Hot proton anisotropies and cool proton temperatures in the outer magnetosphere

摘要

The plasma sheet and ring current ions of the outer magnetosphere typically exhibit an anisotropy such that the perpendicular temperature is greater than the parallel temperature. If such an anisotropy is sufficiently large, the electromagnetic proton cyclotron instability will be excited. This instability is studied using linear Vlasov theory and one-dimensional hybrid simulations for a homogeneous plasma model representative of conditions in the outer magnetosphere. The model includes a hot anisotropic proton component and a cool, initially isotropic proton component. Theory and simulations both predict that there is a threshold hot proton anisotropy for this instability which depends inversely on the parallel (beta) of the hot component. The simulations are also used to examine the nonlinear response of the cool protons to the proton cyclotron instability; the late-time temperature of the cool protons is found to increase as the relative hot proton density increases. Analysis of plasma observations obtained by the Los Alamos magnetospheric plasma analyzer in geosynchronous orbit finds that the hot ion anisotropy is indeed bounded by the predicted (beta)-independent threshold.