## 5 Discussion
### SANAE: Phase Fluctuations without Amplitude Scintillations  
Between 00:00 and 06:00 UT, around 18:00 UT and between 20:00 and 21:00 UT on 8 September the GNSS receiver located at SANAE IV recorded moderate/strong \(\sigma_{\varphi}\) levels with very low (\(S_{4}<0.1\)) or no amplitude scintillation events (Figure 2, panel c and d, respectively). By comparing the phase fluctuations that occurred between 00:00 and 06:00 UT with the magnetic field observations recorded at the same time by the magnetometer co-located with the GNSS receiver (Figure 6), it can be seen that such events occurred when the station is located either poleward or equatorward of the electrojet peak. Indeed, the peaks of \(\sigma_{\varphi}\) are well in agreement with the peaks of the \(Z\) magnetic field component. This means that the phase fluctuations of the recorded GNSS signals seem to occur when the station is located underneath the edge of the electrojet, i.e., the region where instability and turbulence processes are more likely to occur [63; 76; 77; 78; 79; 80]. Thus, the comparison between \(\sigma_{\varphi}\) variations and geomagnetic observations suggests that the observed fluctuations have been caused by variations in plasma dynamics and have, therefore, a refractive nature. In order to confirm such a hypothesis, we compared the \(\sigma_{\varphi}\) variations with the corresponding auroral radiance measurements. Likewise, as shown in Figure 4, Figure 8 shows polar-view maps of the auroral radiance as measured by SSUSI at 01:10 UT (a), 18:07 UT (b) and 20:46 UT (c) on September 8. Such times were selected to investigate the ionospheric background in which the GNSS receiver at SANAE IV base recorded phase fluctuations without amplitude scintillations. The maps display also the auroral oval boundaries location (red dashed contours) as retrieved from the auroral radiance data and the projection to 350 km of altitude of the \(\sigma_{\varphi}\) greater than 0.25 rad (magenta diamonds), recorded during the DMSP passage. In each map, the projection to 350 km of altitude of all the \(\sigma_{\varphi}\) values with an elevation angle \(\alpha=30^{\circ}\) (purple circles) were reported to identify the field of view of the GNSS receiver along the DMSP track. This enables a comparative analysis to show the correlation between phase fluctuations and particle precipitation into the auroral oval. Figure 7: Polar-view maps in AACGM latitude and MLT of SuperDARN observations at 13:38 UT (**a**), 13:40 UT (**b**), 13:42 UT (**c**) and 13:44 UT (**d**) on 8 September 2017. Each map shows the isocontours of the ionospheric potential (red=positive, blue=negative potential) and SuperDARN measurements (black squares) recorded in the two minutes following the time shown at the top of each map. In each map 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 are also shown. In each map, which covers 00:00–24:00 MLT and \(\mid\!50^{\circ}\!\mid-\!\mid\!90^{\circ}\!\mid\) AACGM Lat, the magnetic noon/midnight is at the top/bottom. All maps show that during the investigated periods, the GNSS receiver's field of view (purple circles) was inside the auroral oval. From panel a, it is clear that moderate/strong \(\sigma_{\varphi}\) variations (magenta diamonds) occurred near regions characterized by high values of auroral radiance. This suggests the crucial role of the auroral precipitation in creating electron density gradients of a scale size above the Fresnel's scale for L-band signals, resulting in mostly refractive effects on the phase. Furthermore, the position of the receiver field of view at \(\sim\)01:10 UT (panel a) suggests that the observed events may have been caused by auroral blobs formed when polar cap patches enter the nightside auroral region [14; 81; 82].