A Case Study of Transversely Heated Low­Energy Helium Ions by EMIC Waves in the Plasmasphere

Abstract

The Van Allen Probe A spacecraft observed strong ∼0.5­Hz helium (He+) band and weak ∼0.8­Hz hydrogen (H+) band EMIC waves on April 17, 2018, at L = similar to 4.5-5.2, in the dawn sector, near the magnetic equator, and close to the plasmapause. We examined low­energy ion fluxes observed by the Helium Oxygen Proton and Electron (HOPE) instrument onboard Van Allen Probe A during the wave interval and found that low­energy He+ flux (<10 eV) enhancements occur nearly simultaneously with He­band and H­band EMIC wave power enhancements in a direction mostly perpendicular to the background magnetic field without significant low­energy H+ and O+ flux variations. We suggest that cold He+ ions (<1 eV) are preferentially and transversely heated up 10 eV through the interaction with EMIC waves inside the plasmasphere. The low­Earth orbit spacecraft observed localized precipitations of energetic protons in the upper ionosphere at subauroral latitudes near the magnetic field footprint of Van Allen Probe A. Our observations provide a clear evidence that EMIC waves play an important role in the overall dynamics in the inner magnetosphere, contributing to the high­energy particle loss and low­energy particle energization.