Relationships between the ion flow speed, magnetic flux transport rate, and other plasma sheet parameters

摘要

Eight years of Geotail particle and magnetic field measurements were separated into 12 data sets on the basis of the ion flow speed. The same measurements were separated into 12 other data sets using a magnetic flux transport sorting parameter. Magnetic field lines in the three-dimensional models created using these two sorting methods were dipolar when the flow or transport was fast and stretched into a taillike configuration when the flow or transport was slow. The magnitude of B x measured in the outer central plasma sheet decreased weakly and B z at the neutral sheet increased strongly as the magnetic flux transport rate increased. These observations showed that fast flow flux tubes typically were located near but earthward of the primary region in which localized region 1 sense currents were diverted to the ionosphere. The plasma density was low and the temperature was high when the flow was fast. The particle pressure depended only weakly on flow speed. The average entropy was higher at z = 0 during fast flow events than it was anywhere in the region that could be studied when flows were slow or moderate. The average entropy also decreased as ∣z∣ increased. These observations suggest that the plasma was irreversibly heated by the process that produced the fast flows. Ions and electrons were found, on average, to be remarkably isotropic at the neutral sheet. Scattering through 90° each minute during slow and moderate flow conditions and as rapidly as every 10 s during the fastest flows was needed to maintain this average degree of isotropy. The temperature anisotropy increased away from the neutral sheet, reaching 1.1–1.3 at some point along most field lines. This variation along field lines was attributed primarily to a parallel electric field needed to maintain charge neutrality. The average ion to electron temperature ratio was as low as 5 and as high as 10 in certain regions and under specific flow conditions. These observations showed that electrons and ions were heated or cooled at different rates depending on their locations and bulk flow speeds.