Pressure anisotropy in global magnetospheric simulations: A magnetohydrodynamics model

In order to better describe the space plasmas where pressure anisotropy has prominent effects, we extend the BATS-R-US magnetohydrodynamics (MHD) model to include anisotropic pressure. We implement the anisotropic MHD equations under the double adiabatic approximation with an additional pressure relaxation term into BATS-R-US and perform global magnetospheric simulations. The results from idealized magnetospheric simulations confirm previous studies: pressure anisotropy widens the magnetosheath, increases the density depletion in the vicinity of the magnetopause, enhances the nightside plasma pressure, and introduces an eastward ring current. In addition, we find that the flow speed in the magnetotail is significantly reduced by including pressure anisotropy in MHD simulations. Our model is validated through comparing the simulations to the THEMIS data on both the dayside and nightside of the magnetosphere during quiet times. The comparison to the results from isotropic MHD simulations implies that although anisotropic MHD is comparable to isotropic MHD in matching the measurement, it improves the simulated plasma velocity in some cases.