dc.contributor.author |
Jayachandran, B |
|
dc.contributor.author |
Balan, N |
|
dc.contributor.author |
Rao, P. B |
|
dc.contributor.author |
Sastri, J. H |
|
dc.contributor.author |
Bailey, G. J |
|
dc.date.accessioned |
2008-09-26T16:56:23Z |
|
dc.date.available |
2008-09-26T16:56:23Z |
|
dc.date.issued |
1993 |
|
dc.identifier.citation |
Journal of Geophysical Research, Vol. 98, No. A8, pp. 13741 - 13750 |
en |
dc.identifier.issn |
0148-0227 |
|
dc.identifier.uri |
http://hdl.handle.net/2248/3903 |
|
dc.description |
Open Access |
|
dc.description.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 |
en |
dc.language.iso |
en |
en |
dc.publisher |
American Geophysical Union |
en |
dc.relation.uri |
http://www.agu.org/pubs/crossref/1993/93JA00302.shtml |
en |
dc.subject |
HF doppler radar |
en |
dc.subject |
Solar activity |
en |
dc.title |
HF doppler and ionosonde observations on the onset conditions of equatorial spread F |
en |
dc.type |
Article |
en |