Noontime bite-out (depletion) of NmF2 (peak electron density of the F-2 layer) often occurs in the equatorial ionosphere; it is characterized by a noontime minimum sandwiched by a prenoon peak and a postnoon peak of NmF2. The fountain effect was suggested to be responsible for this abnormal diurnal variation pattern. Noontime bite-out should occur not only at the dip equator but also concurrently at both equatorial north and south according to the fountain effect mechanism. In this study, ionosonde measurements in the equatorial ionization anomaly trough region were selected to investigate the climatology of noontime bite-out. North-south difference of noontime bite-out was presented for the first time. Noontime bite-out occurs near the dip equator in all months. However, NmF2 enhances continuously since early morning till late afternoon at equatorial north (south) in northern summer (winter) months; nevertheless, noontime bite-out still occurs in other months. This cannot be understood only in terms of the fountain effect mechanism. We suggested the reason is seasonally dependent transequator plasma transport induced by neutral meridional winds. Transequator transport works since early morning to modulate NmF2 variation pattern in daytime. Plasma outflow induced by the transequator transport results in significant NmF2 depletion at equatorial north or south before the fountain effect induced bite-out; thus, NmF2 prenoon peak is restrained and noontime bite-out variation pattern is absent.
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