Abstract:
A sporadic E layer with dense ionization blocks the upper ionospheric layers in ionosonde
observations and it is called, blanketing sporadic E (Esb). Although earlier studies have demonstrated
that Esb occurrence is dependent on solar activity, seasons, local time, and equatorial electrojet/counter
equatorial electrojet (EEJ/CEEJ) strength, the physics behind this dependence is not well established
particularly at the dip equatorial stations. Moreover, fewer comprehensive studies are available on Esb.
Present work is a detailed statistical study of Esb occurrence and its characteristics during solar cycle 23
(1996–2006) at Indian dip equatorial station Trivandrum (dip latitude 0.5∘N). In present study, solar flux
dependence of Esb occurrence is clearly evident not only for magnetically quiet but also for the disturbed
periods with maximum during low solar activity. Known seasonal peak Esb occurrence during summer
(May–August) is found to be dominated by larger percentage of total blanketing events (fbEs ≥ 8 MHz)
on magnetically disturbed days as compared to quiet days. We noticed prevalent Esb occurrence in the
postmidnight (00–06 India Standard Time) periods during low solar activity. Besides Esb characteristics, the
present study aims to investigate effect of solar flux on association of Esb and CEEJ. Coexistence of Esb and
CEEJ is emphasized in earlier observational studies. However, any dependence of their association on solar
flux is not yet examined. We find that their association is weaker during low solar activity as compared to
high solar activity in summer, indicating that Esb occurrence is highly likely on CEEJ days during high solar
activity periods.
