Case studies of the impact of high-speed solar wind streams on the electron radiation belt at geosynchronous orbit: Flux, magnetic field, and phase space density

Investigation of electron radiation belt dropouts has revealed the importance of a number of loss processes, yet there remains a lack of quantitative detail as to how these processes wax and wane between events. The overarching aim of this study is to address the issue of electron radiation belt dropouts. This is achieved using in situ observations at geostationary orbit from GOES-13 (pitch angle-resolved electron data and magnetic field measurements) to examine the outer electron radiation belt during three high-speed stream-driven storms. Analysis and interpretation are aided by calculation of the phase space density (PSD) as a function of the three adiabatic invariants. Our results confirm the importance of outward adiabatic transport as a mechanism for causing electron dropouts at geosynchronous orbit; however, study of the pitch angle distributions indicates that other loss mechanisms are also likely to be occurring during these high-speed solar wind stream (HSS)-driven storms. Two of the studied events exhibit similar evolutionary structure in their pitch angle distributions: (i) highly peaked distributions immediately prior to the dropout (ii) sharp transitions between peaked and isotropic and then subsequent butterfly distributions, and (iii) isotropic distributions at minimum flux shortly afterwards (dusk). We also address the difficulty in interpreting PSD calculations by comparing the T96 model magnetic field with that measured by GOES-13. Our results are intended as a first step in quantifying the timeline of events that occur in the radiation belts following the arrival of a HSSparticularly timely given the increase in HSS occurrence expected in the declining phase of the current solar cycle.