Longitudinal structure of the equatorial anomaly in the nighttime ionosphere observed by IMAGE/FUV

[1] The Far Ultraviolet Imager (FUV) on board the IMAGE satellite provides an instantaneous global view of the OI 135.6-nm nightglow with 2 min time resolution. Because the OI 135.6-nm emission from the nighttime ionosphere is determined by the line-of-sight integrated plasma density, the nightglow images are useful for studying the nighttime low-latitude ionosphere globally. With the IMAGE/FUV 135.6-nm observations from March to June 2002, we have examined the global characteristics of the nighttime equatorial anomaly ( EA) by constructing a constant local time map ( LT map), in which pixels within an assigned local time range are extracted from the IMAGE/FUV nightglow images obtained over an observation period of 3 days or more and are put together to compose a global distribution map of emission intensities at that local time. These LT maps show that the development of the EA has a significant longitudinal structure, in which peaks and dips of the crest emission intensity and the crest latitude have about 90 degrees longitudinal separation in the longitude range from 0 degrees to 250 degrees. Although there is not enough data over the American sector, this result suggests that the EA longitudinal structure has a prominent zonal component of the wave number 4. The observed longitudinal structure of the nighttime EA could not be fully explained by factors such as the empirical electric field and neutral wind models, the geomagnetic declination angle, or the displacement of the geomagnetic equator from the geographic equator. To explain the observed longitudinal structure of the EA, in particular, the wave number 4 feature, we may need to consider other forcing, for example, nonmigrating tide originated from the lower atmosphere.