# On the Feasibility of Interhemispheric Patch Detection Using Ground-Based GNSS Measurements
## 1 Introduction
It was confirmed that daily patch modulation occurs with the greatest frequency at 10:00-22:00 UT and 10:00-23:00 UT in the northern and southern hemispheres, respectively [28; 29]. The studies [6; 30] were aimed at an interhemispheric comparison of patch appearance. The results given there are consistent with the maximum of patches that was detected during wintertime for the northern hemisphere. On the other hand, there is no agreement for the southern one, where maxima were observed in the local summer and winter, respectively. This discrepancy was preliminarily explained by the distinct definition of patch (related to different measured values, i.e., plasma density and slant total electron content--STEC [31]), and the areas that were used for their detection. As shown in recent studies aimed at total electron content (TEC) fluctuation [32; 33; 34], the current distributions of permanent GNSS stations allow the interhemispheric detection of high-latitude ionospheric disturbances. Thus, one can consider extending patch analyses with this technique. Up to now, GNSS-based investigations of these polar structures were predominantly performed with regional TEC maps [35; 36], but this methodology has a number of limitations. The first of them is the temporal resolution of maps, which is not high enough to capture the dynamic structures. The second, even more problematic limitation is the lack of geometrical dependency of STEC patch enhancement on elevation angle [30; 37]. As a result, the application of any mapping function used for vertical TEC modeling implicates the distortion of structures in the final ionospheric maps of the ionosphere. In order to overcome these limitations, we propose using relative STEC values derived from a time series of geometry-free combination (\(L_{4}\)). This approach is, to some extent, similar to that applied by [30], where patches were found arc by arc. The parameter applied in our case--the relative STEC value--is the difference between a \(L_{4}\) combination of GNSS signals at each epoch and the background STEC level. In order to compute the latter, we used an iterative algorithm of four-degree weighted polynomial fitting applied to arcs of \(L_{4}\) combination data. The utilization of epoch-wise relative enhancement as a parameter allows following the convections of large-scale ionospheric structures in multi-station mode. The evaluation of the proposed methodology was performed for the St. Patrick storm in March 2015. The paper is organized as follows. The second section presents a brief description of the algorithm that was used to determine the relative STEC values and the data set that was used in experiment. Subsequently, the conditions during the test period are characterized. This was followed by the core section presenting and discussing the results. The section begins with the characteristics of the spatial distribution of GNSS data in both hemispheres, and the applicability of the proposed methodology for patch convection monitoring. This includes also the initial comparison of GNSS-based results with those from the SWARM satellites. The second part of the section analyzes the temporal occurrence of patches and its dependence on geomagnetic conditions. Finally, the summary and conclusions are given in the last section.