报告题目：Simulation of electron acceleration by Kinetic Alfven waves in the topside ionosphere
报告摘要：Kinetic Alfven wave is a major candidate responsible for the auroral electron acceleration. When the perpendicular wave lengths become comparable to either ion acoustic gyro radius or electron inertial length, Alfven waves carry parallel electric field that can directly accelerate electrons leading to aurora. A one dimensional kinetic model is constructed to simulate the electron acceleration by kinetic Alfven waves. The electrons are divided into cold and hot electrons and treated separately. Both components are described by the Vlasov equation. The ions are treated as fluid. In the topside ionosphere, the density of cold electrons from the ionosphere becomes comparable to the density of hot electrons from the magnetosphere. Results from the drift kinetic model capture the mode conversion from kinetic Alfven waves to electron acoustic waves in the topside ionosphere. When the electron acoustic waves propagate into the transition region, where the electron density of ionospheric origin becomes comparable to that of magnetospheric origin, the steep temperature gradient leads to the mode conversion. The electron acoustic waves are short-lived by dissipating their energy into the electron energization, thus revealing a new type of electron acceleration in the topside ionosphere. Besides, the results also show that when Alfven waves encounter the sharp gradient of Alfven speed below the transition region, the electrons accelerated by the waves become super-Alfvenic, and the width of burst structures becomes much wider than the electron inertial length. Consequently, the background electrons carry the oppositely field-aligned current due to plasma oscillation. It is demonstrated that the current carried by the electrons exceeding the wave front is balanced by the reverse current carried by background electrons. This mechanism can be used to reasonably explain observations of the electron bursts accompanied by little net field-aligned current.