HF Doppler radar observations of low‐latitude spread F

[1] HF (5.5 MHz) Doppler radar observations of nonspread F and spread F echoes over Visakhapatnam (17.7°N, 83.3°E; dip 20°) are presented. The echoes appearing suddenly and nearly simultaneously in 16 successive range bins at 7.5 km intervals in association with spread F have been investigated. Two to five episodes of spread F activity were found to appear at intervals of 1–2 hours during individual nights. At the time of onset of spread F conditions, the Doppler velocity for each range bin changed rapidly from a negative maximum to a positive maximum followed by a gradual decrease to a steady ±10–15 m/s or to a large negative velocity and then again to a large positive. At the time of small constant velocity or velocity change from negative to positive, the spread F echoes were weak or even below the detection level of the radar. This disappearance in the higher ranges causes the decrease in range extent of spread F echoes. The positive and negative maximum velocities of spread F were in the range of +70 to −60 m/s. The maximum upward and downward velocity is not the same in all events of spread F activity. The width of the Doppler velocity spectrum for spread F echoes was found to vary with velocity. For zero velocity the width was a minimum of 50 m/s in contrast to 25 m/s for nonspread F events. These features were consistently observed for all spread F incidences. The observed results are compared with already reported HF/VHF observations and are discussed in the light of equatorial plasma dynamics during the growth phase of Rayleigh Taylor instability leading the incidence of spread F.