Geotail measurements compared with the motions of high‐latitude auroral boundaries during two substorms

Abstract

Geotail plasma and field measurements at −95 RE are compared with extensive ground‐based, near‐Earth, and geosynchronous measurements to study relationships between auroral activity and magnetotail dynamics during the expansion phases of two substorms. The studied intervals are representative of intermittent, moderate activity. The behavior of the aurora and the observed effects at Geotail for both events are harmonized by the concept of the activation of near‐Earth X lines (NEXL) after substorm onsets with subsequent discharges of one or more plasmoids down the magnetotail. The plasmoids must be viewed as three‐dimensional structures which are spatially limited in the dawn‐dusk direction. Also, reconnection at the NEXL must proceed at variable rates on closed magnetic field lines for significant times before beginning to reconnect lobe flux. This implies that the plasma sheet in the near‐Earth magnetotail is relatively thick in comparison with an embedded current sheet and that both the NEXL and distant X line can be active simultaneously. Until reconnection at the NEXL engages lobe flux, the distant X line maintains control of the poleward auroral boundary. If the NEXL remains active after reaching the lobe, the auroral boundary can move poleward explosively. The dynamics of high‐latitude aurora in the midnight region thus provides a means for monitoring these processes and indicating when significant lobe flux reconnects at the NEXL.