Simultaneous measurements of magnetotail dynamics by IMP spacecraft

Pressure in the magnetotail is investigated using observations obtained by IMP spacecraft. Using combined field and plasma data, the approximate balance of pressure between the high beta plasma sheet and the low beta tail lobes is demonstrated. The changes in this pressure during substorms and the characteristics of the plasma and field that produce it are discussed. A distance of about 15 earth radii separates an inner region where the plasma sheet thins during the hour before substorm onset from an outer region where the plasma sheet thins within 5 or 10 min of the time of onset. Substorm onset and plasma sheet expansion in the inner region are simultaneous if the spacecraft is near the equatorial plane. This expansion may be delayed as much as a few tens of minutes if the spacecraft is at high latitudes.