Browsing by Author "Sahai, Y."
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Item Restricted Low-Latitude Mesosphere, Thermosphere, and Ionosphere(Hindawi, 2012-05-20) Sahai, Y.; Dabas, R. S.; Otsuka, Y.; Klimenko, M.The objective of this proposed special issue was to provide a forum for papers related to the recent advances in the field of equatorial and low-latitude regions of mesosphere, thermosphere, and ionosphere from observational (groundbased and space-borne), theoretical, and simulation studies. A total of 11 papers are presented, out of which 10 are invited and 1 contributed (F. Ouattara et al.). 2 papers are reviews, one dealing with a network of Fabry-Perot interferometers for large-scale measurements of the neutral winds and temperature in the Earth’s thermosphere (J. J. Makela et al.) and the second dealing with the equatorial F2 layer stratification (M. V. Klimenko et al.)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.