On the variation in the ionospheric response to geomagnetic storms with time of onset

Recent observations from Immel and Mannucci (2013) 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 that the presence of a UT storm onset effect is important for correctly understanding the impact, longitudinal structure, and timing of geomagnetic storms. Using the Global Ionosphere-Thermosphere Model (GITM), we conduct 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 find that the simulations reflect the recent studies indicating that the strongest enhancements of TEC are in the American and Pacific longitude sectors of storms with onsets between 1600 UT and 2400 UT. The underlying mechanisms of the strong TEC increases during storm times in these longitude sectors are also examined. Some of the resulting TEC structures may be explained by changes in the [O]/[N-2] ratio (especially in the high latitudes), 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 appear to be driving the longitude sector variability due to UT storm onset times.