Abstract:
Scintillations on VHF radio signal are sparsely observed during daytime due to unavailability of
strong electron density irregularities in equatorial E or F region. Type I/II irregularities observed at E region
altitudes during the daytime are linked with either two-stream or gradient drift instability. The occurrence
of these irregularities in presence of strong blanketing Es (Esb) can produce weak-moderate scintillations on
VHF signal during daytime. Such sparse daytime VHF scintillations are used in the present study to retrieve
information about E region irregularities, which are generally examined with radar observations. We use
spaced receiver scintillation observations on 251 MHz signal transmitted from geostationary satellite UFO2
(71.2∘E) and recorded at Tirunelveli (8.5∘N, 77.8∘E, dip latitude 0.6∘N). Ionosonde data from Trivandrum
(8.5∘N, 76.6∘E, dip latitude 0.5∘N) during 2003–2005 is used to confirm the association of daytime
scintillations with Esb. The daytime scintillations last for 15–45 min during postnoon hours. Their occurrence
closely matches the peak occurrence time of Esb. For the first time, spatial scale lengths of E region
irregularities are obtained using the technique introduced by Bhattacharyya et al. (2003). The observed
spatial scales are validated using theoretical model. The theoretical model manifests 6–19% density
fluctuations in the E region to produce weak scintillations (0.15 ≤ S4 ≤ 0.4) on 251 MHz. The study reveals
that scale lengths of E region irregularities are smaller on counter equatorial electrojet (CEEJ) days than
non-CEEJ days, which could be resulting from lower electron temperatures in E region on CEEJ days.
