Forecasting the occurrence of ionospheric scintillation activity in the equatorial ionosphere on a day-to-day basis

In the low-latitude ionospheric F region, ambient conditions after sunset are often conducive to the generation of small-scale plasma density structures by the Rayleigh Taylor instability mechanism. Radio wave signals passing through these electron density irregularities experience amplitude fluctuations that lead to radio wave "scintillation". The scintillation index, S4, is a measure of the severity of these amplitude fluctuations. It is well known that the occurrence frequency of scintillation activity depends on solar cycle, season, longitude and local time, with a large day-to-day variability. In this paper, we present the results of a continuing study that utilizes a ground-based digital sounder at the magnetic equator to provide a "forecast" of the subsequent occurrence of scintillation activity and a network of UHF and L-band receivers that measure the S4 index, on a night-to-night basis. To establish the "forecasting" relationship, observations were carried out in 1998 and 1999 in the Peruvian/Chilean longitude sector. The relationship between the magnitude of the vertical E x B drift velocity enhancement just after sunset as determined by the Jicamarca Digisonde and the magnitude of the S4 index subsequently observed an hour or two later by the scintillation receivers at Ancon, Peru and Antofagasta, Chile was established. In most cases at both UHF and L-band frequencies, this relationship is not linear, but there appears to be a "threshold" in E x B drift of similar to 20 m/s. On nights when E x B drift was less than 20 m/s, the UHF S4 index generally was less than 0.5, while on nights when E x B drift exceeded 20 m/s, the S4 index was greater than 0.5. The day-to-day variability of this relationship will be presented and discussed. Since October 1, 2001, the Jicamarca Digisonde has been used to "forecast" the occurrence of UHF S4 > 0.5 or S4 < 0.5 on a night-to-night basis. The validity of the "forecast" is subsequently confirmed by UHF S4 observations at Ancon, Peru and Antofagasta. Chile. The resulting statistics for these nightly S4 forecasts from November 1, 2001 to April 30, 2002 will be presented and discussed. (C) 2004 Elsevier Ltd. All rights reserved.