Origin of the Ionospheric Changes Immediately Before the 15 January 2022 Eruption of the Hunga-Tonga Hunga-Ha’apai Submarine Volcano

Near-field ionospheric total electron content records before and after the 15 January 2022 eruption of the Hunga-Tonga Hunga-Ha’apai submarine volcano were studied using GNSS total electron content data. The data started showing positive departures from the afternoon smooth decreasing trend ~1 h before the eruption. This anomaly is localized around the volcano, i.e., it decays as we go away from the volcano. The signature resembles the one that preceded the 2011 Tohoku-oki earthquake. However, a detailed investigation of ionospheric anomalies from a dense GNSS array in New Zealand showed a large-scale traveling ionospheric disturbance propagating toward Tonga, excited by a moderate geomagnetic storm on the previous day. This suggests that the anomaly around the volcano immediately before the eruption was caused by the arrival of this disturbance at Tonga just at the eruption time.