## 6 Results
### Ionospheric Perturbations of May 2008  
During May 2008, DEMETER recorded a significant number of ionospheric anomalies over the Mediterranean basin. Figure 10 shows an example of DEMETER records for 19 May 2008, displaying a spectrogram of one electric field component up to 20 kHz and the variation of the electron density along the orbit track, in red, intersecting the region extending from Central Europe to Libya between 8:08:30 and 8:10:45 p.m. UT (Figure 11a). Ionospheric disturbances were recorded by DEMETER in concomitance with some eruptive activities that were in progress at the Mt. Etna and Stromboli volcanoes (Aeolian Islands; Italy) [92; 93; 94]. However, \(\mathrm{RST_{ASH}}\) products from AVHRR observations as well as a visual inspection of satellite imagery did not reveal any significant presence of airborne ash. Hence, ionospheric anomalies were not ascribable to volcanic phenomena even because of the  
Figure 8: (**a**) Locations of 17 ground-based Global Positioning System (GPS) receivers (SOPAC, [[http://sopac.ucsd.edu/](http://sopac.ucsd.edu/)]([http://sopac.ucsd.edu/](http://sopac.ucsd.edu/))) and their GPS total electron content (TEC) ground traces during 9:30–10:00 p.m. on 4 November 2006. Blue squares and color curves in the top panel stand for the ground-based GPS receivers and the TEC at its associated ionospheric point, respectively. (**b**) GPS TEC variations along the volcano longitude during 9:30-10:00 p.m. LT on 4 November 2006. Grey and blue curves in the bottom panel denote the GPS TEC in the network and their median within 10–20 longitude (with a 0.1-deg latitudinal grid), respectively. large spatial distance of the DEMETER orbit track from eruptive centers. On the other hand, two intense Saharan dust events affected Europe and the Mediterranean basin in May 2008 (see Section 5.2), prompting us to assess their possible impact on the ionosphere. Figure 11 displays the horizontal (i.e., 2D) distribution of airborne dust (in different shades of brown) over Europe and part of the Mediterranean basin retrieved from nighttime SEVIRI data of 19 and 20 May 2008 using the RST method. In detail, Figure 11a shows the RST map of 19 May at 8:00 p.m. UT, revealing the presence of dust coverage affecting part of the Mediterranean Sea and Eastern Europe. The dust plume, which was presumably spatially more extended than that indicated by Figure 11a, due meteorological clouds masking the underlying dust layer (see black/dark grey pixels on the TIR channel displayed in background), affected part of the Mediterranean Sea and Greece. Figure 11b,  
Figure 9: (**a**) Locations of 15 ground-based GPS receivers and their GPS TEC ground traces during 9:30–10:00 p.m. LT on 6 December 2006; (**b**) GPS TEC variations along the volcano longitude during 9:30–10:00 p.m. on 6 December 2006.  
referring to the map of 20 May at 12:00 a.m. UT, showed that some hours after previous satellite observation, the dust phenomenon still affected the aforementioned areas. To assess the identification of airborne dust in Figure 11, we analyzed the total attenuated backscatter profile at 532 nm, acquired on 20 May between 12:29 and 12:43 a.m. UT along the orbit track of Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO). The latter uses an active lidar instrument and passive infrared/visible imagers to probe the vertical structure and properties of thin clouds and aerosols over the globe [95]. Figure 12 displays the Cloud-Aerosol Lidar with Orthogonal Polarization Lidar (CALIOP) lidar profile, showing the presence of an aerosol layer over the region marked by the black ellipse (see top  
Figure 11: Robust satellite technique (RST) dust maps; pixels in different shades of brown indicate regions less (i.e., \(\otimes_{\Delta TIR}\)(\(x\),\(y\),\(t\)) \(<\) \(-\)2; light brown) and more affected by dust (i.e., \(\otimes_{\Delta TIR}\)(\(x\),\(y\),\(t\))\(<\) \(-\)3; medium brown). (**a**) 19 May 2008 at 8:00 p.m. UT; (**b**) 20 May 2008 at 12:00 a.m. UT.