## 3 Results
### Ionospheric Investigation
The same is for 26 August 2021, which showed a high anomalous value at 45\({}^{\circ}\) North, which is too far and probably due to some auroral activity, even if geomagnetic conditions were quiet. Figure 22: Time series of median daily values of CSES-01 Ne measured in day-time inside 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 day-time Ne. Appendix C compares the CSES-01 Ne with _TEC_ data for the tracks that show some outliers around the La Palma volcano in Figure 23. The first key point is that the CSES-01 Ne ionospheric structures are in agreement with the one depicted by _TEC_ obtained from the GNSS ground station. This confirms the scientific reliability and good quality of CSES-01 Ne data. In Figure 24, we provided an analysis based only on _TEC_ data, processed and normalized, as explained in Section 2.3.2. In particular, the normalized _TEC_ from 23 March 2021 until 19 September 2021 was compared with historical values, searching if it overpasses the two standard deviations threshold. The disturbed days (1 Dst1 > 20 nT and/or ap > 10 nT) were depicted by transparent red boxes, and the geomagnetic indexes time series are also reported in panel B. The geomagnetic index trend and disturbed times are in agreement with the high solar activity identified in Figure 4. Finally, the anomalous values occurred during 21-26 July 2021, 27 August 2021 (even if before and after geomagnetic conditions were perturbed), and 12 and 15 September, i.e., a few days before the volcano eruption. These days coincide with the CO anomalies and could be another indication of the alterations of the geo-layers in preparation for the eruption, even if future studies are necessary to demonstrate the bottom-up propagation effects proposed here as a possible reading of the identified anomalies. _Swarm_ magnetic data were systematically analysed considering a circular area containing the whole volcanic area of the Canarias Islands with a radius of 420 km. Some details of the data processing are provided in Appendix B. The cumulative number of anomalous tracks in the function of time is shown in Figure 25. In Supplementary Materials File S2, all the tracks that present one or more anomalies are shown. All three geomagnetic field components show some increase of anomalies (see Figure 25), i.e., an acceleration of the trend at about three months before the eruption (\(-90\) days). Despite this, the Y-East component is more evident; in fact, no other similar behaviours are depicted in such a period as the rest of the trend is quite linear, as also pointed out by the very close to 1 R\({}^{2}\) coefficient of determination. For example, such a characteristic is confirmed by looking at the single-track residuals in Figure 26. In fact, the peak-peak maximum perturbation of the Y-East component is  
Figure 23: (**A**) Ne latitudinal profiles measured by CSES-01 on the same orbit shown in (**B**) revisited every 5 days. Only the tracks acquired during quiet geomagnetic conditions (1 Dst1 \(\leq\) 20 nT and ap \(\leq\) 10 nT) are plotted. The interquartile range is a box, extreme values are dotted grey, and outliers are plotted as red crosses and calculated every 1\({}^{\circ}\) latitude. about 4 nT/s while the X-North component is about 1.3 nT/s and the Z-Centre component is about 2 nT/s.