Browsing by Author "Lehmacher, G. A."
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Item Restricted 3D Imaging of the OH mesospheric emissive layer(Elsevier, 2010-01-15) Kouahla, M. N.; Moreels, G.; Faivre, M.; Clairemidi, J.; Meriwether, J. W.; Lehmacher, G. A.; Vidal Safor, Erick; Veliz, OscarA new and original stereo imaging method is introduced to measure the altitude of the OH nightglow layer and provide a 3D perspective map of the altitude of the layer centroid. Near-IR photographs of the OH layer are taken at two sites separated by a 645 km distance. Each photograph is processed in order to provide a satellite view of the layer. When superposed, the two views present a common diamond-shaped area. Pairs of matched points that correspond to a physical emissive point in the common area are identified in calculating a normalized cross-correlation coefficient (NCC). This method is suitable for obtaining 3D representations in the case of low-contrast objects. An observational campaign was conducted in July 2006 in Peru. The images were taken simultaneously at Cerro Cosmos (12 09 08.2 S, 75 33 49.3 W, altitude 4630 m) close to Huancayo and Cerro Verde Tellolo (16 33 17.6 S, 71 39 59.4 W, altitude 2272 m) close to Arequipa. 3D maps of the layer surface were retrieved and compared with pseudo-relief intensity maps of the same region. The mean altitude of the emission barycenter is located at 86.3 km on July 26. Comparable relief wavy features appear in the 3D and intensity maps. It is shown that the vertical amplitude of the wave system varies as exp (z/2H) within the altitude range z = 83.5–88.0 km, H being the scale height. The oscillatory kinetic energy at the altitude of the OH layer is comprised between 3 × 10−4 and 5.4 × 10−4 J/m3, which is 2–3 times smaller than the values derived from partial radio wave at 52N latitude.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 Restricted Near-infrared sky background fluctuations at mid- and low latitudes(Springer, 2008-10) Moreels, G.; Clairemidi, J.; Faivre, M.; Pautet, D.; Rubio Da Costa, F.; Rousselot, P.; Meriwether, J. W.; Lehmacher, G. A.; Vidal Safor, Erick; Chau Chong Shing, Jorge Luis; Monnet, G.The emission of the upper atmosphere introduces an additional variable component into observations of astronomical objects in the NIR 700–3,000 nm range. The subtraction of this component is not easy because it varies during the night by as much as 100% and it is not homogeneous over the sky. A program aimed at measuring and understanding the main characteristics of the atmospheric NIR emission was undertaken. A 512 × 512 CCD camera equipped with a RG780/2 mm filter is used to obtain images of the sky in a 36° × 36° field of view. The intensities of a given star and of the nearby region devoid of star in a 439 arcmin2 area are monitored during periods of time of several hours. The sky intensity measured in the 754–900 nm bandpass, reduced to zenith and zero airmass is comprised between mag20 and mag18.5 per arcsecond2. A diminution by a factor of two during the night is frequently observed. Intensity fluctuations having an amplitude of 15% and periods of 5–40 min are present in the images with a structure of regularly spaced stripes. The fluctuations of the NIR sky background intensity are due to (1) the chemical evolution of the upper atmosphere composition during the night and (2) dynamical processes such as tides with periods of 3–6 h or gravity waves with periods of several tens of minutes. We suggest that a monitoring of the sky background intensity could be set up when quantitative observations of astronomical objects require exposure times longer than ~10 min. The publication is illustrated with several video films accessible on the web site http://www.obs-besancon.fr/nirsky/. Enter username: nirsky and password: skynir.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 Stereoscopic imaging of the hydroxyl emissive layer at low latitudes(Elsevier, 2008-05-04) Moreels, G.; Clairemidi, J.; Faivre, M.; Mougin-Sisini, D.; Kouahla, M. N.; Meriwether, J. W.; Lehmacher, G. A.; Vidal Safor, Erick; Veliz, OscarThe hydroxyl nightglow layer is an excellent tracer of the dynamical processes occurring within the mesosphere. A new stereo-imaging method is applied that not only measures the altitude of the airglow layer but also provides a three-dimensional map of the OH-layer centroid heights. A campaign was conducted in July 2006 in Peru to obtain NIR images of the OH nightglow layer which were simultaneously taken for two sites separated by 645 km: Cerro Cosmos (12°09′08.2″S, 75°33′49.3″W, altitude 4630 m) and Cerro Verde Tellolo (16°33′17.6″S, 71°39′59.4″W, altitude 2330 m). Data represented by pairs of images obtained during the nights of July 26–27 and 28–29 are analyzed to yield satellite-type views of the wave field. These are obtained by application of an inversion algorithm. In calculating the normalized cross-correlation parameter for the intensity, three-dimensional maps of the OH nightglow layer surface are retrieved. The mean altitude of the emission profile barycenter is found to be at 87.1 km on July 26 and 89.5 km on July 28. In these two cases the horizontal wavelengths determined are 21.1 and 24.6 km with periods of 18 and 34 min, respectively. A panoramic view of the OH nightglow emission obtained on July 29 at 8 h51–9 h26 UT is presented, in which the overall direction of the waves is found to be N–NW to S–SE, azimuth 150°–330° (counted from South). The wave kinetic energy density at the OH nightglow layer altitude is 3.9×10−4 W/kg, which is comparable to the values derived from partial reflection radiowave data.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.Item Restricted Variability of equatorial mesospheric echoes(Elsevier, 2003-12-18) Lehmacher, G. A.; Kudeki, E.Selected results from recent mesospheric campaigns with the 50-MHz Jicamarca radar in August 1998, December 1999 and January 2000 are reported. The data were acquired with 300 and 450 m rangemresolution and processed with 1 minute time resolution. The focus of this study is on signal-to-noise ratio and its variability on short and long time scales. System changes such as transmitter power were monitored using the signal level near the tropopause. We found that the daily mesopheric signal and its variability increase with solar activity monitored by the sunspot number. Several short-term enhancements of signal power could be linked to solar x-ray flares. Both observations agree with the basic assumption that high electron densities and strong density gradients enhance the illumination of mesospheric turbulence. Daily recurring and descending layers in August is evidence for strong tidal modulation of the dynamic processes that cause the echoes. The good range and time resolution allows to identify many complex structures within the echo layers, including braids and cat's eyes that may be evidence for dynamical instabilities in the mesosphere.