Theoretical study of the high‐latitude ionosphere's response to multicell convection patterns

It is well known that convection electric fields have an important effect on the ionosphere at high latitudes and that a quantitative understanding of their effect requires a knowledge of the plasma convection pattern. When the interplanetary magnetic field (IMF) is southward, plasma convection at F region altitudes displays a two‐cell pattern with antisunward flow over the polar cap and return flow at lower latitudes. However, when the IMF is northward, multiple convection cells can exist, with both sunward flow and auroral precipitation (theta aurora) in the polar cap. The characteristic ionospheric signatures associated with multicell convection patterns were studied with the aid of a three‐dimensional time‐dependent ionospheric model. Two‐, three‐, and four‐cell patterns were considered and the ionosphere's response was calculated for the same cross‐tail potential and for solar maximum and winter conditions in the northern hemisphere. As expected, there are major distinguishing ionospheric features associated with the different convection patterns, particularly in the polar cap. For two‐cell convection the antisunward flow of plasma from the dayside into the polar cap acts to maintain the densities in this region in winter. For four‐cell convection, on the other hand, the two additional convection cells in the polar cap are in darkness most of the time, and the resulting O + decay acts to produce twin polar holes that are separated by a sun‐aligned ridge of enhanced ionization due to theta aurora precipitation. For three‐cell convection, only one polar hole forms in the total electron density, but in contrast to the four‐cell case, an additional O + depletion region develops near noon owing to large electric fields causing an increased O + + N 2 loss rate. These general distinguishing features do not display a marked universal time variation in winter.