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Navegando Artigos de periódico por Autor "Abdu M.A."
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- ArtigoA comparison of two different techniques for deriving the quiet day curve from SARINET riometer dataMoro J.; Denardini C.M.; Correia E.; Abdu M.A.; Schuch N.J.; Makita K. (2012)In this work, an upgrade of the technique for estimating the Quiet Day Curve (QDC) as proposed by Tanaka et al. (2007) is suggested. To validate our approach, the QDC is estimated from data acquired by the Imaging Riometer for Ionospheric Studies (IRIS) installed at the Southern Space Observatory (SSO/CRS/CCR/INPE - MCT, 29°4' S, 53°8'W, 480ma.s.l.), São Martinho da Serra - Brazil. The evaluation was performed by comparing the difference between the QDCs derived using our upgrade technique with the one proposed by Tanaka et al. (2007). The results are discussed in terms of the seasonal variability and the level of magnetic disturbance. Also, the cosmic noise absorption (CNA) images for IRIS data operated at SSO was built using both the techniques aiming to check the implications of the changes in the methods of QDC determination on the CNA that resulted from it. © Author(s) 2012.
- ArtigoCorrelation between the cosmic noise absorption calculated from the SARINET data and the energetic particles measured by MEPED: Simultaneous observations over SAMA regionMoro J.; Denardini C.M.; Abdu M.A.; Correia E.; Schuch N.J.; Makita K. (2013)The cosmic noise absorption is presented in terms of two-dimensional images obtained from the imaging riometers operated at the Southern Space Observatory (geographic coordinate: 29.4° S, 53.1° W), in São Martinho da Serra, Brazil, Concepcion (geographic coordinate: 36.5° S, 73.0° W) and Punta Arenas (geographic coordinate: 53.0° S, 70.5° W) in Chile, which belong to the South American Riometer Network and are located at the central and periphery regions of the South American Magnetic Anomaly. Correlations are performed between the maximum cosmic noise absorption observed at these stations and the energetic electron flux in two energy channels (>30 and >300 keV) and the proton flux in three energy channels (80-240, 800-2500 and >6900 keV) as measured by the Medium Energy Proton and Electron Detector, during a moderate geomagnetic storm that occurred on September 3, 2008. The results show high correlations between the cosmic noise absorption detected at São Martinho da Serra and the flux of protons with energy between 80 and 240 keV, and the flux of electrons with energies higher than 300 keV, while an additional ionization at Concepcion was correlated with electrons of energies higher than 30 keV. The cosmic noise absorption detected at Punta Arenas was probably caused by the increase of the protons flux with energy between 80 and 240 keV. © 2012 COSPAR. Published by Elsevier Ltd. All rights reserved.
- ArtigoD-region recombination coefficients and the short wavelength X-ray flux during a solar flareAnanthakrishnan S.; Abdu M.A.; Piazza L.R. (1973)VLF phase and amplitude measurements were made on five different frequencies at São Paulo, Brazil during a solar flare which occurred on 22nd January 1972. The phase and amplitude measurements during the decay phase of the flare were combined with the full wave solutions of Wait and Spies (1964) to calculate the recombination coefficient in the lower ionosphere. The values thus obtained are lower than those reported by Reid (1970), but are compatible with those reported by Montbriand et al. (1972) during Solar X-ray events. The effective loss rates have been utilized to calculate the ion-production at the maximum of the flare, which in turn has been utilized to calculate the incident X-ray flux as a function of wavelength at the maximum of the flare. Extensions to the calculations are discussed. © 1973.
- ArtigoEquatorial ionosphere responses to two magnetic storms of moderate intensity from conjugate point observations in BrazilAbdu M.A.; Batista I.S.; Bertoni F.; Reinisch B.W.; Kherani E.A.; Sobral J.H.A. (2012)Equatorial ionospheric responses during two magnetic storms of moderate intensity are investigated, for the first time, by conjugate point observations in Brazil. The study focuses on storm-induced changes in the evening prereversal vertical drift, thermospheric trans-equatorial winds, spread F/plasma bubble irregularity development, electron density/plasma frequency heights, the EIA strength, and zonal plasma drifts. It is based on data obtained from five Digisondes operated in Brazil, three of them being part of a conjugate point equatorial experiment (COPEX) involving a dip equatorial and two magnetic conjugate sites at ±12°. The other two were operated at the equatorial ionization anomaly (EIA) trough and crest locations at nearby magnetic meridians. The results bring out, and clarify, many outstanding aspects of the strong influence of storm time electric fields on the equatorial ionosphere at different phases of the two long lasting storm sequences. During both storms prompt penetration electric fields dominated the ionospheric response features as compared to the disturbance wind dynamo effects that were not very conspicuous. An under-shielding (over-shielding) electric field occurring in the evening hours causes enhancement (suppression) of the prereversal vertical drift and post sunset spread F/plasma bubble generation. The same electric fields cause post sunset EIA enhancement and suppression, respectively. Post sunset (post midnight) spread F can develop from under-shielding (over-shielding) electric fields, while it can be disrupted by over-shielding (under-shielding) electric field. Trans-equatorial winds are found to be ineffective to stabilize the post sunset F region against the destabilizing effect of strong prereversal vertical drift. Storm time westward plasma drifts are found to be driven by prompt penetration eastward electric fields (through their effect of inducing vertical Hall electric fields), rather than by a disturbance westward thermospheric wind during these storms.
