Numerical calculations relevant to the initial expansion of the polar wind

Abstract

The collisionless expansion of an H+‐O+ electron plasma into a vacuum was studied because of the relevance to the polar wind. A systematic parameter study was conducted by varying the ion composition, the initial ion‐electron temperature ratio, and the scale length of the density gradient at the plasma‐vacuum interface. The effect of gravity on O+ was also simulated. The character of the expansion is different depending on whether H+ is the major or minor ion. When H+ is the major ion, the density profile in the expansion region is always concave. On the other hand, when H+ is a minor ion, a distinct plateau forms in the H+ density, H+ drift velocity, and electrostatic potential profiles. The longer the density‐gradient scale length at the plasma‐vacuum interface, the longer it took for plateau formation. However, after plateau formation, the expansion proceeds nearly in an identical fashion regardless of the initial density‐gradient scale length. When O+ was fixed to simulate gravity, the lower end of the plateau region remained fixed at the location of the initial plasma‐vacuum interface, whereas when O+ was allowed to expand the H+ plateau region moved along with the moving O+ density front. For all cases, energetic H+ and O+ ions were observed in the expansion region. This suggests that the energization of ionospheric ions through the process of plasma expansion could be one of the mechanisms for creating the energetic ion population of ionospheric origin in the magnetosphere. Such a process would operate over the entire high‐latitude region, not just on auroral field lines. Since the largest density‐gradient scale length at the plasma‐vacuum interface that was considered is much less than typical ionospheric scale lengths, the numerical results cannot be directly applied to the polar wind. However, they illustrate the basic physical processes occuring in the polar wind expansion.