SOURCE-SURFACE MODELING OF PLANETARY MAGNETOSPHERES

In the source-surface approach to field modeling, the magnetosphere is divided conceptually into inner and outer regions (called S and T) by prescribing a cross-magnetospheric surface that marks the tail entran ce. The source surface thus consists of the prescribed magnetopause an d the prescribed tail-entrance surface. In the inner region (S) enclos ed by the source surface, the magnetic field B is expanded formally in a series of analytical functions (e.g., gradients of spherical harmon ics) with coefficients determined by a least-squares fit to the desire d boundary conditions (namely that the field be tangential to the magn etopause and normal to the tail-entrance surface). Field lines in the tail region (T) are constructed geometrically so as to intersect the t ail-entrance surface normally but not to intersect each other or the m agnetopause. Expansion coefficients for region S are determined by min imizing a user-specified linear combination of the mean-square normal component of B on the magnetopause and the mean-square tangential comp onent of B on the tail-entrance surface. This model leads to a neutral line (contour of vanishing normal component of B) on the prescribed c ross-tail surface. The neutral line constitutes the inner edge of the neutral sheet and marks the boundary (separatrix),between closed and o pen magnetic field lines. A mapping of the separatrix along magnetic f ield lines to the planetary surface defines (in the absence of a penet rating interplanetary magnetic field, which can be added later) the po leward boundary of the amoral oval. This mapping accounts well for qui et-time amoral ovals seen in DMSP images. The source-surface model als o yields the position and shape of the neutral sheet in the tail regio n (T), where the strength of B can be calculated (from the normal comp onent of B at the tail-entrance surface) by invoking flux conservation . The current density J in region T can be computed from V x B there.