Sensitivity of EMIC Wave‐Driven Scattering Loss of Ring Current Protons to Wave Normal Angle Distribution

Electromagnetic ion cyclotron 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 electromagnetic ion cyclotron 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 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 equatorial pitch angle protons and overestimates the scattering of high equatorial pitch angle protons by orders of magnitude. Our results suggest that the wave normal distribution of electromagnetic ion cyclotron waves plays an important role in the pitch angle evolution and scattering loss of ring current protons and should be incorporated in future global modeling of ring current dynamics. Electromagnetic ion cyclotron (EMIC) wave is a class of electromagnetic wave that is frequently observed in the Earths' magnetosphere. EMIC waves can cause the loss of ring current protons by scattering them into the atmosphere. Previous attempts to quantify the loss of ring current protons by EMIC waves are mostly limited to the assumption that the waves are propagating exactly along the direction of geomagnetic field line. In this study, we show that this assumption will seriously break down once the waves are obliquely propagating. The obliquity of EMIC waves not only influences the pitch angle evolution of ring current protons but also affects their loss time scales. Our study confirms the importance of including the obliquity of EMIC waves in the modeling efforts of ring current dynamics. The wave normal angle distribution strongly affects the EMIC wave‐driven scattering loss of ring current protonsQuantitative differences in the diffusion coefficients between field‐aligned and oblique waves are presentedPitch‐angle scattering of higher‐energy ring current protons is more sensitive to the variation of wave normal angle distribution