Dayside enhancements of thermospheric O/N 2 following magnetic storm onset

Abstract

One frequently observed effect of thermospheric storms is the reduction of atomic oxygen relative to molecular nitrogen at high and middle latitudes. These composition changes lead to a decrease in thermospheric O I 130.4‐nm emissions in the sunlit hemisphere. Such decreases have been observed by various satellite‐based instruments including the Dynamics Explorer 1 (DE 1) Spin‐Scan Auroral Imager. In contrast, this paper focuses on enhancements of the terrestrial 130.4‐nm dayglow emission observed with DE 1. Following the onset of a geomagnetic storm at 1610 UT on February 5, 1983, an increase (>20%) in the O I 130.4‐nm emission was observed at middle latitudes in the morning sector of the Southern Hemisphere. The increased O I 130.4‐nm emission indicates an increase of atomic oxygen relative to molecular nitrogen. The brightness enhancement coincided with the passage of a large‐scale gravity wave that was observed in measurements from a global network of ionosondes. The global FUV images and ionosonde observations are complemented by a TIMEGCM simulation of the February 1–6 period, which provides a framework for combining the ionosonde and DE 1 observations. The primary mechanisms for the FUV variations considered here are increases in the relative abundance of atomic oxygen in the morning sector caused by (1) the large‐scale gravity wave launched by the onset of magnetic activity and (2) corotation of previously affected parcels onto the dayside. The investigation leads to a physical interpretation of the observed FUV features in terms of the gravity wave effects and a transient Hadley circulation cell. This work represents the first detection of a large‐scale gravity wave from an orbiting FUV imager.