Sensitivity of EMIC wave-driven scattering loss of ring current protons to wave normal angle distribution

Electromagnetic ion
cyclotron (EMIC) waves have long been recognized to play a crucial role in the dynamic
loss of ring current protons. While the field-aligned propagation approximation
of EMIC waves was widely used to quantify the scattering loss of ring current
protons, in this study, we find that the wave normal distribution strongly
affects the pitch-angle scattering efficiency of protons. Increase of peak wave
normal angle or
angular width can considerably
reduce the scattering rates of < 10 keV protons. For > 10 keV protons, the field-aligned propagation
approximation results in a pronounced underestimate of the scattering of intermediate pitch-angle protons and overestimates the scattering of high pitch-angle protons by orders of magnitude. Our
results suggest that the wave normal distribution of EMIC waves plays an
important role in the pitch-angle evolution and scattering loss of ring current
protons and should be carefully incorporated in future global modeling of ring
current dynamics.