FOURIER PARAMETERS OF HELIOSPHERIC CURRENT SHEET AND THEIR SIGNIFICANCE

If the path of the neutral line on the coronal source surface is expressible as a single-valued function (colatitude theta vs longitude phi), then Fourier analysis of ctn theta with respect to phi leads to a simple algorithm for realistically mapping the neutral line outward to model the heliospheric current sheet (HCS) at distances r greater than or similar to 1 AU. To be compatible with MHD, the source surface used for this mapping should be prolate (aligned with dipole axis) rather than spherical. Orientation of the Sun's magnetic-dipole moment is indicated by the m = 1 Fourier amplitude (a(1) sin phi + b(1) cos phi) of ctn theta on the source surface. Physical features (including the neutral line) on a prolate source surface intrinsically map to lower dipole latitudes at r greater than or similar to 1 AU in the heliosphere, and Ulysses observations of a unipolar field at latitudes beyond 30 degrees S (when the neutral line on the source surface still reached 39 degrees S) confirm the expected geometry.