different aspects of which have been widely studied by the community. We present here a very complete set of space and ground-based diagnostics that provide the vertical and latitudinal structures of the ionosphere within the South Atlantic magnetic anomaly (SAMA) region and the contiguous parts of South America and Africa. We show that for both storms, the dusk sector corresponding to the universal time (UT) interval between the fast decrease of the SYM-H index and minimum SYM-H value determines uniquely the longitude interval populated by equatorial plasma bubbles and depletions. Further, we find that the UT of these storms is such that the ionospheric density perturbations occur in the SAMA region, which are most extended in latitude and altitude compared with other regions of the globe. In the dusk sector, the eastward penetration electric field, associated with rapid SYM-H decrease, adds to the postsunset eastward E-field because of the F region dynamo, which may be specially enhanced in this longitude interval because of the increased zonal conductivity gradient caused by energetic particle precipitation. This enhanced E-field at dusk causes a rapid uplift of the ionosphere and sets off plasma instabilities to form bubbles or bite-outs. The decreased ion density seen in the Defense Meteorological Satellite Program (DMSP) in situ data at 840 km indicates that the ionospheric plasma has been lifted above the DMSP altitude and transported away from the region by diffusion along magnetic field lines. Plasma bubbles and bite-outs impact satellite communication and navigation systems by introducing scintillations and steep density gradients. This paper corroborates that intense magnetic storms follow the framework, developed by Su. Basu et al. (2001) for moderate storms, that specifies the longitude interval in which such disturbances are most likely to occur.

In this work we report a statistical analysis of the occurrence frequency of medium‐scale traveling ionospheric disturbances (MSTIDs) observed over Cachoeira Paulista (22.7°S, 45.0°W, −13.2° mag lat), Brazil. The optical signatures of the low‐latitude MSTIDs in the southern hemisphere observed in the OI 630.0 nm emission images can be a single dark band structure or alternating dark/light bands aligned in the northeast‐southwest direction and propagating towards northwest. Because this feature these events were also referred as thermospheric dark band structures. The statistical study is based on 28 events of MSTIDs observed during seven years of optical data, obtained during low, medium, and high solar activities, for geomagnetically quiet nights. We find that the occurrence frequency of the MSTIDs presents a maximum during low solar activity, decreasing during medium solar activity with no occurrences during high solar activity. Also, the occurrence rates are greater near the June‐solstice months.

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.

Abstract. We report here on post-midnight uplifts near the magnetic equator. We present observational evidence from digital ionosondes in Brazil, a digisonde in Peru, and other measurements at the Jicamarca Radio Observatory that show that these uplifts occur fairly regularly in the post-midnight period, raising the ionosphere by tens of kilometers in the most mild events and by over a hundred kilometers in the most severe events. We show that in general the uplifts are not the result of a zonal electric field reversal, and demonstrate instead that the uplifts occur as the ionospheric response to a decreasing westward electric field in conjunction with sufficient recombination and plasma flux. The decreasing westward electric field may be caused by a change in the wind system related to the midnight pressure bulge, which is associated with the midnight temperature maximum. In order to agree with observations from Jicamarca and Palmas, Brazil, it is shown that there must exist sufficient horizontal plasma flux associated with the pressure bulge. In addition, we show that the uplifts may be correlated with a secondary maximum in the spread-F occurrence rate in the post-midnight period. The uplifts are strongly seasonally dependent, presumably according to the seasonal dependence of the midnight pressure bulge, which leads to the necessary small westward field in the post-midnight period during certain seasons. We also discuss the enhancement of the uplifts associated with increased geomagnetic activity, which may be related to disturbance dynamo winds. Finally, we show that it is possible using simple numerical techniques to estimate the horizontal plasma flux and the vertical drift velocity from electron density measurements in the post-midnight period.

Ionospheric vertical sounding observations, using a digital ionosonde, are being carried out on a routine basis at Sao Jose dos Campos (23.2°S, 45.9°W; dip latitude 17.6°S, hereafter referred to as SJC), Brazil, located under the southern crest of the equatorial ionization anomaly (EIA), since August 2000. In this paper, we present and discuss the seasonal variation of gravity wave (GW) and traveling ionospheric disturbance (TID) oscillations in the ionospheric F2 layer during high solar activity (HSA, September 2000 to August 2001) and low solar activity (LSA, January 2006 to December 2006) observed at SJC during different levels of geomagnetic activity. The GW/TID signatures in the F2 layer can be seen in the isofrequency lines of virtual height daily variations for six fixed frequencies (3, 4, 5, 6, 7, and 8 MHz) which show quasiperiodic oscillations (crests and valleys). The crests and valleys when seen in close frequencies present a phase difference (i.e., first it is observed at higher frequency then at lower frequency), indicating a downward phase velocity. These quasiperiodic oscillations induced in the virtual heights are divided into three groups as small amplitude (lower than 40 km), medium amplitude (between 40 km and 60 km), and large amplitude (greater than 60 km). The observations show that GWs/TIDs are much more pronounced at F layer heights during HSA than LSA and the large‐amplitude GWs/TIDs are present normally only during HSA.