Neutral composition effects on ionospheric storms at middle and low latitudes

Abstract

[1] The two-dimensional structure of thermospheric neutral composition, specifically, the atomic oxygen to molecular nitrogen column density ratio, [O/N2], is studied during the 17-24 April 2002 geomagnetic storms to understand the cause of ionospheric storms in regions equatorward of the auroral oval on an instantaneous large scale. The [O/N2] ratio is derived from the dayglow emission ratio of 011356 Å to N2 Lyman-Birge-Hopfield (1600-1800 Å) acquired from the Polar ultraviolet imager (UVI) and the total electron content (TEC), which is used to infer ionospheric storms, is derived from the phase delays of dual-band global positioning satellite (GPS) accumulated around the globe. It is found that the regions of decreasing [O/N2] generally coincided with the regions of depleted TEC during and after the development of the storms. This is consistent with previous theoretical and experimental analysis in which composition changes play a major role in the negative ionospheric storm effects. At lower latitudes, long-lived positive storm effects predicted by empirical and general circulation models were not observed. In fact, there was no noticeable change in [O/N2]. The TEC data also showed no noticeable change, except a few "short-lived," localized positive TEC perturbations. For this particular event, the equatonward expansion of the decreased [O/N2] at the onset of the first storms was estimated to be more than 600 m/s, much faster than the typical thermospheric wind, and reached ∼30°N in 2 hours much equatorward (>25° MLAT) beyond the auroral electrojets. This result suggests that midlatitude (negative) ionospheric storms are caused by direct equatorward penetration of a reduced thermospheric [O/N2], at least during the first few hours of storms. Copyright 2005 by the American Geophysical Union.