Loss of Ring Current and Radiation Belt Particles at the Dayside Magnetopause

Abstract:

Particle loss at the dayside magnetopause, or magnetopause shadowing, is a major loss process for ring current ions and radiation belt electrons. In this process, particles encountering the magnetopause through convective and diffusive transport are no longer trapped and become lost to interplanetary space. Magnetopause shadowing is an important candidate mechanism to explain radiation belt dropout events often seen during magnetic storms. Another process contributing to radiation belt dropouts is loss to the atmosphere when particles get pushed into the loss cone as a result of wave-particle interactions. Electromagnetic Ion Cyclotron (EMIC) waves are particularly important in this regard as they very effectively scatter radiation belt electrons in pitch angle. The free energy required to generate these waves is derived from pitch-angle anisotropy of ring current ions. At the dayside, the combined effect of drift-out loss and drift-shell splitting in a day-night asymmetric magnetic field creates anisotropic pitch-angle distribution of ring current ions; the resulting anisotropy provides a favorable condition for the excitation and growth of EMIC waves. In turn, radiation belt electrons will interact with EMIC waves, causing particle pitch-angle diffusion and subsequent precipitation into the atmosphere. In this presentation, the roles of convective and diffusive transport in dayside loss will be discussed. The relative importance of particle loss due to magnetopause shadowing and loss to the atmosphere resulting from interaction with EMIC waves will also be examined.