Limits on plasma anisotropy in a tail-like magnetic field

Understanding the static balance of forces in a hypothetical stationary, steady-state magnetotail configuration is central to our understanding of planetary magnetotail dynamics, even though the actual planetary magnetotails may rarely if ever attain such a steady state. We are not likely to achieve an understanding of the observed departures from equilibrium if we do not first understand the nature of the equilibrium itself, albeit hypothetical. The condition of magnetohydrostatic equilibrium implies tight constraints on the degree of anisotropy that is supportable in a magnetotail field geometry. If the plasma pressure tensor is assumed to be gyrotropic at the tail midplane (z = 0), then equilibrium requires that it also be nearly isotropic there, with Pperpendicular0/Pparallel0 in the range 1 ± δ2, where δ ∼ 0.1 is the ratio of the normal field component at the symmetry plane to the field strength in the tail lobe. The upper and lower limits are essentially equivalent, respectively, to the marginal mirror and firehose stability conditions evaluated at z = 0, which have been invoked previously to limit the degree of anisotropy in the plasma sheet. 21 refs., 1 fig