We present X-ray observations of the upper atmospheric density disturbance caused by the explosive eruption of the Hunga Tonga-Hunga Ha’apai (HTHH) volcano on 15 January 2022. From 14 January to 16 January, the Chinese X-ray astronomy satellite, Insight-HXMT, was observing the supernova remnant Cassiopeia~A. The X-ray data obtained during Earth’s atmospheric occultations allowed us to measure neutral densities in the altitude range of $\sim$90–150\,km. The density profiles above 110\,km altitude obtained before the major eruption are in reasonable agreement with expectations by both GAIA and NRLMSIS~2.0 models. In contrast, after the HTHH eruption we found a severe density depletion at 500–1,000\,km away from the epicenter, and a relatively weak depletion extending up to $\sim7,000$\,km for over 8\,hr after the eruption. In addition, density profiles showed wavy structures with a typical length scale of either $\sim$20\,km (vertical) or $\sim$1,000\,km (horizontal). This may reflect gravity waves triggered by the volcanic eruption.

Ionospheric plasma blobs have long been studied since it was first reported in 1986. Blobs are localized regions of enhanced plasma with a factor of 2 or 3 above ambient plasma. In this paper, we studied the occurrence of blobs over Nigeria (9.08⁰N, 8.67⁰E geographic coordinates) using the SWARM constellation satellites – ionospheric plasma density dataset specifically. We considered only the nighttime pass of the satellites over Nigeria with time frame 18:00 to 04:59 LT. The satellites passed over Nigeria 126 times in 2019 with 41 cases of plasma blobs. The results show that 58% of the cases were found without bubbles nearby, 29% of the cases were found in the presence of small-scale fluctuations in ionospheric plasma density (henceforth “SSFiI”). From the spectral analysis, the average wavelength, period and the propagating speed of SSFiI are 11 km, 2-4 seconds, and 2.75 – 5.5 km/s, respectively. The rate of change of the electron density inside the blobs associated with SSFiI was ~50% above that of the blobs in the absence of SSFiI. This suggests that bubbles may not be the only prerequisite for the development and dynamics of blobs; and SSFiI may play a significant role in the morphology and dynamics of blobs.

On January 15, 2022, we observed various unusual atmospheric wave events over the South American sector: Atmospheric pressure waves (Lamb mode) around 12:30 to 17:30 UT, Tsunamis along the Chile coast at around 17:00 to 19:00 UT, and related ionospheric disturbances between 11:30 and 20:00 UT. We understand that these events were generated by the Tonga volcanic eruption that occurred at (20.55°S, 175.39°W) in South Pacific Ocean at 04:15 UT. Several Traveling Ionospheric Disturbances (TIDs) were observed before and after the Lamb wave passed over the continent and the arrival of the tsunami on the Chile coast. This is the first time to report the signature of ionospheric disturbances over the South American continent generated by both the Lamb wave and the tsunami that were produced by the huge volcanic eruption.

We present the first comparative statistical study of subauroral arc detachment from the main auroral oval at Athabasca (magnetic latitude = 61.5°N), Canada, for three different types of subauroral arcs: pure red arc, red arc with simultaneous emission in green-line (red+green arc), and STEVE (strong thermal emission velocity enhancement). Based on 15-years (2006-2020) of all-sky imaging observations, this study not only uncovers the occurrence characteristics of different arcs but also provides important insights into the specific geomagnetic conditions under which STEVE develops. Red arc was the most common subauroral arc (139 events), followed by red+green arc (42 events), and STEVE (26 events) was a rare phenomenon. The detachment rate of red and red+green arcs exhibits dependence on both the solar flux and geomagnetic activity. The detachment rate of STEVE was higher during premidnight, whereas red and red+green arcs were higher around the midnight sector. The geomagnetic activity was relatively higher for STEVE, the decrease in the AL index and local X-component magnetic variations were ~2-3 times higher for STEVE as compared to other arcs. STEVE shows a strong association with asymmetric ring current in terms of prominent bay-like enhancement in ASY-H index prior to the STEVE detachment. Such bay-like enhancement was ~4 times higher for STEVE as compared to other arcs. STEVE events were accompanied by dispersionless injection for both electron and proton flux at the geosynchronous orbit. These results unambiguously suggest that STEVE develops after the substorm associated energy injection and significant intensification of asymmetric ring current.

An interesting interaction between equatorial plasma bubbles (EPBs), medium-scale traveling ionospheric disturbance (MSTID), and midnight brightness wave (MBW) were observed at Cachoeira Paulista, Brazil (22.7°S, 45.0°W, magnetic dip latitude ~20°S) by all-sky images of OI 630 nm emission, on the night of September 17-18, 2015. The EPBs were observed moving eastward while the MSTID propagated northwestward with the wavefront aligned to southwest-northeast. The MBW was observed propagating to south-southwest. After interaction with MSTIDs, the EPBs tilted and became larger and narrower. This effect could be associated with the x drift and also with changes in the neutral wind. Furthermore, the MBW also interacted with EPB and filled the EPB edge with plasma.