Bounded anisotropy fluid model for ion temperatures

A bounded anisotropy fluid model is developed which describes the temperature evolution of a collisionless plasma including the effect of pitch angle scattering due to ion cyclotron waves. The model equations accurately describe the proton temperature evolution in the plasma depletion layer, a magnetosheath regime of decreasing plasma density near the magnetopause. As in double adiabatic theory, changes in T⊥ are driven by changes in flux tube area A (∝ 1/B), while changes in T∥ are driven by changes in field line scale length L. In the bounded anisotropy model, if the proton temperature ratio T⊥/T∥ rises above the value 1 + 0.85 β∥p−0.48 (Anderson et al., 1994), where β∥p is the proton parallel beta, energy is transferred from the perpendicular to parallel temperature until this equation is satisfied. This energy exchange represents the effect of ion cyclotron wave pitch angle scattering, which keeps the plasma state near to marginal stability. Equations of the same form, employing bounded anisotropy expressions appropriate to different species, are also applied to He2+ and to electrons. These equations well describe the evolution of the He2+ but do not describe the evolution of the electron temperature, apparently due to the high electron thermal conduction which the model does not include. These results indicate that an energy exchange term may be incorporated into anisotropic fluid equations to simulate the effect of ion cyclotron wave pitch angle scattering in global fluid equations.