Multi-event study of mid-latitude ionospheric responses to substorms using comprehensive ground observations and modeling

Nighttime 630-nm airglow is a red emission at typical altitudes of 200-300 km in the ionospheric F region. The intensity of the emission is proportional to the product of O+ and O2 densities. Thus, ionospheric height variation can be monitored by 630-nm airglow. When the Region 1 current dominates the substorm current wedge during the onset of a substorm, westward electric field penetrates into the mid-latitude ionosphere on the night side. The oblique downward ExB drift associated with this westward electric field causes O+ ions in the F-region ionosphere to penetrate to lower altitudes which has higher O2 density, resulting in an enhancement of the 630-nm airglow. Shiokawa et al. [2000] reported two events that observed this airglow enhancement with two-dimensional all-sky camera images at mid-latitudes. However, there have been no similar reported events, and only one observation station was used for these two events, so the spatial extent and simultaneity of the enhancement were unknown.
In this study, we used all-sky cameras installed at three stations at middle latitudes in Japan to investigate 630-nm airglow enhancement events due to electric field penetration associated with substorms. We report seven such cases based on 11-year data analysis in 2002-2012. This multi-event analysis shows that simultaneous 630-nm airglow enhancement at two or more stations is very rare. Five cases were observed during storms, and three cases were identified as enhancement occurring with the onset of substorms. These results suggest that simultaneous airglow enhancement over Japan requires a large penetrating electric field associated with storm-time substorms. In all 7 cases, a downward motion of the ionosphere was observed during the airglow enhancement. Therefore, these enhancements are considered to be due to the westward electric field in the undershielding state. For the event on October 24, 2003, which was a particularly very intense substorm event, we report a detailed analysis of the mid-latitude ionospheric and thermospheric response using geomagnetic, airglow, GNSS-TEC, ionosonde, and a Fabry-Perot interferometer. We will also compare these ground observations with the MAGE model and discuss the connection between polar and mid-latitude ionosphere and the causes of the penetrating electric field.