therefore essential for a complete picture of energy dynamics in space and astrophysical plasmas. In doing so, one should also keep into account that most heliospheric plasmas are collisionless: since thermalization mechanisms are inefficient, most observed velocity distribution functions (VDFs) depart quite significant from thermal (Maxwellian) distributions, and are better modeled with suprathermal (kappa) distributions. This affects both instability threshold and growth rates (Objective 2). Furthermore, competing/ concurrent instabilities develop together with the tearing instability and participate in particle heating and acceleration (Objective 3). Within this project, we intend to extend existing knowledge of particle energization via tearing- triggered magnetic reconnection to realistic scenarios that factor in the elements mentioned above. We will do so with a two-pronged theoretical and numerical approach.
Humboldt Fellowships
HPC Projects
Interaction of whistler waves and electrostatic processes: The suppression of whistler waves in the near-Sun environment
Humboldt Fellow: Dr. Mahmoud Saad Afify Ali Ibrahim
Whistler waves and their role in regulating the heat flux in the solar wind have triggered a great deal of interest during the last decade. Extensive efforts have been made to investigate many phenomena such as the mechanism behind onset and propagation of parallel and oblique whistler waves, how they modify the electron velocity distribution functions (VDFs) and the role of solar wind expansion in heat flux regulation. The proposed research is motivated by a number of key unsolved questions about the microphysical mechanisms that control the intermittent nature of the whistler heat flux instability, the interaction of the whistler with electrostatic waves (e.g. solitons, Langmuir, and oblique ion-acoustic waves), and the suppression of whistler waves in the near-Sun environment, as recently observed by the Parker Solar Probe (PSP) mission. Addressing the lack of whistler waves very close to the Sun is critical for understanding the evolution of the solar wind energy budget. We will perform analytical studies and kinetic simulations with a fully kinetic code that implements solar wind plasma expansion self-consistently, to investigate the microphysical mechanisms of heat flux regulation very close to the Sun. We expect that our results will help to interpret recent heliospheric observations of heat flux regulating instabilities in the inner heliosphere, that may be of an electrostatic nature. The numerical expertise of the Ruhr-Universität Bochum (RUB) team will perfectly complement my analytical skills in the successful pursuit of this ambitious project.
GCS Large Scale Project: Investigation of suprathermal features in the velocity distribution functions of space and astrophysical plasmas
85 million CPU core/hours @SuperMUC-NG, LRZ
This project aims at addressing the connection between some of the main energy conversion processes in space and astrophysical plasmas and the collisionless, non-thermal nature of most plasmas in these environments. Most heliospheric and astrophysical plasmas are collisionless, which means that collisions are not effective thermalization mechanisms. This also means that non-thermal distribution functions (DFs), such as suprathermal, kappa-distributed, velocity DFs have both the possibility to arise and, when present, persist. And in fact, decades of in situ observations of heliospheric plasmas, and in particular of solar wind and magnetospheric environments, tell us that they are ubiquitously characterized by kappa DFs with varying kappa parameters. When observing suprathermal DFs, one is immediately confronted with two questions: 1) where do they come from? ; and 2) how do they affect plasma processes?
In sub-project 1 we focus on formation of suprathermal tails by tearing instability/ magnetic reconnection in astrophysical relativistic plasmas, and on the possible role of reconnection in the pre-acceleration of suprathermal populations which can justify observed emissions in Active Galactic Nuclei (AGN) jets. In sub-project 2 we move to heliospheric plasmas and investigate how the nature of the DFs affect energy conversion in magnetic reconnection. In this case, it is convenient to work on solar wind or magnetospheric plasmas because we have in-situ observations of DFs and energy conversion rates, which can constrain our choices of parameters. In sub-project 3 we focus on turbulence in the solar wind. We want to understand if and how both large (e.g. energy spectra) and microscopic (e.g. kappa index) aspects of turbulence change if turbulence takes place in the presence of suprathermal DFs.