## 6 Summary and Concluding Remarks
The spatial scale size of irregularities detected in such maps is not small enough to justify the observed amplitude scintillation but the occurrence of echoes back from the ionosphere, recorded by SuperDARN in the field of view of GNSS receivers, confirms the presence of small-scale irregularities (on the order of tens of meters). The investigation highlights that the existence of irregularities with a spatial scale of the order of the first Fresnel L-band radius or below is not a sufficient condition to trigger amplitude scintillation of GNSS signals. According to the understanding we gained from multi-instrument observations, the lack of amplitude scintillations can be attributed to the values of vertical TEC being lower during scintillation events affecting only the phase of GNSS signals (<12 TECu) than, at both sites, during phase and amplitude scintillation events (>20 TECu). Indeed, in agreement with both [PERSON] et al. [95] and [PERSON] and [PERSON] [41], we found that the most intense phase and amplitude fluctuations of the GNSS signals occurred in those regions characterized by intense vertical TEC gradients that occur only when the background TEC is on the order of 20 TECu. The latter, indeed, would ensure that the energy associated with the electric field gradients is sufficient to trigger amplitude scintillation at high latitudes. Our results suggest that a kind of \"threshold\" in the background density seems to exist, above which the amplitude scintillation is favored. However, this aspect deserves further investigations that we leave to future studies. The high level of ionization recorded by both Swarm satellites and GNSS receivers, suggests that the lifetime of small-scale irregularities is greater than the time needed to the ionosphere to locally adjust the dynamics following an increase in conductivity. This observation explains both why significant GPS amplitude scintillations have been rarely observed at high latitudes (e.g., References [20; 33; 96]), and why different profiles of amplitude scintillation indices were recorded at SANAE IV and at Concordia during 8 September 2017. The moderate/intense amplitude scintillations observed at SANAE IV can be ascribed to the TOI fragmentation process, which seems to be more effective for producing small scale irregularities at the cusp due to the increase in particle precipitation and FAC activity driven by the \(B_{\mathrm{z,IMF}}\) negative excursion. Concerning the definition of scintillations as solely due to diffractive effects, the \(\sigma_{\varphi}\) high values, recorded without a simultaneous enhancement of \(S_{4}\), are considered as not being scintillations but phase fluctuations that are due to the inadequacy of the detrending filtering, with a cutoff frequency fixed at 0.1 Hz, to remove fluctuations associated with the high velocity of the ionospheric structures [29]. The behavior of the \(S_{4}\) profile recorded at Concordia is perfectly consistent with the receiver's field of view that looks at the polar cap during the investigated period. Indeed, since the polar cap represents the area encompassing the ground projection of the Earth's magnetic field lines directly connected with the IMF, it is more exposed to both particle precipitation and solar wind variability than the auroral region. Theseconditions greatly augment the variability of plasma dynamics by increasing the likelihood of observing local variations of the ionospheric dynamics sufficient to generate small-scale irregularities [97; 98; 99; 100]. In conclusion, the outcomes of our study can be summarized as follows:  
1. Amplitude scintillation occurred together with phase scintillation at auroral latitudes under condition where the auroral oval expanded to the field of view of the GNSS observatory during geomagnetic storm. 2. Phase scintillation occurred without concurrent amplitude scintillation under conditions when the background electron density was insufficient to produce intense irregularities with scale sizes of the first Fresnel radius. 3.