Computing magnetospheric equilibria with anisotropic pressures

We present initial results from an equilibrium code that has been modified to include the effect of anisotropic pressures. This equilibrium code uses a frictional technique to iterate a set of modified MHD equations to equilibrium. We describe the modifications made to the original isotropic equilibrium code, including changes to the initial conditions that include a procedure to enforce the specified anisotropy with the restriction that the total energy on a flux tube be conserved. This allows ready comparisons with an isotropic version of the code. The initial pressure distribution is obtained from a one‐dimensional curve fit and the empirical model of that specifies the pressure anisotropy as a function of position down the tail. The initial magnetic field is a magnetic field model. As a preliminary exercise with the code, we examined the differences in the equilibrium magnetic field between an anisotropic and an isotropic equilibrium pressure distribution for the same total energy per flux tube under various initial magnetic field configurations. Using anisotropy‐driven instabilities and chaos effects as a guide, we made a further modification by choosing the boundary where the anisotropy transitions to zero in the tail. When the modified boundary was used, this initial investigation found that differences on the magnetic field between the anisotropic and isotropic case can be very sensitive to where we set the isotropy boundary.