The quiet time structure of energetic (35-560 keV) radiation belt electrons

Abstract

Detailed Explorer 45 equatorial observations of the quiet time structure of radiation belt electrons (35–560 keV) for 1.7 ≲; L ≲ 5.2 are presented. Throughout the slot region and outer regions of the plasmasphere the observed pitch angle distributions are found to agree with those expected from resonant interactions with the plasmaspheric whistler mode wave band. Coulomb collisions become the dominant loss mechanism within the inner zone. The overall two-zone structure of the observed radial profiles is found to agree with the equilibrium structure expected to result from a balance between pitch angle scattering losses and radial diffusion from an average outer zone source. This agreement suggests that the dominant quiet time source and loss mechanisms have been identified and evaluated for energetic radiation belt electrons within the plasmasphere. In the outer regions of the plasmasphere (L ∼ 5) the equilibrium structure is observed to be modified by daily flux variations associated with changes in the level of magnetic activity that occur even during relatively quiet times. Within the inner region of the plasmasphere (L ≲ 3.5), electron fluxes are decoupled from these magnetic activity variations by the long time scales (>10 days) required for pitch angle and radial diffusion. Consequently, fluxes of these electrons are observed to remain nearly constant at equilibrium levels throughout the quiet periods examined.