Seasonal Characteristics of the Longitudinal Wavenumber-4 Structures in the Total Electron Contents (TEC) from the TOPEX/Poseidon mission

Abstract

Recently, the longitudinal variation of equatorial ionospheric anomaly (EIA) known as a wavenumber-4 structure have been seen in the nighttime UV airglow emissions observed by FUV/IMAGE and GUVI/TIMED and in the daytime electron density data obtained from ROCSAT-1. It has been speculated that the eastward propagating diurnal wavenumber-3 tide modifies the global temperature and horizontal winds, resulting in the wavenumber-4 pattern of EIA along the longitude at a constant local time. We analyzed the total electron contents (TEC) data from TOPEX/Poseidon mission to investigate the patterns of longitudinal variations of EIA for different solar activities, seasons, and local times. We mapped the TEC data over one solar cycle (1992 ~ 2003) on the geomagnetic coordinate for a fixed magnetic local time sector to see the long period climatology of the longitudinal variation of EIA. From the maps, we chose both regions of -20 ~ -5° and +5 ~ +20° geomagnetic latitude, where EIA is apparent, and made one dimensional plots of EIA variation along the longitude by binning 5°(geographic longitude) × 15°(geomagnetic latitude) TEC data for different solar and magnetic activities at fixed local times. In order to verify how closely the variation of the meridional wind due to tides is related to the wavenumber-4 structure of EIA, we compared the wavenumber-4 components of the longitudinal EIA structures with those from wind structures obtained by WINDII/UARS and TIDI/TIMED for various seasons.