## 5 Discussion
### SANAE: Scintillation Events
These irregularities could be the consequence of density gradients due to the development of Gradient Drift Instability (GDI), driven by field-aligned currents or due to flow shears, which are common in the auroral ionosphere. They also could be direct consequences of cusp dynamics, i.e., precipitation associated with filamentary Field Aligned Currents (FAC) [61]. All of these processes can be responsible for the generation of large-scale inhomogeneities that provide a background on which small-scale instabilities can develop [62; 63]. A corroboration of the presence of plasma fragmentation comes from maps in Figure 5, which display TEC data as a function of AACGM Lat and MLT, with a spatial resolution of 1\({}^{\circ}\) AACGM Lat x 4 min MLT. Each map shows TEC data collected every 30 min following the time shown at the top of each map with the projection to 350 km of altitude of all the \(\sigma_{\varphi}\) values having elevation angle \(\alpha=30^{\circ}\) (purple circles) recorded in the same time interval as the TEC data by the GNSS receiver at SANAE IV. All maps show a clear signature of a Storm Enhanced Density (SED; see, e.g., References [64; 65]) plume extending to a Tongue of Ionization (TOI; see, e.g., References [66; 67; 68; 69; 70]). Therefore, in the regions enclosed in the field of view of the GNSS receiver, TEC values appear generally higher than elsewhere, with TEC values exceeding 20 TECu (see also Figure 2a). In addition, such regions were characterized by irregular distributions of electron density, as highlighted by TEC values of different intensity for adjacent regions (Figure 5). This scenario suggests the presence of a local variability in the plasma dynamics, where intense and spatially localized density gradients exist, favouring the observed scintillation. Figure 5: Polar-view maps in AACGM latitude and MLT, with spatial resolution of 1\({}^{\circ}\) MLt x 4 min MLT, of the Total Electron Content (TEC) at 13:30 UT **(a)**, 14:00 UT **(b)**, 14:30 UT **(c)** and 15:00 UT **(d)** on 8 September 2017. Each map shows vertical TEC data collected in the 30 min following the time shown at the top of each map with the projection to 350 km of altitude of all the \(\sigma_{\varphi}\) values having elevation angle \(\alpha=30^{\circ}\) (purple circles) recorded at SANAE IV in the same time interval as the TEC data. In each map, that covers 00:00–24:00 MLT and 150\({}^{\circ}\) \(-\) \(|\) 90\({}^{\circ}\) \(|\) AACGM Lat, the magnetic noon/midnight is at the top/bottom. To reveal the position of the magnetic observatory with respect to the auroral electrojet current system, observations of the ground geomagnetic field are provided. Figure 6 reports the variation of the values of the magnetic field along the \(H\) and \(Z\) components recorded at the ground magnetic observatory (SNA) located in SANAE IV base. These two magnetic components permit us to reveal the location of the magnetic observatory relative to the auroral electrojet current systems during the selected day. The \(H\) component of the magnetic field, which points toward magnetic north, decreases in the presence of an ionospheric horizontal current flowing westwards and increases in the case of a current flowing eastwards. This means that according to the \(H\) component values assumed during the day, the magnetometer station at SANAE IV base is in the southern hemisphere auroral zone in two different time intervals, i.e., between 00:00 and 07:00 UT and between 13:00 and 24:00 UT. On the other hand, during the time interval 07:00-13:00 UT, it is outside the auroral oval. When the magnetometer station is inside the auroral oval it is located initially underneath the westward auroral electrojet, in correspondence with negative values of the perturbation in the \(H\) component, and subsequently underneath the eastward auroral electrojet, in correspondence with positive values of the perturbation in the \(H\) component. At last, the \(Z\) component permits to estimate the position of the station with respect to the peak of the electrojet.