The nature of the variability of the Total Electron Content (TEC) over Europe is investigated during the 2009 and 2019 Northern Hemisphere (NH) SSW events in this study. As the TEC variability is driven by geomagnetic and lower atmospheric forcing mechanisms, we investigate the dominant drivers and their respective contributions to TEC changes during both SSW events. We simulate the SSWs using the Whole Atmosphere Community Climate Model eXtended version (WACCM-X) and compare the semidiurnal solar and lunar tidal variabilities in the mesosphere-lower thermosphere (MLT) region. Further, in order to assess the mechanisms responsible for the TEC variability during both SSWs, we run numerical experiments using the National Center for Atmospheric Research (NCAR) Thermosphere-Ionosphere Electrodynamics General Circulation Model (TIE-GCM). We constrain the TIE-GCM lower boundary with the WACCM-X fields and carry out simulations both with and without geomagnetic forcing for each of the SSWs. The TIE-GCM simulations allow us to isolate the geomagnetic and lower atmospheric forcing effects on the TEC. We find that there was a major enhancement in daytime TEC over Europe during the 2019 SSW event, which was predominantly geomagnetically forced (~80%), while for the 2009 SSW, the major variability in TEC was accounted for by lower atmospheric forcing.

This paper examines the response of the upper atmosphere to equatorial Kelvin waves with a period of ~3 days, also known as ultra-fast Kelvin waves (UFKWs). The whole atmosphere model GAIA is used to simulate the UFKW events in the late summer of 2010 and 2011 as well as in the boreal winter of 2012/2013. When the lower layers of the model below 30 km altitude are constrained with meteorological data, GAIA is able to reproduce salient features of the UFKW in the mesosphere and lower thermosphere as observed by the Aura Microwave Limb Sounder. The model also reproduces ionospheric response, as validated through comparisons with total electron content data from the GOCE satellite as well as with earlier observations. Model results suggest that the UFKW produces eastward-propagating ~3-day variations with zonal wavenumber 1 in the equatorial zonal electric field and F-region plasma density. Model results also suggest that for a ground observer, identifying ionospheric signatures of the UFKW is a challenge because of ~3-day variations due to other sources. This issue can be overcome by combining ground-based measurements from different longitudes. As a demonstration, we analyze ground-based magnetometer data from equatorial stations during the 2011 event. It is shown that wavelet spectra of the magnetic data at different longitudes are only in partial agreement, with or without a ~3-day peak, but a spectrum analysis based on multipoint observations reveals the presence of the UFKW.

Both ground- and satellite-based airglow imaging have significantly contributed to our understanding of the low-latitude ionosphere, especially of the morphology and dynamics of the equatorial ionization anomaly (EIA). The NASA Global-scale Observations of the Limb and Disk (GOLD) mission focuses on far-ultraviolet airglow images from a geostationary orbit at 47.5°W. This region is of particular interest at low magnetic latitudes because of the high magnetic declination (i.e., about -20°) and proximity of the South Atlantic magnetic anomaly. Nighttime airglow images from GOLD reveal an exciting feature of the EIA. Using observations from 5 October 2018 to 30 June 2020, we characterize a wave-like structure of few thousands of kilometers seen as poleward and equatorward displacements of the nighttime EIA-crests. Initial analyses show that the mesoscale structure is symmetric about the dip equator and appears nearly stationary with time over the night. In quasi-dipole coordinates, maxima poleward displacements of the EIA-crests are seen at about ±12° latitude and around 20° and 60° longitude (i.e., in geographic longitude at the dip equator, about 53°W and 14°W). The wave-like structure presents typical zonal wavelengths of about 6.7x10^3 km and 3.3x10^3 km. The structure’s occurrence and wavelength are highly variable on a day-to-day basis with no apparent dependence on geomagnetic activity. In addition, a cluster or quasi-periodic wave train of equatorial plasma depletions (EPDs) is often detected within the mesoscale structure. We further outline the difference in observing these EPDs from FUV images and in situ measurements during a GOLD and Swarm mission conjunction.

Ionospheric plasma irregularities can be successfully studied with the Swarm satellites. Parameters derived from the in situ plasma measurements and from the topside ionosphere total electron content provide a comprehensive dataset for characterising plasma structuring along the orbits of the Swarm satellites. The Ionospheric Plasma IRregularities (IPIR) data product summarizes these parameters and has already been used in studies related to structuring and variability of ionospheric plasma. We provide a detailed description of the algorithms behind the IPIR data product and demonstrate its use for ionospheric studies.

An exceptionally strong stationary planetary wave with Zonal Wavenumber 1 led to a sudden stratospheric warming (SSW) in the Southern Hemisphere in September 2019. Ionospheric data from ESA’s Swarm satellite constellation mission reveal prominent 6-day variations in the dayside low-latitude region at this time, which can be attributed to forcing from the middle atmosphere by the Rossby normal mode “quasi-6-day wave” (Q6DW). Geopotential height measurements by the Microwave Limb Sounder aboard NASA’s Aura satellite show a burst of Q6DW activity in the mesosphere and lower thermosphere during the SSW, which is one of the strongest in the record. The Q6DW is apparently generated in the polar stratosphere at 30-40 km, where the atmosphere is unstable due to strong vertical wind shear connected with planetary-wave breaking. These results suggest that an Antarctic SSW can lead to ionospheric variability through wave forcing from the middle atmosphere.