On monthly/seasonal/longitudinal variations of equatorial irregularity occurrences and their relationship with the postsunset vertical drift velocities

Monthly variation of global equatorial density irregularity distribution has been obtained with the data taken by ROCSAT-1 at the 600 km topside ionosphere from March 1999 to June 2004 during high to moderate solar activity years. This global distribution of monthly irregularity occurrence rate not only provides the best spatial/temporal distribution in existence so far but also serves to fill the gap of irregularity distribution missing over some eastern Pacific region where no ground observation is available. The 5 1/2-year result of the monthly occurrence pattern indicates a smooth variation across the longitudes contrary to some beliefs that a drastic change in irregularity occurrence pattern has occurred across the eastern Pacific longitudes. Excellent agreement is noted for the current results with Aarons' (1993) conjectured sketch of the global scintillation distribution. Furthermore, the seasonal/longitudinal (s/1) variations of quiet time postsunset vertical drift velocities are found to track closely with the s/1 variations of irregularity occurrences. Linear regression analysis between the vertical drift velocity and the irregularity occurrence rate indicates that the vertical drift velocities at three different longitude zones of magnetic declination have good correlations with the irregularity occurrences for all seasons. This implies that the averaged postsunset vertical drift velocity is a good indicator for the occurrences of equatorial density irregularities in a longitude region of similar magnetic declination. The smooth variation of monthly/seasonal/ longitudinal distributions of the vertical drift velocities that results in the smooth variation of topside density irregularity occurrences implies that the global occurrence pattern of either distribution is controlled by the smooth global variation of the postsunset ionospheric condition, related to the magnetic declination effect and the seasonal variation of the ionospheric density level at the dip equator located with respect to the geographic equator. There seems little need to include the global distribution of instability perturbation seeds from atmospheric disturbances to complete the global distribution of topside irregularity occurrences.