A Comparison of Model Predictions for Plasma Convection in the Northern and Southern Polar Regions

We have presented model calculations to show how the plasma flow distributions in the northern and southern polar regions differ when viewed from a geographic inertial frame. This reference frame was selected because it is the natural frame for geophysical plasma flow measurements, there being well-known velocity corrections for either satellite or ground-based observations. Although the magnetic invariant latitude, magnetic local time reference frame is better suited to studying magnetospheric processes, a transformation from the geographic inertial frame to this magnetic frame requires both a spatial and velocity transformation, and since the latter correction has generally been neglected, we prefer to present our results in the geographic inertial frame. However, we also present some of our results in the magnetic frame, taking account of the complete transformation. Our convection model includes the offset between the geographic and geomagnetic poles, the tendency of plasma to corotate about the geographic poles, and a dawn/dusk magnetospheric electric field mapped to a circle about a center offset by 5° in the antisunward direction from the magnetic pole. We considered both uniform and asymmetric magnetospheric electric field configurations. Our asymmetric electric field distribution contained an enhanced field in the dawnside northern hemisphere in conjunction with an enhanced field in the duskside southern hemisphere. From our study we have found the following: (1) In the geographic inertial frame the plasma flow patterns in both hemispheres exhibit significant variations with universal time because of the relative motion of the geomagnetic and geographic poles. (2) This universal time variation is greater in the southern polar region than in the northern polar region because of the greater displacement between the geomagnetic and geographic poles. (3) For the case of a uniform magnetospheric electric field the universal time dependence of the plasma flow distributions in the two hemispheres is similar, but there is a phase shift of about half a day between them. (4) For the case of an asymmetric magnetospheric electric field this half-day phase shift is still noticeable, but there are significant differences between northern and southern hemisphere convection patterns. (5) The transformation of plasma convection patterns from the geographic inertial to the geomagnetic quasi-inertial frame results in the same convection pattern for both hemispheres for the case of a uniform magnetospheric electric field, but results in different convection patterns for the two hemispheres for the more common case of an asymmetric electric field configuration. (6) Because the magnetospheric electric field distributions in the northern and southern polar regions are generally asymmetric, erroneous conclusions can be drawn about plasma convection patterns if data taken along satellite tracks from the northern and southern polar regions are overlaid. This is true whether the overlaying is done in the geomagnetic quasi-inertial frame or the geographic inertial frame.