Locations of boundaries of outer and inner radiation belts as observed by Cluster and Double Star

Cluster CIS ion spectrograms measured during the period of the recent solar minimum between April 2007 and June 2009, when Cluster was deep in the radiation belts with its perigee as close as L = 2, are analyzed. The analysis is complemented by Double Star TC-1 satellite data from HIA ion spectrograms on perigee passes during the period of May 15, 2007 to September 28, 2007. We demonstrate how the background counts produced by energetic particles of the radiation belts in Cluster CIS and Double Star HIA instruments can be interpreted to obtain the locations of the boundaries of the outer and inner belts. The obtained L-MLT distribution of boundaries reflects the general structure of the radiation belts. Closer examination of the time-dependent L locations of the boundaries reveals several dips to lower L-shells (from L = 6 to L = 4) in the outer boundary location. The importance of the solar wind pressure increases for the Earthward shift of the outer boundary of the outer belt is discussed. The location and thickness of the slot region are studied using the determined inner boundaries of the outer belt and the outer boundaries of the inner belt. It was found that during intervals of low activity in the solar wind parameters, the slot region widens, which is consistent with weaker inward radial diffusion, and also with weaker local acceleration that can occur only at higher L-shells outside the plasmasphere. We conclude that boundaries of radiation belts determined from background measurements on the instruments with energy ranges that do not cover the radiation belts' energies provide valuable additional information that is useful for radiation belts' model development and validation.