The expansion of a low-density solar corona: A one-fluid model with magnetically modified thermal conductivity

A one-fluid model of the coronal expansion, including the reduction in radial heat conduction produced by a spiral interplanetary magnetic field, is extended to the low coronal densities that may occur in the regions of open diverging magnetic field lines, or ‘coronal holes,’ that are regarded as probable sources of the solar wind. At such densities, the ‘cutoff’ in heat conduction at very large heliocentric distances (where the magnetic field becomes nearly azimuthal) has a profound effect on the nature of the expansion. The corona becomes nearly isothermal out to the distance where the flow of plasma dominates the transport of energy. This outward extension of high coronal temperatures leads to large solar wind speeds, approaching those given by Parker's original isothermal model as the coronal density becomes vanishingly small. The model predicts expansion speeds as high as 500 km s−1, with densities in agreement with those observed near the orbit of earth, for a reasonable set of coronal densities and temperatures (e.g., with coronal temperatures no higher than 2.1×106 °K). However, the temperatures (or pressures) predicted at the orbit of earth are substantially higher than those observed; this deficiency of the model could only be removed by incorporation of additional physical effects or processes.