We describe a mechanism to explain the subauroral emission feature called STEVE (Strong Thermal Emission Velocity Enhancement), with a focus on its continuum spectrum. Spacecraft observations show that emissions co‐occur with typically invisible plasma flows known as subauroral ion drifts. If these flows are fast enough, nitrogen molecules are vibrationally excited by collisions with ions, overcoming the activation energy of the N₂+O→NO+N reaction. The resulting NO combines with ambient O, producing NO₂ and spectrally broad light. Importantly, this mechanism also produces N, which reduces the lifetime of NO from hours to seconds and thus explains why the emission is confined to a discrete arc. The predicted emission altitude ( 130 km) and occurrence conditions ( 4‐km/s flows) match well with observations. We simulate this mechanism using a simple photochemical model to demonstrate its validity. This mechanism is initiated by fast ion flows and is thus distinct from auroral and airglow processes.

During a recent 2020 campaign, the Rayleigh lidar aboard the Bâtiment d’Essais et de Mesures (BEM) Monge conducted high-resolution temperature measurements of the upper Mesosphere and Lower Thermosphere (MLT). These measurements were used to conduct the first validation of ICON-MIGHTI temperatures by Rayleigh lidar. A double Mesospheric Inversion Layer (MIL) as well as shorter-period gravity waves was observed. Zonal and meridional wind speeds were obtained from locally launched radiosondes and the newly launched ICON satellite as well as from the European Centre for Medium-Range Weather Forecasts (ECMWF-ERA5) reanalysis. These three datasets allowed us to see the evolution of the winds in response to the forcing from the MIL and gravity waves. The wavelet analysis of a case study suggests that the wave energy was dissipated in small, intense, transient instabilities about a given wavenumber in addition to via a broad spectrum of breaking waves. This article will also detail the recent hardware advances of the Monge lidar that have allowed for the measurement of MILs and gravity waves at a resolution of 5 min with an effective vertical resolution of 926 m.

Vertical shears of horizontal winds play an important role in the dynamics of the 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 between 100–130 km, varying greatly between orbits. This work focuses on these strong shears, and examines their occurrences, horizontal scales and underlying organization. The wind shears can persist for 1000s km horizontally. Over a large data set, no preferred direction for the strong wind shears 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, regardless of the horizontal extent, show a downward progression with local time, following the climatological winds and upward propagating tides.

Isolated geomagnetic storms during solar minimum offer the opportunity to observe the effects of solar wind on Earth's atmosphere without interference from other background changes in the ionosphere‐thermosphere system. Here, we observe the effects of a minor storm on the ionosphere at middle and low latitudes, where disturbances in thermospheric winds and composition can produce storm‐time changes in ionospheric densities. Using data from the NASA Ionospheric Connection Explorer (ICON), we investigate the competing effects of composition changes and neutral wind disturbances in modifying the ionosphere during the 25 January 2021 storm. Large enhancements in plasma densities are seen across the dayside late on 25 January, associated with large changes in meridional winds and significant high‐latitude electric current enhancements as measured by the auroral electrojet index. At the same time, we find evidence of rapid transport of heated air from the southern auroral zone to the equator in the East‐Asian sector with the appearance of deeply reduced O/N₂ in the morning sector and storm‐time northward winds prior to its appearance. We observe significant, rapid erosion of the high plasma densities in the sector where thermospheric disturbance appears over 5 hr of observation. The noon‐time plasma density enhancements are short‐lived and we observe no westward perturbation in the low‐middle latitude, dayside zonal winds over a 5‐day period including the storm. Thus, we attribute the ionospheric changes to prompt penetration and meridional wind forcing, while ruling out the presence of a storm‐time disturbance dynamo and its related ionospheric effects.

Measurements of turbulence and waves were made as part of the Mesosphere-Lower Thermosphere Turbulence Experiment (MTeX) on the night of 25-26 January 2015 at Poker Flat Research Range, Chatanika, Alaska (65 degrees N, 147 degrees W). Rocket-borne ionization gauge measurements revealed turbulence in the 70- to 88-km altitude region with energy dissipation rates between 0.1 and 24mW/kg with an average value of 2.6mW/kg. The eddy diffusion coefficient varied between 0.3 and 134m(2)/s with an average value of 10m(2)/s. Turbulence was detected around mesospheric inversion layers (MILs) in both the topside and bottomside of the MILs. These low levels of turbulence were measured after a minor sudden stratospheric warming when the circulation continued to be disturbed by planetary waves and winds remained weak in the stratosphere and mesosphere. Ground-based lidar measurements characterized the ensemble of inertia-gravity waves and monochromatic gravity waves. The ensemble of inertia-gravity waves had a specific potential energy of 0.8J/kg over the 40- to 50-km altitude region, one of the lowest values recorded at Chatanika. The turbulence measurements coincided with the overturning of a 2.5-hr monochromatic gravity wave in a depth of 3 km at 85km. The energy dissipation rates were estimated to be 3mW/kg for the ensemble of waves and 18mW/kg for the monochromatic wave. The MTeX observations reveal low levels of turbulence associated with low levels of gravity wave activity. In the light of other Arctic observations and model studies, these observations suggest that there may be reduced turbulence during disturbed winters. Plain Language Summary Turbulence remains an outstanding challenge in understanding coupling, energetics, and dynamics of the atmosphere. However, turbulence is recognized as a critical component in our models of terrestrial and space weather. Obtaining routine and accurate measurements of turbulence continues to be a major challenge. We present new rocket-borne measurements of turbulence in January 2015 at Poker Flat Research Range, Alaska. These rocket-borne measurements were coordinated with a suite of ground-based instruments. The rocket-borne instruments captured the small-scale structure of the turbulence. The ground-based measurements documented the meteorological and space weather conditions. We find low levels of turbulence coinciding with a disturbed atmosphere where wave activity is reduced. These finding suggest that there may be systematically low levels of turbulence in the Arctic middle atmosphere, as the Arctic middle atmosphere is routinely disturbed in winter.