# On the Feasibility of Interhemispheric Patch Detection Using Ground-Based GNSS Measurements
## 4 Results and Discussion
### The Analysis of Patch Occurrence with Relative STEC Values. The results given in Figure 4 confirmed the feasibility of continuous patch detection with GNSS data. Thus, this section presents the analysis of these structures' occurrence during the following phases of the storm and its dependence on IMF Bz. Due to the difference in GNSS data distributions for both hemispheres, we performed this analysis for 3\({}^{\circ}\) bands of modified AACGM latitude, according to scheme given in Figure 7. Although the selected areas are not homogenous, which may influence the results to some extent, they provide continuous observations in both cases. Figure 6: The signatures of patches derived from GNSS and SWARM measurements for a depleted ionosphere. The top panel presents the distribution of relative STEC values as a function of MLT and AACGM latitude for elevation masks equal to 30\({}^{\circ}\) and 15\({}^{\circ}\) with a superimposed arc of SWARM data (denoted with dotted and solid black lines). The bottom panel presents temporal profile of SWARM foreground plasma density. Black solid lines correspond to patch structures according to the Polar Cap Products (PCP) project. Figure 7: Sectors used for patch analysis. The summary of such prepared results is demonstrated in Figure 8. It integrates the variations of IMF Bz derived from the OMNI database (top panel) and the epoch-wise maximal values of relative STEC for selected sectors (middle and bottom panels). At this stage, we set the elevation cut-off angle to 30\({}^{\circ}\). The results of analysis were also supported by VTEC values derived from hourly global ionospheric maps (GIM) generated by the European Space Operations Center (ESOC) [54]. The first of them--0.5 VTEC at the pole--was used as an indicator of patch occurrence. Assuming the lack of geometrical dependency of STEC enhancement, this threshold can be identified as the foreground to background ratio for the GNSS technique at the level of 1.5. According to our preliminary comparison with the SWARM dataset, it roughly corresponds to cases when the ratio of plasma density is equal to 2. The latter values show VTEC on the dayside ionosphere at 50\({}^{\circ}\), 60\({}^{\circ}\), and 70\({}^{\circ}\) of the AACGM latitude, respectively. Analyzing the occurrence of polar patches on 17 March 2015, one can observe two periods of high patch activity, i.e., \(\sim\)6:00-10.00 UT and \(\sim\)16:00-24:00 UT. Both of them correspond to negative IMF Bz, which is consistent with the current knowledge in this field [7; 17]. In the case of the first period, the structures near the North Pole rarely exceeded 10 TECu. However, the much more intense patches, \(\sim\)30 TECu, were detected for low-elevated GNSS data (Figure 9). This confirms the feasible and adverse impact of data distribution on the derived results. Taking into account the low-elevated data, the structures that were observed in the first phase reached over 30 TECu and 60 TECu in the northern and southern hemispheres, respectively. In both cases, the strongest enhancement was related to the sequence of three patches (\(\sim\)8.30-10.30 UT), for which propagation above the North Pole is clearly depicted in Figure 9. For the southern hemisphere (the middle panel of Figure 8), the two first structures are merged due to the difference in signatures for particular satellites. The corresponding disproportion for both analyzed regions is also observed in the SWARM data set, provided that not all of the structures have been classified as patches due to the mentioned latitudinal mask. This patch activity appears to be related to high level of dayside plasma (depicted in Figure 8 as VTEC) and feeding of polar ionosphere with the rapid extension of the auroral oval that was reported in [46]. The interhemispheric comparison of dayside VTEC from GIM does not explain such strong differences in patch enhancement.