Mass-loading at interplanetary shocks

Abstract

The interaction of the solar wind with the atmospheres of nonmagnetized and weakly magnetized bodies, such as found at comets and the planets Venus and Mars, is currently an area of great interest in space plasma physics. Photoionization of the atmospheric coma surrounding a comet or a weakly magnetized planet leads to ‘‘mass-loading’’ of the impinging solar wind. A common velocity for the different plasma components is established rapidly via pitch-angle scattering, so ensuring that the bulk plasma flow is described adequately at the hydrodynamic level. On the basis of magnetohydrodynamic (MHD) models, two outstanding problems can be addressed. The first concerns the existence, strength, and properties of shocks in a mass-loading environment, and the second concerns the location of the bow shock. This paper concentrates on the former issue. A general theoretical description of mass-loading shocks in the heliosphere is presented and the differences between mass-loading shocks and classical nonreacting MHD shocks elucidated. In a formal sense, mass-loading shocks are qualitatively similar to combustion shocks, except that mass-loading induces the flow to shear. Mass-loading fronts represent a notably new and interesting class of shocks, which, although found frequently in the solar system, both at the head of comets and, under suitable conditions, upstream of weakly and nonmagnetized planets, has not yet been investigated in great detail.