Browsing by Author "Nogueira, P. A. B."
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Item Restricted Equatorial electrojet responses to intense solar flares under geomagnetic disturbance time electric fields(American Geophysical Union, 2017-01-12) Abdu, M. A.; Nogueira, P. A. B.; Souza, J. R.; Batista, I. S.; Dutra, S. L. G.; Sobral, J. H. A.Large enhancement in the equatorial electrojet (EEJ) current can occur due to sudden increase in the E layer density arising from solar flare associated ionizing radiations, as also from background electric fields modified by magnetospheric disturbances when present before or during a solar flare. We investigate the EEJ responses at widely separated longitudes during two X‐class flares that occurred at different activity phases surrounding the magnetic super storm sequences of 28–29 October 2003. During the 28 October flare we observed intense reverse electrojet under strong westward electric field in the sunrise sector over Jicamarca. Sources of westward disturbance electric fields driving large EEJ current are identified for the first time. Model calculations on the E layer density, with and without flare, and comparison of the results between Jicamarca and Sao Luis suggested enhanced westward electric field due to the flare occurring close to sunrise (over Jicamarca). During the flare on 29 October, which occurred during a rapid AE recovery, a strong overshielding electric field of westward polarity over Jicamarca delayed an expected EEJ eastward growth due to flare‐induced ionization enhancement in the afternoon. This EEJ response yielded a measure of the overshielding decay time determined by the storm time Region 2 field‐aligned current. This paper will present a detailed analysis of the EEJ responses during the two flares, including a quantitative evaluation of the flare‐induced electron density enhancements and identification of electric field sources that played dominant roles in the large westward EEJ at the sunrise sector over Jicamarca.Item Restricted Longitudinal variation in Global Navigation Satellite Systems TEC and topside ion density over South American sector associated with the four‐peaked wave structures(American Geophysical Union, 2013-12-10) Nogueira, P. A. B.; Abdu, M. A.; Souza, J. R.; Bailey, G. J.; Batista, I. S.; Shume, E. B.; Denardini, C. M.Recent observations of the low‐latitude ionospheric electron density revealed a four‐peaked longitudinal structure in the equatorial ionization anomaly when plotted at a constant‐local‐time frame. It was proposed that neutral wind‐driven E region dynamo electric fields due to nonmigrating tidal modes are responsible for this pattern. We examine the four‐peaked structure in the observed topside ion density and its manifestation as longitudinal structures in total electron content (TEC) over South America. The strong longitudinal variation in TEC characterized by larger value over Brazilian eastern longitude sector as compared to that over the Peruvian western longitude is modeled using the Sheffield University plasmasphere‐ionosphere model (SUPIM) aiming to identify the control factors responsible for the longitude variation. We found that the SUPIM runs using as input the existing standard models of vertical drift, and thermospheric winds do not explain the TEC longitudinal structure. Realistic values of these control parameters were generated based on the strong vertical drift longitudinal variation as determined from magnetometer and Digisonde data and appropriately adjusted winds (horizontal wind model). These realistic vertical drifts together with the modified thermospheric wind, when used as input to the SUPIM, are found to satisfactorily explain the longitudinal differences in the TEC and topside ion density (Ni) over South America. The study shows that the TEC in the whole latitude distribution is larger over the east coast than over the west coast of South America and that the vertical drift and thermospheirc winds control the longitudinal four wave structure in the TEC and Ni.Item Restricted Modeling the equatorial and low‐latitude ionospheric response to an intense X‐class solar flare(American Geophysical Union, 2015-03-11) Nogueira, 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.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.Item Restricted Strong longitudinal difference in ionospheric responses over Fortaleza (Brazil) and Jicamarca (Peru) during the January 2005 magnetic storm, dominated by northward IMF(American Geophysical Union, 2012-08-29) Santos, A. M.; Abdu, M. A.; Sobral, J. H. A.; Koga, D.; Nogueira, P. A. B.; Candido, C. M. N.In this study we investigate the response of the equatorial F layer to disturbance zonal electric field associated with IMF (interplanetary magnetic field) variations dominated by a strong northward Bz episode during the magnetic storm that occurred on 21 January, 2005. We compared the results obtained from Digisondes operated at Fortaleza, Brazil (Geogr. 3.9 S, 38.45 W; dip angle: 11.7 ) and Jicamarca, Peru (Geogr. 12.0 S, 76.8 W; dip angle: 0.64 ). A large auroral activity (AE) intensification that occurred at 1715 UT produced a large F-layer peak height increase (from 300 km to 600 km) over Jicamarca with no noticeable simultaneous effect over Fortaleza. Then the Bz turning northward at 1940 UT with a rapid change in AE that was accompanied by a large decrease of F layer height and total suppression of the PRE over Fortaleza with no simultaneous effect over Jicamarca. Strong increase in the AE index (from 400 to 1000 nT) with superimposed oscillations, under Bz North, that soon followed was associated with increases in both the F layer height and the vertical drift velocity over Fortaleza (at 2130 UT), with no corresponding signatures over Jicamarca. These remarkable contrasting responses to prompt penetration electric field (PPEF) as well as to disturbance wind dynamo electric field (DDEF) and other effects observed at the two locations separated only by 2 h in LT in the South American sector are presented and discussed in this paper. Effects on spread-F development and foF2 behavior during this storm event are also addressed in this work.