# On the Feasibility of Interhemispheric Patch Detection Using Ground-Based GNSS Measurements
## 4 Results and Discussion
### The Feasibility of Patch Monitoring Using Ground-Based GNSS Receivers
They provide the majority of ionospheric samples, which correspond to the rotating bands observed in Figure 4. The zone in the northern hemisphere is wider due to the better meridional spread and distribution of the permanent stations. The situation in the southern part is slightly worse. The distance between the Transantarctic mountains implicates the restriction of the sounded area to \(\sim\)85\({}^{\circ}\) of AACGM latitude. On the other hand, the singular receivers that are located on the coastline of Antarctica (see Figure 2) provide some information at the opposite side of the polar ionosphere, which may be useful for the detailed study of patch convection. This feasibility is predominantly visible for the strongest structures on 17 March 2015. The interhemispheric comparison of patch signature continuity confirmed the more favorable measurement conditions near the North Pole. Almost all of the structures that are detected there are observed for the entire 10\({}^{\circ}\) band of modified ACCGM latitude. Moreover, even very weak patches with STEC enhancement at the level of a few TECu appear to be correctly identified. The continuity of signatures is not always the case for the southern hemisphere (e.g., 19:00-22:00 UT in Figure 4b or 12:00-14:00 UT in Figure 4f). This discrepancy between both areas seems to be related to the difference in the receiver distributions in the Greenland and Transantarctic mountains. The approximately linear shape in the second case may influence patch tracing. One can assume that the parallel orientation of the GNSS sub-network and patch propagation is advantageous (all of the stations sequentially observe the particular structure), whereas in perpendicular exposure, simultaneous detection is expected. Thus, in such a case, the effectiveness of the GNSS technique depends on the current satellite constellation, which under very adverse conditions may not be achieved. This unfavorable distribution of stations is probably also responsible for the blurring of structures that was detected near the South Pole. The noteworthy findings are provided by the comparison of patch detection with ground-based GNSS and SWARM data. Looking at Figure 4, one can observe that the results for both methods are only partially compliant. For some periods--such as 16:00-20:00 UT as presented in Figure 4a or 10:00-12:00 UT as given in Figure 4f--there is a good coincidence, but this is not always the case. The main reasons for discrepancies are the orientation of SWARM orbits and the sounding of different areas of the polar ionosphere by both techniques. In order to depict both of these effects, we selected two representative examples. The first of them, which is given in Figure 5, is related to the sequence of three patches (\(\sim\)60-90 TECu in a slant direction) that occurred in the southern hemisphere during the main phase of the storm (16:30-17:40 UT on 17 March). The top panel of Figure 5 presents the relative STEC values as a function of MLT and AACGM latitude, with the latter extended to 60\({}^{\circ}\) and traces of two arcs of SWARM satellites. The bottom one provides the temporal variations of foreground plasma density for the SWARM series. Analyzing these results, it should be remarked that the auroral oval for this period was extended to 60\({}^{\circ}\)[46]. As is shown, both techniques allow the identification of extreme structures, and the results are compatible. In the case of LEO satellites, the detection occurs at 70-73\({}^{\circ}\) of AACGM latitude, i.e., below the latitudinal limit as applied for automatic detection equal to 77\({}^{\circ}\)[6]. Thus, these structures were not classified as patches in such a mode of processing. Certainly, it does not reduce the effectiveness of the SWARM results, but rather highlights that the monitoring of the polar ionosphere for the particular passage of an LEO satellite depends on its orbit orientation and the area that was adopted for patch detection.