# 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  
According to the distributions of permanent GNSS receivers in both hemispheres as given in Figure 2, it seems to be clear that the polar patches should, at least to some extent, be detectable. Thus, the starting point is the analysis of the spatial distribution of epoch-wise relative STEC values. Figure 3: Time series of the selected geomagnetic indices during 15–19 March 2015 (based on OMNI database: [[http://omniweb.gsfc.nasa.gov](http://omniweb.gsfc.nasa.gov)]([http://omniweb.gsfc.nasa.gov](http://omniweb.gsfc.nasa.gov))). In order to evaluate the ground-based GNSS measurements, they were combined with the plasma density that was derived from SWARM. This satellite mission of the European Space Agency consisted of three LEO satellites: Alpha (A), Bravo (B), and Charlie (C). SWARM A and C fly side by side on a nearly circular orbit with an inclination of 87.35\({}^{\circ}\) at an altitude of 462 km. The parameters of the SWARM B orbit are 87.75\({}^{\circ}\) and 510 km, respectively. The same instruments are installed onboard each satellite, which allowed precise in situ measurements of the Earth's magnetic field, upper atmosphere, and ionosphere, with high temporal resolution [52]. The dataset provided by this constellation is currently an object of intensive investigations. In our work, we adopted the results of the Polar Cap Products (PCP) project founded by European Space Agency and handled by Department of Physics at the University of Oslo ([[https://www.mn.uio.no/fysikk/english/research/projects/swarm/](https://www.mn.uio.no/fysikk/english/research/projects/swarm/)]([https://www.mn.uio.no/fysikk/english/research/projects/swarm/](https://www.mn.uio.no/fysikk/english/research/projects/swarm/))), which is acknowledged. The main goal of the PCP was the proposal and further application of an algorithm aimed at an automatic detection of polar patches using data from SWARM Langmuir probes. Its results, covering over four years, include foreground and background plasma densities that were derived using the percentile filtering technique, and are available at the site of the project [6]. The comparison of patch detection using both methods is presented in Figure 4. It demonstrates the distributions of relative STEC values and SWARM data as a function of UT and modified AACGM latitude. The latter parameter corresponds to the point of latitude being a projection of real observations on the noon-midnight axis of magnetic local time (MLT). Despite some disadvantages related to a lack of distinction between the dusk and dawn sides, such an approach allows the depiction of the spatial distribution of measurements as well as its daily temporal modification. At this stage, we used only the data with an original AACGM latitude higher than 77\({}^{\circ}\), which is consistent with the methodology that was employed in [6]. The conversion of GNSS data to an MLT-AACGM latitude reference frame was performed with the methodology proposed in [53] using an ionosphere single-layer model (350 km). At this point, we set the cut-off angle at the level of 30\({}^{\circ}\). The SWARM dataset was preprocessed in a similar manner; however, in Figure 4, only the points with a density ratio higher than two were given. Looking firstly only at the spatiotemporal distribution of ground-based GNSS data, one can assume that the application of epoch-wise relative STEC values enables tracing patch convection  
Figure 4: The comparison of patch detection using GNSS-based relative STEC values and SWARM in situ data (black dots). AACGM latitudes are derived with the projection of measurement points onto the noon–midnight axis of magnetic local time (MLT). Only the points with a density ratio higher than two are shown in the case of SWARM data. across the polar cap. Comparing Figures 2 and 4, it is clear that this feasibility is mostly related to the sub-network of receivers that are located in the Greenland and Transantarctic mountains, respectively.