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We present observations and analysis of seven horizontal wind profiles obtained by the trimethyl aluminum (TMA) tracer method on 27 March 2012 over the Atlantic ocean near Wallops Island, Virginia (37.9°N, 75.4°W). Payloads were launched in order to produce quasi‐simultaneous trails separated by tens to hundreds of kilometers. Tracer positions evolving in time and space were triangulated from three locations along the Atlantic seaboard and wind profiles between 90 and 140 km calculated. The wind profiles present a coherent wind structure dominated by very strong diurnal and semidiurnal tides up to 110 km and an upward propagating inertia‐gravity wave between 110 and 140 km. Properties such as horizontal and vertical wavelength could be extracted from the simultaneous observations at separate locations. A statistical analysis of the wind differences was performed to estimate power‐law coefficients of the second structure function at mesoscales. They show scale‐independence in the region of the largest wind shears, 100–110 km, and a scaling coefficient characteristic for isotropic wind fluctuations above and below this region. Plain Language Summary: We present seven upper atmospheric wind profiles (90–140 km) measured with the tracer method. The winds were observed at seven different locations spaced over 500 km along a line from the Mid‐Atlantic coast. The winds show very similar structures below 110 km, presumably due to very large atmospheric tidal waves. A very distinct, smaller wave motion is evident above 110 km, because it was sampled at different locations. In some regions the variability of the winds show signs of large‐scale turbulence. Key Points: Seven simultaneous wind profiles in the lower thermosphere, horizontally separated by up to 600 km, are dominated by strong tidesA large‐scale upward propagating inertia‐gravity wave is evident in the observationsThe region between 100 and 110 km shows the largest mesoscale variability of winds and wind shears, independent of separation scale [ABSTRACT FROM AUTHOR]

Range‐time‐intensity (RTI) plots of 50 MHz radar backscatter detected at Jicamarca from the 150‐km region of the equatorial ionosphere exhibit necklace‐shaped multilayered structures first reported by Kudeki and Fawcett (1993, https://doi.org/10.1029/93GL01256). The backscatter layers also exhibit quasi‐periodic intensity fluctuations with periods of about 5–15 min and are separated from adjacent layers by thin and undulating regions of no detectible power returns. A study of the fluctuating backscatter layers and undulating gap regions will be presented using VIPIR ionosonde data taken at the Jicamarca Radio Observatory simultaneously with high‐resolution 50‐MHz radar backscatter data. VIPIR virtual reflection height variations in time are noted to match the RTI gap‐region undulations very closely at selected VIPIR frequencies (or, equivalently, electron densities at reflection heights). This matching enables assigning "true heights" to VIPIR virtual height contour maps, and a joint study of the contour maps with the 50‐MHz radar RTI maps strongly suggests that correlated fluctuations and undulations observed in VIPIR and 50‐MHz radar data are indicative of gravity wave‐induced variations in the 150‐km region ionosphere. Accordingly, a complete explanation of the 150‐km echo phenomenon will need to include gravity wave coupling and forcing effects in the enhancement and suppression processes that can account for the observed fluctuations and gap‐region features of necklace‐shaped 150‐km echo maps. Key Points: VIPIR ionosonde and 50 MHz radar data are used jointly to investigate the equatorial 150‐km irregularitiesThe 150‐km echo layers seen in 50 MHz data are aligned with VIPIR isodensity contours in the region rippled by gravity wave modulationsThe 150‐km irregularity models need to account for gravity wave modulation and forcing effects [ABSTRACT FROM AUTHOR]

MST (mesosphere, stratosphere, troposphere) radar observations at Jicamarca use four antenna beams, one vertical, others tilted to the east, west, and south, to detect the scattered pulse returns from mesospheric heights (∼55–85 km). Doppler shifts of scattered returns, estimated by fitting the observed signal spectra by generalized Gaussian‐shaped models, are used to estimate mesospheric wind vectors. At some heights two spectral peaks are seen in which case a dual‐peaked model is fitted the spectrum. Dual peaks are more common for returns from the east and west tilted beams with stronger sidelobes. When sidelobe‐caused peaks are dominant and are mistaken for mainlobe peaks, wind errors occur since the estimation algorithm uses the pointing angle of the mainbeam. To avoid such errors we implemented a clustering‐based machine learning procedure to identify and use only the mainbeam components of dual‐peaked spectra. Wind estimates made before and after the procedure will be presented to assess the improvements achieved by this new method to be used routinely in Jicamarca mesospheric wind measurements and applied to past MST data. Key Points: Radar antenna beam sidelobe contaminations cause dual‐peaked Doppler spectra to be detected in Jicamarca radar MST probingSorting the mean Doppler shifts and related power of detected spectral peaks into mainlobe and sidelobe sets is required in wind estimationWe use weighted least squares fits of dual‐peaked spectra followed by sorting and classification steps to obtain bias free wind estimates [ABSTRACT FROM AUTHOR]

