In situ evidence of the modification of the parallel propagation of EMIC waves by heated He⁺ ions

With observations of the Van Allen Probe B, we report in situ evidence of the modification of the parallel propagating electromagnetic ion cyclotron (EMIC) waves by heated He⁺ ions. In the outer boundary of the plasmasphere, accompanied with the He⁺ ion heating, the frequency bands of H⁺ and He⁺ for EMIC waves merged into each other, leading to the disappearance of a usual stop band between the gyrofrequency of He⁺ ions (Ω_He+) and the H⁺ cutoff frequency (ω_H+co) in the cold plasma. Moreover, the dispersion relation for EMIC waves theoretically calculated with the observed plasma parameters also demonstrates that EMIC waves can indeed parallel propagate across Ω_He+. Therefore, the paper provides an in situ evidence of the modification of the parallel propagation of EMIC waves by heated He⁺ ions.