On 15 January 2022, the submarine volcano on the southwest Pacific island of Tonga violently erupted. Thus far, the ionospheric oscillation features caused by the volcanic eruption have not been identified. Here, observations from the Super Dual Auroral Radar Network (SuperDARN) radars and digisondes \change{are}{were} employed to analyze ionospheric oscillations in the Northern Hemisphere caused by the volcanic eruption in Tonga. Due to the magnetic field conjugate effect, the ionospheric oscillations were observed much earlier than the arrival of surface air pressure waves, and the maximum negative line-of-sight (LOS) velocity of the ionospheric oscillations exceeded 100 m/s in the F layer. After the surface air pressure waves arrived, the maximum LOS velocity in the E layer approached 150 m/s. A maximum upward displacement of 100 km was observed in the ionosphere. This work provides a new perspective for understanding the strong ionospheric oscillation caused by geological hazards observed on Earth.

The Jiamusi (JME) radar is the first high-frequency coherent scatter radar independently developed in China. In this study, we investigate the statistical characteristics of the occurrence rate of F-region ionospheric irregularities between 40°N and 65°N geomagnetic latitude by using the Jiamusi radar data from March 2018 to November 2019. Diurnal and seasonal variations in scattering echoes and their dependence on geomagnetic conditions are statistically investigated. It is shown that the local time of the peak echo occurrence rate varies depending on the season, i.e., approximately 20-22.5 magnetic local time (MLT) in summer, 17.5-20.5 MLT in equinox, and 16-17.5 MLT in winter, which is closely associated with the time of sunset. The echo occurrence rate also increases with the enhancement of the geomagnetic index. To further understand the mechanism of these features, we simulate the distribution of the gradient drift instability indicator (∇n • V E /n) by using the Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIEGCM). The analysis results indicate that the gradient drift instability is an important mechanism for irregularities in this region. Key points: 1. Diurnal and seasonal variations in the scatter occurrence rate and their dependence on Kp conditions are statistically analyzed. 2. The distribution of the gradient drift instability indicator (∇n • V E /n) is simulated by using the TIEGCM. 3. The gradient drift instability is an important mechanism for the high occurrence rate of echoes in the midlatitude region.