Comparing fluid and particle‐in‐cell solutions for the polar wind

Abstract

Theoretical descriptions of the terrestrial polar wind have taken a variety of forms. Fluid equations have long been used to provide a description of the plasma bulk flow parameters in the polar wind. More recently, particle‐in‐cell (PIC) models have been applied to the polar wind, yielding a description of far‐from‐Maxwellian features of the flow. A global, time‐dependent, fluid model of the coupled ionosphere and polar wind has recently made possible the description of plasma flow along a multitude (>1000) of magnetic flux tubes as they move under the combined influence of convection and corotational electric fields. In this paper, fluid and PIC solutions for the time‐dependent polar wind are compared for a variety of realistic flux‐tube trajectories, with both the trajectories and the PIC boundary conditions supplied by the global model described above. The two formulations are generally seen to be in excellent agreement in their description of the H + component of the polar wind. For O + the agreement is good at low altitudes (below 3000 km), but the two descriptions tend to diverge at high altitudes for reasons related to both the fluid and the PIC models. The results of this study indicate that fluid and PIC models of the polar wind are, to a high level of accuracy, interchangeable with regard to their predictions for the behavior of the plasma densities and drift velocities. This is true for a wide range of altitudes and for a broad spectrum of geophysical conditions.