Comparison of IRI-2001 with TOPEX TEC measurements

Abstract

During the last decade, the TOPEX/Poseidon mission has provided a wealth of data pertaining to total electron content (TEC) measurements over the oceans, where conventional measurements are sparse. In this study, a comprehensive comparison of the TOPEX TEC measurements with the recent version of the International Reference Ionosphere (IRI-2001) was performed. The study covered solar cycle, seasonal, geomagnetic activity, and longitudinal variations. First, it was found that both the IRI and TOPEX TEC show a negligibly small geomagnetic dependency, regardless of the solar activity and seasonal conditions. For solar activity, however, not only the TECs from the IRI and TOPEX measurements, but also the difference between them, strongly depend on the solar activity. The comparison also shows that the daytime low-latitude ionosphere from the IRI always develops earlier than the corresponding TOPEX measurements, appearing as an overestimate of IRI TEC at around 7 magnetic local time (MLT) in the morning. With respect to the annual and seasonal variations of TEC, the TOPEX TEC shows stronger annual and semiannual anomalies than the IRI TEC both at low latitudes and at upper mid-latitudes. However, the IRI and TOPEX TECs show a similar seasonal anomaly only for high solar activity. Finally, the longitudinal variations of the IRI TEC show good agreement with the TOPEX measurements for low solar activity, but for high solar activity, large discrepancies occur in the Pacific sector.