Testing the theory of radiation belt electron loss by hiss and electromagnetic ion cyclotron waves

Abstract

Hiss, chorus and electromagnetic ion cyclotron waves (EMIC wave) are three major wave modes that are widely investigated and included in the radiation belt electron models to explain electron precipitation. The quasi-linear theories of electron loss through pitch angle diffusion by hiss and EMIC waves were proposed in 1970s. Since then the testing of the theories is still going on though some progresses had been made. Comparison of theoretical predictions to electrons distribution at loss cone is one effective way to evaluate the theories. The main obstruction of loss cone testing was from the lack of measurements of the electron loss cone distribution with enough pitch angle and energy resolution and simultaneous wave activities at the heart of radiation belt. This thesis is devoted to testing the hiss and EMIC waves diffusion theories from the perspective of the electron loss cone distribution by utilizing the previously unnoticed overlap of UARS and CRRES missions in 1991. The conclusions are as following: (1) Two cases showing the consistency between quasi-linear theory of hiss diffusion and observed loss cone distribution are found. (2) In 25 out of 38 cases, hiss wave power is far insufficient in precipitating the large amount of electrons observed. Loss mechanisms other than hiss, chorus and EMIC waves are needed to account for the discrepancy. (3) Three EMIC wave events were investigated. In the first case, the isotropic distribution (sign of strong diffusion) is caused by process other than EMIC wave diffusion demonstrating that simultaneous presence of EMIC wave and electron precipitation does not guarantee any connection between the two. Large discrepancy between quasi-linear theory of EMIC wave diffusion and observed electron loss cone distribution is found from the second case. The strong depletion of electrons by EMIC waves predicted by the current theory was not found. In the third case the resonant energy goes beyond the instrument limit of electron detectors thus no conclusion is drawn.