Interaction of a nonuniform solar wind with the local interstellar medium

Abstract

Results from a fully time dependent three‐dimensional gasdynamic model of the interaction of the solar wind with the local interstellar medium are presented. Both subsonic and supersonic interstellar winds are considered, while the mediating effects of interstellar neutrals, magnetic fields, and cosmic rays are ignored. In accord with solar minimum observations by Ulysses, the solar wind properties are assumed to depend on heliolatitude. Two large, long‐lived polar coronal holes, one in the northern and the other in the southern hemisphere, are assumed to produce a hot, low‐density, high‐speed wind which bounds a cooler, higher‐density, low‐speed ecliptic wind. The solar wind boundary conditions for the simulation are drawn directly from published Ulysses data [Phillips et al., 1995, 1996]. Results from these calculations are compared to simulations which adopt isotropic solar wind conditions. For the parameters used in these simulations (which correspond to a solar wind ram pressure increase of 1.5 from the ecliptic plane to solar pole), the termination shock is found to be elongated along the solar polar axis and weakly time dependent. The elongation results in an increased flow in the ecliptic plane compared to that over the solar poles. With the increased flow in the ecliptic plane comes enhanced pressure and density gradients which leads to the generation and amplification of turbulent vortices in the heliotail. The rotational axes of the heliotail vortices are perpendicular to the ecliptic plane. Overall, it is found that a modest increase in solar wind ram pressure with heliolatitude has a pronounced effect on the global structure of the termination shock and heliopause.