Evolution of chorus waves and their source electrons during storms driven by corotating interaction regions

Abstract

During Corotating Interaction Region (CIR)‐driven storms, relativistic electron fluxes in the outer radiation belt decrease in the main phase, followed by a gradual increase in the recovery phase. Recent studies have shown that whistler mode chorus waves play an important role in accelerating the seed electron population to relativistic energies in the outer radiation belt. However, the evolution of chorus waves and their source electrons responsible for the wave excitation during storm various phases is not well understood. In this study we select 72 CIR‐driven storm periods over the interval 2007–2011 to perform superposed epoch analysis of the evolution of chorus waves and source electrons in the L* and MLT range extensively observed by THEMIS during various phases of CIR‐driven storms. The wave amplitudes and the occurrence of chorus waves substantially increase and peak in the main phase and gradually decrease in the following recovery phase at L* < ∼7. The phase space density of source electrons at L* < ∼7 also increases in the main phase followed by a gradual decay in the recovery phase, showing a remarkable consistency with the evolution of chorus waves. Importantly, the evolution of the chorus wave activity and source electron population is highly dependent on the preceding interplanetary magnetic field (IMF)Bzorientation. Our results suggest that the increased activity of chorus waves and source electrons may contribute to the enhanced radiation belt electron flux in the recovery phase of CIR‐driven storms, but an additional mechanism is probably required to explain the main phase dropout.