[1] Vertical coupling in the low-latitude atmosphere-ionosphere system driven by the 2-day wave in the tropical MLT region has been investigated. The problem is studied from an observational point of view. Three different types of data were analyzed in order to detect and extract the 2-day wave signals. The 2-day wave event during the period from 1 December 2002 to 28 February 2003 was identified in the neutral winds by radar measurements located at four tropical stations. The 2-day variations in the ionospheric electric currents (registered by perturbations in the geomagnetic field) and in the F-region electron densities were detected in the data from 23 magnetometer and seven ionosonde stations situated at low latitudes. Two features for each kind of wave were investigated in detail: the variation with time of the wave amplitude and the zonal wave number. The results show that the westward propagating global 2-day wave with zonal wave number 2 seen in the ionospheric currents and in F-region plasma is forced by the simultaneous 2-day wave activity in the MLT region. The main forcing agent in this atmosphere-ionosphere coupling seems to be the modulated tides, particularly the semidiurnal tide. This tide has a larger vertical wavelength than the diurnal tide and propagates well into the thermosphere. The parameter that appears to be affected, and thus drives the observed 2-day wave response of the ionosphere, is the dynamo electric field.

The mesospheric sodium data, obtained between 1975 and 1987 at São José dos Campos (23°S, 46°W) with a laser radar, have been analyzed in order to identify the appearance of thin sporadic sodium layers. In this search, a total of 65 events were identified. The average height of the peaks is 95.0 km. The ratio of the maximum peak density to the average layer density is normally 2.5 to 3.0, but values as high as 7 have been observed in the most outstanding cases. The events last from a few minutes to several hours, although durations of 1–2 hours are more typical. The events occur more often during periods of large meteor showers, especially in August. The diurnal variation shows an increasing number of observed peaks from 1500 LT to midnight and remains almost constant from midnight to 0600 LT. In 52 out of 54 days for which sodium and ionosonde data are available there was a sporadic E layer nearly coincident with the sodium cloud. The coincidence is good for short‐lived sporadic layers, but a substantial increase in sporadic E critical and blanketing frequencies normally precedes the long‐lasting and broader ones. These results are compatible with the suggestion that the enhanced layers are produced by the wind shear distortion of sodium clouds originating in meteor deposition, but we cannot rule out the possibility of an ion conversion mechanism.

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Calculations using the Sheeld University plasmasphere ionosphere model have shown that under certain conditions an additional layer can form in the low latitude topside ionosphere. This layer (the F3 layer) has subsequently been observed in ionograms recorded at Fortaleza in Brazil. It has not been observed in ionograms recorded at the neighbouring station SaÄ o Luis. Model calculations have shown that the F3 layer is most likely to form in summer at Fortaleza due to a combination of the neutral wind and the E Â B drift acting to raise the plasma. At the location of SaÄ o Luis, almost on the geomagnetic equator, the neutral wind has a smaller vertical component so the F3 layer does not form.

Equatorial ionospheric sounding has been carried out at São Luís (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 São Luís. 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.