A Kinetic Model for Electron Heating in Magnetic Reconnection Exhausts
Blake Wetherton, University of Wisconsin-Madison
Magnetic reconnection provides energy to heat particles in the ambient plasma and is generally thought to cause superthermal electrons observed in solar flares and Earth's magnetosphere. Various models exist to explain this electron energization, including multiple x-line models that invoke a series of energizations to eventually generate the nonthermal distribution. This requires a volume-filling set of flux ropes, and while such ropes have been observed in the magnetosphere, there is little evidence that they should be volume-filling. As such, there should be efficient heating processes on the level of single x-lines. Parallel beams of cold electrons are often seen streaming into the reconnection region, accelerated inward by a parallel potential. We investigate a method of electron bulk heating wherein energy is exchanged through the parallel potential into bulk streaming energy in beams and is then converted thermally through an effective scattering process. This scattering process is based on the breakdown of the electron magnetic moment as an adiabatic invariant in the reconnection exhaust, which causes the distribution to be independent of the magnetic moment in that region. A simplified differential equation has been derived to explain this thermalization, though it depends on several parameters related to the efficiency of first- and second-order Fermi acceleration that do not yet have well-constrained values and dependencies. This model could potentially be extended to a scale-invariant model that could be useful for large-scale systems that are not amenable to kinetic simulation, such as the solar corona, a significant goal of reconnection studies. First results from VPIC simulations designed to provide constraints for these parameters across a range of upstream conditions will be presented.
Abstract Author(s): Blake Wetherton, William Daughton, Jan Egedal