The equator anomaly region as seen by the TOPEX/Poseidon satellite

Abstract

Satellite altimeter observations of ice and sea surface heights are affected by the retardation/refraction of the altimeter signal in the ionospheric plasma. Altimeter data therefore need to be correct for the ionospheric influence using a ionospheric model like the International Reference Ionosphere. Another possibility is the use of a dual-frequency instrument like the TOPEX altimeter on the TOPEX/Poseidon satellite. By measuring the heights above the Earth surface at two frequencies the ionospheric electron content (IEC) between the ground and the satellite can be determined and thus the ionospheric correction (which is proportional to IEC) can be properly accounted for in the altimeter data analysis. Dual-frequency altimeter thus in effect provide a IEC measurement as a byproduct of the sea surface height observation. The ionospheric data base established by TOPEX/Poseidon since its launch in August 1992 provides a unique opportunity to study the distribution of IEC above the oceans with a 10-day repeat cycle, a region covered only by very few prior observations. The highly variable equator anomaly region is of particular interest since it is the region where globally the highest IEC values occur. We compare the TOPEX IEC with IEC values calculated from the IRI model for different satellite passes representing different conditions in terms of local time, latitude, longitude, and season. Our study points to the possible improvements of IRI that could be accomplished with the TOPEX ionospheric data base.