## 3 The September 2017 Geomagnetic Storm: An Overview  
The period under investigation is featured by a geomagnetic storm, which was caused by the intense solar activity that occurred between September 4 and 10, 2017. The main source of the storm was the Active Region AR2673 (Catania sunspot group 46), which produced four X-class eruptions including the strongest of Solar Cycle 24 on September 6, 2017, with a flare intensity of X9.3.  
An Interplanetary Coronal Mass Ejection (ICME) was observed to start on September 6. This triggered a class G4 storm (severe geomagnetic storm--NOAA Kp = 8, Ap = 106) whose main phase was on September 7 and 8, 2017 [52; 53]. In addition, between September 5 and 8, increased values of the protons flux with energy > 10 MeV were recorded, resulting in several proton events [52]. As reported in [PERSON] et al. [54], the perturbed period was characterized by a first interplanetary shock followed by a magnetic cloud on September 7 and a second interplanetary shock followed by a second magnetic cloud on September 8. Here we focus on the second shock and magnetic cloud.  
Figure 1 displays the velocity and dynamic pressure of the solar wind (SW) and its Interplanetary Magnetic Field (IMF), along with the high-latitude geomagnetic response in terms of _AE_, _AL_ and _AU_ indices. According to the red curve in panel a, on 8 September \(B_{\rm z,IMF}\) shows two periods of pronounced negative values. The first started with the minimum value recorded during the day (~33 nT), occurred between 00:00 and 02:30 UT, which was the reminisice of the first magnetic cloud. The second started at around 11:30 UT and ended around 18:10 UT, when \(B_{\rm z,IMF}\) settled near weakly negative values. During the second period of negative values, shortly before 15:00 UT, two fast fluctuations of \(B_{\rm z,IMF}\) around zero occurred. At the same time, a peak (with a maximum of ~10 nPa) in the SW dynamic pressure (red curve in panel b) was recorded. The negative excursions of the \(B_{\rm z,IMF}\) component resulted in intense particle precipitation at high latitudes that produced enhancements in the substorm activity. Indeed, during southward \(B_{\rm z,IMF}\), intense eastward and westward auroral electrojets ([55] and references therein) were recorded, as testified by the trend of the upper and lower auroral electrojet current indices, respectively (_AU_ and _AL_: green and red curves, respectively, in panel c). This suggests the presence of favorable  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{4}{c}{**Satellite Observations**} \\ \hline
**Satellite** & **Height of Flight** & **Instrument Type** & **Measurements** & **Sampling Time** \\ \hline Defense Meteorological & & Special Sensor & emission from N2 & \\ Satellite Program (DMSP) satellites & \(\sim\)830 km & & Demographic Imager & LBHL band (165–180 & 15 s for each scan \\ Swarm A and B & \(\sim\)445 km and \(\sim\)510 km & (SSUSI) & nm) & \\ satellites & \(\sim\)445 km and \(\sim\)510 km & & & \\ respectively & Langmuir probes & Electron density & 0.5 s \\ \hline \hline \end{tabular}
\end{table}
Table 2: Satellite, height of flight, instrument type, measurements and sampling time of the space-based observations used in this study.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{4}{c}{**Ground-based Observations**} \\ \hline
**Instrument** & **Location** & **Station ID** & **Measurements** & **Sampling Time** \\
**Type** & & & & \\ \hline Fluxgate & Concordia & DMC & \(\rm H,Z\) & 1 min \\ magnetometers & SANAE IV & SNA & \(\rm H,Z\) & 1 s \\ Super Dual & High latitudes & & Convection & \\ Auroral Radar & Mid latitudes & & velocity and & 2 min \\ Network & Polar Cap & & spectral width & \\ \hline \hline \end{tabular}
\end{table}
Table 3: Instrument type, location, station identifier, measurements and sampling time of the ground-based observations used in this study.  
conditions to the formation of ionospheric plasma irregularities causing scintillations on the GNSS signals [56].