Search for: All records

Abstract. Medium-scale gravity waves (MSGWs) are atmospheric waves with horizontal scales ranging from 50 to 1000 km that can be observed through airglow all-sky images. This research introduces a novel algorithm that automatically identifies MSGWs using the keogram technique to study the waves over the Antarctic Peninsula. MSGWs were observed with an all-sky airglow imager located at the Brazilian Comandante Ferraz Antarctic Station (CF, 62° S), near the tip of the Antarctic Peninsula. Several preprocessing techniques are necessary to extract the parameters of MSGWs from the airglow images. These include projecting the images into geographical coordinates, applying a flat-field correction, performing consecutive image subtraction, and employing a Butterworth filter to enhance the visibility of the MSGWs. Additionally, a wavelet transform is used to identify the primary oscillations of the MSGWs in the keograms. Subsequently, a wavelet transform is also used to reconstruct the MSGWs and obtain the fitting coefficients of phase lines. The fitting coefficients are then used to calculate the MSGW parameters and assess the quality of the results. Simulations with synthetic images containing typical propagating gravity waves were conducted to evaluate the errors generated during the MSGW calculations and to determine the threshold for the fitting parameters. This methodology processed a year's worth of data in less than 1 h, successfully identifying most waves with errors lower than 5 %. The observed wave parameters are generally consistent with expected results; however, they show differences from other observation sites, exhibiting larger phase speeds and wavelengths. 

We present in this work a method for estimation of equatorial plasma bubble (EPB) mean zonal drift velocities using keograms generated from images of the OI 6300.0 nm nightglow emission collected from an equatorial station–Cariri (7.4° S, 36.5° W), and a mid-latitude station–Cachoeira Paulista (22.7° S, 45° W), both in the Brazilian sector. The mean zonal drift velocities were estimated for 239 events recorded from 2000 to 2003 in Cariri, and for 56 events recorded over Cachoeira Paulista from 1998 to 2000. It was found that EPB zonal drift velocities are smaller (≈60 ms−1) for events occurring later in the night compared to those occurring earlier (≈150 ms−1). The decreasing rate of the zonal drift velocity is ≈10 ms−1/h. We have also found that, in general, bubble events appearing first in the west-most region of the keograms are faster than those appearing first in the east-most region. Larger zonal drift velocities occur from 19 to 23 LT in a longitude range from −37° to −33°, which shows that the keogram method can be used to describe vertical gradients in the thermospheric wind, assuming that the EPBs drift eastward with the zonal wind. The method of velocity estimation using keograms compares favorably against the mosaic method developed by Arruda, D.C.S, 2005, but the standard deviation of the residuals for the zonal drift velocities from the two methods is not small (≈15 ms−1). 

Amplitude growth rates of quasi-monochromatic gravity waves were estimated and compared from multiple instrument measurements carried out in Brazil. Gravity wave parameters, such as the wave amplitude and growth rate in distinct altitudes, were derived from sodium lidar density and nightglow all-sky images. Lidar observations were carried out in São Jose dos Campos (23 ∘ S, 46 ∘ W) from 1994 to 2004, while all-sky imagery of multiple airglow layers was conducted in Cachoeira Paulista (23 ∘ S, 45 ∘ W) from 1999–2000 and 2004–2005. We have found that most of the measured amplitude growth rates indicate dissipative behavior for gravity waves identified in both lidar profiles and airglow image datasets. Only a small fraction of the observed wave events (4% imager; 9% lidar) are nondissipative (freely propagating waves). Our findings also show that imager waves are strongly dissipated within the mesosphere and lower thermosphere region (MLT), decaying in amplitude in short distances (<12 km), while lidar waves tend to maintain a constant amplitude within that region. Part of the observed waves (16% imager; 36% lidar) showed unchanging amplitude with altitude (saturated waves). About 51.6% of the imager waves present strong attenuation (overdamped waves) in contrast with 9% of lidar waves. The general saturated or damped behavior is consistent with diffusive filtering processes imposing limits to amplitude growth rates of the observed gravity waves.