Browsing by Author "Sobral, J. H. A."
Now showing 1 - 10 of 10
Results Per Page
Sort Options
Item Open Access Abnormal evening vertical plasma drift and effects on ESF and EIA over Brazil-South Atlantic sector during the 30 October 2003 superstorm(American Geophysical Union, 2008-11) Abdu, M. A.; De Paula, E. R.; Batista, I. S.; Reinisch, B. W.; Matsuoka, M. T.; Camargo, P. O.; Veliz, Oscar; Denardini, C. M.; Sobral, J. H. A.; Kherani, E. A.; De Siqueira, P. M.Equatorial F region vertical plasma drifts, spread F and anomaly responses, in the south American longitude sector during the superstorm of 30 October 2003, are analyzed using data from an array of instruments consisting of Digisondes, a VHF radar, GPS TEC and scintillation receivers in Brazil, and a Digisonde and a magnetometer in Jicamarca, Peru. Prompt penetrating eastward electric field of abnormally large intensity drove the F layer plasma up at a velocity 1200 ms−1 during post dusk hours in the eastern sector over Brazil. The equatorial anomaly was intensified and expanded poleward while the development of spread F/plasma bubble irregularities and GPS signal scintillations were weaker than their quiet time intensity. Significantly weaker F region response over Jicamarca presented a striking difference in the intensity of prompt penetration electric field between Peru and eastern longitudes of Brazil. The enhanced post dusk sector vertical drift over Brazil is attributed to electro-dynamics effects arising energetic particle precipitation in the South Atlantic Magnetic Anomaly (SAMA). These extraordinary results and their longitudinal differences are presented and discussed in this paper.Item Restricted Disturbance zonal and vertical plasma drifts in the Peruvian sector during solar minimum phases(American Geophysical Union, 2016-02-11) Santos, A. M.; Abdu, M. A.; Souza, J. R.; Sobral, J. H. A.; Batista, I. S.In the present work, we investigate the behavior of the equatorial F region zonal plasma drifts over the Peruvian region under magnetically disturbed conditions during two solar minimum epochs, one of them being the recent prolonged solar activity minimum. The study utilizes the vertical and zonal components of the plasma drifts measured by the Jicamarca (11.95°S; 76.87°W) incoherent scatter radar during two events that occurred on 10 April 1997 and 24 June 2008 and model calculation of the zonal drift in a realistic ionosphere simulated by the Sheffield University Plasmasphere‐Ionosphere Model‐INPE. Two main points are focused: (1) the connection between electric fields and plasma drifts under prompt penetration electric field during a disturbed periods and (2) anomalous behavior of daytime zonal drift in the absence of any magnetic storm. A perfect anticorrelation between vertical and zonal drifts was observed during the night and in the initial and growth phases of the magnetic storm. For the first time, based on a realistic low‐latitude ionosphere, we will show, on a detailed quantitative basis, that this anticorrelation is driven mainly by a vertical Hall electric field induced by the primary zonal electric field in the presence of an enhanced nighttime E region ionization. It is shown that an increase in the field line‐integrated Hall‐to‐Pedersen conductivity ratio urn:x-wiley:21699380:media:jgra52445:jgra52445-math-0001, which can arise from precipitation of energetic particles in the region of the South American Magnetic Anomaly, is capable of explaining the observed anticorrelation between the vertical and zonal plasma drifts. Evidence for the particle ionization is provided from the occurrence of anomalous sporadic E layers over the low‐latitude station, Cachoeira Paulista (22.67°S; 44.9°W)—Brazil. It will also be shown that the zonal plasma drift reversal to eastward in the afternoon two hours earlier than its reference quiet time pattern is possibly caused by weakening of the zonal wind system during the prolonged solar minimum period.Item Restricted Effects of the intense geomagnetic storm of September–October 2012 on the equatorial, low- and mid-latitude F region in the American and African sector during the unusual 24th solar cycle(Elsevier, 2015-12-30) Jesus, R. de; Fagundes, P. R.; Coster, A.; Bolaji, O. S.; Sobral, J. H. A.; Batista, I. S.; Abreu, A. J. de; Venkatesh, K.; Gende, M.; Abalde, J. R.; Sumod, S. G.The main purpose of this paper is to investigate the response of the ionospheric F layer in the American and African sectors during the intense geomagnetic storm which occurred on 30 September–01 October 2012. In this work, we used observations from a chain of 20 GPS stations in the equatorial, low- and mid-latitude regions in the American and African sectors. Also, in this study ionospheric sounding data obtained during 29th September to 2nd October, 2012 at Jicamarca (JIC), Peru, São Luis (SL), Fortaleza (FZ), Brazil, and Port Stanley (PST), are presented. On the night of 30 September–01 October, in the main and recovery phase, the h´F variations showed an unusual uplifting of the F region at equatorial (JIC, SL and FZ) and mid- (PST) latitude stations related with the propagations of traveling ionospheric disturbances (TIDs) generated by Joule heating at auroral regions. On 30 September, the VTEC variations and foF2 observations at mid-latitude stations (American sector) showed a long-duration positive ionospheric storm (over 6 h of enhancement) associated with large-scale wind circulations and equatorward neutral winds. Also, on 01 October, a long-duration positive ionospheric storm was observed at equatorial, low- and mid- latitude stations in the African sector, related with the large-scale wind circulations and equatorward neutral winds. On 01 and 02 October, positive ionospheric storms were observed at equatorial, low- and mid-latitude stations in the American sector, possibly associated with the TIDs and an equatorward neutral wind. Also, on 01 October negative ionospheric storms were observed at equatorial, low- and mid-latitude regions in the American sector, probably associated with the changes in the O/N2 ratio. On the night of 30 September–01 October, ionospheric plasma bubbles were observed at equatorial, low- and mid- latitude stations in the South American sector, possibly associated with the occurrence of geomagnetic