Recent Advances in Polar Cap Density Structure Research

In the polar cap region, high-density ionospheric structures, such as the large-scale tongue-of-ionization and mesoscale polar cap patches, are frequently observed, which are of significant space weather concerns. In this chapter, recent progress on these polar cap high-density ionospheric structures is reviewed with an emphasis on the polar cap patch. Topics include the statistical occurrence rate of patches, plasma structure and characteristics inside patches, dynamic horizontal and vertical transport of high-density ionospheric structures, as well as the mechanisms and variability of optical patch emissions. For each topic, future works are envisioned following the progress reviews.