The importance of ion composition for radiation belt modeling

The banded structure of Electromagnetic Ion Cyclotron (EMIC) wave spectra and their resonant interactions with radiation belt electrons depend on the cold ion composition. However, there is a great deal of uncertainty in the composition in the inner magnetosphere due to difficulties in direct flux measurements. Here we show that the hydrogen and helium band wave spectra are most consistent with a helium and oxygen composition of a few percent. Less than 10% of hydrogen band wave intensity is consistent with a high helium fraction of ∼20%. Similarly, only ∼20% helium band wave intensity is consistent with a oxygen torus ion composition. Furthermore, we find that the decay of the ultra-relativistic electrons in the radiation belts by EMIC waves depends on the ion composition. The decay is most sensitive to the helium fraction, and the strongest agreement with Van Allen Probes data is found when the helium fraction is a few percent. We suggest that more observations of the cold ion composition would significantly help understand and set constraints on the decay of ultrarelativistic electrons in the radiation belts.