## 4 Discussion
### Source of the High Amount of SO\({}_{2}\) Detected in 1991
In the following weeks, the large SO2 plume was distributed in all equatorial regions with lower concentrations but larger than standard distribution, and around 12 July 1991 it reached the La Palma location, recording the highest ever value measured from 1980 to 2021 for this area. An animated gif with the daily evolution of SO\({}_{2}\) map distributions in June and July is available in the online Supplementary Materials. Figure 28: Cumulative trends in the lithosphere, atmosphere, and ionosphere of the anomalies or events. The lithosphere layer is represented by the seismic events (cross) and their cumulative Benioff strain. The atmosphere is represented by the cumulative amount in terms of sigmas of the ground magnetic uncorrelated windows (one band for each component) and the confirmed SO\({}_{2}\), DMS, AOT, and CO anomalies. The ionosphere shows the cumulative number of anomalies identified by CSES (Ne), _Swarm_ (Y-East magnetic field component), and TEC. The ionospheric trend started at about \(-130\) days because no anomalies had been identified before. \begin{table}
\begin{tabular}{c c c c} \hline
**Parameter** & **Anomalous Day [2021]** & **Amount of the Anomaly 1** & **Visual Check of the Anomaly** \\ \hline SO\({}_{2}\) & 15 August & 50.00\% & They seem emitted by the volcano (anomaly clearly above volcano) \\ \hline \multirow{6}{*}{DMS} & 17 May & 79.76\% & \multirow{6}{*}{Not emitted by the volcano, but value closer to the island. It could be a secondary effect.} \\  & 18 May & 20.81\% & \\  & 19 May & 97.22\% & \\  & 20 May & 95.44\% & \\  & 21 May & 484.99\% & \\  & 22 May & 400.04\% & \\  & 23 May & 130.18\% & \\ \cline{2-3}  & 8 June & 66.11\% & \\  & 9 June & 484.08\% & \\  & 10 June & 417.27\% & \\  & 11 June & 270.24\% & \\  & 12 June & 246.10\% & \\ \cline{2-3}  & 15 August & 80.31\% & \\  & 16 August & 190.41\% & \\  & 17 August & 291.80\% & \\  & 18 August & 231.11\% & \\  & 19 August & 174.06\% & \\ \hline \end{tabular}
\end{table} 
Table 1: List of the anomalous atmospheric days. For each day, the evaluation of the amount of exceedance of atmospheric parameters in terms of standard deviations is provided. A comment based on the map of the specific day or group of days is also inserted. A common vision of the three geo-layers, as presented in Figure 28, is essential to search for possible interactions between them. In this work, we search for possible effects from the lithosphere to the ionosphere (bottom-up effects) even if the opposite effect (top-down) is also possible, for example, in the case of the impact of solar pulsation on Earth's environment like Pi2 [88]. Still, they are out of the scope of this paper. We noted that the atmospheric anomalies identified between \(-130\) and \(-120\) of CO, AOT, and DMS correspond to two anomalous days in terms of high electron density observed by CSES in the ionosphere. Furthermore, the significant increase in the atmospheric trend (dominated by DMS anomalies) at \(-100\) days is followed at about \(-90\) days by a particular magnetic activity detected by _Swarm_ satellite in the ionosphere, with concomitance anomalies from GUI ground geomagnetic observatory that could correspond to the migration of the disturbance from the atmosphere to the ionosphere. A delay of one week is in agreement with previous studies. It can be explainable only by a mix of mechanical and chemical processes in the atmosphere that slowly migrates to the ionosphere, such as the model proposed by [PERSON] and [PERSON] [2] or the one illustrated in Figure 2A. Another possible fluctuation of the ionosphere with simultaneous variations in the atmosphere was detected by an increase in night- and day-time of CSES-01 electron density compatible with the _TEC_ map, between \(-80\) and \(-40\) days with respect to the eruption, together with high values of aerosol and CO in the atmosphere.