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 Te) 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.
4.2 STATISTICAL OCCURRENCE RATE OF POLAR CAP PATCHES
The occurrence rate of polar cap patches has been traditionally studied using both ground‐ and space‐based observations. An individual instrument that is fixed on the ground can provide long‐term data sets, but the results may only be applied to a limited geographic location due to the known longitudinal or UT dependence of patch occurrence (Coley & Heelis, 1998). In the recent years, in situ density measurements by low‐Earth‐orbiting (LEO) satellite (Chartier et al., 2018; Spicher et al., 2017), ground‐based 2‐D GPS TEC (David et al., 2016), and upward‐looking TEC measured by LEO satellite (Noja et al., 2013) have been utilized to characterize the polar cap patch occurrence rate, and the relevant findings based on these different measurement techniques are summarized below. While we embrace these new capabilities, it is also important to keep in mind the advantages and deficiencies of these techniques. Among all these measurements, TEC maps have the largest field of views spanning from regional to continental scales, while their coverage and resolution in the polar cap is somewhat limited because of the relatively low inclination angles of GNSS satellites, and the data gaps within the polar cap hinders identification of patches of small scale. Often, TEC maps are produced every 5 to 10 mins or even longer, which prohibits the analysis of dynamic patch evolution on a shorter timescale. On the other hand, upward‐ looking TEC measurement from LEO satellites integrates the density from the LEO satellite altitudes to the GNSS satellite, and thus is likely to miss a fraction of the F‐region density.
It is widely held that patches have a higher occurrence rate in winter than in summer. However, Noja et al. (2013) identified more patches in the summer Southern Hemisphere using the upward looking GPS TEC data from CHAMP. This finding was later confirmed by Chartier et al. (2018). Using the Langmuir probe in situ electron density data from multiple Swarm satellites, Chartier et al. (2018) also found that patches occur more frequently in both winter hemispheres (Fig. 4.2a–d). However, patches identified using upward‐looking GPS TEC data revealed that patches occur more frequently in December, in both hemispheres, rather than the winter hemisphere. The TEC data also presented a more clear solar cycle dependence of patch occurrence (Fig. 4.2e–h). A further in‐depth analysis by the authors identified the reason for the discrepancy: the background density in the winter Southern Hemisphere is extremely low and the traditional patch identification method of labeling regions where a doubling of the ambient density is present as patches can incorrectly identify very small density fluctuations as patches.
Figure 4.2 Polar cap patches detected by two different algorithms based on in situ density and upward‐looking TEC between August 2014 and July 2017 in each hemisphere. December and June solstices are indicated by vertical red and blue dashed lines (from Chartier et al., 2018).
Using ground‐based GPS TEC data between 2009 and 2015, David et al. (2016) studied the occurrences of high‐density structures, including both TOIs and patches, as a function of season and UT (Fig. 4.3) in the Northern Hemisphere. There is a clear “hole” in the winter season between ~05 and ~12 UT, during which the magnetic pole is tilted toward the nightside. This finding confirms the earlier numerical modeling results in Sojka et al. (1994) and supports the idea that the dayside solar EUV‐produced plasma is the major plasma source for the polar cap patches. The other possible patch plasma source, that is, particle precipitation, is not expected to have such UT dependence. Similarly, Yang et al. (2016) compared the averaged TEC patterns obtained between 00 and 11 UT and 12 and 23 UT during solar maximum in the mlat/MLT coordinates, and clearly revealed this UT dependence as well.
Figure 4.3 Seasonal and UT variations of the TOI or Patch to background ratio
(from David et al., 2016; Reproduced with permission of John Wiley and Sons).
In addition, David et al. (2016) reported that a majority of the patches or TOI in their database are during low Kp rather than high Kp and, thus, they suggested that their occurrence is not controlled