Multiple-Event Study of Substorm Electric Field Penetration Into Middle Latitudes Based on Simultaneous Observation of 630-nm Airglow Enhancements at Three Stations

Abstract

It is known that convection electric fields develop in the polar region associated with substorm onset, and a westward electric field penetrates to lower latitudes on the night side. Two-dimensional observations of 630-nm airglow enhancements associated with the westward electric field can be conducted using all-sky cameras at middle latitudes. We investigated seven simultaneous 630-nm airglow enhancement events using all-sky cameras at Rikubetsu (43.5°N, 143.8°E), Shigaraki (34.9°N, 136.1°E), and Sata (31.0°N, 130.7°E) in Japan. A multi-event multi-point study of 630-nm airglow enhancement has been conducted for the first time. These seven events were found from 1,800 nights in 11 years, from 2002 to 2012, indicating that simultaneous 630-nm airglow enhancements over Japan were very rare. Five of these events occurred during storms, suggesting that simultaneous enhancement requires a large penetrating electric field associated with storm-time substorms. In three events, the enhancement was identified as occurring simultaneously with clear substorm onsets, while the other four events were also associated with high-latitude magnetic disturbances. In all seven events, we observed downward motion of the ionosphere associated with the airglow enhancement. These results indicate that they are due to the westward electric field in the substorm undershielding state. In two events, the time at the end of the 630-nm airglow enhancement correlated well with the increase in ionospheric altitude observed by ionosondes and an FM-CW radar. This increase in ionospheric altitude and termination of the airglow enhancement could be caused by eastward electric field penetration associated with overshielding in the magnetosphere.