Longitudinal structure in the ionosphere due to UT onset time of a geomagnetic storm

Recent observations have indicated that geomagnetic storms cause larger enhancements in the ionospheric plasma density and total electron content (TEC) in the American sector than anywhere else on the planet. This suggests the presence of a UT storm onset effect that is important for correctly understanding the impact, longitudinal structure and timing of geomagnetic storms. Using the Global Ionosphere-Thermosphere Model (GITM) we have conducted a modeling experiment of the August 2011 geomagnetic storm by modifying the storm arrival time (UT) in Earth's daily rotation and examining the subsequent system response. We found that the simulations reflect the recent studies indicating the strongest enhancements of TEC are in the American and Pacific longitude sectors of storms that onset between 1600 UT and 2400 UT. The underlying mechanisms of the strong TEC increases during storm times in these longitude sectors were also examined. Some of the high-latitude TEC structure may be explained by changes in the [O]/[N2] ratio, but it is unable to explain all of the variability in the equatorial regions. Storm time neutral winds and vertical ion motions coupled to Earth's asymmetrical geomagnetic topology do appear to be driving the longitude sector variability due to UT storm onset times.