- ArtigoLatitudinal dependence of cosmic noise absorption in the ionosphere over the SAMA region during the September 2008 magnetic stormMoro J.; Denardini C.M.; Abdu M.A.; Correia E.; Schuch N.J.; Makita K. (2012)In this work we present and discuss some results of latitudinal dependence in the cosmic noise absorption (CNA) as observed by the South American Riometer Network (SARINET) operated in the South American Magnetic Anomaly (SAMA) region, during a moderate intensity geomagnetic storm that occurred on 3 September 2008. In our analysis, we used the data acquired by the imaging riometers installed at So Martinho da Serra (SSO - geographic coordinate: 29.4°S, 53.1°W), Concepcion (CON - geographic coordinate: 36.5°S, 73.0°W) and Punta Arenas (PAC - geographic coordinate: 53.0°S, 70.5°W) and by the single beam riometer installed at Trelew (TRW - geographic coordinate: 43.1°S, 65.2°W). A comparison among the selected riometer data showed that the mean CNA was more pronounced at SSO, which is the site located nearest to the center of the SAMA, but the second highest value was found at the farther station. Also, a second-order polynomial curve fitting was performed in order to establish an empirical relationship between the mean CNA and the total intensity of the geomagnetic field at the riometer stations.
- ArtigoLongitudinal and day-to-day variations of equatorial spread F occurrence from observations over south AmericaCueva R.Y.C.; Valladares C.E.; De Paula E.R.; Abdu M.A.; Paulino I.; Batista I.S.; Takahashi H.; Raulin J.P. (2017)© 2017 Sociedade Brasileira de Geofísica.In this work we studied the longitudinal and day-to-day variability of equatorial spread F occurrence over South America. Digisonde from the equatorial stations of São Luís and Jicamarca, and Total Electron Content (TEC) data from several GPS receivers were used to analyse the ionospheric conditions conductive to ionospheric plasma irregularity generation during the solar minimum months of September 2009 to January 2010. To support this analysis an Automatic-Bubble- Detection-Algorithm was used to detect abrupt decreases that plasma bubbles introduce in the TEC values. The bubble occurrence pattern over SA observed in this work maximizes on September-October (equinox) in the west and November-January (December solstice) in the east-longitudes. However, on a day-to-day basis bubble signatures are also detected all over SA during this period. Besides being observed during days with pronounced prereversal enhancement (PRE), as expected, the TEC depletions were also observed during days without PRE, during equinox at Jicamarca and December solstice at São Luís. This unexpected occurrence of TEC depletions (TECds) suggests that seeding mechanisms, like periodic structures are present and modify the onset conditions of bubble-events. In this report are shown evidences of wave-like structures previous to bubble development.
- ArtigoModeling the equatorial and low-latitude ionospheric response to an intense X-class solar flareNogueira P.A.B.; Souza J.R.; Abdu M.A.; Paes R.R.; Sousasantos J.; Marques M.S.; Bailey G.J.; Denardini C.M.; Batista I.S.; Takahashi H.; Cueva R.Y.C.; Chen S.S. (2015)©2015. American Geophysical Union. All Rights Reserved.We have investigated the ionospheric response close to the subsolar point in South America due to the strong solar flare (X2.8) that occurred on 13 May 2013. The present work discusses the sudden disturbances in the D region in the form of high-frequency radio wave blackout recorded in ionograms, the E region disturbances in the form of the Sq current and equatorial electrojet intensifications, and the enhancement and decay in the ionospheric total electron content (TEC) as observed by a network of Global Navigation Satellite Systems receivers, the last of these manifestations constituting the main focuses of this study. The dayside ionosphere showed an abrupt increase of the TEC, with the region of the TEC increase being displaced away from the subsolar point toward the equatorial ionization anomaly (EIA) crest region. The decay in the ΔTEC following the decrease of the flare EUV flux varied at a slower ratio near the EIA crest than at the subsolar point. We used the Sheffield University Plasmasphere-Ionosphere Model to simulate the TEC enhancement and the related variations as arising from the flare-enhanced solar EUV flux and soft X-rays. The simulations are compared with the observational data to validate our results, and it is found that a good part of the observed TEC variation features can be accounted for by the model simulation. The combined results from model and observational data can contribute significantly to advance our knowledge about ionospheric photochemistry and dynamics needed to improve our predictive capability on the low-latitude ionospheric response to solar flares.
- ArtigoSome trends in the red-shift distribution of quasi-stellar objects and related peculiar galaxiesBasu D.; Abdu M.A. (1972)Gaps in the red-shift distribution of quasi-stellar objects and related peculiar galaxies have been studied using 205 sources. The result indicates certain definite trends in the distribution of the gaps but does not suggest any periodicity when the entire sample is considered. © 1972 D. Reidel Publishing Company.