Date of Award

Fall 2015

Project Type

Dissertation

Program or Major

Physics

Degree Name

Doctor of Philosophy

First Advisor

Li-Jen Chen

Second Advisor

Roy B. Torbert

Third Advisor

Charles J. Farrugia

Abstract

Magnetic reconnection converts energy stored in magnetic fields to plasma kinetic energy by accelerating and heating the plasma, and is believed to be the underlying mechanism of many energetic phenomena in space. Electron distribution functions exhibit the effects of electron energization by the reconnection process. Using CLUSTER data, we have studied electron distributions in the inflow and outflow regions of magnetotail reconnection. Based on comparisons of CLUSTER measurements with PIC simulation results, we discuss the energization mechanisms. The inflow electron distributions can be characterized by their temperature anisotropy into three distinct categories: (1) anisotropic with Tepara>Teperp, (2) isotropic with Tepara=Teperp, and (3) hybrid with a lower energy anisotropic population exhibiting Tepara>Teperp with a higher energy isotropic population. The first two categories are likely associated with different temporal stages of reconnection while the third category may result from reconnection onset within the plasma sheet. Electron distributions show distinct anisotropic features in different regions throughout the reconnection exhaust. Near the electron diffusion region (EDR), distributions exhibit a temperature anisotropy of Teperp>Tepara. The electron distribution becomes isotropic between the EDR and magnetic field pile-up region. The parallel and perpendicular components of the distribution function in the pile-up region are enhanced in different ways by different mechanisms. Acceleration by the reconnection electric field during electrons' meandering orbits in the EDR, curvature and $\nabla B$ drift forces, and pitch angle scattering all contribute to form the distinct anisotropic structures of the distributions. In an effort of understanding a special type of dense electron distribution in the exhaust region, we explore the 3D structure of reconnection. The 3D magnetic field reconstruction shows that the dense distribution is associated with 3D magnetic nulls. Electron energization in 3D reconnection requires further investigation.

Download

Share

COinS