## 7 Discussion  
In this work, we investigated the impact of volcanic/Saharan dust events on the ionosphere, analyzing a number of independent observations. Ionospheric perturbations recorded by DEMETER on 4 November and 6 December 2006 occurred during some intense eruptive events of Mt. Etna. Further, an ash plume, dispersing over the Mediterranean Sea more than 100 km from the eruptive center, affected the same region where, some hours later, DEMETER recorded a significant increase of the ion density. Since volcanic ash may electrically interact with the atmosphere (e.g., [34; 96]) and considering that gas emissions, heat flux, and volcanic explosions may also disturb the ionosphere [4; 97], the impact of those eruptions on DEMETER observations appears plausible. In support of this hypothesis, the sea region characterized by the ion density increase was spatially close to the Mt. Etna area. Indeed, ionospheric perturbations recorded by DEMETER were within the distance threshold (VEI \(\leq\) 3) of 500 km from eruptive centers identified by other authors to assess the possible relation between ionospheric anomalies and eruptive activities [4; 5]. Those outcomes fit with results of previous papers, which independently investigated the possible relation between ionospheric perturbations and eruptive activities (e.g., [5; 75] and the references herein). In each instance of this study, we can attribute the variation of the ion density to the modification of the GEC, which is present between the Earth's surface and the bottom of the ionosphere [98]. In particular, large amounts of heavy ion clusters, which replace the light ions within the area of a volcano eruption, drastically change the air column conductivity. This leads to the modification of parameters of the GEC, which is responsible for electromagnetic coupling between the ground and ionosphere. It creates the local anomalies of electron concentration and electron and ion temperature within the ionosphere (e.g., [11; 99]). DEMETER observations, showing anomalies in the electrical field, coincided with the timing of thermal enhancements on 4 November and 6 December 2006, as revealed by OLR anomalies. The cross-correlation of OLR at TOA and ionospheric anomalies is evidence for the presence of the lithosphere-atmosphere-ionosphere coupling chain triggered by Mt. Etna volcanic activity occurring in 2006. Moreover, due to the plasma characteristics of the quasi-neutrality, which means the number of ions is about equal to that of electrons, the DEMETER ion and the ground-based  
Figure 15: Latitude–longitude (LLT) maps at global fixed local time 8:00 p.m. LT extracted from the GIMs along various longitudes in reference to: (**a**) 19 May 2008; (**b**) 27 May 2008. GPS TEC simultaneously increased over the Mt. Etna area. Those agreements indicated that the electromagnetic environment around the volcano area was modified during the eruptive periods investigated here. Regarding the ionospheric disturbances of May 2008, which cannot be associated with Mt. Etna and/or Stromboli eruptions (see previous paragraph), they were possibly determined by two massive dust outbreaks occurring in that period. Those dust events were identified using data from SEVIRI, which offers a synoptic view and a high frequency of observation adequate to monitor spatially distributed and rapidly evolving phenomena. Specifically, RST maps showed that airborne dust affected the same region where DEMETER recorded an increase of the electron density and lightning strokes during May 2008. In this case, it is certainly difficult to attribute a part of the lightning strokes to electrostatic discharges in the dust plume because their intensities could not be so important. Therefore, we can hypothesize that either the dust phenomenon contributed to increasing the number of lightning strokes in the atmosphere or it changed the properties of the bottom of the ionosphere in such a way that the waves were not so attenuated when they crossed the ionosphere to reach the satellite.