## 6 Results
### Ionospheric Perturbations of November-December 2006
Etna eruptive activity possibly perturbed the ionosphere. As a further evidence of this hypothesis, Figure 6a displays the continuous monitoring of  
Figure 4: DEMETER data recorded on 6 December 2006 between 8:43:00 and 8:49:00 p.m. UT. On the top, electric VLF spectrogram from 0 up to 20 kHz with its intensity color coded according to the scale; on the bottom, variation of the ion O+ density (the densities of the other ions are much lower), with the parameters displayed below having the same meaning as Figure 1.  
Figure 3: AVHRR channel 4 (thermal infrared (TIR)) image of 4 November 2006 at 8:44 p.m. UT in LAEA projection; ash pixels detected by RST\({}_{\text{ASH}}\) (with \(\otimes_{\Delta TIR}(x,y,t)<-2\)\(AND\)\(\otimes_{\Delta TIR}(x,y,t)>1\)) are depicted in red and magnified at the top right-side of the figure (see yellow square). OLR over the monitored volcanic area, obtained from the analysis of NOAA-15 AVHRR nighttime data (with 1 \(\times\) 1-degree resolution) during the period of 1 September\(-\)31 December 2006. The plot shows the thermal enhancement time (red columns) at TOA (at 8:00 p.m. LT) during the Mt. Etna eruptions (grey boxes). Mean daily nighttime values (black curve) show the climatological trend (decrease) for the winter season. Distinctive anomalies were observed on 26 October; 6 and 23 November; and 2, 6, and 14 December. For comparison, the same calculations for one year later (September-December 2007) of OLR daily nighttime values (yellow columns) were included. No significant anomalies were observed for 2007, except for the single large anomaly recorded in September 2007, which was probably associated with the significant eruption registered for the same period. For the rest of the period in 2007, sporadic and less intense eruptive activity occurred; therefore, no significant thermal anomaly at TOA was registered. As an additional parameter for this analysis, we computed the correction into the ACP. This parameter shows the ionization rate change in the lower atmosphere [75]. The 2006 ACP for the same period (pinks line) followed the OLR trend and in values were smaller than 2007, the period with no eruptions (cyan). Thus, the heat released from the volcano events associated with eruption activities, showing synchronization with thermal anomaly occurrences, altered the ACP in 2006. Additionally, Figure 6b,c show the 2D maps of the anomaly intensities represented by the OLR _E_-index. Only positive values are shown, revealing that more intense OLR anomalies were located over the Mt. Etna area. Finally, the OLR thermal anomaly generation initiated a chain of other effects in atmosphere, reaching the altitudes of the upper atmosphere and ionosphere. The temporal evolution of the chemical potential shown in Figure 7a for the period 10 October-30  
Figure 5: RST\({}_{\text{ASH}}\) product generated from AVHRR data of 6 December 2006 at 12:37 p.m. UT. Ash pixels are depicted in red (the visible channel shown in background was used in detection phase for better filtering clouds; e.g., [91]). The blue line is the DEMETER orbit ground track. December 2006 revealed the maximum value four days before the eruption onset marked by the red arrow. Those measurements were taken exactly at the Mt. Etna location. In addition, Figure 7b displays the 2D distributions of ACP over Southern Italy for the period 30 October-1 November, when the mean peak in the time series of \(\Delta U\) was recorded. On 30 October at 6:00 p.m. UT, a yellow spot affected the monitored volcano. On 31 October at 12:00 a.m. UT, it extended in longitude over almost all of Sicily. The same day at 6:00 a.m. UT, the signal intensity increased; at 12:00 p.m. UT, we observed the signal only over the volcanic area, but at the same time, \(\Delta U\) increased over Southern Italy. Then, on the next two intervals, the signal expanded again to disappear at 12:00 p.m. UT on 1 November. Figure 6: (a) Daily nighttime National Oceanic and Atmospheric Administration (NOAA)-AVHRR outgoing longwave-Earth radiation (OLR) anomalous values over Mt.