Evolvement of the Ring Current During a Magnetic Storm: TC‐2 Neutral Atom Imager Observations

Abstract

AbstractA series of ENA images were obtained by the NUADU instrument aboard TC‐2 during a strong, long duration magnetic storm that occurred in November 2004. This paper follows the evolution of the ring current during this magnetic storm. It is shown that the southward IMF Bz component plays a key role in facilitating ion injection from the magnetotail into the inner magnetosphere with consequent enhancement of the ring current. Ion injection stopped soon after the IMF turned from southward to northward. Also during ion injection episodes, the ion drift paths opened and ring current ions were lost at the noon to dusk magnetopause soon after they were injected at the dusk side region. As a result, the ring current intensified after the occurrence of an open‐to‐closed transformation of the ion drift paths, rather than at the stage when ion injection was strong. In the main phase of the storm, ion injection took place between local times about 17:00~22:00, leading to an extremely asymmetric ring current. In contrast in the recovery phase of the storm, the ion injection region reached the post‐midnight sector. We attributed this phenomenon to a significant positive IMF By component. A symmetrical ring current was formed after the drift paths changed back to a closed configuration due to a decrease in tail convection. When the storm entered its late recovery phase, the ring current faded away while the Dst still indicated a strong magnetic disturbance. This suggested an important contribution to Dst from the cross tail current.