Browsing by Author "Lehmacher, Gerald A."
Now showing 1 - 4 of 4
Results Per Page
Sort Options
Item Restricted Height variation of gaps in 150‐km echoes and whole atmosphere community climate model electron densities suggest link to upper hybrid resonance(American Geophysical Union, 2020-01) Lehmacher, Gerald A.; Wu, Haonan; Kudeki, Erhan; Reyes, Pablo M.; Hysell, David L.; Milla, MarcoRadar 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.Item Restricted Mesospheric wind estimation with the Jicamarca MST radar using spectral mainlobe identification(American Geophysical Union, 2019-12) Lee, Kiwook; Kudeki, Erhan; Reyes, Pablo M.; Lehmacher, Gerald A.; Milla, MarcoMST (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.Item Open Access Turbulent kinetic energy dissipation rate and eddy diffusivity study in the tropical mesosphere using Jicamarca radar data(Clemson University, 2006-05) Guo, Liyu; Lehmacher, Gerald A.The MST radar at Jicamarca Radar Observatory JRO is a powerful radar that can detect atmospheric turbulence on the Bragg scale of 3 m in the daytime mesosphere 60-85 km Since 2002 the radar was operated for a few days each year in the mode that collecting 1 minute Doppler spectra in four beam directions and 150 m resolution The spectral widths along with GSWM MSIS and SABER temperatures have been used to compute the kinetic energy dissipation rate due to atmospheric turbulence Eddy diffusivities K have also been calculated A small contamination due to beam broadening beam width 0 7 degree has been removed For most days median kinetic energy dissipation values of 1-10 mW kg increase with height consistent with the results from other VHF radars The variability during each day is large Turbulent dissipation rates and eddy diffusivities for individual layers and the day-to-day variability are discussed in relationship with the observed wind shear and estimated Richardson number.Item Restricted VIPIR and 50 MHz radar studies of gravity wave signatures in 150‐km echoes observed at Jicamarca(American Geophysical Union, 2020-08) Reyes, Pablo M.; Kudeki, Erhan; Lehmacher, Gerald A.; Chau, Jorge L.; Milla, MarcoRange‐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.