The dynamics of non-thermal electrons generated during electron-only magnetic reconnection

Particle acceleration has been a long-studied topic of research in astrophysical processes as diverse as solar/stellar flares and geomagnetic storms. A key component of such processes is magnetic reconnection (e.g., the CSHKP model of solar flares includes magnetic reconnection), which causes the conversion of magnetic energy to particle kinetic energy in a plasma and also causes the acceleration of charged particles in the current sheet associated with reconnection. In the case of zero-guide-field reconnection for collisionless systems, acceleration in the current sheet can explain the generation of non-thermal particle populations quite well. However, the dynamics of such populations are complicated by non-zero guide fields and our understanding of such systems is incomplete. Such is the case with measurements performed in the PHAse Space MApping (PHASMA) experiment at West Virginia University. In PHASMA, two, one-meter-long flux ropes generated by pulsed plasma guns reconnect on temporal and spatial scales relevant for electron dynamics only, thereby precluding the influence of ion motion and resulting in "electron-only" magnetic reconnection. Measurements of the non-thermal component of the electron energy distribution function parallel to the guide field (performed with an energy analyzer) and the corresponding thermal component (measured by the parallel Thomson scattering system on PHASMA) for a guide field to reconnecting field ratio of 10 show the following features: a three-component energy distribution of non-thermal electrons located away from the X-point with particles as energetic as 13 times the thermal speed (in tandem with heating at the X-point), dispersion of non-thermal electrons propagating towards the X-point during reconnection, and the development of a knee in the electron velocity distribution function towards the end of reconnection. These features disappear at higher guide fields (ratios of 15 and 25). We posit that the disappearance of these non-thermal features at higher guide fields is caused by the influence of the in-plane components of the parallel electric field, though no such explanation is available for the knee observed in the velocity distribution function.