Day-to-day variability of equatorial and low latitude F-region ionosphere in the Indian zone

The F-region variability of the ionosphere during high solar activity periods have been studied using different hourly foF2 data from Indian stations, Thiruvananthapuram (8.5 degrees N, 76.8 degrees E), Kodaikanal (10.2 degrees N, 77.5 degrees E) near the Dip equator and Delhi (28.6 degrees N, 77.2 degrees E) located north of the equatorial ionization anomaly (EIA). During the study three types of changes have been observed, viz., single day abnormality, alternate day periodic fluctuations and long-term (annual/seasonal) periodic variations in foF2 values. The magnitude of the variability depends on the latitude of the observing station and it has been found that the day-to-day changes in the foF2 values are not always correlated with the corresponding change in solar and magnetic activities. The short as well as the long-term variations in the critical frequency of the F layer are found to be mainly controlled by the equatorial electrojet strength (EEJ). Some short-term changes are also identified due to the variation in magnetic activity. It has been noticed that foF2 values over Delhi shows a direct (positive) statistically significant correlation with the EEJ whereas equatorial stations generally have negative correlation coefficients which is much lower as comparison to Delhi. Study suggests that for equatorial and low latitude location electrojet strength is a useful parameter for the prediction of day-to day changes in the ionosphere as well as latitudinal distribution of plasma. The predicted electron density for a particular day, few hours advanced, can be further used for various application of radio communication. (C) 2006 Elsevier Ltd. All rights reserved.