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

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.