Seasonal and longitudinal variation of large‐scale topside equatorial plasma depletions

Abstract

We examine the large‐scale properties of three classes of equatorial topside plasma depletions referred to as channels. In order of increasing zonal width and internal large‐scale substructuring, we study simple channels, multichannels, and superchannels. We find that peaks in channel activity are centered on 0° (African sector) and 180° (Western Pacific sector). Superchannels are found to strongly dominate in the African sector, whereas simple and multichannels dominate the Pacific sector. Within the African sector, the relative proportions of the three channel classes remain stable in all seasons. In the Pacific sector, simple and multichannels are dominant in all seasons of activity, though small but significant seasonal variations in channel widths and spacings are seen. Structure at a zonal scale size of about 100 km is found to be present in all channels examined. Observational and model evidence suggests that 100‐km density structure represents channels, such that all multichannels and superchannels consist of multiple contiguous simple channels. Since seeds that initiate channel formation necessarily influence large‐scale channel features, we utilize our data to draw conclusions regarding bottomside seeding conditions in the various seasonal and longitudinal sectors. Specifically, we find that strong, long‐wavelength (∼1000 km) seeds appear much more frequently in the African sector, resulting in the prevalence of superchannels there. When long‐wavelength seeds appear in the Pacific sector, they result in periodic trains of simple and multichannels. A bottomside preseeded at a scale size near 100 km accounts for the ubiquity of 100‐km scale size in the AE‐E data.