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Observations of daytime f2-layer stratification under the southern crest of the equatorial ionization anomaly region


Ionospheric vertical sounding observations are being carried out at Sao Jose dos Campos (23.2o S, 45.9o W; dip latitude 17.6oS), Brazil, under the southern crest of the equatorial ionization anomaly (EIA), since August 2000. In this paper, we present and discuss the observations of daytime F2 layer stratification near the crest of EIA, for the first time, under magnetically quiet high solar activity conditions. Three examples and an year of statistics are presented. The F2 layer stratification and F3 layer were observed between 10:40-11:45 UT on 31 December 2000, between 13:30-14:30 UT on 01 January 2001, and between 13:15-15:15 UT on 11 February 2001. The statistics during September 2000-August 2001 shows that the F3 layer occurs only on 66 days (18% occurrence), and it occurs only during September-February (spring-summer), with maximum occurrence in September-October and longest duration in February. The F2 layer stratification seems to be associated with gravity waves (GW), which have periods of about 30-60 minutes, downward phase velocities of about 60-140 m/s, and vertical wavelengths of about 200-500 km. The presence of powerful gravity waves in a vertically extended F-layer seems to stratify the F2 layer and produce F3 layer. Since the stratifications are observed during geomagnetically quiet periods, the source of the gravity waves are most likely to be associated with local troposphereical disturbances and not with high latitude disturbances. (AU)

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