The effect of heat flux on pressure evolution in the magnetosheath

Abstract

An analysis of anisotropic pressure ( p ⊥ ≠ p ∥ ) evolution in the quasi-perpendicular magnetosheath is carried out based on a two-dimensional hybrid code simulation. The validity of different fluid models is evaluated by comparing the prediction of the models with the values of pressure observed in the hybrid code simulation. The Double Adiabatic model, the bounded anisotropy model, which incorporates local energy exchange from p ⊥ to p ∥ to model the effects of ion cyclotron waves, and a two way local energy exchange model have all been tested with and without including the effect of the heat flux terms q ∥ and q ⊥ (the third velocity moment). The heat flux terms yield a flow of perpendicular or parallel heat along magnetic field lines due to particle velocity distributions that are asymmetric with respect to the direction along the magnetic field and non-Maxwellian with respect to the parallel velocity. The two way energy exchange with heat flux terms does the best job of accounting for the pressure evolution in the hybrid code simulation. An attempt to model the parallel heat flux q ∥ using lower fluid moments was made, but this was not successful. These results show the inadequacy of fluid equations to model the hybrid code results. However, the effects we have examined here may not be as significant in the real magnetosheath, because the specific hybrid simulation examined here does not have a realistic separation of time scales of interest.