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Title: HF doppler and ionosonde observations on the onset conditions of equatorial spread F
Authors: Jayachandran, B
Balan, N
Rao, P. B
Sastri, J. H
Bailey, G. J
Keywords: HF doppler radar;Solar activity
Issue Date: 1993
Publisher: American Geophysical Union
Citation: Journal of Geophysical Research, Vol. 98, No. A8, pp. 13741 - 13750
Abstract: 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
Description: Open Access
ISSN: 0148-0227
Appears in Collections:IIAP Publications

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