storm.Item Restricted Equatorial electrojet 3-M irregularity dynamics during magnetic disturbances over Brazil: results from the new VHF radar at São Luı́s(Elsevier, 2003-10) Abdu, M. A.; Dinardini, C. M.; Sobral, J. H. A.; Batista, I. S.; Muralikrishna, P.; Iyer, K. N.; Veliz, O.; De Paula, E. R.Data collected during the first two observational campaigns, conducted in August 1998 and December 1999, using the new coherent back-scatter radar, developed at INPE, that became operational at the magnetic equatorial site at São Luı́s, (2.33S,44.2W), Brazil, are analyzed in this paper. The spatial and temporal distribution of 3-m irregularity power in the form of range–time–intensity maps and spectral distribution in the form of spectrograms are analyzed for ‘quiet’ conditions and during geomagnetic storm disturbances. The analysis has brought out some new findings, besides confirming some of the already known storm response features of the EEJ and its plasma instabilities. Among the highlights of the results are: the electrical coupling between the equatorial and auroral electrojets is important even on a ‘quiet’ day, and gets very strong during magnetic storm disturbances; disturbance prompt penetration electric field, and the delayed electric field from disturbance dynamo, control the 3-m plasma wave development and inhibition in different degrees during the storm main phase and recovery phase; the amplitudes of the disturbance are larger during the morning hours than in the afternoon, in agreement with theoretical models; the height dependence of the relative dominance of the type-1 and type-2 waves generated by disturbance electric field is different from that of quiet conditions, the relative power of the type-2 getting enhanced at higher levels in the former case. A few other results are also discussed in this paper.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 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 Low‐ and mid‐latitude ionospheric electric fields during the January 1984 GISMOS Campaign(American Geophysical Union, 1990-03) Fejer, B. G.; Kelley, M. C.; Senior, C.; De la Beaujardiere, O.; Holt, J. A.; Tepley, C. A.; Burnside, R.; Abdu, M. A.; Sobral, J. H. A.; Woodman Pollitt, Ronald Francisco; Kamide, Y.; Lepping, R.An alternative approach for estimating E region density profiles using radio occultation total electron content (ROTEC) measurements is presented. In this approach, the F region contribution to the measured ROTEC is removed using the estimated F region from an assimilative model of ionospheric density. E region density profiles are then obtained from a numerical inversion of the residual ROTEC, which is assumed to be the E region contribution to the ROTEC. The proposed technique has been applied to radio occultation measurements made by the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC), while the F region specification is obtained from the Ionospheric Data Assimilation Four-Dimensional (IDA4D) algorithm. Examples of E region profiles obtained with this approach are presented and compared with nearby radar measurements at the magnetic equator. The results indicate that accurate estimates of the E region peak height and density can be obtained with this approach. This technique may be applicable to the estimation of E region conductivities with the global coverage provided by the radio occultation measurements.Item Restricted Sporadic E layer development and disruption at low latitudes by prompt penetration electric fields during magnetic storms(American Geophysical Union, 2013-04-11) Abdu, M. A.; Souza, J. R.; Batista, I. S.; Fejer, B. G.; Sobral, J. H. A.An investigation of low‐latitude sporadic E layers during magnetic storms shows that the formation and disruption of these layers are strongly controlled by the magnetospheric electric fields that penetrate to equatorial ionosphere. It is observed that a prompt penetration electric field (PPEF) of westward polarity that dominates the nightside ionosphere can cause formation of sporadic E layers near 100 km, while a PPEF of eastward polarity that dominates the dayside and eveningside can lead to disruption of an Es layer in progress. It is shown that a vertical Hall electric field, induced by the primary zonal PPEF, in the presence of the storm‐associated enhanced conductivity of the night E layer, can be responsible for vertical ion velocity convergence sufficient to influence the Es layer formation. A downward polarity of the Hall electric field leads to Es layer formation, while an upward polarity causes the Es layer disruption. An interplay of magnetic storm associated prompt penetration electric field and energetic particle precipitation is evident in the observed Es layer response features in the region of the South Atlantic/American magnetic anomaly reported here.Item Restricted Storm time equatorial plasma bubble zonal drift reversal due to disturbance Hall electric field over the Brazilian region(American Geophysical Union, 2016-05-18) Santos, A. M.; Abdu, M. A.; Souza, J. R.; Sobral, J. H. A.; Batista, I. S.; Denardini, C. M.The dynamics of equatorial ionospheric plasma bubbles over Brazilian sector during two magnetic storm events are investigated in this work. The observations were made at varying phases of magnetic disturbances when the bubble zonal drift velocity was found to reverse westward from its normally eastward velocity. Calculation of the zonal drift based on a realistic low‐latitude ionosphere modeled by the Sheffield University Plasmasphere‐Ionosphere Model showed on a quantitative basis a clear competition between vertical Hall electric field and disturbance zonal winds on the variations observed in the zonal velocity of the plasma bubble. The Hall electric field arising from enhanced ratio of field line‐integrated conductivities, ΣH/ΣP, is most often generated by an increase in the integrated Hall conductivity, arising from enhanced energetic particle precipitation in the South American Magnetic Anomaly region for which evidence is provided from observation of anomalous sporadic E layers over Cachoeira Paulista and Fortaleza. Such sporadic E layers are also by themselves evidence for the development of the Hall electric field that modifies the zonal drift.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.