## 5 Discussion
### SANAE: Scintillation Events
Indeed, when a ground station is located directly below the peak of the electrojet, most of the ground perturbation is recorded in the \(H\) component with minimal or no perturbation along the \(Z\) component. When, as in the case considered here, there is a magnetic perturbation in both the \(H\) and \(Z\) components, this means that the ground station is located either poleward or equatorward of the electrojet peak but it is still underneath the bulk of it. Between about 13:30 and 15:30 UT, corresponding to the time interval in which the GNSS receiver recorded amplitude scintillations (Figure 2d), the magnetic station crosses the two different auroral electrojets moving from the westward to the eastward one, which is a clear signature of a transition from dawn to dusk side of the cusp. In fact, between about 07:30 and 11:30 UT, that is when \(B_{\mathrm{z,IMF}}\) was around zero (see red curve in panel a of Figure 1), the variation in the Earth's magnetic field recorded at SNA was almost zero, as expected from a sub-auroral station under geomagnetically quiet conditions. On the  
Figure 6: \(H\) and \(Z\) magnetic field components recorded at SNA magnetic station located in SANAE IV base on 8 September 2017.  
contrary, when \(B_{\rm z,IMF}\) turned strongly southward at about 11:45 UT (red curve in panel a of Figure 1), and the auroral oval and the ionospheric convection cell expanded equatorward, SNA became a cusp/auroral station, and intense fluctuations of geomagnetic field were observed. Such transition, characterized by strong magnetic field variations, together with the observed TEC values (>30 TECu) greater than those generally recorded in quiet conditions (<20 TECu), leads to speculate about the setting, in this region, of a very structured FAC activity, where filamentary currents accompany an increase in electron precipitation. The presence of filamentary FACs, often associated with multifractal and turbulent behaviors of magnetic field variations (e.g., Reference [71]), could induce local electric field gradients able to alter the amplitude of an electromagnetic signal which crosses such region. This scenario agrees with [PERSON] et al. [72] who found the cusp as the sole GNSS scintillation hotspot. In addition, it is possible to suppose the presence of different plasma regimes in which irregularities of the order of the first Fresnel radius or even below, can form, which is compatible with the observed amplitude scintillations. Figure 7 shows further evidence that support this hypothesis. In detail, each map in Figure 7 shows the isocontours of the ionospheric potential (red = positive, blue = negative potential) as derived from SuperDARN convection velocity measurements and extended to the whole auroral region by means of an empirical model [73; 74]. Moreover, the locations of actual SuperDARN measurements (black squares) recorded in the two minutes following the time shown at the top of each map are also shown. Each map also reports the projection to 350 km of altitude of all the \(\sigma_{\varphi}\) values recorded by SNA0 with elevation angle \(\alpha=30^{\circ}\) (purple circles) in the same time interval as for SuperDARN observations. All the maps show that between 13:38 and 13:44 UT the overall convection pattern in the Southern Hemisphere is driven by the huge and continuous negative values of \(B_{\rm z,IMF}\) with convection cells tilted towards dawn because of the positive \(B_{\rm y,IMF}\) values (panel a of Figure 1, red and blue curve, respectively). In addition, the location of the GNSS receiver's field of view (purple circles) with respect to the configuration of the ionospheric potential cells and to the SuperDARN measurements (black squares) suggests that amplitude scintillations occurred at the cusp and close to the ionospheric regions in which SuperDARN recorded echoes back from the ionosphere. It is worth mentioning that SuperDARN echoes originate when a transmitted signal encounters plasma density irregularities of spatial scale in the order of tens of meters, mainly caused by the GDI.