Production of energetic electrons during magnetic reconnection

J. F. Drake, W. Thongthai and M. A. Shay, University of Maryland
M. Swisdak, Naval Research Laboratory

The energization of electrons during magnetic reconnection is
explored. The production of energetic electrons has been documented in
observations of solar flares, reconnection in the Earth's
magnetosphere and in laboratory experiments yet the understanding of
these widespread observations remains poor. The parallel electric
field that develops during reconnection controls the acceleration of
electrons in situations with an ambient guide field. The structure of
this parallel electric field differs greatly from earlier
ideas: during reconnection a deep cavity in the electron and ion
density forms that extends through the x-line along one of the
magnetic separatrices. This density cavity enables the parallel
reconnection electric field to remain finite over an extended region,
forming an acceleration cavity that controls the strength of the
current layer that defines the dissipation region. The acceleration of
electrons in a single pass through this cavity, however, does not
explain the observed powerlaw spectra of energetic electrons seen in
some data. In particle simulations a distinct high energy tail,
extending well above the rest energy, forms on the electron energy
distribution. These very energetic electrons are found to arise from
multiple encounters with acceleration cavities. The simulations
provide evidence that reconnection with a guide field is dominated by
the formation of many islands and that electron energization
results from multiple accelerations. In this picture the surprising
amount of energy going into electrons in comparison with ions is first
because of the significant length of the acceleration cavities (large
numbers of electrons enter the cavities) and because of their high
mobility -- they can rapidly interact with many cavities to reach high
energy.