Short‐period gravity waves are ubiquitous in the mesosphere, but the vertical structures of their perturbations are difficult to observe. The Jicamarca 50‐MHz very high frequency radar allows observations of winds and turbulent scatter with high temporal and vertical resolution. We present a case of a quasi‐monochromatic gravity wave with period 520 (±40) s that is likely ducted below a southward wind jet between 68 and 74 km. Above this layer of evanescence, a northward wind enables it to emerge into a more stable layer, where it is refracted to a short vertical wavelength of 2.2 (±0.2) km; data show evidence of weak nonlinearity, and possible overturning or partial reflection from higher altitudes, above the observable region, in the form of a standing wave structure in vertical velocity at approximately 75 km. Based on the dispersion relation, and with help of a two‐dimensional model, we determine that most likely the wave is propagating northward and is being ducted below and tunneling through the regions of evanescence created by the wind flow and typical mesospheric thermal structure. This is the first time that such an event has been identified in the Jicamarca mesospheric echoes, and it is distinct from Kelvin‐Helmholtz billows also commonly seen with this sensitive radar—instead apparently revealing tunneling of the gravity wave through ambient winds. Key Points: Quasi‐monochromatic gravity waves are observed propagating in the equatorial middle mesosphere, likely in a Doppler ductThe waves extend through an upper evanescent layer, propagating in a second stable layer above the ductDisplacement of radar scattering layer and measured vertical velocity match the oscillatory motion of a wave near reflection [ABSTRACT FROM AUTHOR]

Four mesosphere-lower thermosphere temperature and turbulence profiles were obtained in situ within ~ 30 min and over an area of about 100 by 100 km during a sounding rocket experiment conducted on 26 January 2015 at Poker Flat Research Range in Alaska. In this paper we examine the spatial and temporal variability of mesospheric turbulence in relationship to the static stability of the background atmosphere. Using active payload attitude control, neutral density fluctuations, a tracer for turbulence, were observed with very little interference from the payload spin motion, and with high precision (< 0:01%) at sub-meter resolution. The large-scale vertical temperature structure was very consistent between the four soundings. The mesosphere was almost isothermal, which means more stratified, between 60 and 80 km, and again between 88 and 95 km. The stratified regions adjoined quasi-adiabatic regions assumed to be well mixed. Additional evidence of vertical transport and convective activity comes from sodium densities and trimethyl aluminum trail development, respectively, which were both observed simultaneously with the in situ measurements. We found considerable kilometer-scale temperature variability with amplitudes of 20K in the stratified region below 80 km. Several thin turbulent layers were embedded in this region, differing in width and altitude for each profile. Energy dissipation rates varied between 0.1 and 10mWkg-1, which is typical for the winter mesosphere. Very little turbulence was observed above 82 km, consistent with very weak small-scale gravity wave activity in the upper mesosphere during the launch night. On the other hand, above the cold and prominent mesopause at 102 km, large temperature excursions of C40 to C70K were observed. Simultaneous wind measurements revealed extreme wind shears near 108 km, and combined with the observed temperature gradient, isolated regions of unstable Richardson numbers (0 < Ri < 0:25) were detected in the lower thermosphere. The experiment was launched into a bright auroral arc under moderately disturbed conditions (Kp ~ 5). [ABSTRACT FROM AUTHOR]

Radar echoes from the daytime lower F region near the magnetic equator, so‐called 150‐km echoes, have been puzzling researchers for decades. Neither the mechanisms that generate the enhanced backscatter at very high frequencies (typically 30–50 MHz), the sharp lower cutoff height, the intricate layering with multiple echo layers separated by narrow gaps, nor the modulation of the echoes by short‐period gravity waves is well understood. Here we focus on the diurnal variation of the echo layers—specifically, certain wide gaps in the vertical structure—which apparently descend in the morning, reach their lowest altitude near local noon, and ascend in the afternoon, sometimes described as necklace structure based on the appearance of the layers in range‐time‐intensity diagrams. Analyzing high‐resolution data obtained with the Jicamarca radar between 2005 and 2017, spanning more than one solar cycle, we find that (a) wide gaps and narrow lines occur in vertically stacked, systematically repeating pattern; (b) the gap heights vary with season and solar cycle; and (c) the gap heights can be associated with specific contours of plasma frequencies or electron densities. The last two findings are supported by simultaneous observations of VIPIR ionosonde reflection heights and by comparison of gap heights with electron density contours obtained with the WACCM‐X 2.0 global model. Finally, the wide gaps appear to coincide with the double resonance condition, where the upper hybrid frequency equals integer multiples of the electron gyrofrequency. This may explain why field‐aligned plasma irregularities are suppressed and enhanced radar backscatter is not observed inside the gaps. Key Points: Specific gaps and lines in the 150‐km echo structure correspond to specific electron density contoursThe height of the gaps varies with season and solar activityWide gaps where echoes are suppressed likely correspond to regions of double resonance conditions [ABSTRACT FROM AUTHOR]

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°N, 147°W). Rocket‐borne ionization gauge measurements revealed turbulence in the 70‐ to 88‐km altitude region with energy dissipation rates between 0.1 and 24 mW/kg with an average value of 2.6 mW/kg. The eddy diffusion coefficient varied between 0.3 and 134 m2/s with an average value of 10 m2/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 0.8 J/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 85 km. The energy dissipation rates were estimated to be 3 mW/kg for the ensemble of waves and 18 mW/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. Key Points: Low levels of turbulence in the upper mesosphere are observed in a disturbed Arctic atmosphere following an SSWLow levels of turbulence coincide with low levels of gravity wave activityTurbulence is found in both the topside and bottomside of MILs [ABSTRACT FROM AUTHOR]