Periodic Oscillations of Doppler Frequency Excited by the Traveling Ionospheric Disturbances Associated with the Tonga Eruption in 2022

Abstract

Abstract The explosive eruption of the Hunga Tonga-Hunga Ha’apai volcano on 15 January 2022 generated atmospheric waves traveling around the Earth, which caused ionospheric disturbances on various spatio-temporal scales. A HF Doppler (HFD) sounding system in Japan detected characteristic ionospheric disturbances showing periodic oscillations in the Doppler frequency with a period of ~ 4 minutes. In this study, such periodic oscillations were examined by comparing HFD data with those of GNSS (Global Navigation Satellite System) Total Electron Content (TEC) observation. The observed periodic oscillations in the Doppler Frequency were characterized by a S-letter shaped variation, implying the passage of the Traveling Ionospheric Disturbances (TIDs) through the reflection points of the HFD sounding system. It was also found that the periodic oscillations occurred prior to the arrival of the tropospheric Lamb wave excited by the Tonga eruption. From the GNSS TEC data, the TIDs causing the periodic oscillations were excited by the tropospheric Lamb waves at the conjugate point in the southern hemisphere, namely, the electric field perturbations due to the Lamb waves in the southern hemisphere mapped onto the sensing area of the HFD system in the northern hemisphere along the magnetic field lines. The periodic oscillations were observed only in the path between Chofu transmitter and Sarobetsu receiver, whose the radio propagation path is almost aligned in the north-south direction. This suggests that the TID has a structure elongating in the meridional direction. The variation in the Doppler frequency was reproduced by using a simple model of the TID propagation and the resultant motion of the reflection point. As a result, the vertical motion of the reflection point associated with the periodic oscillations was estimated to be about 1 km. It is known that ~ 4 minute period variations are sometime observed in association with earthquakes, which is due to resonances of acoustic mode waves propagating between the ground and the lower ionosphere. Therefore, a similar resonance structure in the southern hemisphere, that was caused by the passage of the tropospheric Lamb wave excited by the Tonga eruption, is the plausible source of the TIDs detected in the northern hemisphere.