The radial gradient of relativistic electrons at geosynchronous orbit

Earth's outer electron radiation belt is a highly dynamic region, driven externally by the solar wind and controlled within the magnetosphere by numerous source and loss processes. Critical to assessing the importance of the different source and loss processes and to developing accurate physics-based models of the radiation belts is knowing phase space density and its radial gradient in terms of adiabatic invariants. In this paper, we demonstrate a new technique for determining the radial gradient of relativistic electron phase space density at geosynchronous orbit. This technique utilizes the fact that the GOES geosynchronous satellites are located at different geomagnetic latitudes because of their separation in longitude, even though both are located on the geographic equator. From simultaneous measurements at different geomagnetic latitudes, and therefore in different L shells, we are able to obtain the local radial gradient of phase space density. We have restricted these initial calculations to a single value of magnetic moment (M = 6000 MeV/G) and to equatorially mirroring particles. In order to minimize the sensitivity of our results to the magnetic field model used, we have analyzed data from a time period with quiet geomagnetic conditions, when the relativistic electron flux was elevated and steady. We find that during this time period the radial gradient of phase space density was positive. This indicates that the direction of radial diffusion was inward, transporting electrons from outside to within geosynchronous orbit.