Comparative study of outer‐zone relativistic electrons observed by Akebono and CRRES

Abstract

Simultaneous observations by CRRES and Akebono in the near‐ and off‐equatorial inner magnetosphere are used to examine the pitch angle dependence of the relativistic electron variation in the outer zone during the period from 24 August to 17 September 1990. This period includes two interesting time intervals, namely, a magnetic storm interval with a net reduction of outer‐zone electron flux and a nonstorm interval with a gradual increase of the relativistic electron population. The Akebono and CRRES data show the coherent behavior of the relativistic electron fluxes between the two satellites in the region 4 < L < 6, consistent with the “global coherence” of the outer‐zone variation suggested by previous multisatellite observations. On the other hand, the electron fluxes in the region 3 < L < 4, corresponding to the center of the outer zone after the magnetic storm from 26 August 1990, show a remarkable difference between near‐ and off‐equatorial electron fluxes: at L = 3.25, during the storm recovery phase, the flux increase lasts longer at Akebono (which measures small equatorial pitch angles) than at CRRES (which measures larger equatorial pitch angles), and after the storm, the decay of relativistic electron fluxes is faster for the latter than the former. The former longer‐lasting flux increase at Akebono than at CRRES is explicable with pitch angle evolution from a pancake‐like to a more isotropic pitch angle distribution with a timescale of a few days. The latter difference in the decay timescale after the storm is caused by the energy‐dependent electron loss, i.e., faster decay for lower‐energy electrons in the 1–2.5 MeV energy range. These results indicate that evolution of the pitch angle distribution can violate the “global coherence” of relativistic electron behavior at least in some cases, depending on the wave mode(s) responsible for the pitch angle diffusion of electrons, and suggest the importance of equatorial observations in the center of the outer zone.