## 4 Discussion  
### Source of the High Amount of SO\({}_{2}\) Detected in 1991  
It is surprising to note that in 1991 the value of SO\({}_{2}\) showed a clear peak more intense than all the other 40 years, as is visible in Figure 19. The first interpretation of this behaviour would be an emission from La Palma or other Canarias-volcano systems in the atmosphere. Still, there are no reports of eruptions or emissions in this area for that year. To search for the source of this particularly high peak of SO\({}_{2}\), we plotted the worldwide distribution maps of SO\({}_{2}\) in Figure 27. Here we reported only six representative days, but in online Supplementary Materials, it is possible to reproduce an animated gif with all the days from 1 st June until 31 July 1991. We clearly immediately noted that the whole equatorial region is affected by the SO2 plume covering the entire Earth's circumference at the equator. Backtracking the origin of such a large emission of SO\({}_{2}\) in the atmosphere, we found that it was due to a vast and violent explosion one month prior of the Pinatubo volcano. In fact, on 11 June 1991, the distribution of SO\({}_{2}\) can be considered \"standard\" with a maximum value of about 1.4 10\({}^{-4}\) kg/m\({}^{2}\) (as seen from the maximum of the colour bar). The distribution is also above several regions of the world, probably due to natural and human sources. On the day after, i.e., 12 June 1991, the maximum value increased by about one order of magnitude, reaching 10\({}^{-3}\) kg/m\({}^{2}\). This is clearly above the Pinatubo volcano, which had an eruption estimated of Volcano Explosive Index VEI = 6 in June 1991 [85, 86]. In the following weeks, the SO\({}_{2}\) plume was transported by the winds and slowly diluted in all the equatorial regions, as visible from the picture in Figure 27 and in the animated gif in Supplementary Materials. Our findings are in agreement with the global effects induced by the Pinatubo 1991 explosion reported by [PERSON] et al. [87]. Figure 26: _Swarm_ Bravo track 25 with magnetic anomaly that was recorded on 18 June 2021 above La Palma volcano at 18:50:29 UT (17:37:02 LT). Geomagnetic indices were Dst = \(-\)10 nT and ap = 5 nT. Subpanels (**A–C**) represent the residuals of X-North, Y-East, and Z-Centre (vertical) components of the geomagnetic field. The residual of the total intensity of the geomagnetic field is shown in (**D**), The research area with a radius of 380 km centred on the volcano (the green star) is represented by a yellow circle. The brown line is the satellite orbit projection over the ground map provided in (**E**). Lithosphere, Atmosphere, and Ionosphere Bottom-Up Interactions in the Six Months Preceding the 19 September 2021 La Palma Volcano Eruption  
In the following Figure 28, we provided a common view of the identified anomalies in the lithosphere, atmosphere, and ionosphere during the six months before the eruption of \"La Palma\" volcano on 19 September 2021. Considering that not all the identified anomalies are related to the La Palma volcanic activity, we excluded the anomalies in this section that are unlikely to be related to such an event. However, we still kept the one with a doubt source. For this purpose, we resumed all the identified anomalous days in Table 1, also providing the amount of exceedance of the threshold in terms of sigma and a comment for each group of anomalies based mainly on the visual investigation of the corresponding parameter maps. Figure 27: World SO\({}_{2}\) map distributions for 6 selected days of June and July 1991 at 23 UT. For 11 July 1991, the distribution was a typical one, while from 12 June 1991 the Pinatubo volcano emitted a large amount of SO2 into the atmosphere, and the maximum concentration in these analyses was found on 17 June 1991.