Origins of Highly Structured Distribution Functions in Magnetic Reconnection Exhausts: Understanding Electron Acceleration and Heating

Abstract:

Electron velocity distribution functions (VDFs) during reconnection with negligible guide field from particle in cell (PIC) simulations and Cluster observations are studied to further understand electron acceleration and heating. Until recently, electrons in the exhaust of reconnection with negligible guide field were thought to be isotropic. PIC simulation results with zero guide field reveal that near the time of peak reconnection, VDFs become highly structured in magnetic islands and open exhausts. Ring, arc, and counterstreaming populations are generic and lasting constituents of exhaust electron VDFs. Analyses of particle trajectories indicate that a number of mechanisms including Fermi acceleration, the parallel potential, and adiabatic heating contribute to the energization of exhaust electrons. Near the electron diffusion region (EDR), exhaust electrons exhibit large Te⊥ due to ring and arc populations of electrons accelerated in the EDR. Farther away from the EDR, the VDFs show a mixture of electrons from the EDR and those crossing the separatrix from the inflow. Pitch angle scattering is effective near the exhaust midplane, away from the EDR and before reaching the magnetic pileup region, producing isotropic, high-energy electrons, while the low energy exhaust electrons exhibit the anisotropy Te// > Te⊥ characteristic of the inflow. The work done on the electrons by the perpendicular electric field between the end of EDR and the magnetic pileup region is due to Fermi acceleration which leads to a net increase in the electron's parallel velocity. For the net increase of electrons' v⊥ beyond the EDR, pitch angle scattering effectively converts v// gained by acceleration from the parallel potential into v⊥. Electron's v⊥ further increases downstream through adiabatic heating from the increasing magnetic field in addition to less efficient pitch angle scattering. The parallel potential and the magnetic bottle together determine the trapped-passing boundary of the distribution, and thus the apparent temperature. Thus far, we find no evidence that the parallel potential accelerates passing electrons as suggested by a prior work. Further study is underway to quantify the overall amount of energy which goes into heating electrons and generating counterstreaming beams in the magnetotail.