Study on the pitch angle diffusions of energetic electrons caused by EMIC waves in the Earth's radiation belts

Abstract

Energetic electrons in the Earth’s radiation belts have not only a great effect on space technology and ground-based systems but also have important physics, such as their formation, acceleration, and losses. Electromagnetic Ion Cyclotron (EMIC) waves generated through the temperature anisotropy of hot ring current ions resonantly interact with these energetic electrons, leading their pitch angles to the loss cone and subsequently causing precipitation into the atmosphere. This wave-particle interaction can be dealt with through theoretical and statistical approaches, specifically the Fokker-Planck diffusion equation. Hence, this dissertation presents a comprehensive study of the pitch-angle diffusion and scattering of energetic electrons caused by EMIC waves with the calculation of the pitch-angle diffusion rates, loss timescales and numerical simulations. We analytically calculated the bounce-averaged pitch-angle diffusion coefficients and precipitation timescales of electrons related to He-band and H-band EMIC waves at the prevalent regions of EMIC waves and the EMIC-driven precipitation of magnetic L shell parameters when equal to 4, 6, and 8 with respect to the geomagnetic activity using quasi-linear Fokker-Planck formulae, the Tsyganenko 04 (T04) magnetic field model, and a wave spectra model stemming from observations. The results provide pitch-angle diffusion coefficients enhanced by up to several orders of magnitude and lower electron energies resonant with EMIC waves compared to those based on an earlier dipole model. We suggest that a realistic field model such as the T04 model should be used for the calculation of bounce-averaged diffusion coefficients. A reduction in the magnetic field strength is the main cause of the enhanced diffusion coefficients and resonant energies estimated with the T04 model relative to those determined when using the dipole model. The bounce-averaged diffusion coefficients were highly proportional to the inversion of the equatorial magnetic field strength, and we found that scaling the diffusion coefficients with the equatorial magnetic field strength is a good approximation to account for the effect of a realistic field model during the diffusion modeling process. The energetic electron flux was numerically simulated using the Radiation Belt Environment (RBE) model, incorporating the pitch-angle diffusion caused by EMIC waves for the geomagnetic storm event of 23 - 27 October 2002. This result showed remarkable decreases in the energetic (1.8 - 9 MeV) electron flux during the storm event, compared to those without EMIC waves. Furthermore, the energetic electron flux has a comparable levels with those observed from SAMPEX spacecraft. We hence verified that EMIC waves cause the loss of energetic electrons in the radiation belts by the pitch-angle diffusion of these energetic electrons. We expect that our study will contribute to better space weather forecasting and climate modeling.