Equatorial F-layer heights, evening prereversal electric field,and night E-layer density in the American sector: IRI validation with observations Original Research Article

The equatorial F-layer height variations resulting from the variabilities in the zonal electric fields and winds and associated variability in ionospheric dynamo strength are important factors in determining the distribution and structuring of the electron density of the equatorial ionization anomaly (EIA) region. Especially, the evening enhancement in the F-layer heights and the associated prereversal enhancement in the zonal electric field due to the F-layer dynamo are believed to provide the most basic precondition for the equatorial spread F/ plasma bubble irregularity (ESF) generation. A realistic description by the International Reference Ionosphere (IRI) of the quiet time equatorial F-layer heights is therefore of fundamental importance for applications related to the studies of the ESF and EIA variabilities. The existing IRI description scheme (that uses the CCIR coefficients) appears to represent the equatorial F-layer peak density (NmF2/ƒoF2) better than the peak height (hmF2) and the heights of specific densities, the largest disagreement with observations being verified during the evening hours. Digisonde data from the three permanent stations in Brazil: São Luis (2.33°S, 44.2°W, dip angle: −.5°); Fortaleza (3.9°S, 38.45°W, dip angle: −9°); and Cachoeira Paulista (22.6°S, 315°E; dip angle: −28°) and from Jicamarca (12°S, 76.9°W; dip latitude: 1°N) in Peru have been analysed, to determine the quiet time mean behavior of the key F-layer parameters as a function of local time, season, and solar activity. These are complemented by data from the three conjugate point stations: Boa Vista (02.8°N; 60.66°W, dip angle: 22.5°) in the north and Campo Grande (20.45°S; 54.65°W, dip angle: −22.5°) in the south, and an equatorial station, Cachimbo (9.47°S; 54.83°W, dip angle: −3.9°) that were operated during the 2002 COPEX (Conjugate Point Experiment) campaign conducted in Brazil. The data for São Luis and Jicamarca are used to evaluate the longitudinal differences in the prereversal F-layer vertical drift, arising from the large magnetic declination angle difference that characterize these Brazilian and Peruvian longitude sectors. An attempt is made to characterize and quantify any systematic difference that exists between the mean behavioral patterns of the critical parameters as described by the IRI and those observed, with an objective to improve the IRI prediction capability.