Warm Plasmaspheric Ions in the Ring Current Overlap Region

Abstract:

Warm ions in the outer plasmasphere are a few eV to ~10 eV, with pancake or isotropic pitch angle distributions. They are believed to be the heated, warm-energy tail of the otherwise cold plasmaspheric ion distribution. They have been observed mostly in the duskside bulge region and within plumes, and in the outer L-shells of the main plasmaspheric torus. Densities of warm plasmasphere ions are usually 1 to 10 cm^–3 in the duskside bulge region, and as high as ~100 cm^–3 in the main torus. The heating mechanism responsible for warm plasmasphere ions is still unknown. Some studies suggest warm ion pancakes result from perpendicular heating of refilling ion beams. Other work indicates warm pancakes can be produced by heating of cold pancakes. One candidate heating mechanism is ion cyclotron waves in the ring current overlap region. To investigate the statistical properties of warm plasmasphere ions, we compute density and temperature moments from dayside ion data obtained by the Magnetospheric Multiscale (MMS) Hot Plasma Composition Analyzer (HPCA) during 1 September 2015 to 31 March 2016 (7 months). We determine the statistical distributions of warm plasmaspheric ions: density, temperature, spatial location, ion species, ion concentration, and pitch angle. We use a test particle code to investigate whether ion cyclotron waves can provide the heating necessary to produce warm plasmaspheric ions.