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Abstract Numerical forecasts of plasma convective instability in the postsunset equatorial ionosphere are made based on data from the Ionospheric Connections Explorer satellite (ICON) following the method outlined in a previous study. Data are selected from pairs of successive orbits. Data from the first orbit in the pair are used to initialize and force a numerical forecast simulation, and data from the second orbit are used to validate the results 104 min later. Data from the IVM plasma density and drifts instrument and the MIGHTI red‐line thermospheric winds instrument are used to force the forecast model. Thirteen (16) data set pairs from August (October), 2022, are considered. Forecasts produced one false negative in August and another false negative in October. Possible causes of forecast discrepancies are evaluated including the failure to initialize the numerical simulations with electron density profiles measured concurrently. Volume emission 135.6‐nm OI profiles from the Far Ultraviolet (FUV) instrument on ICON are considered in the evaluation. 

Abstract The Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) was launched aboard NASA’s Ionospheric Connection (ICON) Explorer satellite in October 2019 to measure winds and temperatures on the limb in the upper mesosphere and lower thermosphere (MLT). Temperatures are observed using the molecular oxygen atmospheric band near 763 nm from 90–127 km altitude in the daytime and 90–108 km in the nighttime. Here we describe the measurement approach and methodology of the temperature retrieval, including unique on-orbit operations that allow for a better understanding of the instrument response. The MIGHTI measurement approach for temperatures is distinguished by concurrent observations from two different sensors, allowing for two self-consistent temperature products. We compare the MIGHTI temperatures against existing MLT space-borne and ground-based observations. The MIGHTI temperatures are within 7 K of these observations on average from 90–95 km throughout the day and night. In the daytime on average from 99–105 km, MIGHTI temperatures are higher than coincident observations by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on NASA’s TIMED satellite by 18 K. Because the difference between the MIGHTI and SABER observations is predominantly a constant bias at a given altitude, conclusions of scientific analyses that are based on temperature variations are largely unaffected. 

Abstract Measurements from the Ionospheric Connections Explorer satellite (ICON) form the basis of direct numerical forecast simulations of plasma convective instability in the postsunset equatorialFregion ionosphere. ICON data are selected and used to initialize and force the simulations and then to test the results one orbit later when the satellite revisits the same longitude. Data from the IVM plasma density and drifts instrument and the MIGHTI red‐line thermospheric winds instrument are used to force the simulation. Data from IVM are also used to test for irregularities (electrically polarized plasma depletions). Fourteen datasets from late March 2022, were examined. The simulations correctly predicted the occurrence or non‐occurrence of irregularities 12 times while producing one false positive and one false negative. This demonstrates that the important telltales of instability are present in the ICON state variables and that the important mechanisms for irregularity formation are captured by the simulation code. Possible refinements to the forecast strategy are discussed. 

Abstract The Hunga‐Tonga Hunga‐Ha'apai volcano underwent a series of large‐magnitude eruptions that generated broad spectra of mechanical waves in the atmosphere. We investigate the spatial and temporal evolutions of fluctuations driven by atmospheric acoustic‐gravity waves (AGWs) and, in particular, the Lamb wave modes in high spatial resolution data sets measured over the Continental United States (CONUS), complemented with data over the Americas and the Pacific. Along with >800 barometer sites, tropospheric observations, and Total Electron Content data from >3,000 receivers, we report detections of volcano‐induced AGWs in mesopause and ionosphere‐thermosphere airglow imagery and Fabry‐Perot interferometry. We also report unique AGW signatures in the ionospheric D‐region, measured using Long‐Range Navigation pulsed low‐frequency transmitter signals. Although we observed fluctuations over a wide range of periods and speeds, we identify Lamb wave modes exhibiting 295–345 m s⁻¹phase front velocities with correlated spatial variability of their amplitudes from the Earth's surface to the ionosphere. Results suggest that the Lamb wave modes, tracked by our ray‐tracing modeling results, were accompanied by deep fluctuation fields coupled throughout the atmosphere, and were all largely consistent in arrival times with the sequence of eruptions over 8 hr. The ray results also highlight the importance of winds in reducing wave amplitudes at CONUS midlatitudes. The ability to identify and interpret Lamb wave modes and accompanying fluctuations on the basis of arrival times and speeds, despite complexity in their spectra and modulations by the inhomogeneous atmosphere, suggests opportunities for analysis and modeling to understand their signals to constrain features of hazardous events. 

Abstract The spectral line profile of the atomic oxygen O¹D₂—³P₂transition near 6300 Å in the airglow has been used for more than 50 years to extract neutral wind and temperature information from the F‐region ionosphere. A new spectral model and recent samples of this airglow emission in the presence of the nearby lambda‐doubled OH Meinel (9‐3) P₂(2.5) emission lines underscores earlier cautions that OH can significantly distort the OI line center position and line width observed using a single‐etalon Fabry‐Perot interferometer (FPI). The consequence of these profile distortions in terms of the emission profile line width and Doppler position is a strong function of the selected etalon plate spacing. Single‐etalon Fabry‐Perot interferometers placed in the field for thermospheric measurements have widely varying etalon spacings, so that systematic wind biases caused by the OH line positions differ between instruments, complicating comparisons between sites. Based on the best current determinations of the OH and O¹D line positions, the ideal gap for a single‐etalon FPI wind measurements places the OH emissions in the wings of the O¹D spectral line profile. Optical systems that can accommodate prefilters with square passbands less than ∼3 Å in the optical beam can effectively block the OH contamination. When that is not possible, a method to fit for OH contamination and remove it in the spectral background of an active Fabry‐Perot system is evaluated. 

Abstract Midlatitude thermospheric wind observations from the Michelson Interferometer for Global High‐resolution Thermospheric Imaging on board the Ionospheric Connections Explorer (ICON/MIGHTI) and from the ground‐based Boulder, Urbana, Millstone Hill and Morocco Fabry‐Perot interferometers (FPIs) are used to study a distinct solar local time (SLT) evolution in the nighttime wind field around the December solstice period. Our results show, to the best of our knowledge for the first time, strong non‐migrating tides in midlatitude thermospheric winds using coincident from different observing platforms. These observations exhibited a structure of strong (∼50–150 m/s) eastward and southward winds in the pre‐midnight sector (20:00–23:00 SLT) and in the post‐midnight sector (02:00–03:00 SLT), with a strong suppression around midnight. Tidal analysis of ICON/MIGHTI data revealed that the signature before midnight was driven by diurnal (D0, DE1, DE2, DW2) and semidiurnal (SE2, SE3, SW1, SW4) tides, and that strong terdiurnal (TE2, TW1, TW2, TW5) and quatradiurnal (QW2, QW3, QW6) tides were important contributors in the mid‐ and post‐midnight sectors. ICON/MIGHTI tidal reconstructions successfully reproduced the salient structures observed by the FPI and showed a longitudinal dual‐peak variation with peak magnitudes around 200°–120°W and 30°W–60°E. The signature of the structure extended along the south‐to‐north direction from lower latitudes, migrated to earlier local times with increasing latitude, and strengthened above 30°N. Tidal analysis using historical FPI data revealed that these structures were often seen during previous December solstices, and that they are much stronger for lower solar flux conditions, consistent with an upward‐propagating tidal origin.