Date of Award

Fall 2015

Project Type

Dissertation

Program or Major

Physics

Degree Name

Doctor of Philosophy

First Advisor

Lynn M Kistler

Second Advisor

Li-Jen Chen

Third Advisor

Roy B Torbert

Abstract

We investigate three aspects of magnetic reconnection where kinetic processes play a strong role: hot O+ and cold ion behaviors in magnetopause reconnection, their effect on the reconnection rate, and electron heating during magnetotail reconnection. At the magnetopause, we analyze observed velocity distribution functions (VDFs) and find that hot O+, despite its large gyroradius, almost fully participates in the reconnection outflow with a demagnetization-pickup process. Finite Larmor radius effects are apparent, controlling how far the ions extend to the magnetosheath side. For cold ions, if entering the central diffusion region, they behave like hot ions; otherwise, they convect with the magnetic field adiabatically. How these species behave determines their effect on the reconnection rate. We compare the observed reconnection rate with predictions of the fluid-based Cassak-Shay formula for 8 events. The measured rate does correlate with the predictions when all magnetospheric and magnetosheath populations are included, but the correlation is better when just magnetosheath populations are used. This indicates possible deviations from the Cassak-Shay theory caused by the kinetic effects of the different populations. The diffusion region aspect ratio does not show a clear dependence on the O+ abundance, density asymmetry or guide field. To understand the electron heating, using a particle-in-cell simulation,we divide the reconnection exhaust into four sub-regions based on electron temperatures and VDFs. The same defining distributions are found in observations. The associated acceleration mechanisms are determined by tracing particles through the simulation fields. Electrons obtain initial energization from the electron diffusion region (EDR) electromagnetic fields and the parallel potential, and pitch angle scattering isotropizes the distribution. Further downstream, electrons with initial high v∥ (v⊥) are mainly accelerated with the curvature (gradient-B) drift opposite to the out-of-plane electric field, generating distinct populations in VDFs. We estimate the heating coefficient, rh = kBΔTe/miv2 Ai, using a simple model of the outflowing EDR distribution. The electron heating in 11 magnetotail reconnections shows rh∼1.5%-2.6% with considerable variations caused by the magnetotail pressure unloading, in reasonable agreement with the simulation results. Thus, both for heavy ions and electrons, we find the key to understanding the reconnection dynamics is in interpreting the individual particle behavior.

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