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TU Berlin

Kinetic instabilities mediating particle acceleration in collisionless 3D magnetic reconnection

HPC Project: Kinetic instabilities mediating particle acceleration in collisionless 3D magnetic reconnection.

Project ID: pjr.1602-008 (PRACE, DECI-13th call)
Supercomputer cluster: PDC/KTH, Stockholm (Sweden)
Duration of the project: 02.2016-08.2017
Mcore-hours granted: 3
Principal Investigator: Prof. Dr. Jörg Büchner
Researchers: Patricio Muñoz
Numerical code: ACRONYM


Magnetic  reconnection is an essential process for conversion from magnetic into kinetic energy in many space environments. Although it has been studied extensively in the fluid (MHD) framework and in 2D, that it is not true for a fully kinetic 3D case. The kinetic approach, justified due to the collisionless nature of many space plasmas, allows to study, i.e.: electron acceleration generated from micro-instabilities, with important  observational signatures.  Previous works have shown that magnetic reconnection can be an efficient source of non-thermal electrons in the relativistic regime, although the conditions for that in non-relativistic cases, more suitable for spaces plasma in the Solar Sytem, are not clear yet. On the other hand, a 3D geometry allows current aligned instabilities able to produce enhanced micro-turbulence and influence the electron dynamics in magnetic reconnection. However, most of the simulation studies of electron acceleration in magnetic reconnection have remained 2D due to computations constraints, but it is uncertain if the same mechanisms will operate in realistic 3D configurations. For these reasons, our aim is to study electron acceleration in non-relativistic magnetic reconnection with fully kinetic Particle-in-Cell 3D numerical simulations.  Note that these simulations are computationally more expensive than their relativistic counterparts.  We propose to clarify the mechanisms generating these non-thermal populations, by relating them with the signatures of kinetic micro-instabilities expected under several parameter ranges. This will allow to improve the understanding of  the micro-physics of these essential kinetic plasma processes, with far-reaching consequences for phenomena like solar flares, magnetic substorms and similar processes in the solar wind.


By means of 3D PIC simulations, we found a new two-stage mechanism of electron acceleration near X-lines of 3D collisionless guide-field magnetic reconnection in the non-relativistic regime. The first stage consists of a pre-acceleration by parallel electric fields (aligned to the magnetic field). The second stage takes place during the nonlinear stage of reconnection, where electric and magnetic fields become filamentary structured due to streaming instabilities. This filamentation produces an additional curvature-driven acceleration in the guide-field direction. The spectrum of energetic electrons can be described as a power law with an spectral index of ~ -1.6.

In addition, we also analyzed the self-generated kinetic turbulence by 3D PIC-code simulations in both frequency and wavenumber domain. Our investigations reveal reconnection rates and kinetic turbulence spectra with some common features to those obtained in current in-situ spacecraft observations (e.g., MMS) as well as in laboratory experiments (MRX, VTF, Vineta-II). During the course of evolving reconnection, kinetic scale turbulence is characterized by a broadband power-law spectrum beyond the lower-hybrid and up to the electron frequencies. In the frequency space, the spectral index is close to -2.8 at the reconnection X-line. The kinetic turbulence at this location is caused by electron streaming instabilities and shear flows, which develop preferentially for thin enough current sheets. This kind of turbulence is correlated with enhanced reconnection rates.



Muñoz, P. A., & Büchner, J. (2018). Kinetic turbulence in fast 3D collisionless guide-field magnetic reconnection. Physical Review E, 98(4), 043205. https://doi.org/10.1103/PhysRevE.98.043205

Muñoz, P. A., & Büchner, J. (2018). Two-stage electron acceleration by 3D collisionless guide field magnetic reconnection.  The Astrophysical Journal, 864(1), 92. https://doi.org/10.3847/1538-4357/aad5e9

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