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 ((He+)) and the H+ cutoff frequency ((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 (He+). Therefore, the paper provides an in situ evidence of the modification of the parallel propagation of EMIC waves by heated He+ ions.