## 2 Background  
Electrification in dust/sand clouds during storms is a well-known phenomenon in the low terrestrial atmosphere. This phenomenon has been intensively studied because it possibly represents the unique source of electric discharges in the rarefied atmosphere of planets such as Mars [12].  
The data showed that dust and sand storms are electrically active. [PERSON] [13] noted that charged particles and aerosols may be separated by gravity within the column of a dust devil. The author observed a field of the order of 20 kV/m above the ground. Charge densities were measured during large dust devils over the Sahara region by [PERSON] [14]; in New Mexico by [PERSON] [15]; and in the Southwestern United States, as well as in India, by [PERSON] [16; 17]. Wind velocity is a key parameter. [PERSON] et al. [18] determined an electric field of 166 kV/m at a 1.7-cm height above a sand dune under the condition of a uniform wind speed of 12 m/s. More recently, [PERSON] and [PERSON] [19] measured the horizontal \(E\)-field in dust devils, finding values up to 120 kV/m (other references can be found in [20]). [PERSON] et al. [21] performed a wind tunnel experiment simulation for blown sand electrification. They found that, at certain speeds, the maximum average intensity of an electric field at 16 m couldreach 200 kV/m or more. [PERSON] [22] claimed that inside thunderstorm clouds, the field rarely exceeded 400 kV/m, and that an average value could be of the order of 100 kV/m. Therefore, it is likely that electrostatic discharges could occur at the time of sandstorms.  
Concerning volcanic eruptions, these phenomena produce two different types of electromagnetic perturbations in the upper atmosphere. On the one hand, they are sometimes accompanied by lightning strokes, and on the other hand, their effects are similar to explosions (i.e., acoustic-gravity waves are emitted). Volcanic eruptions may perturb the environment up to the ionosphere, owing to the propagation of acoustic-gravity waves (their amplitude increases with the decrease of the density in altitude). Other authors reported some ionospheric variations in the records of HF (High Frequency) Doppler and TEC (Total Electron Content) data, in association with the explosions of Mount Pinatubo on 15 June 1991 [2]. Lightning activities in volcanic plumes have been observed for a long time. There have been, in fact, many reports concerning some spectacular events (e.g., [23; 24; 25; 26; 27; 28; 29]), including the April-May 2010 Eyjafjallajokull eruption, which strongly perturbed air traffic (e.g., [30]). [PERSON] et al. [31] showed that the number of whistlers (waves associated to the lightning strokes) observed at Dunedin in New Zealand could be correlated with volcanic activities occurring in the Aleutian Islands, which are the magnetically conjugated region of Dunedin. However, lightning activity is not always observed during volcanic eruptions, and some studies have been undertaken to understand this phenomenon. [PERSON] et al. [32] performed some experiments to simulate the electrification mechanism of ash. They showed that the dominant charging process occurs during the fracture of silicate particles (see also [33]). [PERSON] and [PERSON] [34] suggested that the ash fragmentation mechanism is a key parameter. During the aforementioned Eyjafjallajokull eruption, some authors noticed a change in the lightning activity [35]. They attributed this effect to a temperature change at the top of the plume. [PERSON] and [PERSON] [36] intensively studied the occurrence of lightning activity as a function of many parameters (e.g., Volcanic Explosivity Index--VEI, ash plume height, magma composition, latitude of volcanoes, and season during many different eruptions), emphasizing the important role of water vapor. Finally, another work demonstrated the impact of dust aerosols on the ionosphere because of an electromagnetic coupling mechanism based on a drop of column electric conductivity [11].