MMS Observations of Warm‐Ion (E < 100 eV) Heating Inside Plasmaspheric Plumes

Abstract

AbstractNear‐Earth plasma surges sunward during enhanced magnetospheric convection (driven by dayside magnetic reconnection) to form a pathway feature called a plasmaspheric plume. This study uses Magnetospheric Multiscale (MMS) observations to investigate the heating of warm ions (H+ and He+) inside plumes. We identify 287 plume events using targeted in‐situ‐observational criteria and plasmapause test particle simulations. One plume‐crossing event by MMS shows that the scalar temperatures of warm He+ increase from ∼10 to ∼40 eV during simultaneous observations of helium‐band electromagnetic ion cyclotron (EMIC) waves, while H+ temperature is almost constant. Next, we analyze all individual observations to investigate warm‐ion heating and its possible association with EMIC waves. The statistical results show that EMIC waves associated with the plume have left‐hand polarizations and small normal angles and the temperatures of warm He+ are always higher than those of warm H+. Also, the minimum resonant energies for H+ are mostly above 100 eV, whereas 17% of warm He+ observations show that they can interact with the H+‐band EMIC waves via cyclotron resonance. These observations suggest that while EMIC waves play a role, there is an unknown mechanism for warm‐ion heating inside the plume that requires more investigation.