IRI related studies in Argentina: Topside profile and electron content

Abstract

This paper reviews and extends studies of the ability of the International Reference Ionosphere (IRI) to make predictions of the topside electron density (N) and the total electron content for low latitude locations. Members of the Argentine scientific community have performed the reported studies. Topside Profile : Measurements of N obtained with the Hinotori and Taiyo satellites were used to check the ability of the IRI to predict monthly average value of N at 600 km of altitude (N 600) in the low latitude region and, also, that corresponding to a particular day and time so-called “instanteneous N value”. The comparison showed that the best agreement between IRI derived average N 600 values and measurements was obtained for a station near the magnetic equator and, in general, for the period of minimum ionisation for stations not near the magnetic equator. When ground ionosonde data obtained at Tucumán (26.9° S, 294.6°E; mag. latitude:−15.5) and Huancayo (12.0° S, 284.7° E; mag. latitude:−0.7) were used as input to the model the predicted instantaneous N 600 values were better than those obtained using either the CCIR or URSI options. Additional studies, covering different heights in the F region at low latitudes for low solar activity, showed that the IRI gives better N predictions than those obtained for high solar activity. Electron Content : Measurements of vertical total electron content (VTEC) over Tucumán, Havana (23.1° N, 277.5° E, mag. latitude: 34.2) and Arequipa (16.5° S, 289.0° E; geoma. Lat.:−5.1) obtained from geosynchronous and GPS satellites signals during high solar activity periods, have been used to study the IRI performance as predictor of TEC. Both versions, IRI-90 and IRI-95, overestimate VTEC at the southern crest of the equatorial anomaly for hours around the daily minimum, and underestimate it the rest of the day. This suggests that the main reason for the disagreement is the shape of the N profile assumed by the model. Good predictions were obtained for hours of maximum ionisation over Arequipa in some cases. For Havana, which is not under the northern peak of the equatorial anomaly, the IRI generally gives good predictions for the period around the daily minimum, and for all hours in some cases. Moreover, IRI has been used to calculate the ionospheric slant electron content (SEC) for the ATS 6 geosynchronous satellite — Boulder (40.13° N, 254.76° E) ratio signal path. The comparison with measurements obtained during low solar activity showed cases with good agreement among predictions and measurements. But, in general, the model underestimates SEC. The need for additional studies covering more stations and conditions to complete the validation of IRI model is stressed.