Factors controlling the shape and size of the post-terminator magnetopause

Abstract

The position and shape of the post-terminator magnetospheric boundary is studied as a function of downtail distance, solar wind dynamic pressure, and the z-component of the IMF. We have used 360 orbits of ISEE-2 for which plasma data are available to determine when the spacecraft was in the lobe of the magnetotail. The lobe pressure has then been calculated from the total magnetic field measured by ISEE-2, and simultaneous calculations of the solar wind pressure components (dynamic, thermal, and magnetic) are made from IMP-8 measurements. Perpendicular pressure balance between the magnetotail lobes and the solar wind is used to calculate a set of flare angles for the magnetotail boundary. The flare angle of the magnetotail is found to increase for southward IMF, but the variation for northward IMF is much weaker, indicating that the magnetotail size increases appreciably only for southward IMF. The flaring angle has been integrated to get the average shape and size of the magnetotail, matching the boundary to the average dayside magnetopause shape at the terminator. The magnetotail shape and size has been determined as a function of the solar wind condition. We have compared this model with other empirical models of the magnetotail size and shape, as well as a regression to IMP-6 magnetotail boundary crossings. Comparisons are made for nominal as well as extreme solar wind conditions. We find that the tail size depends more strongly on the solar wind dynamic pressure than predicted by the Roelof and Sibeck [1993] model, but is in excellent agreement with the dependence obtained by Lui [1986]. This tail model has been used to examine several sudden impulse events, comparing the tail magnetic flux content for those events which trigger substorms and for those that do not.