Abstract:
An unseasonal development of Equatorial Plasma Bubble (EPB)/Equatorial Spread-F (ESF) activity in a wide longitudinal
sector over India and Southeast Asia during the post-sunset hours of 28 July 2014 has been investigated in detail using
the multiple independent observations in this paper and a companion paper by (Prog Earth Planet Sci 5:10, 2018). This
post-sunset ESF/EPB event was preceded by a substantial elevation of the equatorial F-layer due to strong pre-reversal
enhancement (PRE) during the season which is climatologically unfavorable for strong PRE and post-sunset EPBs. In
this paper, it is found that a strong equatorward meridional neutral wind at off-equatorial E-region and enhanced
longitudinal gradient of equatorial electrojet appear to be the responsible candidates for the enhanced PRE and EPB
development on 28 July 2014. These changes in the local wind system and PRE are found to have an apparent
connection with the planetary wave forces of lower atmospheric origin. Both the post-sunset height rise (PSSR) and
lower thermospheric meridional neutral wind consistently exhibit quasi-2-day planetary wave-like oscillations during
20–31 July 2014 with maximum amplitudes on 28 July 2014. This study emphasizes the role of planetary wave forces
from the lower atmosphere that can alter the local neutral wind system and E-region conductivities that may lead to
the development of strong PRE and EPB activity which were otherwise climatologically unfavorable for post-sunset
EPB/ESF development.
