HF DOPPLER AND IONOSONDE OBSERVATIONS ON THE ONSET CONDITIONS OF EQUATORIAL SPREAD F

The relative importance of height, vertical drift velocity, and electr on density gradient of the postsunset bottomside (5.5 MHz) equatorial F region for the onset of spread F is studied using simultaneous HF Do ppler radar and ionosonide observations. The study conducted for the p eriods January-March of 1984 and 1985 shows that the height of the F l ayer, determined by the time history of the prereversal enhancement of the drift velocity, is the deciding factor for the onset of equatoria l spread F (ESF) with little contribution from the electron density gr adient. Maximum growth rate of linear collisional Rayleigh-Taylor inst ability occurs at the time of peak height rather than at the time of p eak velocity confirming that, for the onset of ESF, the layer should a ttain a threshold height. The threshold (group) height of the 5.5 MHz layer falls from approximately 450 km in 1984 (mean F10.7 equals 120) to approximately 350 km in 1985 (mean F10.7 equals 70); the correspond ing evening peak upward drift velocities decrease from about 30 m s-1 in 1984 to about 20 m s-1 in 1985. The significant fall of the thresho lds with the declining solar activity is due to the decrease in the io n-neutral collision frequency with declining solar activity; the fall of the thresholds is reflected in large decreases in the intensity and duration of the spread F.