On the Relative Constancy of the Solar Wind Mass Flux at 1 AU

Abstract

Employing solar wind measurements from the Advanced Composition Explorer and Ulysses, photospheric magnetic data, and conservation laws along open field lines, we confirm that the energy and mass flux densities at the Sun increase roughly linearly with the footpoint field strength, B ₀. This empirical result has a number of important physical implications. First, it supports the assumption that the magnetic field is the source of the heating in coronal holes. Second, because B ₀ may vary by over 2 orders of magnitude, depending on how close the footpoint is located to active regions, the heating rate in coronal holes varies over a very wide range, with active-region holes being characterized by much stronger heating and much larger mass fluxes at low heights than the large, weak-field polar holes. Third, the variation of the mass flux density at 1 AU remains very modest because the mass flux density at the Sun and the net flux-tube expansion both increase almost linearly with B ₀, so that the two effects offset each other.