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Exploring kinetic physics in space plasmas using PIC simulations
Date: Friday, July 15th
Time: 2:00 pm
Place: B343 Sterling or
Speaker: Harsha Gurram, Physics PhD Graduate Student
Abstract: This dissertation investigates two major long-standing problems in space physics with the aid of kinetic simulations. Our first study delves into magnetic reconnection, a fundamental process in magnetized plasmas wherein global magnetic topology is modified and the built-up magnetic stress is transformed into plasma flows and heating. This energy conversion process is thought to be an important mechanism for particle energization in space. Here, we analyzed fully kinetic simulation results of magnetic reconnection to study how the released energy and associated signatures propagate away from the reconnection site. In contrast to previous studies, where, the Hall magnetic field structures were carried away by kinetic Alfv\'{e}n waves (KAWs). Our kinetic simulation implemented at a large numerical domain and with open boundary conditions permits large-amplitude SAWs to be excited by the reconnection dynamics. Due to the dispersive nature of the KAWs, they eventually get damped, and SAWs become the main carrier of the energy away from the reconnection site. These reconnection-driven SAWs are observed to propagate distances $\gg 9R_e$ unattenuated carrying sufficient energy and may act as a primary energy source to drive the white aurora.

Our second study examines the effects of Coulomb collisions in solar-wind heating. The temperature of the solar wind plasma expanding from the hot solar corona does not decrease with the distance as fast as predicted by the adiabatic expansion law. The heating of the solar-wind electrons results from the energy exchange of the fast electrons propagating from the corona along the background magnetic field (the beam or strahl) and the electrons trapped between the electric potential and magnetic mirror walls (the core). The level of the trapped population is a result of two competing processes—particle influx from the streaming population due to pitch-angle scattering and particle losses through the boundary due to energy diffusion. As scattering rates are a free parameter in our study, we can determine how the scattering rates affect the electron distributions, and in turn temperature of the solar wind electrons. Using 1D cylindrical simulations we found the electron temperature profiles scaled with the ratio $\nu{ee}/\nu{ei}$, higher the $\nu{ee}/\nu{ei}$ higher the electron temperatures. The dependency of the electron temperatures and trapped electron population on $\nu{ei}/\nu{ee}$ in the kinetic simulations implies that the Coulomb collisions have a significant effect on the electron temperature profiles as suggested by the collisional model.
Host: Jan Egedal
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