## 4 Discussion
### Source of the High Amount of SO\({}_{2}\) Detected in 1991
Among the several parameters, the most interesting seems to be the CO that is represented in Figure 20; in fact, it shows in half of 2010 (first peak on 25 June 2010), particularly anomalies never seen in the 30 years before, which continued to sparsely appear until the first months of 2020. Additionally, the other atmospheric parameters show some anomalous activity in these 11 years, but it is not unique, as they also showed some previous activity. The CO could be linked to the magma flow in the mantle [90], i.e., the anomalous CO emission in 2010, 2012-2013, 2016, and 2020 could suggest some activity in the mantle that brought toward the eruption. Parts of this anomaly could be due to the El Hierro volcano eruption of 10 October 2011, a very close volcano in the same archipelagos. Despite this, a previous study about El Hierro 2011 volcano eruption by [PERSON] et al. [26] identified the unrest of such a volcano starting in July 2011. Therefore, the start of the CO anomalies we depicted in this paper is still before, but we cannot discriminate which volcano eventually could be related to such phenomena. They could likely be considered a proxy of the whole volcanic archipelagos instead of a specific volcano. Finally, the 2010 CO anomalies, together with the soil deformation and deep (-25 km) seismicity reported by [PERSON] et al. [91], could be further evidence of the unrest of the magma in 2009-2010, which they claimed. From this paper, the acceleration of the seismicity started 10 days before the 2021 eruption (see Figure 7C), is very evident, and underlines the magma uplift before the same eruption. Furthermore, it is compatible with GNSS measurements on the ground and deformation estimated from radar satellite by DInSAR (Differential Interferometric Synthetic Aperture Radar) technique by [PERSON] et al. [27], as they reported in their Figure 3I-L.  
Our temporal result suggests a process that started from a depth of about 40 km, i.e., mantle depth, in November 2020 that gradually filled a deeper chamber at about 30 km, also identified by [PERSON] [74]. Then, only in the last days did the magma raise toward \"La Cumbre Vieja\" vent, in agreement with the geochemical investigation of the erupted magma conducted by [PERSON] et al. [92] that identified the erupted magma as originating from the melting of the mantle. Investigation of the Possible Direction of Propagation of the Electromagnetic Waves by Calculating the Pointing Vector  
In order to confirm whether the detected ionospheric anomalies were produced by an electromagnetic wave propagating from bottom to top in the hypothesis of an electromagnetic propagation mechanism (Figure 2B), the pointing vector can help to provide an answer. The details of how the vector has been estimated are provided in Appendix E.  
In Figures 29 and 30, two examples applied to Swarm tracks above La Palma volcano are provided. The first one is selected in the period that has statistically been identified as an acceleration in the cumulative trend of the magnetic Y-East anomalies (see Figure 25), while the second one is more than one month after. In particular, the Swarm Alpha track, 10-17 June 2021 (Figure 29), identified a decrease of centre component relatively close to \"La Palma\" volcano, suggesting an outgoing (upward) electromagnetic wave as visible from the 3D reconstruction with a dominant upward direction. On the other hand, in the Swarm Bravo track 6, acquired on 26 July 2021, shown in Figure 30, there is not a clear indication of upward or downward direction. This could be due to a different physical mechanism that produced such an anomaly, for example, a plasma bubble, as simulated by [PERSON] et al. [64]. In fact, electron density on 26 July 2021 presented particular anomalous values as reported in Figure 23, even if a \"contamination\" from the high auroral activity recorded on this day cannot be totally excluded.