Browsing by Author "Meriwether, J. W."
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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 Open Access Correlative study of neutral winds and scintillation drifts measured near the magnetic equator(American Geophysical Union, 2002-07-18) Valladares, C. E.; Meriwether, J. W.; Sheehan, R.; Biondi, M. A.Measurements of the thermospheric neutral wind at Arequipa, Peru, and observations of the drift of the irregularities at Ancon, Peru, are used to study the coupling that exists between ions and neutrals at equatorial latitudes and the variability of this coupling as a function of the occurrence of scintillations. This study is based on data collected at the Arequipa and Ancon stations between 1996 and 1998. Our comparative analysis indicates that the relative wind-drift values vary depending on season and the solar flux level. We found that during the equinoxes and low solar flux values, the averaged zonal drift is larger than the wind by 15 m s−1, but for solar flux values above 130 units, the average wind exceeds the drift values by 10 to 20 m s−1. We suggest that the occurrence of larger equinoctial drifts can be explained by the existence of altitude gradients in the zonal wind during that season. During the June solstice the zonal wind seems to exceed the irregularity drift by ∼10–20 m s−1 independent of the solar flux. We also find that the meridional wind shows a modest dependence on the scintillation activity during the June solstice. During scintillation events and between 2000 and 2400 LT the averaged meridional wind observed to the south and north of Arequipa exceeds their corresponding no-scintillation values by 20 m s−1. We present likely explanations of this effect.Item Open Access Data‐driven numerical simulations of equatorial spread F in the Peruvian sector: 2. Autumnal equinox(American Geophysical Union, 2014-08-11) Hysell, D. L.; Milla, Marco; Condori, L.; Meriwether, J. W.An ongoing effort to simulate plasma instability in the equatorial ionosphere leading to equatorial spread F (ESF) in the American sector is described. Ionospheric state parameters including plasma number density and vector drift velocity profiles were measured at the Jicamarca Radio Observatory in the period between 20 September and 3 October 2013. Coherent radar backscatter from plasma irregularities was recorded simultaneously, and images of the irregularities were calculated using aperture synthesis methods. Neutral winds were measured by the red line Fabry‐Perot interferometers at Jicamarca and Arequipa, Peru. A fully 3‐D numerical simulation of ionospheric irregularities, initialized and forced using parameterizations derived from measurements and empirical models, was used to reproduce the ESF activity observed. Simulations were able to recover many of the features of the irregularities, although some important anomalies can be noted. ESF events in which the first appearance of radar plumes occurred either very early or very late were not reproduced in simulation and may be indicative of nonlocal influence.Item Open Access Instrumentation and science at Jicamarca and LISN for CAWSES-II TG4(Instituto Geofísico del Perú, 2010) Chau Chong Shing, Jorge Luis; Hysell, D. L.; Valladares, C. E.; Meriwether, J. W.Diapositivas presentadas en el 2010 CEDAR Workshop, University of Colorado, Boulder, CO, 20-25 June 2010.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 Pasado, presente y futuro de estudios de aeronomía usando FPI en Perú(Instituto Geofísico del Perú, 2010) Navarro, L.; Veliz, Oscar; Chau Chong Shing, Jorge Luis; Meriwether, J. W.Diapositivas presentadas en el XIII Encuentro Científico Internacional, (ECI 2010i), realizado del 2 al 6 de agosto de 2010 en la ciudad de Lima.Item Open Access Simultaneous measurements of thermospheric winds and ion drifts at Jicamarca and Arequipa, Peru(Instituto Geofísico del Perú, 2010) Navarro, L.; Veliz, Oscar; Chau Chong Shing, Jorge Luis; Meriwether, J. W.Ponencia presentada en el 2010 CEDAR Workshop, University of Colorado, Boulder, CO, 20-25 June 2010.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.