The explosive eruption of the Hunga-Tonga volcano in the southwest Pacific at 0415UT on 15 January 2022 triggered gigantic atmospheric disturbances with surface air pressure wave propagating around the globe in Lamb mode. In space, concentric traveling ionosphere disturbances (CTIDs) are also observed as a manifestation of air pressure acoustic waves in New Zealand ~0500UT and Australia ~0630UT. As soon as the wave reached central Australia ~0800UT, CTIDs appeared simultaneously in the northern hemispheres through magnetic field line conjugate effect, which is much earlier than the arrival of the air pressure wave to Japan after 1100UT. Combining observations over Australia and Japan between 0800-1000UT, CTIDs with characteristics of phase velocities of 320-390 m/s are observed, matching with the dispersion relation of Lamb mode. The arrival of atmospheric Lamb wave to Japan later created in situ CTIDs showing the same Lamb mode characteristics as the earlier arriving CTIDs.

The Tonga volcano eruption of 15 January 2022 unleashed a variety of atmospheric perturbations, coinciding with the recovery phase of a geomagnetic storm. The ensuing thermospheric variations created rare display of extreme poleward-expanding conjugate plasma bubbles seen in the rate of total electron content index (ROTI) over 100-150°E. This is associated with fluctuations in FORMOSAT-7/COSMIC-2 (F7/C2) ion-density measurements and spread-F signatures in ionograms, reaching ~40°N geographic latitude. This was preceded by an unusually strong pre-reversal enhancement (PRE) in the global ionospheric specification (GIS) electron density profiles derived from F7/C2 observations. The GIS further revealed a decrease of equatorial ionization anomaly (EIA) crest density due to the storm impact. A sharp decrease of E-region conductivity by volcano-induced waves, combined with enhanced F-region wind over EIA with less ion-drag apparently intensified the PRE. The strong PRE and seed perturbations from the volcano-induced waves likely further triggered super plasma bubble activity.

Equatorial plasma bubbles (EPBs) are elongated plasma depletions that can occur in the nighttime ionospheric F region, causing scintillation in satellite navigation and communications signals. EPBs are believed to be Rayleigh-Taylor instabilities seeded by vertically propagating gravity waves. A necessary pre-condition for EPB formation is a threshold vertical ion drift from the E region, which is required to produce the vertical plasma gradients conducive to this instability. Factors affecting the variation of EPBs therefore include magnetic declination, the strength of the equatorial electojet, and the wind dynamo in the lower thermosphere controlling vertical plasma drifts. In most longitude zones, this results in elevated EPB occurrence rates during the equinoxes. The notable exception is over the central Pacific and African sectors, where EPB activity maximizes during solstice. \citet{tsunoda_jgr2015} hypothesized that the solstice maxima in these two sectors could be driven by a zonal wavenumber 2 atmospheric tide in the mesosphere and lower thermosphere. In this study, we find that the post-sunset electron density observed by FORMOSAT-3/COSMIC during the boreal summer from 2007 - 2012 does indeed exhibit a wave-2 zonal distribution, consistent with results expected from elevated vertical ion drift over the Central Pacific and African sectors. Numerical experiments are also carried out which found that forcing from the aforementioned tidal and stationary planetary wave (SPW) components produced wave-2 modulations on vertical ion drift, ion flux convergence, and midnight TEC. The relation between the vertical ion drift enhancements and the midnight TEC enhancements are consistent with the solstice maxima hypothesis.

This work explores the results of applying Square Euclidean and Correlation k-means cluster analysis on ion density irregularity profiles observed by the Advanced Ionospheric Probe (AIP) onboard the Formosat-5 (F-5) satellite from November 2017 to November 2020. The Square Euclidean cluster analysis yielded separate clusters each for ion density irregularities consistent with Equatorial Plasma Bubbles (EPBs) occurring over the southern low-latitude, northern low-latitude and equator. The Correlation k-means cluster analysis only yielded one cluster characterized by ion density irregularities consistent with EPBs occurring over the equator. Thus, this work suggests that a cluster analysis preferably a Square Euclidean Cluster analysis can be used to find and classify ion density irregularities consistent with EPBs. This work also shows that the F-5/AIP can perform multi-year observations of ion density irregularities at ~710 km altitude and at ~2230 pm local-time that are consistent with irregularities due to EPBs.

We investigate ionospheric disturbances using the total electron content (TEC) data retrieved by the three satellites after the foreshock of the 2011 Tohoku Earthquake on 9 March 2011. Co-seismic ionospheric disturbances (CIDs) appeared to extend from an onset point concentrically in all of the satellite data. We have found, however, that the geographic coordinates of the onset points did not coincide if the observed CIDs were assumed to occur at one altitude. Admitting that the altitudes of the onset points are different, we searched for coinciding geographic coordinates of the onset points using the two data sets by changing the altitudes and identified the altitude of the two onset points at 155.4 and 234.9 km, and the onset time at these altitudes. As a result, the vertical velocity of acoustic wave is estimated to be 1.04 km/s from the travel time between the altitudes of 155.4 and 234.9 km. This is 1.4 times higher than the sound velocity calculated using the empirical model NRLMSISE-00. The present study provides a method of determining the source location of the acoustic wave from the ionospheric TEC analysis without using the seismic data.