Navegando por Assunto "Solar cycle"

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    Daily and Monthly Variations of the Equatorial Ionization Anomaly (EIA) Over the Brazilian Sector During the Descending Phase of the Solar Cycle 24
    (Advancing Earth and Space Sciences) Dias, Maukers Alem Lima; Fagundes, Paulo Roberto; Venkatesh, Karnam; Pillat, Valdir Gil; Ribeiro, Brunno Augusto Gomes; Seemala, Gopi K.; Arcanjo, Mateus de Oliveira
    It has been noticed recently that the equatorial ionization anomaly (EIA) in the Brazilian eastern, middle, and western sectors present significant longitudinal variability within a short separation due to the presence of typical magnetic field structure over South America. Therefore, the main motivation of this study is to investigate day‐to‐day and month‐to‐month characteristics of the EIA in three closely spaced longitudinal sectors over the Brazilian region during the year 2016 in the descending phase of the solar activity. These three longitudinal sectors are perpendicular to the magnetic equator at its largest declination over the globe. The EIA showed a semiannual variation in all sectors with a major maximum during the summer, a minimum during the winter, and secondary maximum during spring. In addition, the EIA shows its classical behavior with a trough around the magnetic equator and crests at low latitudes (from ±15° to ±20°). When the EIA is not developed, the maximum ionization is located around the dip equatorial latitudes and the anomaly crest is absent. The number of days within 1 year that showed clear EIA development in the west, middle, and east sectors is 248 (78.2%), 236 (74.4%), and 265 (83.6%), respectively. The undeveloped/absence of EIA is also found often during the months of winter, and the numbers of such days during 2016 in the west, middle, and east are 69 (21.8%), 81 (25.6%), and 52 (16.4%), respectively. Also, the shape, strength, extent, and lifetimes of EIA crest have shown significant variabilities between the three sectors during different seasons. Further, a comparative analysis is carried out on the variations of EIA crest and lifetime between GPS observations and IRI‐2016 model, and the results are discussed.
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    Longitudinal variations of the occurrence of F3 and F4 layers within the southern EIA and their dependence on solar cycle
    (Elsevier) Tardelli, Alexandre; Fagundes, Paulo Roberto; Pezzopane, Michael; Pillat, Valdir Gil
    This investigation presents for the first time the seasonal and solar cycle variations of the daytime F-layer multiple stratifications (F3 and F4 layers) near the southern crest of the EIA in two different longitudinal sectors of South America. To perform the study, the ionograms recorded from 2007 to 2015 at Sao Jose´ dos Campos (23.2 S, 45.9 W), Brazil (eastern sector), and at Tucuma´n (26.9 S, 65.4 W), Argentina (western sector), are considered. Both sites present a frequency of occurrence of the F3 and F4 layers which is directly proportional to the solar activity, and an annual variation with a maximum in spring/summer and a minimum in autumn/winter. The main result that came out from the analysis is that the frequency of occurrence of the F3 and F4 layers is higher in the western sector than in the eastern sector, and this could be attributed to a different gravity waves activity characterizing the two longitudinal sectors. 2021 COSPAR. Published by Elsevier B.V. All rights reserved.