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This chapter has provided a review of multiscale processes in the high‐latitude ionosphere. A large amount of existing data, high‐resolution observations, and modeling capabilities have revealed key properties and their importance in understanding large‐scale processes. As summarized in Figure 3.2, such processes include cusp, PMAFs, polar cap patches/TOIs, auroral arcs, PBIs, streamers, substorm, surges, diffuse aurora, and related flow channels, field‐aligned currents (FACs), and precipitation/conductance. Those can be comparable or larger in magnitude than the large‐scale background, and thus substantially affect local processes. Moreover, mesoscale/small‐scale processes can propagate over long distances and affect processes in adjacent regions. Possible influence of small‐scale processes on global processes has also been indicated. Such feedback studies are still limited and further studies are desired to understand multiscale processes. Particularly, more systematic studies of mesoscale/small‐scale features are necessary to understand how multiscale interaction processes occur.

This chapter also presented an approach to specify instantaneous distribution of mesoscale precipitation over a regional scale. We found that mesoscale precipitation is dynamic and has a substantial impact (~25%–50%) compared with the total precipitation energy input. It is potentially a useful method to provide event‐specific high‐resolution information of precipitation without statistical averaging. On the other hand, measurement capabilities are limited for resolving convection, currents, and density over a similar scale. It is even more challenging to identify distributions of small‐scale parameters. Further advances are necessary to quantify instantaneous distributions of mesoscale/small‐scale features.

From a modeling standpoint, inclusion of mesoscale/small‐scale features would be critically important for improving understanding of local and global processes. However, it is currently not feasible to resolve all scales globally. Statistical parameterization and mesh refinement techniques are necessary and are fast developing. It is also necessary to treat physics in global models such as inertial and time‐dependent momentum effects, kinetic effects, and inductive processes; however, care should be taken when statistical parameterization is used because of their limitations.