GOES Observations of Pitch Angle Evolution During an Electron Radiation Belt Dropout

High Speed Stream (HSS) events exhibit characteristic structure in the solar wind which, when studied in conjunction with in situ observations at geostationary orbit (GEO) from GOES, allows us to examine the temporal evolution of dropouts in the outer electron radiation belt. Using pitch-angle-resolved Magnetospheric Electron Detector (MAGED) data, we study the evolution of perpendicular and parallel electron flux. During the HSS commencing on January 6th 2011, the flux over the entire energy distribution (30-600 keV) takes ~1.5 hours to dropout by two orders of magnitude from its pre-onset level. At this time, the lower energy electrons begin to reappear at GEO; however the 350-600 keV electron flux becomes highly parallel oriented and continues to decrease. Calculating the phase space density as a function of the three adiabatic invariants allows us to further investigate these loss mechanisms. Taking partial moments of the available electron distribution, we observe the number density quickly recovers (~4 hours), as well as the flux of the lower energy channels, however, the highest energy channel takes ~18 hours to recover to an approximately constant elevated level. This indicates that the electrons quickly reappear at GEO following the dropout before being heated over a period of days. This is consistent with the temperature values from GOES, showing an increase after the arrival of the HSS, peaking after ~3 days. This study provides independent confirmation of earlier statistical work and is a first step toward gaining understanding of the electron radiation belt dropout and recovery phenomena, in conjunction with coincident magnetic field measurements.