Ciencias del Geoespacio y Astronomía
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Browsing Ciencias del Geoespacio y Astronomía by Author "Akgiray, A."
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Item Restricted High-resolution observations of mesospheric layers with the Jicamarca VHF Radar(Elsevier, 2007) Lehmacher, G. A.; Guo, L.; Kudeki, E.; Reyes, P. M.; Akgiray, A.; Chau Chong Shing, Jorge LuisWe report new results from the 50-MHz Jicamarca radar in Peru (12°S, 77°W), which is able to observe backscatter from the daytime mesosphere on any given day. Since 2005, the radar has been operated in a high-power MST-ISR mode for 3-day runs four times per year to study the seasonal variation of mesospheric echoes. Doppler spectra are obtained with 1 min and 150-m nominal resolution yielding power, horizontal and vertical winds, and spectral width. The rich echo structures contain braids and billows suggestive of Kelvin–Helmholtz instability (KHI). We present three cases; (1) a short sequence of billows growing in height and becoming unstable; (2) a long train of billows showing high levels of turbulence at two different locations; and (3) a series of billows passing only slowly through the field of view. In all cases, the layers were associated with strong wind shears, mature billows were 1–1.5 km tall, and separation of KH phase fronts was 8–10 km. We compare our observations with OH imager observation and numerical simulations.Item Open Access Radar cross sections for mesospheric echoes at Jicamarca(European Geosciences Union (EGU), 2009-07-06) Lehmacher, G. A.; Kudeki, E.; Akgiray, A.; Guo, L.; Reyes, P.; Chau Chong Shing, Jorge LuisRadar cross sections (RCS) of mesospheric layers at 50 MHz observed at Jicamarca, Peru, range from 10−18 to 10−16 m−1, three orders of magnitudes smaller than cross sections reported for polar mesospheric winter echoes during solar proton events and six orders of magnitude smaller than polar mesospheric summer echoes. Large RCS are found in thick layers around 70 km that also show wide radar spectra, which is interpreted as turbulent broadening. For typical atmospheric and ionospheric conditions, volume scattering RCS for stationary, homogeneous, isotropic turbulence at 3 m are also in the range 10−18 to 10−16 m−1, in reasonable agreement with measurements. Moreover, theory predicts maximum cross sections around 70 km, also in agreement with observations. Theoretical values are still a matter of order-of-magnitude estimation, since the Bragg scale of 3 m is near or inside the viscous subrange, where the form of the turbulence spectrum is not well known. In addition, steep electron density gradients can increase cross-sections significantly. For thin layers with large RCS and narrow spectra, isotropic turbulence theory fails and scattering or reflection from anisotropic irregularities may gain relevance.Item Restricted Turbulent kinetic energy dissipation rates and eddy diffusivities in the tropical mesosphere using Jicamarca radar data(Elsevier, 2007-06-06) Guo, L.; Lehmacher, G. A.; Kudeki, E.; Akgiray, A.; Sheth, R.; Chau Chong Shing, Jorge LuisThe 50-MHz MST radar at Jicamarca Radar Observatory (JRO) 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 certain number of days each year collecting 1-min Doppler spectra in four off-vertical (2.5°) beam directions and 150 m resolution. The spectral widths have been used to compute the kinetic energy dissipation rate ε due to atmospheric turbulence. A small beam broadening effect has been removed from the observed spectral widths. The daily median energy dissipation rates ε increase from 5 to 30 mW/kg between 67 and 80 km, and the eddy diffusivities increase from 3 to 20 m2/s, consistent with similar studies conducted by two other large 50-MHz radars in Japan and India. The energy dissipation rates are about the same magnitude as the ε estimates for low-latitudes from a global model and are larger than the averages from rocket observations at high-latitudes, confirming previous comparisons.