Four cells or two? Are four convection cells really necessary?

Abstract

This paper addresses the question whether a four‐cell convection pattern in the polar cap ionosphere is required by observations, or whether the data are fully explainable by a (perhaps highly distorted) two‐cell convection pattern. We present convection data from Atmosphere Explorer C, which, if only the flow component in the sunward‐antisunward direction were measured, could be explained either as one of two possible distorted two‐cell patterns or as a full four‐cell pattern. However, neither of the distorted two‐cell patterns that are consistent with the sunward‐antisunward flow component can be made consistent with the dawn‐dusk flow component over the entire spacecraft trajectory, without postulating a severe flow kink and extra field‐aligned currents sunward of the spacecraft track. In addition, the zero potential point (which in a four‐cell model would mark the division between the two reverse convection cells) also exactly corresponded to the location of the reversal of the east‐west component in the flow, a feature predicted from the four‐cell model but more difficult to explain in a distorted two‐cell model. Because the pattern was repeated on two consecutive passes, time variations can probably be ruled out as a cause of the sunward flow. Between the two northern hemisphere dayside passes, a southern hemisphere nightside pass also showed a region of sunward flow in the polar cap. The fact that in this case the sunward flow was not confined to the dayside also favors a four‐cell explanation.