Experimental Studies of Particle Acceleration and Heating During Magnetic Reconnection

Abstract

Energy conversion from magnetic energy to particle energy during magnetic reconnection is studied in the collisionless plasma of the Magnetic Reconnection Experiment (MRX). The plasma is in the two-fluid regime, where the ion motion is decoupled from that of the electron within the so-called ion diffusion region.

For ion heating and acceleration, the role of the in-plane (Hall) electric field is emphasized. The in-plane potential responsible for the Hall electric field is established by electrons that are accelerated near the small electron diffusion region. The in-plane electrostatic potential profile shows a well structure along the direction normal to the reconnection current sheet that becomes deeper and wider downstream as its boundary expands along the separatrices where the in-plane electric field is strongest. Since the Hall electric field is 3--4 times larger than the reconnection electric field, unmagnetized ions obtain energy mostly from the in-plane electric field, especially near the separatrices. The Hall electric field ballistically accelerates ions near the separatrices toward the outflow direction. After ions are accelerated, they are heated as they travel into the high pressure downstream region. This downstream ion heating cannot be explained by classical, unmagnetized transport theory, which suggests that the magnetic field should be important due to an effect called re-magnetization.

Electrons are also significantly heated during reconnection. The electron temperature sharply increases across the separatrices and peaks just outside of the electron diffusion region. Unlike ions, electrons acquire energy mostly from the reconnection electric field and the energy gain is localized near the X-point. However, the electron bulk flow energy increase remains negligible. These observations support the assertion that efficient electron heating mechanisms exist around the electron diffusion region and that the generated heat is quickly transported along the magnetic field due to the high parallel thermal conductivity of electrons. Classical Ohmic dissipation based on the perpendicular Spitzer resistivity is too small to compensate the heat flux, indicating the presence of anomalous electron heating.

Finally, a total energy inventory is calculated based on analysis of the Poynting, enthalpy, flow energy, and heat flux in the measured diffusion layer. More than half of the incoming magnetic energy is converted to particle energy during collisionless reconnection. Unlike in the Sweet-Parker model, the outgoing Poynting flux is not negligible, which is due to considerable Hall fields, i.e., the quadrupole out-of-plane magnetic field and the in-plane electric field. The total ion energy gain during reconnection is larger than that of electrons, since the energy gain occurs over a broader region. The total ion thermal energy gain is larger than the increase of the ion flow energy. Finally, the electron thermal energy gain is comparable to the ion thermal energy gain, while the electron flow energy remains insignificant.