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.

This study presents the longitudinal distribution of thermospheric density around the terminator (in the dawn and dusk sectors), using observations collected by the atmospheric density detector onboard the Chinese satellite APOD (Atmospheric density detection and Precise Orbit Determination) from 2017 to 2018. The APOD observations show a significant relative longitudinal variation of thermospheric density with global maxima (Δρrmax) near the geomagnetic pole, especially in the winter hemisphere. The annual maximum of Δρrmax appears in the Southern Hemisphere around the June solstices and reaches 26.3% and 39.6% at dawn and dusk, respectively. Compared with at dawn, Δρrmax occurs at a higher latitude with a larger value at dusk. The auroral heating and meridional wind might play an important role in the longitudinal variation of thermospheric density. We further compare the APOD observations with the NRLMSIS 2.0 model predictions under low solar activity condition. The NRLMSIS 2.0 model reproduces similar longitudinal variations to the observations, with hemispheric asymmetry and local time difference.

In this paper, multi-ground-based instruments, including an all-sky airglow imager (ASAI), a very high frequency (VHF) radar, and eight digisondes, were combined to investigate multi-source perturbations in the evolution of an EPB event that occurred over low latitudes in China. We found this EPB event initially evolved from the bottom-type structures, most likely seeded by the atmospheric gravity wave (AGW) and the collisional shear-type instability (CSI)-inducing perturbations. Once formed, those bottom-type structures further evolved into bifurcated/plume-like structures at the ionospheric topside by the generalized Rayleigh-Taylor instability (RTI). Observed and analyzed are two different perturbation mechanisms of RTIs: one is the prereversal enhancement of the zonal electric field (PRE) inducing-RTI; another is the equatorward wind-inducing RTI around midnight. Accompanied by the PRE-inducing RTI are bifurcated/plume-like structures with a larger poleward (upward) velocity. The PRE could directly elevate the bottom-type structures to the ionospheric topside where the bifurcated/plume-like structures were generated by the RTI process. The near-midnight RTI was trigged by a vertical upward plasma jet caused by a seasonal equatorward wind in a region far away 10°N (20°N) from the geomagnetic (geographic) equator. This equatorward wind-inducing RTI persistently forced topside structures of those developed depletions to form secondary bifurcated/plume-like structures near midnight. Poleward developments of two cluster-type depletions of the EPB event were modulated by a large-scale wave-like structure (LSWS) occurring on the bottomside of the ionosphere. An eastward/westward polarization electric field inside the upwelling/trough region of the LSWS could accelerate/suppress the development of cluster-type depletions.