Outer Radiation Belt Flux and Phase Space Density Response to Sheath
Regions: Van Allen Probes and GPS Observations
Abstract
Turbulent and compressed sheath regions preceding interplanetary coronal
mass ejections (ICMEs) strongly impact electron dynamics in the outer
radiation belt. Changes in electron flux can occur on timescales of tens
of minutes, which is difficult to capture by a two-satellite mission
such as the Van Allen Probes (RBSP). The recently released Global
Positioning System (GPS) data set has higher data density owing to the
large number of satellites in the constellation equipped with energetic
particle detectors. Investigating electron fluxes in a wide range of
energies and sheaths observed from 2012 to 2018, we show that the flux
response to sheaths on a timescale of 6 hours, previously reported from
RBSP data, is reproduced by GPS measurements. Furthermore, GPS data
enables derivation of the response on a shorter timescale of 30 minutes,
which further confirms that the energy and L-shell dependent changes in
electron flux are due to the impact of the sheath. Sheath-driven loss is
underestimated over longer timescales as the electrons recover during
the ejecta. We additionally show the response of electron phase space
density (PSD), which is a key quantity in identifying true loss from the
system and electron energization through wave-particle interactions. The
PSD response is calculated from both RBSP and GPS data for the 6-hour
timescale, as well as from GPS data for the 30-minute timescale. The
response is divided based on the geoeffectiveness of the sheaths
revealing that electrons are effectively accelerated only during
geoeffective sheaths, while loss is commonly caused by all sheaths.