Browsing by Author "Fagundes, P. R."
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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 Open Access First very low frequency detection of short repeated burstds from MAGNETAR SGR J1550−5418(American Astronomical Society, 2010-08) Tanaka, Y. T; Raulin, Jean-Pierre; Bertoni, Fernando C. P.; Fagundes, P. R.; Chau Chong Shing, Jorge Luis; Schuch, N. J; Hayakawa, M.; Hobara, Y.; Terasawa, T.; Takahashi, T.We report on the first detection of ionospheric disturbances caused by short repeated gamma-ray bursts from the magnetar SGR J1550−5418. Very low frequency (VLF) radio wave data obtained in South America clearly show sudden amplitude and phase changes at the corresponding times of eight soft gamma-ray repeater bursts. Maximum amplitude and phase changes of the VLF signals appear to be correlated with the gamma-ray fluence. On the other hand, VLF recovery timescales do not show any significant correlation with the fluence, possibly suggesting that the bursts’ spectra are not similar to each other. In summary, Earth’s ionosphere can be used as a very large gamma-ray detector and the VLF observations provide us wit.Item Restricted Numerical modeling of ionospheric effects in the middle‐ and low‐latitude F region during geomagnetic storm sequence of 9-14 September 2005(American Geophysical Union, 2011-05-27) Klimenko, M. V.; Klimenko, V. V.; Ratovsky, K. G.; Goncharenko, L. P.; Sahai, Y.; Fagundes, P. R.; Jesus, R. de; Abreu, A. J. de; Vesnin, A. M.This study presents the Global Self‐Consistent Model of the Thermosphere, Ionosphere and Protonosphere (GSM TIP) numerical simulations of the 9–14 September 2005 geomagnetic storm effects in the middle‐ and low‐latitude ionosphere. Recent modifications to the GSM TIP model include adding an empirical model of high‐energy electron precipitation and introducing a high‐resolution (1 min) calculation of region 2 field‐aligned currents and a cross‐cap potential difference. These modifications resulted in better representation of such effects as penetration of the magnetospheric convection electric field to lower latitudes and the overshielding. The model also includes simulation of solar flare effects. Comparison of model results with observational data at Millstone Hill (42.6°N, 71.5°W, USA), Arecibo (18.3°N, 66.8°W, Puerto Rico), Jicamarca (11.9°S, 76.9°W, Peru), Palmas (10.2°S, 48.2°W, Brazil), and San Jose Campos (23.2°S, 45.9°W, Brazil) shows good agreement of ionospheric disturbances caused by this storm sequence. In this paper we consider in detail the formation mechanism of the additional layers in an equatorial ionosphere during geomagnetic storms. During geomagnetic storms, the nonuniform in height zonal electric field is generated at the geomagnetic equator. This electric field forms the additional layers in the F region of equatorial ionosphere.Item Open Access Observations and modeling of post-midnight uplifts near the magnetic equator(European Geosciences Union (EGU), 2006-07-03) Nicolls, M. J.; Kelley, M. C.; Vlasov, M. N.; Sahai, Y.; Chau Chong Shing, Jorge Luis; Hysell, D. L.; Fagundes, P. R.; Becker-Guedes, F.; Lima, W. L. C.We report here on post-midnight uplifts near the magnetic equator. We present observational evidence from digital ionosondes in Brazil, a digisonde in Peru, and other measurements at the Jicamarca Radio Observatory that show that these uplifts occur fairly regularly in the post-midnight period, raising the ionosphere by tens of kilometers in the most mild events and by over a hundred kilometers in the most severe events. We show that in general the uplifts are not the result of a zonal electric field reversal, and demonstrate instead that the uplifts occur as the ionospheric response to a decreasing westward electric field in conjunction with sufficient recombination and plasma flux. The decreasing westward electric field may be caused by a change in the wind system related to the midnight pressure bulge, which is associated with the midnight temperature maximum. In order to agree with observations from Jicamarca and Palmas, Brazil, it is shown that there must exist sufficient horizontal plasma flux associated with the pressure bulge. In addition, we show that the uplifts may be correlated with a secondary maximum in the spread-F occurrence rate in the post-midnight period. The uplifts are strongly seasonally dependent, presumably according to the seasonal dependence of the midnight pressure bulge, which leads to the necessary small westward field in the post-midnight period during certain seasons. We also discuss the enhancement of the uplifts associated with increased geomagnetic activity, which may be related to disturbance dynamo winds. Finally, we show that it is possible using simple numerical techniques to estimate the horizontal plasma flux and the vertical drift velocity from electron density measurements in the post-midnight period.Item Restricted The global thermospheric and ionospheric response to the 2008 minor sudden stratospheric warming event(American Geophysical Union, 2012-10-09) Korenkov, Y. N.; Klimenko, V. V.; Klimenko, M. V.; Bessarab, F. S.; Korenkova, N. A.; Ratovsky, K. G.; Chernigovskaya, M. A.; Shcherbakov, A. A.; Sahai, Y.; Fagundes, P. R.; De Jesus, R.; De Abreu, A. J.; Cóndor, P.This paper presents a study of thermospheric and ionospheric response to the 2008 minor sudden stratospheric warming (SSW) event. This period was characterized by low solar and geomagnetic activity. The study was performed using the Global Self-consistent Model of Thermosphere, Ionosphere, and Protonosphere (GSM TIP). Model results were compared with ionosonde data from Irkutsk, Kaliningrad, Sao Jose dos Campos, and Jicamarca. The SSW event was modeled by specifying the temperature and density perturbations at the lower boundary of the GSM TIP (80 km altitude). GSM TIP simulation allowed the reproduction of the lower thermosphere temperature disturbances (the occurrence of the quasi-wave 1 structure at 80–130 km altitude with a vertical scale of 40 km), the negative response of F2 region electron density and the positive response of electron temperature at 300 km during the 2008 minor SSW event. The main formation mechanism of the global ionospheric response is due to the disturbances (decrease) in the n(O)/n(N2) ratio. The change in zonal electric field is another important mechanism of the ionospheric response at low latitudes.