HF doppler and ionosonde observations on the onset conditions of equatorial spread F

Jayachandran, B. ; Balan, N. ; Rao, P. B. ; Sastri, J. H. ; Bailey, G. J. (1993) HF doppler and ionosonde observations on the onset conditions of equatorial spread F Journal of Geophysical Research, 98 (A8). pp. 13741-13750. ISSN 0148-0227

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Official URL: http://www.agu.org/pubs/crossref/1993.../93JA00302...

Related URL: http://dx.doi.org/10.1029/93JA00302


The relative importance of height, vertical drift velocity, and electron density gradient of the postsunset bottomside (5.5 MHz) equatorial F region for the onset of spread F is studied using simultaneous HF Doppler radar and ionosonde observations. The study conducted for the periods January-March of 1984 and 1985 shows that the height of the F layer, determined by the time history of the prereversal enhancement of the drift velocity, is the deciding factor for the onset of equatorial spread F (ESF) with little contribution from the electron density gradient. Maximum growth rate of linear collisional Rayleigh-Taylor instability occurs at the time of peak height rather than at the time of peak velocity confirming that, for the onset of ESF, the layer should attain a threshold height. The threshold (group) height of the 5.5 MHz layer falls from ~450 km in 1984 (mean F10.7 equals 120) to ~350 km in 1985 (mean F10.7 equals 70); the corresponding 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 thresholds with the declining solar activity is due to the decrease in the ion-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.

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