Outward radial diffusion driven by losses at magnetopause

Loss mechanisms responsible for the sudden depletions of the outer electron radiation belt are examined based on observations and radial diffusion modeling, with L *‐derived boundary conditions. SAMPEX data for October–December 2003 indicate that depletions often occur when the magnetopause is compressed and geomagnetic activity is high, consistent with outward radial diffusion for L * > 4 driven by loss to the magnetopause. Multichannel Highly Elliptical Orbit (HEO) satellite observations show that depletions at higher L occur at energies as low as a few hundred keV, which excludes the possibility of the electromagnetic ion cyclotron (EMIC) wave‐driven pitch angle scattering and loss to the atmosphere at L * > 4. We further examine the viability of the outward radial diffusion loss by comparing CRRES observations with radial diffusion model simulations. Model‐data comparison shows that nonadiabatic flux dropouts near geosynchronous orbit can be effectively propagated by the outward radial diffusion to L * = 4 and can account for the main phase depletions of outer radiation belt electron fluxes.