Proton-cyclotron instability induced by the thermal anisotropy of minor ions

Abstract

We study ion cyclotron instabilities due to the thermal anisotropy of minor heavy ions. Very large thermal anisotropics have been observed in O +5 ions in coronal holes [Cranmer, 2001], and as is well known, the thermal anisotropy of an ion species is the free-energy source of ion cyclotron waves of the corresponding species. Nevertheless, as the heavy ion thermal anisotropy in coronal holes develops, the ions are being heated and accelerated, and therefore they acquire a drift velocity relative to the protons. Owing to the drift velocity a new instability branch develops very close to the proton gyrofrequency. In other words, heavy ion cyclotron waves can also trigger proton-cyclotron waves as a result of the drift velocity of the minor heavy ions. We propose that the proton-cyclotron waves generated by this mechanism are absorbed by the protons, and therefore this process can contribute to fast proton heating in coronal holes.