Physics of collisionless reconnection in a stressed X-point collapse

TL;DR

This study investigates collisionless magnetic reconnection in a stressed X-point collapse using kinetic simulations, revealing the roles of electron pressure tensor, ion-electron mass ratio, and energy conversion dynamics.

Contribution

It provides new insights into the physical mechanisms of collisionless reconnection, including the influence of the ion-electron mass ratio and electron pressure tensor effects.

Findings

Reconnection electric field is generated by off-diagonal electron pressure tensor terms.

Reconnection rate is independent of ion-electron mass ratio for large mass ratios.

Energy is partitioned with ~40% to electrons and ~60% to ions within one Alfven time.

Abstract

Recently, magnetic reconnection during collisionless, stressed, X-point collapse was studied using kinetic, 2.5D, fully electromagnetic, relativistic Particle-in-Cell numerical code [D. Tsiklauri and T. Haruki, Phys. Plasmas, 14, 112905 (2007)]. Here we finalise the investigation of this topic by addressing key outstanding physical questions. It has been established here that: (i) reconnection out-of-plane electric field at the magnetic null is generated by the electron pressure tensor off-diagonal terms, resembling to the case of tearing unstable Harris current sheet studied by the GEM reconnection challenge; (ii) For m_i / m_e >> 1 the time evolution of the reconnected flux is independent of ion-electron mass ratio; also, in the case of m_i / m_e = 1 we show that reconnection proceeds slowly as the Hall term is zero; when m_i / m_e >> 1 (i.e. the Hall term is non-zero) reconnection is fast and we conjecture that this is due to magnetic field being frozen into electron fluid, which moves significantly faster than ion fluid; (iii) within one Alfven time, somewhat less than half (40%) of the initial total (roughly magnetic) energy is converted into the kinetic energy of electrons, and somewhat more than half (60%) into kinetic energy of ions (similar to solar flare observations); (iv) in the strongly stressed X-point case, in about one Alfven time, a full isotropy in all three spatial directions of the velocity distribution is seen for super-thermal electrons (also commensurate to solar flare observations).