## 3 Results
### Atmospheric Investigation
#### 3.2.1 Ground Geomagnetic Field  
In this section, we present the investigation of ground geomagnetic field data retrieved from an observatory station relatively close to the volcano, Guimar (GUI), at Tenerife Island compared with the far observatory of Tamanrasset (TAM). The goal is to search for possible variations of the magnetic field induced by the volcano that could likely have reached the close geomagnetic observatory of GUI, as its distance is about 140 km inside the same volcano system group of Las Canarias. The comparison observatory was more than 2400 km away, so it is unlikely to be influenced by the volcano. Instead, a common signal to both the observatories located in the same North hemisphere and quite comparable geomagnetic latitude (GUI: 33.17\({}^{\circ}\)N TAM: 24.37\({}^{\circ}\)N) can be considered a global or external disturbance. The original data were cleaned from the non-available samples (i.e., the samples equal to \(-\)9999), and the data were interpolated at 1 min sampling by a Piecewise Cubic Hermite Interpolating Polynomial (PCHIP, [75]) to avoid the data gap that is fundamental for the data filtering, requiring uniformly distributed data. The available band is from a period of 2 min for the Nyquist theorem [76]; it has been divided into five frequency bands by applying five digital filters. They are selected as a pass-high with a period of 1 h, a pass-band between 1 h and 6 h periods, a pass-band filter between 6 h and 1 day periods, a pass-band filter between 1 day and 1 week periods, and a pass-low filter at 1 week period. The selected period considers some natural phenomena like the tidal modulation over a period of 6 h or the SQ current, a daily (24 h) typical current that influences the magnetic field [77]. Figures 8 and 9, and Figure S1 (in Supplementary Materials File S1) report the geomagnetic data filtered in the five frequency bands from GUI (blue curves) and TAM (orange curves) observatories for X (North), Y (East), and Z (Centre) geomagnetic components, respectively. It is possible to note that the two observatories are generally in agreement, and this is their response to the global perturbation. In particular, the X-North component shows very similar behaviour in all the investigated bands. This could be due to the main orientation of the geomagnetic field that horizontally is very close to this component, so a small local perturbation cannot significantly affect the X component but could be visible in the Y, which has an absolute value of about ten times lower. In fact, Y and Z (Figures 8 and 9) show some slight differences between the two observatories. When the  
TAM observatory shows some higher signal, this could also be due to local perturbations like the Sahara dust storm, but it is out of the scope of this work. With red circles and arrows, some differences when GUI shows some different (more) signal than TAM have been pointed out. Such signals are likely to be local in the Canarias archipelago, and they could be due to the preparation of \"La Palma\" volcano eruption on 19 September 2021. Furthermore, the Z, vertical component at GUI observatory, shows a trend of decreasing its intensity as shown in Figure 9 by the superposed linear fit (dashed blue line) and its slope equal to \(-0.067\) nT/day. On the other observatory of TAM, the same linear fit show a slight increase of \(0.033\) nT/day vertical magnetic field. Such a local decrease of the magnetic field in the vertical component could be produced by the demagnetization of the rocks induced by the warming caused by the uplift of the magma in the analogy of what [PERSON] et al. [32] found for the Kuchi-erabu-jima volcano in Japan. In order to validate by an objective criterium the identified anomalies in GUI observatory by visual inspection, a correlation coefficient has been computed inside a moving window.