Estimation of pitch angle diffusion coefficients for EMIC waves with realistic field models

EMIC waves, one of the responsible mechanisms for precipitation of MeV electrons, frequently occur with high wave power near the plasmapause. We estimated the pitch angle diffusion rates related to the He band EMIC waves and the corresponding lifetimes at L=4 and L=6 for the cases of an intense storm and a quiet time, respectively, using the Tsyganenko 04 (T04) magnetic field model. The result shows enhanced pitch angle diffusion rates compared to those, by an order of magnitude, based on the dipole model. The corresponding lifetime is estimated to be less than a few sec for the case of an intense storm and less than ~10 sec for a quiet time. The reduction of the magnetic field strength is the main cause of the enhanced diffusion rates when estimations are made with the more realistic T04 model. We also calculated the diffusion rates for the H band EMIC waves and found the diffusion rates are much smaller than those associated with the He band EMIC waves. Hence, the He band is the dominant mode of the EMIC waves that are related to the precipitation of the relativistic electrons.