## 6 Results  
### Ionospheric Perturbations of November-December 2006  
Figure 1 shows the DEMETER data of 4 November 2006 recorded along the satellite orbit track intersecting part of Italy, the Mediterranean Sea, and North Africa (see Figure 1a). Specifically, Figure 1b displays an electric field spectrogram up to 20 kHz (see top panel) showing the increase of the whistler intensities (each vertical line corresponds to a lightning stroke and is called a whistler) over the Mediterranean Sea. In the spectrogram, the horizontal lines above 15 kHz were due to the VLF ground-based transmitters. The increase of whistler intensities recorded by DEMETER revealed an important increase of the ion density at the Mt. Etna latitude, as shown in the bottom panel of Figure 1b. Figure 2 displays the AVHRR imagery of November 4 2006 at 20:44 UT, which was close in time to the aforementioned DEMETER observation, magnified over the target area. The figure shows that some image pixels, i.e., those located over Mt. Etna area (see pixels in bright tones), were significantly more radiant than the neighbor ones in the MIR band. RST\({}_{\text{VOLC}}\) considered most of those pixels as anomalous (see the red ones on hot spot map shown in the figure inset), revealing the occurrence of an intense thermal volcanic activity at the time of DEMETER observation. Figure 3 displays the RST\({}_{\text{ASH}}\) map from same AVHRR data, showing that weak ash emission also possibly occurred on that day (see pixels depicted in red within the yellow square indicating Mt. Etna area). It is worth noting that RST\({}_{\text{ASH}}\) flagged some ash pixels also SE of Sicily (Southern Italy). Since those pixels were sparsely distributed, we can hypothesize that a thin dust layer also affected the scene. Figure 4 displays the DEMETER data of 6 December 2006. In particular, the top panel represents the spectrogram of an electric component up to 20 kHz, whereas the bottom panel shows the variation of the density of the ion O+ density along the satellite orbit. Looking at the figure, one can see some lightning activities (i.e., vertical lines in the spectrogram), but above all, an increase of the ion density when the satellite was at the latitude of the Mt. Etna area (i.e., at around 8:45 p.m. UT). Figure 5 shows the RST\({}_{\rm ASH}\) product of 6 December at 12:37 p.m. UT revealing the presence of an ash plume, which was partially identified by satellite because of low ash concentration (e.g., [90]), dispersing from Mt. Etna area towards Mediterranean Sea. The plume ideally crossed the DEMETER ground track (see the blue line in Figure 5) some hours before the observation of ionospheric disturbances at 8:45 p.m. UT,  
Figure 1: (**a**) Orbit track of Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions (DEMETER) on 4 November 2006 with indication of time of the pass at the Mount Etna latitude. (**b**) On the top, electric VLF (Very Low Frequency) spectrogram from 0 up to 20 kHz, with its intensity color coded according to the scale on the right, referring to DEMETER data recorded between 8:42:00 and 8:48:00 p.m. UT; on the bottom, variation of the O+ ion density (the densities of the other ions are much lower) with the parameters displayed below representing the time in UT and LT, the geographic latitude and longitude, and the McIlwain parameter L.  
Figure 2: Advanced Very High Resolution Radiometer (AVHRR) channel 3 (medium infrared (MIR)) image of 4 November 2006 at 8:44 p.m. UT in Lambert Azimuthal Equal Area (LAEA) projection, with indication of Mt. Etna area (see blue square). The right top panel of the figure displays the hot spot map reporting anomalous pixels (in red) detected by RST\({}_{\rm VOLC}\) (with \(\otimes_{MIR}(x.y.t)>3\)_AND_\(\otimes_{MIR-TIR}(x.y.t)>3\)). shown in Figure 4. It is worth mentioning that the volcanic activity was still in progress at the time of DEMETER observation, as indicated by the RST\({}_{\text{VOLC}}\) map of 8:02 p.m. UTC, which is not shown here. The analyses performed above indicated that Mt.