Empirical Cross-satellite Calibration of THEMIS SST Measurements Based on Electron Phase Space Density Conjunctions

Radial profiles of the equatorial electron phase space density (PSD) are required in order to accurately determine radiation belt sources and losses, and study the rate of cross-field diffusion. A main question in radiation belt dynamics that can be immediately studied even with limited radial coverage is whether large scale magnetopause motions in combination with Dst effects can result in shadowing and outward radial diffusion, or whether they are inconsequential for the observed flux increases at recovery phase. With the availability of a multi-year dataset with a number of storm occurrences on THEMIS, extending over a large range of L-shells, we can explore this problem using reanalysis tools. Since THEMIS energetic particle instrument has significant background and scaling effects from radiation, that are currently still being removed with particle tracing tools, we perform an empirical cross-satellite calibration of the instrument fluxes, based on electron PSD conjunctions between the THEMIS SST and LANL-01A SOPA for a half year period starting from July 1, 2007. Systematic errors in the THEMIS SST measurements and cross-instrument calibration factors are evaluated for the 11 SST energy channels from 40 keV to 2159 keV. Consistency in PSD conjunctions between the five THEMIS probes indicates that each SST instrument responds quite similarly to the ambient electron radiation environment. Our results further show that compared to the LANL-01A SOPA measurements, the THEMIS SST underestimates the electron fluxes within a factor of 2 for the 40 - 140 keV energy channels and overestimates the electron fluxes within a factor of 3 for the 204 - 2159 keV channels. Integrating the cross-satellite calibrated SST fluxes with the SOPA data from LANL-01A and 1989-048, we analyze the response of radiation belt electrons to a sudden solar wind pressure enhancement event. The electron PSD conjunctions between the THEMIS probes and the two geostationary satellites show good agreement, suggesting a reasonable empirical cross-satellite calibration of the SST measurements. The reconstructed radial profile of electron PSD based on data assimilation also indicates a clear correlation between the electron PSD dropouts and the solar wind pressure pulse, probably mainly due to the combination of magnetopause shadowing and outward radial diffusion.