MIRROR AND ION-CYCLOTRON ANISOTROPY INSTABILITIES IN THE MAGNETOSHEATH

Magnetosheath plasmas are typically characterized by a proton beta > 1 and a proton temperature anisotropy T(perpendicluar-to)/T(parallel-to) > 1. Such a plasma is unstable to both the ion cyclotron anisotropy and mirror instabilities, and evidence for both of these growing modes has been found in the magnetosheath. In this paper, we use one-dimensional hybrid simulations to investigate the kinetic properties of these two instabilities. We find that, for moderate values of ion beta and temperature anisotropy, the two instabilities produce similar levels of turbulence, but that the cyclotron instability apparently produces higher fluctuation levels than the mirror instability for high beta or anisotropy. Although both instabilities saturate by reducing the ion temperature anisotropy, mirror waves accomplish this through the use of the wave magnetic fields, whereas cyclotron waves do so by using the wave electric fields in resonant interactions with the ions. Consequently, mirror waves preferentially affect those ions with large magnetic moment, while cyclotron waves affect a broader range of ions and thereby dominate the isotropization process even when the mirror mode is somewhat stronger. We also investigate the utility of transport ratios as mode identifiers, and find that the Alfven ratio in particular can be useful in distinguishing between the two modes.