## 3 Results
### Ionospheric Investigation  
Electron density of CSES has been monitored in the volcanic area of the Canarias Islands system, considering a circle centred on La Palma volcano with a radius of 420 km. Considering that the ionosphere status is very different at night- and day-time for the influence of solar irradiation that induced, for example, the classic fountain effect around the magnetic equator [84], we separated their analyses. Figure 21 reports the night-time detrended values measured by CSES at about 2 AM, and Figure 22 reports those of daytime, acquired at about 2 PM. For each day in quiet geomagnetic time (\(\,\mathrm{I}\,\mathrm{D}\mathrm{s}\mathrm{t}\mathrm{1}\leq 20\,\mathrm{n} \mathrm{T}\) and \(\,\mathrm{a}\mathrm{p}\leq 10\,\mathrm{n}\mathrm{T}\)), the median of the Ne samples in the circular area is computed and represented after detrending, as explained in Appendix A. The figures report the indications of the anomalous positive days with the indication of how many days before the La Palma volcano eruption were recorded and the amount of anomaly. We noted that both analyses identified an increase in electron density about four months before the eruption (\(-128\) days for day-time and \(-121\) days for night-time). Figure 21: Time series of median daily values of CSES-01 Ne measured in night-time inside the circled area of 420 km radius centred on La Palma volcano. A detrend of the time series has been applied, as explained in Appendix A. Time series median plus or minus 1.5 times the interquartile range are overplotted as thresholds to define the anomalous days in terms of night-time Ne. Considering that the CSES satellite every five days passes in exactly the same orbit, here we analysed the orbit more closely to La Palma volcano for night-time measurements that are less affected by variation of solar activity. All such Ne latitudinal profiles are superposed in Figure 23. Every 1\({}^{\circ}\) latitude, the interquartile and eventual outliers were computed and denoted by red crosses. It is evident that two passes show a very high value of night-time electron density just southward of the La Palma volcano latitude (denoted by a vertical black line). They were acquired on 22 July 2021 and 1 August 2021. On 22 July 2021, the Auroral Electrojet (AE) index showed a very high geomagnetic activity at poles with a value that even overpassed 1000 nT. Despite this, the passage of CSES-01 above La Palma volcano was before the AE values reached such a high value, even if not completely calm. Despite this, from the comparison provided in Appendix C with the _TEC_ map of the same day, the anomaly depicted by CSES-01 is confirmed with _TEC_ from GNSS stations, but it seems to be a global phenomenon. Consequently, it is less probable that the source of the anomaly was the volcano. A completely different scenario was for 1 August 2021; in fact, the AE index presented low values for the whole day. Furthermore, the _TEC_ map shown in Appendix C of the following day identifies a local anomaly above the region of La Palma volcano; this provides more chances that the Ne increase depicted by CSES-01 on 1 August was a local phenomenon that may be linked to the preparation of the volcano eruption. It is noteworthy that the Swarm satellite on 26 July and 2 August 2021 detected some magnetic anomalies, as reported in Supplementary Materials File S2. This further confirms the reliability of CSES-01 data and underlines the ionosphere's perturbed status in such time. Outliers close to La Palma volcano latitude were also observed on 2 July 2021. Even if less intense than the previous track, the latitude of the peak is the closest to the volcano among all the anomalous tracks, so we consider it a strong indication of possible relation with the volcano activity. Another peak was recorded on 10 September 2021, but its distance (more than 10\({}^{\circ}\)) seems quite unrealistic to be due to the pre-eruption volcano processes.