  Magnetic reconnection can convert magnetic energy into kinetic energy of
non-thermal electron beams. Those accelerated electrons can, in turn, cause
radio emission in astrophysical plasma environments such as solar flares via
micro-instabilities. The properties of the electron velocity distribution
functions (EVDFs) of those non-thermal beams generated by reconnection are,
however, still not well understood. In particular properties that are necessary
conditions for some relevant micro-instabilities. We aim at characterizing the
EVDFs generated in 3D magnetic reconnection by means of fully kinetic
particle-in-cell (PIC) code simulations. In particular, our goal is to identify
the possible sources of free energy offered by the generated EVDFs and their
dependence on the strength of the guide field. By applying a machine learning
algorithm on the EVDFs, we find that: (1) electron beams with positive
gradients in their 1D parallel (to the local magnetic field direction) velocity
distribution functions are generated in both diffusion region and separatrices.
(2) Electron beams with positive gradients in their perpendicular (to the local
magnetic field direction) velocity distribution functions are observed in the
diffusion region and outflow region near the reconnection midplane. In
particular, perpendicular crescent-shaped EVDFs (in the perpendicular velocity
space) are mainly observed in the diffusion region. (3) As the guide field
strength increases, the number of locations with EVDFs featuring a
perpendicular source of free energy significantly decreases. The formation of
non-thermal electron beams in the field-aligned direction is mainly due to
magnetized and adiabatic electrons, while in the direction perpendicular to the
local magnetic field it is attributed to unmagnetized electrons.

Plasma Physics;Solar and Stellar Astrophysics