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Abstract. The need to nowcast and forecast scintillation for the support of operational systems has been recently identified by the interagency National Space Weather Program. This issue is addressed in the present paper in the context of nighttime irregularities in the equatorial ionosphere that cause intense amplitude and phase scintillations of satellite signals in the VHF/UHF range of frequencies and impact satellite communication, Global Positioning System navigation, and radar systems. Multistation and multifrequency satellite scintillation observations have been used to show that even though equatorial scintillations vary in accordance with the solar cycle, the extreme day-to-day variability of unknown origin modulates the scintillation occurrence during all phases of the solar cycle. It is shown that although equatorial scintillation events often show correlation with magnetic activity, the major component of scintillation is observed during magnetically quiet periods. In view of the day-to-day variability of the occurrence and intensity of scintillating regions, their latitude extent, and their zonal motion, a regional specification and shortterm forecast system based on real-time measurements has been developed. This system, named the Scintillation Network Decision Aid, consists of two latitudinally dispersed stations, each of which uses spaced antenna scintillation receiving systems to monitor 250-MHz transmissions from two longitudinally separated geostationary satellites. The scintillation index and zonal irregularity drift are processed on-line and are retrieved by a remote operator on the Internet. At the operator terminal the data are combined with an empirical plasma bubble model to generate three-dimensional maps of irregularity structures and two-dimensional outage maps for the region.

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.