## 6 Results
### Ionospheric Perturbations of May 2008
Red (top panel) and green (bottom panel) lines on top and bottom panels are the ground track of DEMETER and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) orbits, respectively. The SEVIRI TIR channel, at 11-\(\upmu\)m wavelength, is displayed in the background. Figure 10: DEMETER data recorded on 19 May 2008 between 8:04:00 and 8:13:00 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 right; on the bottom, variation of the electron density. panel), where RST flagged a number of dusty pixels. The aerosol subtype profile reported in the bottom panel (see Figure 12b) indicated that dust was dominant (see yellow areas), corroborating information retrieved from SEVIRI data. It should be remarked that an increase of the electron density and many whistlers (vertical lines), with increased intensities corresponding to the lightning strokes in the atmosphere, characterized the dusty region intersected by the orbit track of DEMETER on 19 May at around 8:08 p.m. UT (see Figure 10). Thus, we can suppose that the dust plume was electrically active and capable of perturbing the ionosphere, such as the following dust event of 26-31 May 2008. Indeed, desert dust affected the same region of the Mediterranean Sea where DEMETER (the orbit track of which is depicted in red in Figure 13a) recorded, on 27 May, an increase in the electron density and thunderstorm activity (see Figure 13b). Also, similar ionospheric disturbances were recorded in the following days when the dust phenomenon was still in progress. In addition, the GIM TEC significantly enhanced over the area affected by Saharan dust both on 19 and 27 May 2008, as indicated by dotted circles in Figure 14, where the top panel is the raw data and the bottom one is the associated STD (Standard Deviation). In detail,  
Figure 12: **(a)** Cloud-Aerosol Lidar with Orthogonal Polarization Lidar (CALIOP) lidar profile of 20 May 2008 acquired between 12:29 and 12:43 a.m. UT, along the orbit track of CALIPSO reported in green in Figure 11, showing the presence of an aerosol layer extending up to about 6 km above sea level (a.s.l.) (see black ellipse); **(b)** aerosol subtype profile. a marked and continuous increase in the electron density was recorded between 8:55 and 8:59 p.m. UT (i.e., just when DEMETER passed over the dusty area identified by RST using SEVIRI data). To investigate the impact of Saharan dust on the ionosphere, we also examined the longitude-time-TEC (LTT) along the 0\({}^{\circ}\) E longitude. In addition, we developed TEC for the global fixed local time at 10:00 p.m. for cross-comparison with DEMETER observations. It can be noted that the GIM TEC significantly increased over the Saharan region during the investigated period (i.e., both on 19 and 27 May 2008) (see both panels in Figure 15). Since the global fixed local time is the TEC at the same local time on a particular day being isolated, the local time effects were removed, allowing us to cross-compare the GIM TEC with the sun-synchronous orbit observations of DEMETER at about 8:00-10:00 p.m. LT, globally. Figure 13: (a) RST map of 27 May 2008 at 9:00 p.m. UT, orange and brown pixels as well as the red line have the same meaning as in Figure 11; (b) DEMETER data recorded on 27 May 2008 between 8:53:00 and 9:01:00 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 right. On the bottom, variation of the electron density with the parameters displayed below representing the time in UT and LT, the geographic latitude and longitude, and the McIlwain parameter L.  
Figure 14: (a) Longitude–time–TEC (LTT) plots extracted from the global ionosphere map (GIM) along the 0\({}^{\circ}\) E longitude; (b) standardized plot. The mean is computed at each location and each time point for the observation period of 8 May–6 June 2008. Figure 15: _Cont._