Observations of thermospheric neutral winds and temperatures obtained during a geomagnetic storm on 2 October 2013 from a network of six Fabry‐Perot interferometers (FPIs) deployed in the Midwest United States are presented. Coincident with the commencement of the storm, the apparent horizontal wind is observed to surge westward and southward (toward the equator). Simultaneous to this surge in the apparent horizontal winds, an apparent downward wind of approximately 100 m/s lasting for 6 h is observed. The apparent neutral temperature is observed to increase by approximately 400 K over all of the sites. Observations from an all‐sky imaging system operated at the Millstone Hill observatory indicate the presence of a stable auroral red (SAR) arc and diffuse red aurora during this time. We suggest that the large sustained apparent downward winds arise from contamination of the spectral profile of the nominal thermospheric 630.0 nm emission by 630.0 nm emission from a different (nonthermospheric) source. Modeling demonstrates that the effect of an additional population of 630.0 nm photons, with a distinct velocity and temperature distribution, introduces an apparent Doppler shift when the combined emissions from the two sources are analyzed as a single population. Thus, the apparent Doppler shifts should not be interpreted as the bulk motion of the thermosphere, calling into question results from previous FPI studies of midlatitude storm time thermospheric winds. One possible source of contamination could be fast O related to the infusion of low‐energy O+ ions from the magnetosphere. The presence of low‐energy O+ is supported by observations made by the Helium, Oxygen, Proton, and Electron spectrometer instruments on the twin Van Allen Probes spacecraft, which show an influx of low‐energy ions during this period. These results emphasize the importance of distributed networks of instruments in understanding the complex dynamics that occur in the upper atmosphere during disturbed conditions.

Abstract. Measurements of equatorial thermospheric winds, temperatures, and 630 nm relative intensities were obtained using an imaging Fabry–Perot interferometer (FPI), which was recently deployed at Bahir Dar University in Ethiopia (11.6° N, 37.4° E, 3.7° N magnetic). The results obtained in this study cover 6 months (53 nights of useable data) between November 2015 and April 2016. The monthly-averaged values, which include local winter and equinox seasons, show the magnitude of the maximum monthly-averaged zonal wind is typically within the range of 70 to 90 ms−1 and is eastward between 19:00 and 21:00 LT. Compared to prior studies of the equatorial thermospheric wind for this local time period, the magnitude is considerably weaker as compared to the maximum zonal wind speed observed in the Peruvian sector but comparable to Brazilian FPI results. During the early evening, the meridional wind speeds are 30 to 50 ms−1 poleward during the winter months and 10 to 25 ms−1 equatorward in the equinox months. The direction of the poleward wind during the winter months is believed to be mainly caused by the existence of the interhemispheric wind flow from the summer to winter hemispheres. An equatorial wind surge is observed later in the evening and is shifted to later local times during the winter months and to earlier local times during the equinox months. Significant night-to-night variations are also observed in the maximum speed of both zonal and meridional winds. The temperature observations show the midnight temperature maximum (MTM) to be generally present between 00:30 and 02:00 LT. The amplitude of the MTM was  ∼  110 K in January 2016 with values smaller than this in the other months. The local time difference between the appearance of the MTM and a pre-midnight equatorial wind was generally 60 to 180 min. A meridional wind reversal was also observed after the appearance of the MTM (after 02:00 LT). Climatological models, HWM14 and MSIS-00, were compared to the observations and the HWM14 model generally predicted the zonal wind observations well with the exception of higher model values by 25 ms−1 in the winter months. The HWM14 model meridional wind showed generally good agreement with the observations. Finally, the MSIS-00 model overestimated the temperature by 50 to 75 K during the early evening hours of local winter months. Otherwise, the agreement was generally good, although, in line with prior studies, the model failed to reproduce the MTM peak for any of the 6 months compared with the FPI data.

