## 1 Introduction  
Ionospheric scintillation is the most intriguing and challenging effect observed on L-band signals (100 MHz-4 GHz), such as those used by the Global Navigation Satellite System (GNSS). Scintillations are rapid and random fluctuations of the trans-ionospheric signals received by ground-based and space-borne instruments. They are induced by ionospheric irregularities, which are defined as regions of enhanced or depleted plasma density relative to background encountered by the electromagnetic wave in its travel through the ionosphere (e.g., References [1, 2]). Electron density gradients result in refractive index variations, which give rise to refraction and diffraction processes distorting the original wave front of the received signals. Refraction induces mainly phase fluctuations. The received phase changes because the electromagnetic wave enters a medium of either increased or decreased phase velocity. Diffraction implies that the impinging electromagnetic wave enters the ionosphere with a spatially uniform phase and exits the ionosphere with a spatially irregular phase. These irregular phases may combine either constructively or destructively, giving rise to amplitude and phase scintillations (e.g., References [2; 3; 4]). This diffraction pattern is produced by ionospheric irregularities near the first Fresnel scale, which, in the case of GNSS signal transmitted at L1 = 1575.42 MHz frequency, is in the range 300-400 m for irregularity residing in the F-region ionosphere [3]. The regions mostly affected by scintillation phenomena are the equatorial and the auroral/polar latitudes [5; 6; 7]; however, the physical mechanisms involved in the formation of the irregularities are significantly different in the two regions. At high latitudes, the main phenomena triggering GNSS signal degradation are as follows: (i) large-scale electron density gradients (auroral blobs, polar cap patches, etc. ); (ii) precipitating energetic particles within the auroral oval and cusp; and (iii) variation of the plasma velocity [7; 8; 9; 10; 11; 12; 13; 14]. Geomagnetic forcing and solar activity control such events. Indeed, under high geomagnetic activity, the solar wind-magnetosphere-ionosphere coupling leads to an increased formation of ionospheric irregularities that, consequently, can result in an increased probability of GNSS signal disruptions [15; 16; 17; 18; 19; 20; 21; 22; 23]. To quantify the scintillation, the \(S_{4}\) and the \(\sigma_{\varphi}\) indices are derived from GNSS receivers to characterize the amplitude and the phase fluctuations, respectively. \(S_{4}\) represents the normalized standard deviation of the amplitude over 1 min, while \(\sigma_{\varphi}\) is the standard deviation of the detrended phase over 1 min [24]. Since to derive scintillation indices, it is necessary to remove a long-term trend--consisting of the Doppler shift due to satellite receiver relative motion, slowly varying background ionosphere and some hardware effects--usually a sixth-order Butterworth filter with a 0.1 Hz cutoff frequency is used (see References [24; 25; 26]). However, since amplitude and phase fluctuations in a received electromagnetic wave may be diffractive or refractive, the choice of this cutoff frequency is not unique and should be made carefully; otherwise, the refractive variations are also included (see, e.g., Reference [27] and references therein). This leads to case of \"phase without amplitude scintillation\", i.e., phase fluctuations misinterpreted as phase scintillations, because the value of the cutoff frequency does not affect the determination of the amplitude scintillation index, but it does affect the phase scintillation index very much (see References [27; 28; 29] and references therein). This is especially true at high latitude (see, e.g., References [8; 11; 22; 23; 30; 31]), where a very high and variable ionospheric drift velocity (~100 to 1500 m/s; see, e.g., Reference [32]) exists.