The analysis and modeling of microburst electron precipitation using pitch-angle diffusion theory.

摘要

This dissertation presents a study of the microburst electron precipitation, characterized by quasi-periodic bursty precipitation of electrons in the morningside auroral zone, with a typical duration of {dollar}sim{dollar}.2-.3 seconds. Most of the existing knowledge about this process came from the study of secondary effects like bremsstrahlung X-rays, with little information about the primary electrons which is necessary in order to comprehend the underlying physics. A Nike-BlackBrant V sounding rocket, designed to investigate this phenomenon, was launched on May 6, 1993, from the Poker Flat launching station, Fairbanks, Alaska. This experiment was the first to measure the microburst characteristics in detail, and to look at particles and waves simultaneously. This work was motivated by previously suggested correlation between microbursts and VLF wave activity.; As a starting point, this dissertation begins with a brief descript ion of the geomagnetic environment and a summary of the present knowledge about microbursts. The rocket instrumentation, launch conditions, and an overview of the acquired data are presented in the following chapters. The process of pitch-angle diffusion in the velocity space, due to the cyclotron resonance interaction between electrons and whistler waves, has been considered as a potential mechanism responsible for microbursts. The nature of this interaction is discussed, followed by numerical models developed in the framework of this theory.; We find that an enhancement in the diffusion coefficient D(t) can cause a microburst-like structure in particle precipitation. The microburst width is approximately equal to the quarter-bounce period {dollar}Tsb{lcub}B{rcub},{dollar} and is fairly insensitive to the duration of D(t). This result could possibly explain why microbursts have a characteristic width. Our simulation shows the rise time of the burst is proportional to {dollar}Tsb{lcub}B{rcub},{dollar} which agrees with the observed data. The variation of the shape of the burst can be explained as a consequence of different forms of D(t). We have also examined the possibility of self-induced wave growth. This wave enhancement was favored by a smaller value of total electron density {dollar}Nsb{lcub}e{rcub}{dollar} and a larger value of energetic electron density {dollar}Nsb{lcub}w{rcub}{dollar}, and was also found to be dependent on the length of the interaction region and electron energy. In summary, the pitch-angle diffusion process is suggested to be the primary mechanism behind the microburst electron precipitation.