THE PROTON CYCLOTRON INSTABILITY AND THE ANISOTROPY BETA-INVERSE CORRELATION

Spacecraft observations in the strongly compressed subsolar magnetosheath show an inverse correlation between the proton temperature anistropy (T (perpendicular-to p)/T (parallel-to)p > 1 where perpendicular-to and parallel-to denote directions perpendicular and parallel to the background magnetic field) and the parallel proton beta (beta (parallel-to) p). This manuscript uses one-dimensional hybrid simulations of the proton cyclotron anisotropy instability in homogeneous electron-proton plasmas to study this correlation which may represent a limited closure relation for fluid theories of anisotropic space plasmas. The emphasis is on driven simulations which increase the temperature anisotropy by periodically reducing the magnetic-field-aligned velocities of the protons. The late-time states from ensembles of both initial value and driven simulations yield very similar expressions for the proton anisotropy/beta (parallel-to p) inverse correlation, and provide a basis for explaining differences between sheath observations from different spacecraft. The driven simulations also yield expressions for the maximum instability growth rate and the fluctuating field energy as functions of beta(parallel-to)p and a parameter characterizing the anisotropy driver.