Observation of day-to-day variability in equatorial spread-F

Complete text of publication follows. It is well established that Rayleigh -Taylor instability mechanism is fundamental for the generation of large scale equatorial ionospheric irregularities. The pre-reversal enhancement (PRE) of zonal electric field, which uplifts the ionosphere after the sunset, is one of the most important ingredients in the day-to-day variability of equatorial spread-F (ESF). Although, ESF have been studied since late 1930s, nevertheless it continues to attract considerable attention because still debate is going on regarding the onset conditions and their influence on trans-ionospheric radio-wave communications, location and navigation systems in the equatorial and low latitude regions. However, up to now there is not a good explanation for the day-to-day ESF variability during geomagnetically quiet and disturbed conditions. Observations from multi-instruments both ground (optical, ionosondes, and radars) and space (satellites) have been utilized for these studies. In this paper we show that a planetary wave type oscillation affects the PRE and then consequentially the variability of ESF. Also, the strength of the PRE plays an important role in the generation of 'fresh' spread-F, where 'fresh' spread-F are those observed between 19:00 and 21:00 LT (post-sunset conditions).