Abstract
The equinox transition (ET) in the thermosphere is analyzed on the basis of the hmF2 parameter for two southern hemisphere mid‐latitude ionosonde stations with 152° longitudinal separation (Buenos Aires and Canberra). The period under study covers August 1 to October 31, 1984, thus including the Equinox Transition Study core period. Such a sample is necessary for the correct interpretation of long‐lasting trends. ET features are observed at both locations, but important longitudinal differences are evident. Harmonic analysis of the data, performed to separate 24‐, 12‐, and 8‐hour oscillations shows reasonable agreement with meridional tidal wind models, particularly with respect to phases. Results indicate that the ET is a gradual phenomenon not restricted to the core period but starting before it. With respect to the longitudinal differences, the most prominent one is the difference in the amplitude of the 12‐hour oscillation, which is of greater magnitude over Buenos Aires than Canberra by at least a factor of 3. The 8‐hour oscillation at both locations shows a progressive increase in amplitude fluctuations after the core period with a poor correlation between sites, thus suggesting an important contribution due to local origin.
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