Single crest phenomenon in the equatorial ionospheric anomaly region and its longitudinal distribution caused by nonmigrating tides

When the fountain effect is weak so the double crest structure is not fully developed in the equatorial ionospheric anomaly region, only one peak of electron density is probably observed, which is called single crest phenomenon. In this study, we statistically present the occurrence of the single crest phenomenon during year 2001-2010, based on the electron density data measured by the CHAllenging Minisatellite Payload. The single crest appears more frequently at 08:00-10 :00 and 16 :00-19 :00 local time, showing longitudinal dependence that is more obvious at 10 :00-18 :00. Around equinox, wave number 4 longitudinal structure is observed, and around winter solstice, wave number 3 is the predominant component. The longitude where the single crest frequently occurs in general corresponds to the place where the double crests structure is weak. We suggest that, the occurrence of the single crest phenomenon is affected by the DE2 and DE3 components of nonmigrating tides that modulate the wind field and thus the atmospheric dynamo electric field. At the longitude where the dynamo zonal electric field is weak, the resultant fountain effect is not strong enough to produce the double crest structure, then the ionosphere will have only one crest in response to the direct solar ionization. The single crest phenomenon can therefore be used to study the nonmigrating tides and related electrodynamics in the equatorial ionospheric anomaly region.