The effect of magnetic topology on particle acceleration in a three-dimensional reconnecting current sheet: a test-particle approach

Electron and proton acceleration by a drifted super-Dreicer electric field is investigated in a strongly compressed non-neutral reconnecting current sheet (NRCS). The guiding field is assumed to be constant within a reconnecting current sheet (RCS) and parallel to the direction of the drifted electric field. The other two magnetic field components, transverse and tangential, are considered to vary exponentially and linearly with distances from the X-nullpoint. The proton and electron energy spectra are calculated numerically in a model RCS with different magnetic field topologies by solving an equation of motion in the test-particle approach with some test with a particle-in-cell (PIC) approach. Three kinds electric field generated inside a RCS are considered: a drifted electric field caused by the plasma inflows formed during a magnetic reconnection process; a polarization electric field induced by the accelerated protons and electrons; and a turbulent electric field induced by instabilities generated by accelerated particles. Electron and proton densities, and energy spectra inside a RCS and at ejection are found to be strongly affected by the magnetic field topology: for stronger magnetic fields the spectra are softer having a small higher-energy cutoff while for weaker magnetic fields the spectra are harder with much larger upper cutoff energies. Depending on the magnetic component ratios and drifted electric field magnitude, particles are found to be ejected either as quasi-thermal flows with very high temperatures or as focused power-law beams. A polarization field is found to reduce the acceleration time inside a RCS and to increase the energy gained by particles at acceleration by a pure drifted electric field by a few orders of magnitude. The turbulent electric field induced by the two beam instabilities of the same kind of particles leads to a significant increase in the number of particles with higher energies resulting in a flattening of their energy spectra.