Particle Heating and Acceleration During Magnetic Reeconnection in a Laboratory Plasma
Authors: Jongsoo Yoo, et. al.
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-
uid regime, where the motion of the ions is decoupled from that of the electrons
within the ion di usion region.
Our experimental data shows that the in-plane (Hall) electric eld plays a key role in ion heating
and acceleration. The electrostatic potential that produces the in-plane electric fi eld is established
by electrons that are accelerated near the electron di usion region. The in-plane pro le of this
electrostatic potential shows a "well" structure along the direction normal to the reconnection
current sheet. This well becomes deeper and wider downstream as its boundary expands along the
separatrices where the in-plane electric fi eld is strongest. Since the in-plane electric fi eld is 3-4
times larger than the out-of-plane reconnection electric field, it is the primary source of energy
for the unmagnetized ions. With regard to ion acceleration, the Hall electric fi eld causes ions
near separatrices to be ballistically accelerated toward the out
flow direction. Ion heating occurs
as the accelerated ions travel into the high pressure downstream region. This downstream ion
heating cannot be explained by classical, unmagnetized transport theory; instead, we conclude
that ions are heated by re-magnetization of ions in the reconnection exhaust and collisions. Two dimensional
(2-D) simulations with the global geometry similar to MRX demonstrate downstream
ion thermalization by the above mechanisms.
Electrons are also signi cantly heated during reconnection. The electron temperature sharply
increases across the separatrices and peaks just outside of the electron difusion region. Unlike
ions, electrons acquire energy mostly from the reconnection electric fi eld, and the energy gain
is localized near the X-point. However, the increase in the electron bulk
flow energy remains
negligible. These observations support the assertion that efficient electron heating mechanisms
exist around the electron difusion region and that the heat generated there is quickly transported
along the magnetic fi eld due to the high parallel thermal conductivity of electrons. Classical Ohmic
dissipation based on the perpendicular Spitzer resistivity is too small to balance the measured heat
flux, indicating the presence of anomalous electron heating.
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Submitted to: Physics of Plasmas (January 2014)
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Download PPPL-4979 (pdf KB pp)
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