Outer radiation belt boundary location relative to the magnetopause: Implications for magnetopause shadowing

Relativistic electron fluxes of the outer radiation belt often decrease rapidly in response to solar wind disturbances. The importance of the magnetopause shadowing (MPS) effect on such electron losses has yet to be quantified. If the MPS is essential for outer radiation belt electron losses, a close relationship between the outer edge of the outer belt and the magnetopause standoff distance is expected. Using GOES and THEMIS data, we examined earthward movement of the outer edge of the outer belt during electron loss events at geosynchronous orbit and its correlation with the magnetopause standoff distance. In events with significant earthward movement, we found a good correlation. There were no clear correlations in events without significant earthward movement, however. Comparing the observational results with a test particle simulation, the observed dependence between the outer edge and the magnetopause standoff distance is consistent with the MPS effect.