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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 by geomagnetic activity level. This result seems counterintuitive, but as shown in Liu et al. (2016) and Zou et al. (2014), the evolution of SED plume or TOI depends on the interplay between the convection electric field and thermospheric winds. There are cases in which the SED plume/TOI do not extend into the polar cap during geomagnetic storms. This is further complicated by the fact that global‐scale thermospheric composition change with increased molecular species can occur during the negative storm phase and, thus, extremely low ionospheric densities may occur while the geospace is still under perturbed condition. Therefore, the relationship between TOI/patch and geomagnetic activities may not be described by a simple linear relation.

It has been well known that the IMF direction and magnitude largely control the ionospheric convection pattern and their variations can segment large‐scale high‐density structures into smaller‐scale patches (Anderson et al., 1988; Lockwood & Carlson, 1992; Rodger et al., 1994; Valladares et al., 1996, 1998; Zhang et al., 2013a). Studies have been performed trying to understand the patch occurrence rate dependence on the IMF conditions. Spicher et al. (2017) found that patches occur more often in the Northern Hemisphere postnoon/prenoon sector for negative/positive B_y condition, while the trend is mirrored in the Southern Hemisphere. This result is consistent with the cusp location dependence on the IMF B_y, confirming that the dynamics in the cusp region is responsible for the patch segmentation. The superposed epoch analysis carried out by Noja et al. (2013) shows that enhanced IMF B_z preceded the patches, suggesting that enhanced convection is important for the patch formation. In the Jin et al. (2018) paper, ESR was selected to observe patches within 3 hours surrounding the noon MLT in order to minimize the time between their formation near the dayside cusp and their detection at ESR. This study confirmed the preference of patch formation during southward IMF B_z, and also revealed the IMF B_y influence on the patch location.

In this section, we briefly reviewed the recent results about the statistical occurrence rate of polar cap patches and its dependence on season, UT, geomagnetic activity, and IMF conditions. In the Southern Hemisphere, the seasonal occurrence rate of patch differs depending on its identification mechanisms, whether it is based on in situ density measurement or integrated TEC. This discrepancy suggests that caution is needed when identifying the patches using the traditional doubling electron density method. Criteria reflecting that patch is a high‐density structure should be included as well, such as a requirement of the patch density being higher than the average background density. Also, the occurrence rate of TOI/patch shows no clear relationship with the geomagnetic activity level indicated by Kp. More detailed analysis is needed in the future to take into consideration the storm phases and thermosphere composition changes. In addition, further statistical studies are needed to understand the most probable IMF conditions right at the time when the patches are produced at the dayside cusp region, in order to single out the major segmenting mechanism of patch.