Physics
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufmsm11b0427s&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #SM11B-0427
Physics
0654 Plasmas, 2708 Current Systems (2409), 2724 Magnetopause, Cusp, And Boundary Layers, 2728 Magnetosheath, 3210 Modeling
Scientific paper
This work, based loosely on the draping of magnetic field lines around a conducting sphere or cylinder, provides an analytical construction of the magnetosheath's B field for arbitrary IMF direction. The basic procedure is to create a generic expression for the equation of a solar-wind streamline. This procedure is easily visualized for the case of a hemispherically capped cylindrical magnetopause, except that "cap" (which extends ~ 11 RE upstream and ~ 65 RE downstream from Earth) is better represented as half of a prolate ellipsoid [e.g., Sibeck et al., JGR, 96, 5489-5495, 1991], matched to a cylinder of radius ~ 28.4 RE. The corresponding equation of a magnetosheath streamline, most conveniently expressed in ellipsoidal coordinates, is replaced upstream from the bow shock by the equation of a straight line parallel to the solar-wind velocity, which is regarded as locally uniform until it reaches the bow shock. Since the easily constructed Euler potentials (essentially a flux variable α and an azimuth β) that label streamlines must be continuous across the bow shock, they serve also to label streamlines in the magnetosheath, whereupon the magnetosheath plasma's momentum-flux field is given by grad α x grad β. This is most easily visualized for IMF parallel to the solar-wind velocity, in which case magnetic field lines are also streamlines of the plasma flow-velocity field. Results for B perpendicular to the solar-wind velocity can be obtained [Spreiter et al., Planet. Space Sci., 14, 223-253, 1966] by threading field lines through points of equal accrued travel time from corresponding straight field lines upstream of the prescribed (here paraboloidal) bow shock. Results for arbitrary IMF direction can be obtained through weighted superposition of results for B in the two limiting cases. This model for B is not current-free, but neither is the real magnetosheath. This model for B is also not unique, being dependent on a physics-based but ultimately "cooked-up" expression for the equation of a generic streamline. However, the model seems flexible enough to provide a realistic representation of the magnetosheath's B field upon suitable adjustment of the model's parameters. In particular, adjustment of the global "shape" parameter Q (incorporated into the analytical model for this presentation) improves the similarity of overall streamline shapes to past findings [e.g., Spreiter et al., 1966] from fluid simulations.
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