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.

Data from the Midcourse Space Experiment Tilere is evidence (R. H. Picard, J,'unes H. Brown (MSX) has provided the first observations of thunderstorm-personal communications, 1997) that gravity wave generated gravity waves imaged from space. Gravity wave structures are present in a number of MSX MWIR images. theory predicts that isolated, sufficiently convective This evidence is based on morphology, length scales, thunderstorms can launch waves mid create a unique power specUa and MWIR mdimive properties. The present intensity pattern of concentric circles on a radiating sinface paper shows that in particular cases the sources of these of constant altitude above such a storm. banong the MSX waves can be established as thunderstorms. This source constant-nadir-angle mid-wave hlfmred (MWIR) identification was prompted by the work of Taylor anti observations, two instances of such patterns have been Hapgood [1988] who showed that a pattern of concentric identified. It was conf'mned frown •neteorological satellite circles in ground-based observations of nightglow images that highly convective isolated thunderstorms emissions in the mesopause region was caused by va• occurred at the locations and ti•nes expected. isolated thunderstorm that occurred six hours prior to the nightglow observation.

[1] Long-term thermospheric neutral density trends near 400 km altitude are analyzed using high accuracy satellite drag measurements over the common time period 1970 -2000. Data coverage is over all latitudes and local times and an extensive range of solar and geomagnetic conditions. Densities are compared to empirical models that remove known variations related to solar activity, latitude, local time, day of year and altitude. An average unmodeled secular neutral density decrease of 1.7% per decade is detected. This result is qualitatively consistent with predictions of thermospheric cooling related to anthropogenic causes deduced by theoretical models, and in general agreement with global cooling estimates determined from previous analyses of satellite orbital decay.

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