The Distant Magnetotail: Its Structure, IMF Dependence, and Thermal Properties

Abstract

Magnetic and thermal structures of the distant magnetotail as surveyed recently by the GEOTAIL spacecraft are reviewed. An analysis of the distant (-220 Re<x<-150 Re) tail structure is synthesized based on a coordinate system whose x-axis is taken to be parallel to the hourly direction of the solar wind measured in the upstream region. In this coordinate system, the distant magnetotail is found to be cylindrical, with almost the same dimension (about 50∼55 Re) in the y and z directions for average IMF conditions. The most prominent feature of the distant magnetotail is that the tail current sheet is twisted under the presence of IMF B y . The twist angle is largcr for northward IMF than for southward IMF, showing that significant amounts of magnetospheric field lines are reconnected with the solar wind field lines even when the IMF is directed northward. The plasma mantle which is known to be formed in the high-latitude magnetosphere near the Earth is twisted in the distant magnetotail and forced against the plasma sheet owing to the large torque exerted by the IMF. Examination of individual ion distribution functions near the boundary suggests that the plasma mantle is often located adjacent to the plasma sheet. Sometimes an accelerated ion beam is superposed on the mantle plasma, suggesting that mantle field lines are reconnected at the plasma sheet boundary. An analysis of the magnetic field component normal to the twisted neutral sheet supports the view that reconnection occurs within the distant neutral sheet even during quiet times when the IMF is directed northward.