VHF radio scintillations at Rajkot-A station near the crest of the equatorial anomaly in India

Abstract

Amplitude scintillations of FLEETSAT signals at 244 MHz at Rajkot for a period of increasing solar activity during 1987-89 are used to bring out, for the first time, the salient features of scintillation occurrence, scintillation depth, and rate of fluctuation of amplitude near the crest of the Appleton anomaly in the Indian zone. Scntillation occurs predominantly in the pre-midnight period during equinox and winter months while in summer it occurs predominantly in post-midnight period. Scintillation occurrence increases with solar activity in winter and equinox but decreases during summer. Scintillation is suppressed by magnetic disturbances during winter and equinox but the trend is reversed in summer. It is correlated with range type Spread-F in pre-midnight and frequency type Spread-F in post-midnight. Scintillation is generally strong (> 10 dB) in winter and equinox while in summer it is weak (< 5 dB). Scintillation is fast (> 10 fades per min) during pre-midnight and slow (< 5 fades per min) during post midnight especially in summer. Daytime scintillation is generally rare and weak, and is associated with sporadic-E, particularly during low sunspot summer. Certain types of periodic amplitude fluctuations also occur predominantly during daytime in low sunspot summer months. The observed features of nighttime scintillation are consistent with F-region vertical drift measurements at the equator (Trivandrum) and support the view that equinox and winter scintillations at Rajkot are of equatorial origin while the summer events may not be. Strong (> 10dB) and slow (< 1 fade/min) scintillation occurring in the post-midnight period and continuing into the next day is observed as a new feature needing further study.