SELF-CONSISTENT PLASMA PRESSURE TENSORS FROM THE TSYGANENKO MAGNETIC-FIELD MODELS

The high-pressure plasma trapped in the geomagnetic tail strongly shapes the surrounding magnetic field. Force balance in a steady state requires a balance between the divergence of the total particle momentum stress tensor and the divergence of the magnetic field momentum stress tensor which is the Lorentz force. This condition of stress balance allows strict constraints on the total particle pressure tensor to be derived from the magnetic field structure. The K(p) parameterized empirical magnetospheric fields of the Tsyganenko model with local two-dimensional approximations are used to derive the pressure tensor components implied for the central plasma sheet. The small anisotropies p(perpendicular-to)/p(parallel-to) in the gyrotropic pressure tensor required for equilibrium in the central plasma sheet are calculated from the nonpotential values found for the j x B force. The required ratio p(perpendicular-to)/p(parallel-to) approaches the marginal stability criterion for the magnetohydrodynamic (MHD) mirror mode and the Alfven ion cyclotron mode, and we suggest that these instabilities may play a role in establishing the equilibrium structure of the geomagnetic tail. From this perspective the equilibrium and stability constraints for the maximum allowed anisotropy suggest that the Tsyganenko magnetotail field parameterization needs to be modified to support self-consistent gyrotropic equillibria.