Dominant Longitudinal Wave-1 Pattern of the Equatorial Topside Ionosphere in the Early Morning Sector

The low-latitude ionosphere has complex longitude structures under the effects of various space environment factors such as non-migrating tides and the geomagnetic field. In this study, a distinctive wave-1 dominated longitude pattern occurring in the early morning sector was revealed from the ROCSAT-1 observation for the low-latitude topside ionosphere. This longitude pattern consists of a primary structure near 210°–240°E and a secondary structure near 60°–90°E, both characterized by enhanced ion density (N_i) and weakened ion temperature (T_i) around the dip equator. It steadily appears in all seasons at different solar activity levels, differing from the daytime longitude variation that is dominated by seasonally dependent wave-4 and wave-3. This longitude pattern is not a continuation of nighttime longitude structure; it forms at early morning hours as a result of larger N_i increment and T_i decrement induced by equatorial upward O⁺ transport at those longitudes. Equatorial N_i and T_i longitude variations turn to be anti-correlated as this longitude pattern appears. Longitude variation of the equatorial vertical plasma drift, which is dominated by strong longitudinal wave-1 accompanying with weaker wave-2 in the early morning sector, is responsible for this longitude pattern. Especially, the difference in the turning time of equatorial vertical plasma drift from downward to upward reaches ∼2 hr at different longitudes. Interestingly, the strong wave-1 in the early morning sector likely has different drivers from those of daytime wave-1, which are dominated by the non-migrating tides SW1 and SW3; wave-wave interaction is a possibility.