Magnetosphere-Ionosphere Coupling Between North-South Propagating Streamers and High-Speed Earthward Flows

摘要

The magnetosphere-ionosphere coupling between the high-speed earthward flows and the north-south oriented streamers in the ionosphere has been discussed for decades, but to date, no one has examined the formation of that coupling from the start of the streamer to the end. We investigate the formation and development of the magnetosphere-ionosphere coupling using THEMIS observations of high-speed earthward flows within the magnetotail, simultaneous auroral all-sky images, and ionospheric equivalent current maps derived from ground magnetometer measurements. We show the formation of a downward field-aligned current on the dawnside of the north-south streamer and the upward current on the duskside, as well as the vortices within the equivalent currents around the field-align-like currents from the poleward boundary to the equatorward edge of the auroral oval for two substorm streamer events. By removing the background Birkeland current system, we can determine the current densities uniquely associated with the streamer current wedge and demonstrate that the downward and upward currents within the streamer are approximately balanced for one event. Furthermore, we find that the longitudinal size of the streamer current wedge is more transient and localized, and does not change, whereas the substorm current wedge is larger and expands during the first part of the substorm.