Observations of the warm plasma cloak and an explanation of its formation in the magnetosphere

Abstract

[1] Previous studies of the magnetospheric plasma populations have concentrated on the low-energy (1 eV) plasma of the plasmasphere, the more energetic (1-100 keV) plasma of the plasma sheet and ring current, and the high-energy (approximately MeV) plasma of the radiation belts. A compilation of satellite measurements over the past 30 years augmented by recent observations from the Polar-TIDE instrument has revealed a new perspective on a plasma population in the middle magnetosphere. This population consists of ions with energies of a few eV to greater than several hundred eV which display a characteristic bidirectional field-aligned pitch angle distribution. Measurements from the ATS, ISEE, SCATHA, DE, and POLAR satellites establish the characteristics of this ''warm plasma cloak'' of particles that is draped over the nightside region of the plasmasphere and is blown into the morning and early afternoon dayside sector by the sunward convective wind in the magnetosphere. The satellite observations combined with the predictions of an ion trajectory model are used to describe the formation and dynamics of the warm plasma cloak.

Formation of the oxygen torus in the inner magnetosphere: Van Allen Probes observations

“…Chappell et al [] shed new light on a plasma population with energies of 10 eV–3 keV in the inner magnetosphere, compiling satellite measurements of low‐energy ions over the past 30 years and conducting a statistical analysis of the thermal ion fluxes at <400 eV measured by the Polar satellite. This low‐energy plasma population was called the “warm plasma cloak,” because it was draped over the nightside region of the plasmasphere and spread out to higher L shells in the morning and early afternoon sectors.…”

Chorus intensity modulation driven by time‐varying field‐aligned low‐energy plasma

“…This pitch angle distribution is similar to that of the low-energy electrons shown in Figure 1d, suggesting that those ions and electrons have the same source region. Such field-aligned ions have been related to ion outflows from the ionosphere [Horwitz et al, 1982;Chappell et al, 2008]. Unlike the low-energy electrons, those ions are covered fully within the ESA energy range well above the spacecraft potential, and thus, the density moment can be calculated more accurately.…”

“…Here the ESA instrument does not resolve ion species, and all ions are treated as H + . This assumption seems reasonable based on the statistics by Chappell et al [2008], who showed that low-energy ions in the postmidnight sector outside the geosynchronous orbit are mainly H + . Figure 3 shows that the most oscillations in the low-energy ion density are correlated with the chorus intensity modulation.…”

The Earth: Plasma Sources, Losses, and Transport Processes

“…They noticed another trend: the median temperatures of all three species were quite warm (10-100s eV) and traced out a path from pre-midnight through the dawn side, consistent with particles in the warm plasma cloak, with evidence of another path of the nightside warm ions along the dusk side. The dawnside trend implied that the heavy ions, likely to originate from the nightside ionosphere, could make it to the equator, gain moderate energy from injections or waves, and then become part of the cloak, which was discussed but not directly observed by Chappell et al (2008).…”

“…As this population convects inward, it drifts eastward due to the corotation electric field, remaining outside the closed drift paths of the plasmapause. Chappell et al (2008) named this population the warm plasma cloak, due to its observed features that showed it to be a bi-directional streaming population of warm (∼10 eV to few keV) plasma draped over the plasmasphere that was being blown sunwards by convection. This population co-exists with the more energetic ring current.…”