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Ionospheric density varies significantly and affects the propagation of radio signals that pass through the ionosphere or are reflected by it. One example of these effects is the loss of phase lock and range errors in Global Navigation Satellite Systems (GNSS) signals. Because our modern society increasingly relies on ground‐to‐ground and ground‐to‐space communications and navigation, understanding the sources of the ionospheric density variability and monitoring its dynamics during space weather events is of great practical importance (e.g., Jin et al., 2014, 2017; Moen et al., 2013; Zhang et al., 2018).

Due to the geometry of the Earth's magnetic field, the solar wind has direct access to the high‐latitude polar cap region because the magnetic field lines there are open. The convection electric fields in the solar wind map directly down to the polar cap along the equipotential magnetic field lines (Lyons & Williams, 1984). The shape and size of the polar cap change considerably with varying solar wind and the polarity and magnitude of interplanetary magnetic field (IMF). Typically, the open‐closed field line boundary (OCB) of the polar cap expands to lower latitudes during southward IMF condition and retreats back to higher latitudes during northward IMF condition. Particles in the solar wind and magnetosheath can move along the magnetic field line down to the polar cap ionosphere and form the polar rain (Fairfield & Scudder, 1985). In the polar cap region, the most important high‐density structures in the F region and topside ionosphere are polar cap patches and storm‐enhanced density (SED) plumes and/or tongues of ionizations (TOIs) (Whitteker et al., 1978).

During enhanced convection and most often during a geomagnetic storm, a ridge of electron‐density enhancement often occurs in the midlatitude and subauroral regions, termed an SED (Foster, 1993). Occasionally, an SED can extend to higher latitudes as a plume‐like and continuous high‐density structure into the cusp and the polar cap, where it is often termed TOI or SED plume (Foster, 1984). On the other hand, polar cap patches (Weber et al., 1984) refer to islands of high‐density plasma within the polar cap, which have typical scale size of 100–1,000 km. An excellent review about the polar cap patch was provided by Carlson (2012), and therefore progresses after the year of 2012 are reviewed in this chapter. The criteria often used to identify the patches are that their density is at least twice that of the surrounding plasma (Crowley, 1996). Various instruments have been used to monitor the polar cap density structures, such as ground‐based coherent and incoherent scatter radars (ISRs), LEO satellites, and optical imagers. In recent years, numerous ground‐based GNSS receivers have been deployed and synoptic maps of the total electron content (TEC) in the ionosphere have been generated to provide continental‐scale view of the ionospheric density distributions (Coster et al., 1992; Mannucci et al., 1998; Rideout & Coster, 2006). Figure 4.1 shows two polar‐view plots of the dynamic density variations present in the polar cap during two intense geomagnetic storms in the solar cycle 24 (Zou et al., 2014). The left figure shows a well‐developed SED and plume but the plume did not extend into the polar cap, while the plume in the right figure indeed extended into the polar cap and had clear density variability inside.

Figure 4.1 Example of dynamic polar cap density structures (from Zou et al., 2014;

Reproduced with permission of John Wiley and Sons).

The difference between the SED plume/TOI and polar cap patch lies in their structural size with the former/latter being of larger/smaller scale size. In addition, it has been suggested that the SED plume/TOI is the major plasma source for the patches (Foster et al., 2005). Intermittent bursts of cold (low T_e) SED plasma streaming into the polar cap in the noontime cusp ionosphere were observed with the Chatanika (Foster & Doupnik, 1984) and Sondrestrom (Foster et al., 1985) ISRs. However, particle precipitations in the cusp (Rodger et al., 1994) and those within the polar cap are also proposed to be able to create patches, but of weaker effect (Hosokawa et al., 2016; Oksavik et al., 2006).

This brief review focuses on recent progresses of polar cap patches and SED plumes/TOIs. In the following sections, we review the statistical occurrence rate of patches, plasma structure, and characteristics inside patches, as well as dynamic horizontal and vertical transport of the high‐density plasma. We also review various mechanisms and the variability of patch optical signatures. Future research directions follow progress review for each topic. The formation and evolution of the SEDs, which are closely related with patches and TOI, are reviewed separately in Chapter 7 in this book and are not included in this chapter.