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Abstract: In this study, we present multi-instrument observations of a strong positive phase of ionospheric storm, which occurred on September 10, 2005 during a moderate geomagnetic storm with minimum D st=−60nT and maximum K p=6–. The daytime electron density measured by the Millstone Hill incoherent scatter radar (42.6°N, 288.5°E) increased after 13UT (∼8LT) compared with that before the storm. This increase is observed throughout the daytime, lasts for about 9h, and covers F-region altitudes above ∼230km. At the altitude of 300km, the maximum increase in N e reaches a factor of 3 by 19:30–20:00UT and is accompanied by a ∼1000K decrease in electron temperature, a ∼100–150K increase in ion temperature, and a strong upward drift. Observations by Arecibo ISR (18.3°N, 293.3°E) reveal similar features, with the maximum increase in electron density reaching a factor of 2.5 at 21:30UT, i.e. 1.5–2h later than over Millstone Hill. The GPS TEC data show that the increase in electron density observed at Millstone Hill and Arecibo is only a part of a global picture reflected in TEC. The increase in TEC reaches a factor of 2 and covers middle and low latitudes at 19UT. At later times this increase moves to lower latitudes. A combination of mechanisms were involved in generation of positive phase. The penetration electric field resulted in N e enhancements at subauroral and middle latitudes, the TAD/TID played an important role at middle and lower latitudes, and increase in O/N2 ratio could contribute to the observed positive phase at middle and lower latitudes. The results show the importance of an upward vertical drift at ∼140–250km altitude, which is observed for sustained period of time and assists in the convergence of ionization into the F-region. [Copyright &y& Elsevier]

Abstract: In this paper, we use the modified GSM TIP model to explore how the thermosphere–ionosphere system in the American longitudinal sector responded to the series of geomagnetic storms on September 9–14, 2005. Comparison of modeling results with experimental data at Millstone Hill, USA (42.6°N, 71.5°W), Ramey, Puerto Rico (18.3°N, 66.8°W) and Jicamarca, Peru (11.9°S, 76.9°W) has shown a good agreement of ionospheric disturbances in the F-region maximum height. We examine in detail the formation mechanisms of these disturbances at different latitudes and describe some of the important physical processes affecting the behavior of the F-region. In addition, we consider the propagation of thermospheric wind surge and the formation of additional layers in the low-latitude ionosphere during geomagnetic storms. [Copyright &y& Elsevier]