Though the Kelvin‐Helmholtz instability (KHI) has been extensively observed in the mesosphere, where breaking gravity waves produce the conditions required for instability, little has been done to describe quantitatively this phenomenon in detail in the mesopause and lower thermosphere, which are associated with the long‐lived shears at the base of this statically stable region. Using trimethylaluminum (TMA) released from two sounding rockets launched on 26 January 2018, from Poker Flat Research Range in Alaska, the KHI was observed in great detail above 100 km. Two sets of rocket measurements, made 30 min apart, show strong winds (predominantly meridional and up to 150 ms−1) and large total shears (90 ms−1 km−1). The geomagnetic activity was low in the hours before the launches, confirming that the enhanced shears that triggered the KHI are not a result of the E‐region auroral jets. The four‐dimensional (three‐dimensional plus time) estimation of KHI billow features resulted in a wavelength, eddy diameter, and vertical length scale of 9.8, 5.2, and 3.8 km, respectively, centered at 102‐km altitude. The vertical and horizontal root‐mean‐square velocities measured 29.2 and 42.5 ms−1, respectively. Although the wind structure persisted, the KHI structure changed significantly with time over the interval separating the two launches, being present only in the first launch. The rapid dispersal of the TMA cloud in the instability region was evidence of enhanced turbulent mixing. The analysis of the Reynolds and Froude numbers (Re = 7.2 × 103 and Fr = 0.29, respectively) illustrates the presence of turbulence and weak stratification of the flow.

Presented here are several cases of midnight temperature maximum (MTM) observations using the Millstone Hill incoherent scatter radar (ISR) and Arecibo ISR. The MTM, a temperature enhancement in the upper atmosphere (at~300 km altitude), is a poorly understood phenomenon as observations are sparse. An upward propagating terdiurnal tide and coupling between atmospheric regions may play a large part in the generation of the MTM, yet this phenomenon and its implications are not fully understood. Two nights (6 March 1989 and 12 July 1988) show clear cases of the MTM occurring between 30 and 34°N with amplitudes of~100 K and at~18°N with amplitudes of~40 K. The MTMs occurred later at the higher latitude. Experiments in 2013 also show a clear MTM at 34°and 36°N from 250 to 350 km altitude. The ionospheric measurements presented here demonstrate a new application of a well-established technique to study atmospheric parameters and allow us to study the latitudinal extent of the MTM. The results provide evidence of the phenomenon occurring at latitudes and altitudes not previously sampled by radar techniques, showing that the MTM is not just an equatorial process, but one that can easily reach midlatitudes. Simultaneous measurements with a Fabry-Perot interferometer allow us to compare the neutral temperatures with the ion temperature. Overall, these are key observations that point to large-scale effects that can help constrain model outputs at different heights and latitudes.

Abstract. Fabry–Perot interferometer (FPI) measurements of thermospheric temperatures and winds show the detection and successful determination of the latitudinal distribution of the midnight temperature maximum (MTM) in the continental mid-eastern United States. These results were obtained through the operation of the five FPI observatories in the North American Thermosphere Ionosphere Observing Network (NATION) located at the Pisgah Astronomic Research Institute (PAR) (35.2∘ N, 82.8∘ W), Virginia Tech (VTI) (37.2∘ N, 80.4∘ W), Eastern Kentucky University (EKU) (37.8∘ N, 84.3∘ W), Urbana-Champaign (UAO) (40.2∘ N, 88.2∘ W), and Ann Arbor (ANN) (42.3∘ N, 83.8∘ W). A new approach for analyzing the MTM phenomenon is developed, which features the combination of a method of harmonic thermal background removal followed by a 2-D inversion algorithm to generate sequential 2-D temperature residual maps at 30 min intervals. The simultaneous study of the temperature data from these FPI stations represents a novel analysis of the MTM and its large-scale latitudinal and longitudinal structure. The major finding in examining these maps is the frequent detection of a secondary MTM peak occurring during the early evening hours, nearly 4.5 h prior to the timing of the primary MTM peak that generally appears after midnight. The analysis of these observations shows a strong night-to-night variability for this double-peaked MTM structure. A statistical study of the behavior of the MTM events was carried out to determine the extent of this variability with regard to the seasonal and latitudinal dependence. The results show the presence of the MTM peak(s) in 106 out of the 472 determinable nights (when the MTM presence, or lack thereof, can be determined with certainty in the data set) selected for analysis (22 %) out of the total of 846 nights available. The MTM feature is seen to appear slightly more often during the summer (27 %), followed by fall (22 %), winter (20 %), and spring (18 %). Also seen is a northwestward propagation of the MTM signature with a latitude-dependent amplitude. This behavior suggests either a latitudinal dependence of thermosphere tidal dissipation or a night-to-night variation of the composition of the higher-order tidal modes that contribute to the production of the MTM peak at mid-latitudes. Also presented in this paper is the perturbation on the divergence of the wind fields, which is associated with the passage of each MTM peak analyzed with the 2-D interpolation. Keywords. Ionosphere (mid-latitude ionosphere)