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.