Ionospheric Disturbances During the 4 December 2015, Mt. Etna Eruption

This study presents the detection and characterization of co-volcanic ionospheric disturbances (CVIDs) associated with Mt. Etna's large-scale lava fountain (Italy). Leveraging a dense and proximal GNSS network, we identify local Total Electron Content (TEC) perturbations extending up to $\sim$200 km south/southwest of the vent. The observed anomalies exhibit quasi-periodic signatures with amplitudes of $\sim$0.6 TECU, periods of 15–25 min, and horizontal propagation velocities of 135–300 m$\cdot$s⁻¹, with dominant spectral power in the 0.5–1.5 mHz range, consistent with internal gravity waves. These signatures emerge gradually, 20–30 min after the seismo-acoustic onset of the eruption, coinciding with the rise of the volcanic plume. Eruption chronology is independently constrained using seismo-acoustic and thermal/visible imagery. Detection robustness is ensured via complementary spectral analyses (FFT and Empirical Mode Decomposition) and confirmed across multiple GNSS stations. The results suggest that open-conduit eruptive dynamics may facilitate sustained, gravity-dominated atmospheric forcing, generating subtle TEC disturbances that are otherwise difficult to detect under natural ionospheric variability. Comparison with prior studies, that did not detect such signals, highlights the critical role of near-field GNSS coverage. These findings contribute to a better understanding of CVID typologies and open new avenues for integrating ionospheric observations into multi-sensor volcanic monitoring frameworks. Further multi-event analyses are needed to generalize the proposed mechanisms and assess their utility in hazard forecasting.