Observations of daytime F2-layer stratification under the southern crest of the equatorial ionization anomaly region



[1] Ionospheric vertical sounding observations are being carried out at Sao Jose dos Campos (23.2°S, 45.9°W; dip latitude 17.6°S), 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 a year of statistics are presented. The F2-layer stratification and F3-layer were observed between 10:40 and 11:45 UT on 31 December 2000, between 13:30 and 14:30 UT on 1 January 2001, and between 13:15 and 15:15 UT on 11 February 2001. The statistics during September 2000 to August 2001 shows that the F3-layer occurs only for 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 (GWs), which have periods of about 30–60 min, 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 the F3-layer. Because the stratifications are observed during geomagnetically quiet periods, the source of the gravity waves are most likely to be associated with local tropospheric disturbances and not with high-latitude disturbances.