Observations of Fast Local Loss of High‐Energy Ring Current Protons Associated With Deepening Local Minimum in Phase Space Density

Abstract The development of a deepening local minimum in phase space density (PSD)‐ profile indicates fast local loss potentially caused by wave‐induced scattering. The identification and characterization of proton PSD deepening minima are important for investigating the ring current loss and overall dynamics. Using multiyear Van Allen Probes observations, we analyze ∼10–100s keV proton PSD and report >100 keV proton deepening PSD minima for the first time. The overall occurrence rates of proton deepening local minimum peaks at ∼3%, mainly located at = 4.5–5.0 near the plasmapause. The occurrence rate increases with the decrease of AL index and increase of solar wind dynamic pressure. The theoretical resonance energy of protons with typical He‐band electromagnetic ion cyclotron (EMIC) waves agrees with the energy of protons with deepening PSD minima. Thus, EMIC waves are the likely cause of the deepening PSD minimum and contribute to the fast local loss of ring current protons.