A document by L. Claire Gasque. Click on the document to view its contents.

Day-to-day variability in thermospheric composition is driven by solar, geomagnetic and meteorological drivers. The ratio of the column density of atomic oxygen and molecular nitrogen (O/N\textsubscript{2}) is a useful parameter for quantifying this variability that has been shown to exhibit close correspondence to F-region electron density, total electron content and upper atmospheric transport. Therefore, understanding the variability in O/N\textsubscript{2} gives an insight into the geophysical variability of other relevant ionospheric and thermospheric parameters. The relative contributions of these drivers for thermospheric variability is not well known. Here we report a new analysis of the variability in O/N\textsubscript{2} to identify the sources of variability in a 55-day time period. Principal Component Analysis (PCA) was performed on thermospheric O/N\textsubscript{2} column density ratio from days 81 to 135 of 2020 from NASA’s Global-scale Observations of the Limb and Disk (GOLD) mission. We find that geomagnetic activity is the major source of variability in O/N\textsubscript{2} column density ratio, followed by solar-driven transport and meteorological driving from the lower atmosphere. The first component (PC1) showed a strong correlation to Kp index and IMF, and geomagnetic storm effects are seen in the wavelet analysis of PC1’s weights. The fifth component (PC5) showed a strong quasi-6-day oscillation(Q6DO). The higher explained variance ratio of PC1 suggests a stronger effect of geomagnetic activity relative to meteorological forcing from planetary scale waves. The methodology of the present study also demonstrates how PCA can be used to isolate and rank different sources of variability in other IT parameters.

The eruption of the Hunga Tonga-Hunga Ha’apai volcano on 15 January 2022 triggered atmospheric waves at all altitudes. The NASA Ionospheric Connection Explorer (ICON) and ESA Swarm satellites were well placed to observe its impact on the ionospheric wind dynamo. After the Lamb wave entered the dayside, Swarm A observed an eastward and then westward equatorial electrojet (EEJ) on two consecutive orbits, each with magnitudes exceeding the 99.9th percentile of typically observed values. ICON simultaneously observed the neutral wind (90-300 km altitude) at approximately the same distance from Tonga. The observed neutral winds were also extreme (>99.9th percentile at some altitudes). The covariation of EEJ and winds is consistent with recent theoretical and observational results, indicating that the westward electrojet is driven by strong westward winds in the Pedersen region (~120-150 km). These observations imply that the dynamo is a key mechanism in the ionospheric response to the Tonga disturbance.

The 15 Jan 2022 Hunga Tonga-Hunga Ha’apai volcano eruption drove global atmospheric waves that propagated into space and impacted the ionosphere. Here we show immediate large-scale electrodynamic effects of the eruption using observations from NASA’s Ionospheric Connection Explorer. We report extreme zonal and vertical ExB ion drifts thousands of kilometers away from Tonga within an hour of the eruption, before the arrival of any atmospheric wave. The measured drifts were magnetically connected to the ionospheric E-region just 400 km from Tonga, suggesting that the expanding wavefront created strong electric potentials which were transmitted along Earth’s magnetic field. A simple theoretical model suggests that the observed drifts are consistent with an expanding wave with a large (>200 m/s) neutral wind amplitude. These observations are the first direct detection in space of the immediate electrodynamic effects of a volcanic eruption and will help constrain future models of impulsive lower atmospheric events.

Vertical shears of horizontal winds play an important role in the dynamics of the middle and upper atmosphere. Prior observations have indicated that these shears predominantly occur in the lower thermosphere. MIGHTI observations from the Ionospheric Connection Explorer indicate that strong wind shears are a common feature of the lower thermosphere and vary greatly between orbits. This work focuses on these strong shears, and examines their occurrences, horizontal scales and underlying organization. No preferred wind shear direction is found. The shears that persist for a short horizontal extent are slightly larger in amplitude and more numerous than those that persist across large horizontal scales. The altitude at which the strongest shears occur often shows a downward progression with local time, following the climatological winds. Consistent with prior observations, the strongest shears are often seen just below the wind maximum, which follows the downward phase propagation consistent with upward propagating tides.