MMS Observations of Dayside Warm (Several eV to 100 eV) Ions in the Middle and Outer Magnetosphere

Abstract

AbstractWarm (several eV to 100 eV) ions are an important, and still poorly understood, component of the magnetospheric plasma environment. We present the first comprehensive statistical analysis of several distinct populations of warm ions in the dayside middle and outer magnetosphere. We analyze 7 months (1 September 2015–31 March 2016) of Magnetospheric Multiscale Hot Plasma Composition Analyzer data comprising 734,200 moments (density, temperature) and energy‐dependent pitch angle distributions (PADs) of three major ion species (H+, He+, and O+) with energy ≤100 eV. PADs are represented by an energy‐averaged index that characterizes the shape of the PAD as field‐aligned (FA), pancake, or isotropic. We use filtering by temperature, pitch angle, and concentration to distinguish different populations, and obtain new and more complete information about average density, temperature, PADs, and composition. Our analysis explores two known populations of warm ions: the warm plasmasphere (WPS) and the warm cloak/trough (C/T). The WPS is a higher‐temperature, higher‐L extension of the duskside plasmaspheric bulge, containing mostly trapped (pancake/isotropic) ions with an H+:He+:O+ order of ion dominance. The C/T contains mostly FA warm ions with a dawnward (duskward) temperature gradient for H+ (He+ and O+), lower densities, and an H+:O+:He+ order of ion dominance. The WPS‐C/T overlap contains a mixture of the two populations (e.g., FA He+ in WPS, trapped O+ in C/T). Pancake (FA) PADs are correlated with higher (lower) density/temperature. Our analysis also identifies warm ions in the low‐energy plasma sheet. Our work consolidates and systematically extends the characterizations of warm ions reported in previous studies.