PLASMA EXPANSION AND EVOLUTION OF DENSITY PERTURBATIONS IN THE POLAR WIND - COMPARISON OF SEMIKINETIC AND TRANSPORT MODELS

Comparisons are made between transport and semikinetic models in a stu dy of the time evolution of plasma density perturbations in the polar wind. The situations modeled include plasma expansion into a low-densi ty region and time evolution of localized density enhancements and cav ities. The results show that the semikinetic model generally yields sm oother profiles in density, drift velocity, and ion temperature than t he transport model, principally because of ion velocity dispersion. Wh ile shocks frequently develop in the results of the transport model, t hey do not occur in the semikinetic results. In addition, in the semik inetic results, two ion streams, or double-humped distributions, frequ ently develop. In the transport model results the bulk parameters, at a given time, often have a one-to-one correspondence in the locations of their local minima or maxima. This is a consequence of the coupling of the fluid equations. There is, however, no such relationship among the moments produced by the semikinetic model where the local moment maxima and minima are often shifted in altitude. In general, incorpora tion of enhanced heat fluxes in the transport model leads to somewhat improved agreement with the semikinetic results.