Fourier Parameters of Heliospheric Current Sheet and Their Significance

Abstract

If the path of the neutral line on the coronal source surface is expressible as a singlevalued function (colatitude θ vs longitude φ), then Fourier analysis of ctn θ with respect to φ leads to a simple algorithm for realistically mapping the neutral line outward to model the heliospheric current sheet (HCS) at distancesr≳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 them=1 Fourier amplitude (a ₁ sin φ+b ₁ cos φ) of ctn θ on the source surface. Physical features (including the neutral line) on a prolate source surface intrinsically map to lower dipole latitudes atr≳1 AU in the heliosphere, and Ulysses observations of a unipolar field at latitudes beyond 30°S (when the neutral line on the source surface still reached 39°S) confirm the expected geometry.