The Co-Volcanic Ionospheric Disturbances at Mt.Etna, Italy

Global Navigation Satellite Systems (GNSS), originally developed for positioning and navigation, are now widely used for ionospheric sounding. Due to the dispersive nature for radio waves, the ionosphere affects the GNSS signals as a function of its electron density. By this condition the total electron content (TEC) of the ionosphere can be estimated from GNSS data. TEC fluctuations can be related, from above, to space weather dynamics or, from below, to geodynamic events and then GNSS-TEC applications represent an innovative technique to improve the studies on natural hazards. Volcanic eruptions perturb the atmosphere by changing temperature, pressure and density of the surrounding air. In particular, the thermodynamic forcing from explosive eruptions can generate acoustic-gravity waves that, due to the exponential decrease in air density and to the conservation of kinetic energy, propagate upward until to reach the electron density layers of the ionosphere. The match between the acoustic-gravity waves and the ionospheric layers induces electron density oscillations affecting the satellitereceiver line of sight of GNSS. This type of oscillations can be visualized in terms of TEC fluctuations that are called co-volcanic ionospheric disturbances (CVIDs) and they represent a key observational signature of the coupling processes between lithosphere, atmosphere and ionosphere. Based on the GNSS-TEC application, the coupling processes between lithosphere and ionosphere have been studied into regional scale at Mt. Etna (Italy). Leveraging a dense and proximal GNSS network around the volcano, co-volcanic ionospheric disturbances are observed during three largescale lava fountains that occurred on April 12th 2012, December 4th 2015 and July 4th 2024. The associated TEC perturbations occur in the near field, within 300 km of Mt. Etna, with an estimated apparent velocity around 200 m·s-1 and dominant frequency around 1 mHz. The spectral signature is consistent with the propagation of internal gravity waves, by suggesting that the paroxysmal activity of Mt. Etna is coupled with the ionospheric electron density layers through the lower-middle atmosphere. The results provide an unprecedented spatiotemporal characterization of CVIDs at Mt. Etna based on GNSS observations. Differences in signal travel times among the investigated events suggest that lower-middle atmospheric dynamics, including background wind conditions, may significantly modulate the ionospheric response to the explosive activity of Mt. Etna. The observed CVIDs seem to be typical for Mt. Etna, in such a way that these findings provide a potential GNSS-TEC framework to implement the monitoring of Mt. Etna and the characterization of the lithosphere-atmosphere-ionosphere coupling processes at the regional scale.