EMPIRICAL CONVECTION MODELS FOR NORTHWARD IMF

It is clear that polar cap convection during times of northward IMF is more structured and of lower mean speed than at times of southward IM F. This, coupled with the fact that the polar cap is smaller, means th at empirical models are more difficult to construct with certainty. It is also clear that sunward flow deep in the polar cap is often observ ed, but its connection with the rest of the flow pattern is controvers ial. At present, empirical models are of three types: 'statistical' mo dels wherein data from different days but with similar IMF conditions are averaged together: 'pattern recognition' models, which are built u p by examining individually hundreds of passes to derive a 'typical' p attern which embodies features frequently observed; and 'assimilative' models, which use data of different types and from as many locations as possible, but all taken at the same time, in order to derive a snap shot (or series of snapshots) of the entire pattern. Each type of mode l has its own difficulties. Statistical models, by their very nature, smooth out now features (e.g. the convection reversal, and the locus o f sunward flow deep in the polar cap) which are not found at precisely the same invariant latitudes and magnetic local times on different da ys. Pattern recognition models are better at reproducing small-scale f eatures, but the large-scale pattern can be a matter of interpretation . Assimilative models (such as AMIE) hold out the best hope for creati ng instantaneous, global convection patterns; however, the analysis te chnique tends to be most irregular (and least reliable) in the regions which are not well covered by in situ data. It appears that, at least at times, a four cell model with sunward flow at the highest and lowe st latitudes, and antisunward flow in between, is consistent with the observations. At other times, the observations may be consistent with a two-cell convection pattern, but which includes significant meanders within the polar cap.