O and N₂disturbances in the<i>F</i>region during the 20 November 2003 storm seen from TIMED/GUVI

Abstract

[1] The ratio of height-integrated atomic oxygen number density, [O], to molecular nitrogen number density, [N2], defined as Σ[O]/[N2], has been used as an indicator of the thermosphere-ionosphere coupling during geomagnetic storms. However, the disturbance in [O]/[N2] in the F region can be different from the disturbance in Σ[O]/[N2], and knowledge of the storm-time behaviors of [O] and [N2] is necessary to precisely describe the thermospheric neutral composition disturbance and its effect on the ionosphere. In this study, we examine the separate roles of [O] and [N2] in modifying the F region [O]/[N2] and Σ[O]/[N2] by analyzing far ultraviolet images made by Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics (TIMED)/Global Ultraviolet Imager (GUVI) during the severe 20 November 2003 geomagnetic storm. The GUVI observation shows that the fractional storm-time change in [N2] is greater than that in [O] in the F region in areas where Σ[O]/[N2] is both decreased and increased. Therefore the disturbance in [O]/[N2] in the F region is primarily determined by the change in [N2]. The reduction in [O]/[N2] in the F region during the storm period is consistent with the reduction in Σ[O]/[N2]. However, only a minor change of the F region [O]/[N2] is observed in the region where the increase in Σ[O]/[N2] is observed. The TEC increase observed at the locations of the Σ[O]/[N2] increase in the American-Atlantic sectors is associated with the poleward extension of the equatorial ionization anomaly rather than with the neutral composition change in the F region.