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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]

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]