“hot” patches with higher electron temperature have also been observed and further studies are needed to distinguish the classical cold and hot patches, in particular, whether they are generated due to different mechanisms or produced by similar mechanisms but evolve under different precipitating particle and field‐aligned current environments. In the future, it may be more sensible to treat the relatively lower‐density patches and higher‐density patches separately, since they might be produced by different mechanisms, and the similarities and differences of their evolution, as well as their relationship with the hot and cold patches, are of great interest.
In the polar cap region, the dynamic evolution of the high‐density structures is mainly controlled by the convection pattern and their fate after they enter the polar cap would be determined by a sequence of convection patterns controlled mainly by the IMF. The patches exit the polar cap nearly symmetrically around midnight, but indeed show clear preference for dusk or dawn sectors under positive or negative IMF By. Besides the horizontal transport, significantly enhanced type‐1 and type‐2 ion upflow fluxes associated with these high‐density structures have also been observed. The impact of large but intermittent ion upflow/outflow fluxes associated with polar cap patches and TOI on magnetospheric dynamics is of great interest and would require close collaboration between the aeronomy and magnetospheric physics communities.
Enhanced 630 nm airglow emissions due to recombination are often used to image the polar cap high‐density structures. However, besides the recombination‐induced 630 nm emission, other mechanisms can also lead to variations in 630 nm emission, such as soft electron precipitation, thermal excitation, and lifting or descending of the F‐region height. Therefore, care is needed when interpreting the 630 nm emission variations, and it is better to be combined with other diagnosis tools, such as electron temperature. The relative contributions of those mechanisms to the total patch emission under various conditions and for different types of patch should be further explored.
ACKNOWLEDGMENTS
S. Zou would like to acknowledge NASA grant NNX14AF31G, NSF grant AGS 1400998 and NASA grant 80NSSC20K1313. G. W. Perry acknowledges the support from the Natural Science and Engineering Research Council of Canada (NSERC) Discovery Grant RGPIN/06069‐2014.
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