Field‐Aligned Flux Intensity of Warm Plasma Cloak Ions in the Near‐Earth Plasma Sheet: THEMIS Observations

Abstract

To understand the favorable local and external conditions under which particle fluxes of warm ions (∼10 eV–1 keV) in the near‐Earth plasma sheet can acquire higher field‐aligned intensities (FAIs), we statistically analyzed 19 years of satellite observations in the equatorial magnetosphere. FAIs can reach substantially higher values in the 0–6 MLT quadrant than others, and at locations of smaller radial distances. For the plasma sheet warm ions immediately outside the plasmapause, higher FAIs are more frequently observed when the plasmapause is smaller and thus the plasma sheet is located closer to the Earth, where the local conditions show larger magnetic field strengths, shorter field‐line lengths, and higher plasma sheet electron fluxes, while the external conditions show the solar wind conditions being favorable for driving more active geomagnetic conditions, as indicated by lower SYM‐H and higher AE* values. The changes of warm ion fluxes leading to higher FAI values are due to increases in phase space densities along field‐aligned directions, as well as adiabatic effects associated with changes in magnetic field strength and field‐line length. The dominant particle source providing such an increase in field‐aligned warm ions during stronger geomagnetic conditions is likely to be an enhancement in ionospheric outflow coming out of auroral latitudes in response to intensified electron precipitation.