L.M. Lima

Signatures of ultra fast Kelvin waves in the equatorial middle atmosphere and ionosphere

[1]xa0In the equatorial atmosphere, oscillations with periods of 3 to 4 days have been observed in the meteor radar zonal wind at Cariri (7.4°S, 36.5°W), in the ionospheric minimum virtual height hF and the maximum critical frequency foF2 at Fortaleza (3.9°S, 38.4°W), and in the TIMED/SABER satellite temperature data in the stratosphere-mesosphere. Wavelet analyses of these time series reveal that the 3–4-day oscillation was observed for all of these data during the period from March 1 to 11, 2005. From the characteristics of the downward phase propagation (wavelength of ∼40 km), longitudinal and latitudinal extension, we conclude that this oscillation must be a 3.5–day Ultra Fast Kelvin (UFK) wave. This is the first report of clear evidence of propagation of a UFK wave from the stratosphere to the ionosphere. The UFK wave could have an important role in the day-to-day variability of the equatorial ionosphere evening uplift.

Evidence on 2–4 day oscillations of the equatorial ionosphere h′F and mesospheric airglow emissions

[1]xa0Equatorial ionospheric sounding has been carried out at Sao Luis (2.6°S, 44.2°W). The upper mesosphere-lower thermosphere (MLT) airglow OI5577, O2b(0,1) and OH(6,2) emissions and OH rotational temperature have been observed at Cariri airglow observatory (7.4°S, 36.5°W) within a distance of approximately 1000 km to the east of Sao Luis. Both observation sites are located in the equatorial region of South America. Spectral analyses of the ionospheric F-layer bottom height (h′F) and airglow emission intensity reveal that there are quasi 2- and 4-day period oscillations in their temporal variations. This might indicate that planetary scale oscillations, Rossby-gravity waves (or inertial-gravity waves) and Ultra Fast Kelvin waves, are present in the ionosphere. This is the first time that the planetary scale waves in the MLT region and in the ionosphere are discussed by airglow and ionospheric observations, respectively.

Possible influence of ultra-fast Kelvin wave on the equatorial ionosphere evening uplifting

Equatorial 3.5-day ultra-fast Kelvin wave was observed in the MLT zonal wind measured by meteor radar at Cariri (7.4°S, 36.5°W, geomag. 8°S) and in the stratosphere-mesosphere temperature structures from the TIMED/SABER data. The ionospheric F-layer bottom-side virtual height hT’F and the critical frequency foF2 observed at Fortaleza (3.9°S, 38.4°W, geomag. 5°S) also showed similar oscillation structures, indicating an influence of the Kelvin wave in the F region height and modulation of E × B uplifting during the evening period. Consequently the ionospheric spread F onset time was also modulated in the same period, around 4 days.

Atmospheric density and pressure inferred from the meteor diffusion coefficient and airglow O2b temperature in the MLT region

Atmospheric density and pressure in the upper mesosphere-lower thermosphere (MLT) region, around 90 km, are inferred from the meteor trail ambipolar diffusion coefficients, D, and simultaneously observed airglow O2b rotational temperatures. For the present study simultaneous observation data from the meteor radar and SATI imaging spectrometer taken at Shigaraki MU radar observatory (34.9°N, 136.1°E) were used. From the 18 winter nights of data, it is observed that in most of the cases nocturnal variation of the O2 temperature has a good correlation with D at 90 to 92 km. The inferred densities at 90 km showed a negative correlation with temperature variation, suggesting a constant pressure process. The O2 emission intensity shows a good correlation with the temperature, and negative correlation with the density variation. The OH rotational temperature and D at 87 km also showed similar results to the case of the O2 temperature.