Browsing by Author "Carrasco, A. J."
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Item Restricted Equatorial F region evening vertical drift, and peak height, during southern winter months: A comparison of observational data with the IRI descriptions(Elsevier, 2006-05-03) Abdu, M. A.; Batista, I. S.; Reinisch, B. W.; Sobral, J. H. A.; Carrasco, A. J.The equatorial F region evening vertical drift, due to pre-reversal electric field enhancement, is an important condition for the spread F/plasma bubble irregularity generation, that is more frequent during summer-equinoctial months over South America. A comparative study of these vertical drifts with their IRI representations was presented at the Grahamstown IRI 2003 workshop. During southern winter months the post-sunset ESF development is relatively infrequent over South America due to the generally weaker intensity of the sunset zonal electric field, which, however, is critical in determining the equatorial spread F (ESF) development under magnetospherically disturbed conditions. Therefore a detailed understanding of the characteristics of the evening F layer vertical drift, hmF2 and foF2 during southern winter months is important for developing/improving their representations in the IRI scheme. In this paper we have undertaken a study of these parameters over the Brazilian equatorial sites, Sao Luis (2.33S, 44.2W, dip angle: −0.5°, declination angle: 21W°) and the low latitude site, Cachoeira Paulista (22.6°S, 315°E; dip angle: −32°) in comparison with their existing representations in the IRI. The study is made as a function of the solar flux varying from the solar activity minimum to maximum conditions. Some of the results in the Brazilian longitude sector are compared with results from Jicamarca (12°S, 76.9°W; dip latitude: 1°N, declination angle: ∼3°E) in Peru, separated by a large difference in magnetic declination angle. The magnetic equatorial and the low latitude stations analyzed here are all located in the southern geographic hemisphere. Systematic patterns of difference between the observed characteristics of these parameters and their IRI representations are identified for eventual corrections to their existing representations in the IRI model. The study has yielded further important clues towards a better understanding of the possible mechanism for the infrequent ESF occurrence in winter over South America, and especially over Brazil.Item Restricted Equatorial F-layer heights, evening prereversal electric field, and night E-layer density in the American sector: IRI validation with observations(Elsevier, 2004) Abdu, M. A.; Batista, I. S.; Reinisch, B. W.; Carrasco, A. J.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 (N m F 2 ƒ o F2) better than the peak height (h m F2) 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: Sao Luis (2.33S, 44.2W, dip angle: − . 5); Fortaleza (3.9S, 38.45W, dip angle: − 9); and Cachoeira Paulista (22.6S, 315E; dip angle: − 28) and from Jicamarca (12S, 76.9W; dip latitude: 1N) 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 (02.8N; 60.66W, dip angle: 22.5) in the north and Campo Grande (20.45S; 54.65W, dip angle: − 22.5) in the south, and an equatorial station, Cachimbo (9.47S; 54.83W, 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.