Temporal behavior of density perturbations in the polar wind

Abstract

The temporal evolution of density perturbations in the supersonic, collisionless polar wind is presented by solving the time‐dependent hydrodynamic equations. When the perturbations occur in the form of extended density depletions, the temporal evolution shows some characteristics of expanding plasmas seen in numerical simulations of small‐scale plasmas, including ion acceleration. When the density depletion is relatively small, the expansion is headed by a shock. On the other hand, when the extended density depletion is very strong, the expansion is headed by a pair of forward and reverse shocks. Such shocks are common features of transients in the solar wind. The extent of ion acceleration also depends on the magnitude of the density depletions. Proton accelerations up to about 40 eV (∼90 km/s) have been seen. In a nonflowing plasma a rarefaction wave propagates in a direction opposite to that of the expansion. However, in the case of the polar wind the rarefaction front is convected along with the wind, and its traversal determines the time for the reestablishment of the steady state situation. Perturbations in the form of localized density enhancements and depletions have also been studied. Such perturbations are found to be propagated upward in the direction of the polar wind, but they undergo a considerable modification, evolving into forward‐reverse shock pairs. Soon after a localized density depletion is created, it breaks into two secondary holes which propagate as rarefaction waves. The forward‐reverse shock pair is sandwiched between these waves. In the case of density bumps, the forward shock precedes the front of accelerated ions, while the reverse shock precedes it in the case of a density hole. These results are relevant to expanding astrophysical and space plasmas as well as to plasma expansion in certain ionospheric modification experiments.