# 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 concentration of plasma in the southern hemisphere is only slightly  
Figure 8: Variations of IMF Bz (top panel) and maximal relative STEC values for sectors given in Figure 7 (middle and bottom panels). In the latter case, VTEC on the dayside ionosphere and at the poles are superposed. higher (up to 10%), whereas the last structure observed there was approximately two times stronger. This inconsistency is probably related to the infeasibility of the detection of polar structures with hourly GIM. The second period (\(\sim\)16:00-24:00 UT on 17 March 2015) with an accumulation of large-scale structures confirms the disparity of enhancement observed for both hemispheres. In this case, the maximal STEC values for the southern hemisphere were almost three times the size of those of the northern hemisphere. Unfortunately, the large number of structures leads to their strong merging. This effect requires further investigation, but it should be able to be reduced through the selection of particular satellites. On the other hand, this period can be considered as the best-case scenario with reference to the number of samples on the dayside ionosphere. Thus, in order to clarify the interhemispheric differences, we used the relative STEC values derived from all of the stations given in Figure 2. Such results (Figure 10), covering AACGM latitudes above 45\({}^{\circ}\), clearly demonstrate the propagation of plasma being a source of polar patches. As one can see, the stream on the northern hemisphere was characterized by relative STEC enhancement rarely exceeding 20 TECu, whereas on the opposite side, it reached 40 TECu and more. Taking into account the spatially extended effect in the second case, one can assume that the observed disproportion in polar structures is a consequence of the difference in the dense stream of dayside plasma. The second sequence of patches seems to be originated from the same region of high-density plasma as the first; however, it was fed from the dusk sector of the mid-latitude ionosphere. This also explains the relatively low level of noon VTEC for this period. The much less intense patches between \(\sim\)10:00 and 16:00 UT resulted from the reorientation of IMF Bz, changing the high-latitude convection pattern, and the poleward shift of auroral oval. The single structures that were observed within are a few times weaker, and did not exceed 15 TECu. Figure 10: The distribution of relative STEC values as a function of AACGM latitude and MLT obtained for both hemispheres (18:00–20:00 UT on 17 March 2015). Figure 9: Variations of maximal relative STEC values according to the northern hemisphere sector given in Figure 7, as derived for two employed elevation cut-off angles. The occurrence of patches in the two periods indicated above is basically consistent with the regional maps of the ionosphere that were provided in [50], which were generated using MAP GPS software [55]. However, the structures that were observed in the latter case featured VTEC not exceeding 20 TECu. This difference seems to be related to two factors. The first of them is STEC-VTEC conversion, which reduces the enhancement for low-elevated observations. The second reason is the applied indicator. We adopted the maximal epoch-wise STEC values, while the VTEC values for each of the 1\({}^{\circ}\) by 1\({}^{\circ}\) bins in [50] are the results of adjustment. The opposite conditions were observed for the period \(\sim\)00:00-14:00 UT on 18 March, which were characterized by northwardly-oriented IMF and a depleted ionosphere. The results for the northern side depicted an extremely quiet ionosphere (Figure 8), which was partly related to poor data distribution. The application of additional low-elevated data revealed the occurrence of four patches with STEC enhancement at the level of five to seven TECu (Figure 9). These met the applied criterion and were classified as patches in the SWARM data as well.