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TU Berlin

Kinetic simulations of electron-scale turbulent high-beta and asymmetric collisionless magnetic reconnection

HPC Project: Kinetic simulations of electron-scale turbulent high-beta and asymmetric collisionless magnetic reconnection

Project ID: pr27ta
Supercomputer cluster: LRZ/SuperMUC-NG, Garching (Germany)
Duration of the project: 03.2018-01.2022
Mcore-hours granted: 32,56
Principal Investigator: Prof. Dr. Jörg Büchner
Project partner institute: University of Bergen (Norway)
Researchers: Patricio Muñoz (TU Berlin), Jan Benáček (TU Berlin), Prof. Michael Hesse (U. of Bergen)
Numerical code: ACRONYM

 

Abstract
Collisionless magnetic reconnection is supposedly the most efficient mechanism of magnetic energy conversion into plasma kinetic energy, heat and particle acceleration in the Universe. Its properties can be investigated in situ, however, only in the Earth's magnetosphere. For the first time the current high-resolution observations of the magnetospheric multi-spacecraft mission MMS now allows to resolve the electron kinetic plasma scales. This way in the turbulent high-beta plasma of the magnetosheath surrounding the Earth’s magnetosphere electron-scale thin current sheets where discovered which are assumed to undergo reconnection. Further asymmetric reconnection was discovered through the boundary of the magnetosphere, the magnetopause, through which plasmas of different origin are transported, which mix. These two discoveries are most relevant for astrophysical plasmas since they correspond to plasma and  field conditions which are typical also for remote places in the Universe. The observations are, however, theoretically not well understood, yet. Their understanding requires fully kinetic numerical simulations which we plan to carry out in the framework of this proposal. In particular we are going to investigate the efficiency of reconnection through current sheets in turbulent high-beta plasmas as observed in the magnetosheath as well as the plasma transport through asymmetric current sheets in dependence on the plasma inflow conditions and compositions: In fact the properties of reconnection for large thermal (compared to the magnetic) pressure (high plasma-beta) are not well understood as well as reconnection through asymmetric current sheets. We will address these problems via 2.5 D Particle-in-Cell (PIC) code implementing appropriate parameters and geometries. This way we will not only contribute to the investigation of these newly discovered kinds of magnetic reconnection, providing theoretical support for ongoing and future in-situ measurements, but we will also improve the understanding of the basic physics of collisionless reconnection in a larger variety of astrophysical plasmas.

 

To strengthen the capabilities of remote diagnostics of astrophysical plasma processes we also investigated the simulation of the interaction of accelerated electrons and positrons streaming in pulsar magnetospheres, possibly associated with magnetic reconnection, where they are supposed to cause the observed radio-emissions of neutron stars (sub-project 2). The mechanisms behind this pulsar radio emission are still not understood. We focus on the interaction of electron-positrons bunches formed in the neutron star magnetospheres by the sparking effect. The bunches may mutually interact and produce streaming instabilities and soliton-like waves, which emit coherent radio waves. However, their emission properties are unknown.

 

Results sub-project 1: magnetic reconnection in turbulent current sheets
We carried out 2D simulations of a thermal plasma perturbed by long-wavelength Alfvénic fluctuations. A number of different cases were simulated in order to test for the influence of the plasma-beta, the simulation box size, the mass ratio, and the number of particles. The largest simulations had a domain size of 410x410 ion skin depths resolved by 9600x9600 grid points. The electron and ion plasma-? was varied from 0.1 to 2. The ion to electron mass ratio was set to 25 or 100. The number of particles per cell was varied from 300 to 2700, leading up to a total of 0.5x10^11 particles for the largest utilized simulation box. As the times goes by wave-wave interactions from the initial perturbations led to the formation of a fully developed turbulence cascade. Current sheets are formed out of this turbulence through which magnetic reconnection occur One of our most important results was the quantification of the deviations of a particle distribution function from a thermal Maxwellian distribution function. Those deviations were quantified by a quantity called electron non-gyrotropy, which basically measures the dominance of the non-diagonal electron pressure terms. We found that the non-gyrotropy was concentrated in and near the current sheets that undergo magnetic reconnection. This quantity as well as the maximum values of the current density diminishes for a high plasma-beta, indicating that in those plasmas turbulent magnetic reconnection operates with a lower efficiency. A higher mass ratio (from 25 to 100) leads to an increase of the maximum values of the current density as well of the non-gyrotropy. This highlights the need to correctly resolve the electron scales (i.e. the use of a more realistic mass ratio) in order to correctly capture the physics of this system.

Results sub-project 2: pulsar radio emission
We carried out two different types of 1D relativistic PIC simulations by means of specific and optimized high-order solvers and shape functions. We analyzed the evolution of the relativistic streaming instabilities as a mechanism for pulsar radio emission. Moreover, we found that streaming instabilities may form solitary-like waves that were proposed as one of the most promising coherent radio sources but never found before in simulations. We also investigated for the first time the process of coherent radio emission during the nonlinear interaction of electron-positron bunches. We found that the main parameter influencing the bunch evolution is the initial drift between electrons and positrons. Only a very small drift allows faster generation of waves with few orders higher energy densities. We also estimated the bunch coherent radio emission properties by postprocessing of the simulational data and applying the linear acceleration emission mechanism. We found that the emission of interacting bunches with initial drifts between species is very similar to pulsar observations, in particular the predicted spectrum

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