Multisatellite measurements of relativistic electrons: Global coherence

In this paper we report on multisatellite measurements of relativistic electrons (energies of greater than or similar to2 MeV) in the outer zone using detectors on board SAMPEX, HEO, GOES, and Polar satellites. These satellites are in distinct orbits around the Earth, ranging from polar low-Earth to geosynchronous orbits. The data thus comprise a broad sampling of the relativistic electron populations of various pitch angles, local times, and energy range and of time resolutions ranging from tens of minutes to a day. By a quantitative intercomparison of these multisatellite electron measurements collected over a period of 2 years (from 1998 to 1999), we find that the relativistic electron populations exhibit a remarkable degree of coherence throughout the outer zone. The correlation coefficients between fluxes measured by different satellites as a function of lag time and at different L shells, are highest at or very near zero lag. The strong correlation between electron fluxes measured at different altitudes is seen over a range of L shells. This suggests that the magnetospheric processes responsible for electron acceleration and decay in the outer zone may be of a global nature. The highest correlation being at nearly zero lag suggests that either the underlying acceleration mechanisms are largely pitch angle independent or that the isotropization timescales for these electrons are quite short (of the order of half a day at most). Our observations thus provide a valuable constraint on the various theoretical models that have been proposed for the acceleration and decay of the relativistic electron population in the outer zone.