Meridional wind effect on anti-solar activity correlation of equatorial density irregularity distribution

Contrasting longitudinal variation in equatorial spread-F (ESF) density irregularity distribution has been observed by ROCSAT-I during high solar activity year of 2000 against moderate solar activity year of 2003. The monthly averaged solar flux intensity F_10.7 was higher every month in 2000 than in 2003, but the irregularity occurrence rate did not always correlate with solar activity at every longitude in a season. For the equinox seasons, the occurrence rate was higher at every longitude in 2000 than in 2003. Higher occurrence rate was also observed in 2000 than in 2003 during a solstice season at longitudes of the so-called ESF longitudes. On the other hand, higher occurrence rates were observed in 2003 at the so-called low-ESF longitudes from 230° to 10° during the June solstice season and from 90° to 260° during the December solstice season, where the magnetic flux tube makes a large-angle alignment with the sunset terminator during the respective solstice season. Such anti-solar activity correlation of ESF irregularity occurrences at these low-ESF longitudes could be due to transequatorial meridional wind-induced instability suppression agent which becomes more effective to offset the Rayleigh-Taylor instability growth rate in a solstice season during a high solar activity period than during a low solar activity period.