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

Warm (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. Plain Language Summary: The plasmasphere is often defined as the cold and dense innermost region of the magnetosphere. The ring current may be defined as the torus of hot ions encircling the planet at several Earth radii of geocentric distance. Warm plasma (several eV to 100 eV) is an intermediate‐energy population. In this paper, 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 data from the Magnetospheric Multiscale mission. Several‐eV to 100‐eV ions constitute a large reservoir of mass and momentum that can influence numerous basic, system‐level aspects of the magnetosphere. Warm ions are also a "bridge population" whose intermediate‐energy dynamics reveal heating and energization processes that trap ions to enable plasmaspheric refilling and transform very cold, ionospheric‐origin plasma into an integral component of the stormtime plasma sheet and ring current. Key Points: Statistical analysis of dayside warm (≤100 eV) ions uses 734,200‐point database of density, temperature, and pitch angle (PA) distributionsFiltering by temperature, PA, and concentration helps distinguish and separate different warm ion populationsThis analysis consolidates and systematically extends the characterizations of the warm plasmasphere and the warm cloak/trough (C/T) [ABSTRACT FROM AUTHOR]