Abstract:
A comprehensive investigation of spread F irregularities over the Indian sector has been carried
out using VHF radar and ionosonde observations. Two different categories of spread F observations, one
where the onset of the range spread F (RSF) was concurrent with the peak h′F (category 1) and another where
the RSF onset happened ~90 min after the peak h′F time (category 2), are presented. RSF in category 2 was
preceded by the presence of oblique echoes in ionograms, indicating the irregularity genesis westward of
Sriharikota. The average peak h′F in category 1 was ~30 km higher than that in category 2 indicating the
presence of standing large-scale wave structure (LSWS). Occurrence of the blanketing Es during 19:30 to
20:30 Indian Standard Time in category 1 (category 2) was 0% (>50%). Model computation is also carried
out to further substantiate the observational results. Model computation indicates that zonal variation of
low-latitude Es can generate zonal modulation in the F layer height rise. It is found that the modulation of the
F layer height, linked with the low-latitude Es, assists the equatorial spread F onset by modifying both the
growth rate of the collisional Rayleigh-Taylor (R-T) instability and also its efficiency. A predominant presence
of low-latitude Es has been observed, but the increase in the F layer height and the R-T instability growth in
the evening hours will maximize with complete absence of low-latitude Es. A new mechanism for the
generation of LSWS and its implications on R-T instability is discussed.
