Van Allen Probe Observations of Electric Field Irregularities within the Plasmasphere Including Examples of Very High Altitude Spread-F

Abstract:

Broadband ELF electric field irregularities (DC to 1 kHz) are commonly observed in the plasmasphere as discrete packets revealed by the DC and wave double probe experiments on the two Van Allen Probe (VAP) satellites. We have reviewed seven years (6556 orbits) of EMFISIS/EFW data to investigate how this ELF electric field turbulence is distributed in the plasmasphere as a function of local time, altitude, and magnetic activity. In general, the irregularities are divided into two different “populations” -- low altitude (< 4000 km) and high altitude (> 1 Re) irregularities. In both cases, they are strongest at night, although some daytime events were recorded in the plasmasphere.

For the low altitude population, the irregularities are strongest near the satellite perigees near 600 km yet frequently extend to a few thousand km. These ELF spectra and accompanying DC electric field perturbations only appear at night and are classified as high altitude Spread-F turbulence often with associated depletions of the ambient plasma density, as inferred from the s/c potential measurements.

At higher altitudes, distinct regions of irregularities are observed with and without large scale plasma density structures. These irregularities are strongest above ~1 Re (6000 km) and increase in number and amplitude with altitude, extending to the VAP apogees of 30,000 km. These irregularities are associated with plasmasphere structure and turbulence. Distinct from the high altitude spread-F, their driving mechanisms have not been identified. Magnetic perturbations were generally absent in association with these irregularities.

We focus on the high altitude spread-F whose flux tubes extend across the mid-latitude ionosphere. A key aspect of interpreting these observations relies on converting the spacecraft potential perturbations to thermal plasma density variations. In association with the electric field signatures, we also identify magnetic field irregularities using the VAP search coil data, finding that they appear with some, but not all, of the spread-F events. We interpret these magnetic structures as filamentary currents on the walls of plasma depletions and also with Alfven waves. We summarize our findings and interpret the observations in terms of current theories of high altitude spread-F and plasmaspheric structures.