Magnetospheric sash dependence on IMF direction

The magnetospheric sash is a ribbon of weak field shaped like a horseshoe w ith its open ends adjacent to the north and south dayside, magnetopause cus ps and its closed end forming the cross-tail current sheet. The clock angle of the sash in the dawn-dusk meridian plane (as seen from the sun) rotates from 0 degrees to 90 degrees as the clock angle of the interplanetary magn etic field (IMF) rotates from 0 degrees to 180 degrees. We use a global MHD simulation to obtain the sash clock angles for IMF clock angles of 45 degr ees, 90 degrees, and 135 degrees. Remarkably, the results are very close to the clock angle of the magnetic null points obtained by superposing a unif orm field representing the IMF on a dipole Field representing the earth. Co ntours of magnetic field strength on cross sections perpendicular to the so lar wind flow direction show how the sash evolves tailward from the dayside cusps.