Ion cyclotron waves: Direct comparison between ground‐based measurements and observations in the source region

Abstract

Simultaneous measurements of ion cyclotron waves (ICW's) were performed on GEOS spacecraft and in the vicinity of their magnetic footprints with the French Mobile station. The detailed comparison between the two sets of data shows that while ICW's having F < F He , the He + gyrofrequency at the equator, generally propagate to the ground, only 50% of those generated above F He can reach the ground station. It is shown that these results are in good agreement with the conclusions that Rauch and Roux [1982] drew on the basis of measurements reported by Young et al. [1981]. In an He + ‐rich plasma, ICW's with F > F He suffer a reflection where the frequency locally matches the local bi‐ion hybrid frequency. We extend the calculations of Rauch and Roux and calculate, as a function of the He + concentration, the tunnelling of ICW's through the stopband induced by the presence of minor He + ions. It is shown that the transmission coefficient strongly depends upon the wave frequency for a given He + abundance ratio. The results obtained are shown to be supported by existing observations.