## 2 Data and Methods  
The present investigation is mainly based on GNSS observations, recorded by selected ground-based receivers located in Antarctica at Concordia station and SANAE IV base (Upper Atmosphere Physics and Radiopropagation Working Group, [34]). The geographic and Altitude Adjusted Corrected GeoMagnetic (AACGM, [35]) coordinates of these stations are reported in Table 1 together with their locations and owners. Each station is equipped with a Septentrio PolaR5 Ionospheric Scintillation Monitoring Receiver (ISMR), which is a multi-frequency, multi-constellation receiver dedicated to ionospheric monitoring and space weather applications [36; 37]. In particular, the ISMR is able to provide access to raw phase and post-correlation In-phase (I) and Quadrature (Q) components sampled at 50 Hz not only from the United States GPS system of navigation satellites, but also from the Russian GLONASS, the European Galileo, from the Chinese Beidou, the QZSS and satellites of the Satellite Based Augmentation System (SBAS) constellations. From the 50 Hz samples, ISMRs provide, among others, RINEX data and values of the scintillation indices at different time intervals for all visible satellites and at multiple frequencies. In this work, 1-min data of phase (\(\sigma_{\varphi}\)) and amplitude (\(S_{4}\)) scintillation indices [25] at L1 (GPS) and E1 (Galileo) central frequency (1575.42 MHz) are considered. To detrend the carrier phase, used in the \(\sigma_{\varphi}\) we adopted the standard scheme based on a sixth-order Butterworth filter with a 0.1 Hz cutoff frequency bearing in mind that \(\sigma_{\varphi}\) includes phase fluctuations due to ionospheric refraction not related to scintillation and is dependent on the plasma convection velocity (see, e.g., References [27; 29]). This choice, indeed, does not affect our discussions that are based on the different behavior of \(S_{4}\) and \(\sigma_{\varphi}\) according to the different scale sizes of the probed irregularities. To reduce the impact on measurements of longer paths through the ionosphere, we vertically projected scintillation indices according to the method described in [PERSON] et al. [7; 11] by fixing the height of the Ionospheric Piercing Point (IPP) at 350 km according to [PERSON] et al. [16; 17]. In addition, following [PERSON] et al. [38] we applied a mask of \(30^{\circ}\) on the elevation angle of the satellites to reduce the impact of non-scintillation related tracking errors (such as multipath). According to the convergence time of the Butterworth filter, we included in the analysis only data characterized by a lock time greater than 240 s [39]. To investigate the ionospheric scenario leading to the observed scintillations in terms of the occurrence of electron density irregularities, we collected and processed raw GNSS measurements provided by the permanent stations of the University NAVSTAR Consortium (UNAVCO), located in polar, auroral and subauroral regions of the Southern Hemisphere, including also measurements recorded by the DMC0 and SNA0 receivers. In particular, we processed GNSS measurements to obtain calibrated Total Electron Content (TEC) data. We performed the TEC calibration by using the technique by [PERSON] et al. [40] and [PERSON] et al. [41]. We also calculated the Rate of TEC change (ROT) that allows inferring the scale sizes of the ionospheric irregularities [8; 42; 43] and also retrieving information on their dynamics. Since in situ and ground-based observations can enhance the understanding of the ionospheric features associated with signal scintillations, we resorted to a wide variety of additional data sources. For instance, we analyzed and interpreted data from the Special Sensor Ultraviolet Spectrographic Imager (SSUSI) on board the Defense Meteorological