[1] We report on the climatology of equatorial thermospheric winds and temperatures based on Fabry-Perot interferometer measurements of Doppler shifts and Doppler broadenings of the 630.0 nm spectral emission from the Cajazeiras observatory located in the northeastern part of Brazil (6.89°S, 38.56°W). These results apply to the lower thermosphere region near 240 km and were obtained during a period of weak solar activity with the solar flux typically within the range of 72 and 82 solar flux units. Examination of the monthly averaged meridional thermospheric winds for 1 year of measurement from October 2009 to September 2010 found the wind direction to be equatorward during summer months throughout the early evening hours with maximum speeds reaching ∼65 ms −1 . During winter months, the early evening meridional wind direction reversed to poleward with similar speeds. This result is attributed to the cross-hemispheric flow from the summer to winter hemisphere. Superimposed upon this wintertime meridional wind flow was an equatorward surge shortly before midnight. This surge shifted to earlier local times in the transition from the vernal equinox to summer and to later local times between the summer solstice and the autumnal equinox; this flow feature is attributed to tidal wave forcing. The temperature observations exhibited the expected behavior, with the midnight temperature maximum showing a greater amplitude, ∼120 K, in the vernal equinox with somewhat weaker amplitudes, ∼75 K, seen at earlier local times during the summer. Also observed was a phase lag of 60 to 90 min between the appearance of the equatorward meridional wind flow and occurrence of the midnight temperature maximum peak.

We describe an analysis procedure for estimating the thermospheric winds and temperatures from the multi-order two-dimensional (2D) interferograms produced by an imaging Fabry-Perot interferometer (FPI) as imaged by a CCD detector. We also present a forward model describing the 2D interferograms. To investigate the robustness and accuracy of the analysis, we perform several Monte Carlo simulations using this forward model for an FPI that has recently been developed and deployed to northeastern Brazil. The first simulation shows that a slight cross-contamination at high temperatures exists between neighboring orders in the interferogram, introducing a bias in the estimated temperatures and increasing errors in both the estimated temperatures and winds when each order is analyzed in full. The second simulation investigates how using less than an entire order in the analysis reduces the cross contamination observed in the first set of simulations, improving the accuracy of the estimated temperatures. The last simulation investigates the effect of the signal-to-noise ratio on the errors in the estimated parameters. It is shown that, for the specific FPI simulated in this study, a signal-to-noise ratio of 1.5 is required to obtain thermospheric wind errors of 5 m/s and temperature errors of 20 K.

Previous theoretical models showed that, in the middle geomagnetic latitudes the oscillations in the Doppler frequency shift lag and lead the H component of ULF pulsations field on the ground, owing to the advection and compression mechanisms by 90°, respectively. On March 24, 1991, measurements obtained from a CW-HF Doppler sounding system and a fluxgate magnetometer show phase differences of 15°-77° between the Doppler frequency shift oscillations and the Hcomponent of ULF pulsations field at ground level, which indicates that the Doppler velocity arises from predominant changes due to the compression mechanism.

We present an evaluation of global‐scale equatorial and low latitude ionospheric disturbances in response to the weak‐to‐moderate disturbed conditions that marked the SUNDIAL/ATLAS 1 twelve‐day campaign of March 22 to April 2, 1992. Ionosonde data from South American and Indian‐Asian longitudes are analyzed to examine first the climatological (the average) pattern of the critical F region parameters (the layer peak density and height) in comparison with the empirical International Reference Ionospheric model, and then to characterize the day‐to‐day variabilities aiming at identifying potential causal contributions from either disturbance zonal and meridional winds or magnetospheric and disturbance dynamo electric fields. Included in this analysis are data from South American midlatitude locations which are used to determine meridional winds using an adaptation of the servo analysis technique in the Field Line Integrated Plasma (FLIP) model. We have made an assessment of the causal mechanism of the day‐to‐day variabilities as arising from latitude dependent disturbance meridional winds, and from electric fields produced by disturbance zonal winds and disturbance dynamo. While the contribution from disturbance meridional winds decreases from middle to equatorial latitudes, that of the electric fields maximizes around the equator. In particular, first‐time evidence based on ionosonde data is provided for a disturbance dynamo electric field in the equatorial ionosphere. It is found that there are two time intervals of maximum ionospheric variability resulting from the weak to moderate magnetospheric disturbance conditions that prevailed during the campaign: one near the evening/postsunset hours and the other in the postmidnight‐sunrise hours over low latitudes. At midlatitudes, a broad maximum of the response occurs from premidnight to morning hours. We provide a comparison of results for the South American and Indian‐Asian longitudes and a discussion of the competing roles of the disturbance zonal and meridional winds, and magnetospheric and disturbance dynamo electric fields as a function of latitude.

The propagation of acoustic gravity waves has been observed by an investigation of their effects upon the ionosphere over distances ranging up to 3400 km and extending from mid-latitudes (45.4øN) to low latitudes (14.7øN) in east Asia. Recognizable wave structures were observed mainly on low sounding frequency, virtual height hf and to a lesser extent foF2 time variations, obtained from ionograms taken at 10 ionosonde stations, sounding at 5-min intervals throughout the period October 15-19, 1985. Investigations were mainly confined to the nighttimes, since during the day large latitudinal gradients of ionization (associated mainly with the equatorial anomaly) together with movements of the equatorial anomaly crest obscured the observation of waves. In general, inferred propagation of the waves was southward, but three cases of northward propagation were observed, the former having a range of periods 40-210 min, while the latter were consistently about 70 min. The waves structures, mostly, differed depending upon whether they were located to the north or south of Yamagawa (31.2øN). There was some evidence of the predomination of longer-period waves to the south, the shorter-period waves observed to the north being apparently damped out in their propagation southward. The southward propagating waves exhibited upward energy propagation, while at Hong Kong (22.3øN) there was some evidence of downward phase propagation with a shift to lower periods at lower heights. It has not been possible to identify the origins of the detected waves, but it is clear that sources existed within and outside the latitude range covered by the ionosonde sounding stations (14.7ø-45.4øN). munications, Tokyo. '•Telecommunication Training Institute, Taipei. •Manila Observatory. larly prevalent during magnetic storms (see review by Hunsucker [1982]). The so-called "large-scale" AGWs of period 30 m•n to 3 hours have been observed to propagate equatorward over distances of several thousand kilometers, giving rise to periodic ionospheric variations differing in phase at meridional, spaced stations [e.g., Morgan, 1983]. The "medium-scale" AGWs of period 15 min to 1 hour, though, seem to propagate over much shorter distances (less than 1000 km) and may have as their sources meteorological disturbances I-Waldock and Jones, 1986]. Previous investigations of AGWs have been mostly confined to mid and high latitudes. Recently, the detected wave propagation characteristics have been obtained from simultaneous measurements at closely separated stations, for example, using ionosonde sounding at 150 km spacing [Morgan, 1983; Tedd and Morgan, 1985] and using HF Doppler at 80 km spacing [Waldock and Jones, 1987]. Traveling ionospheric disturbances (TIDs) have been detected, moving over long distances (3000 km), from recognizable "kinks" in ionograms [e.g., Heisler, 1963]. Although such disturbances may have been associated 867 868 WALKER ET AL.' IONOSPHERIC WAVES IN EAST ASIA WALKER ET AL.: IONOSPHERIC WAVES IN EAST ASIA 869