A study on seasonal and latitudinal variations of Fresnel frequency and drift velocity of amplitude scintillation over Indian sector.

The studies on ionospheric irregularities due to VHF and L band scintillations have great importance because of its application to GPS systems, communication, remote sensing etc. Many researchers are studying the impacts of ionospheric irregularities, their dependence on local time and seasonal aspects to understand the causative mechanisms behind these irregularities. Power spectral studies on scintillation is a very good tool to study the range of scale sizes of irregularities and their relative contribution to the scattered intensity. In the present work, the seasonal and latitudinal variations of power spectral parameter such as Fresnel frequency, drift of scintillation events were studied using the data derived from the receivers at equatorial trough station Trivandrum, near equatorial crest stations Hyderabad and Bangalore, far equatorial off crest stations Delhi and Kolkata during the year 2005. The seasonal and latitudinal variations of amplitude scintillation (S4 index), TEC depletion, were also studied. The data is obtained from receivers at these stations, as part of GAGAN(GPS Aided Geo Augmented Navigation)system. Strong scintillation events were not very frequent during the period of study. Scintillation occurrence is more in the pre midnight hours compared to post midnight hours and also scintillation event is more during equinox compared to other seasons. The latitudinal and range wise distributions of S4 index depict that during equinox most of the scintillations lie within the range 0.16-0.20 for Trivandrum; within 0.46-0.50 for Bangalore; within 0.21-0.25 for Hyderabad, Kolkata and Delhi. It is observed that the Fresnel frequency varied between 0.123Hz to 0.368 Hz during the period of study. The observed drift velocity varied between 33 m/s -138 m/s during the period of study. For stations Trivandrum, Hyderabad and Bangalore the most probable value of drift velocity is within the range 60-80 m/s during equinoctic period. During summer for station at Hyderabad most probable value of drift velocity is within the range 40-60 m/s, shows a decrease during summer. For the station Trivandrum and Kolkata most probable distribution is in within the range 80-100 m/s during summer. Observations at Bangalore shows an equal distribution in the ranges 40-60 m/s, 60-80 m/s and 80-100 m/s during summer. The preferential range of drift velocity falls within the range 60-100 m/s, which agrees with the previous results. In addition to this, the LT distributions of scintillation events were also compared. [ABSTRACT FROM AUTHOR]