Combined Effect of EMIC Waves and Magnetosonic Waves on Rapid Loss of MeV Electrons in Outer Radiation Belt

Abstract:

Electromagnetic ion cyclotron (EMIC) waves can cause rapid loss of relativistic electrons in the outer radiation belt by pitch angle scattering, especially for >2 MeV electrons. The rapid pitch angle scattering is limited to the low pitch angle electrons and cannot affect ~90 degree electrons. However, normal pitch angle distribution (PAD) of electron flux with peaks at 90 degree pitch angle is generally observed in the outer radiation belt. Magnetosonic (MS) waves in the outer radiation belt can scatter ~90 degree pitch angle electrons to lower pitch angles and lead to the formation of electron’s butterfly PAD. This paper studies the combined effect of EMIC waves and MS waves on the loss of the outer belt relativistic electrons during a minor storm on 16 November 2013 by combining Van Allen Probe measurements with test particle simulations. During the pre-storm period strong MS waves were observed by Probe A. Meanwhile normal PAD of 2.1 MeV electrons was measured by relativistic electron and proton telescope (REPT) on Probe A. When Probe B orbit was passing through the same area during the storm main phase, MS waves still existed but with weak intensity, while strong EMIC wave with ~1 nT amplitude were observed. Butterfly pitch angle distribution of 2.1 MeV electrons was seen to be formed at L = ~5-6. Four hours later, stronger EMIC waves were measured by Probe A and the fluxes of 2.1 MeV electrons at L=~5-6 showed great losses at all pitch angle sectors. The computed pitch angle diffusion rates show that the MS waves can produce the observed butterfly pitch angle distributions (flux peaks at 50-60 degree) for 2.1 MeV electrons. This indicates that ~90 degree pitch angle electrons are scattered to lower pitch angle by MS waves to form the butterfly PAD, and the observed strong EMIC waves then can resonate effectively with these butterfly distributed electrons and cause the electron loss in ~hours. Therefore, we suggest that although MS waves themselves cannot cause electron loss, they play a significant role by scattering ~90 degree pitch angle electrons to lower degrees, providing source particles to react with EMIC waves and then are lost from the outer radiation belt. Thus, by combining the effects of both MS waves and EMIC waves, we can explain not only the loss of the low pitch angle electrons, but also the loss of the electron core population.