The proton cyclotron instability and the anisotropy/β inverse correlation

Spacecraft observations in the strongly compressed subsolar magnetosheath show an inverse correlation between the proton temperature anisotropy (T{sub {perpendicular}p}/T{sub {parallel}p} > 1 where {perpendicular} and {parallel} denote directions perpendicular and parallel to the background magnetic field) and the parallel proton {beta}({beta}{sub {parallel}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}{sub {parallel}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}{sub {parallel}p} and a parameter characterizing the anisotropy driver. 50 refs., 5 figs.

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