AN EXAMINATION OF THE EFFECT OF DIPOLE TILT ANGLE AND CUSP REGIONS ONTHE SHAPE OF THE DAYSIDE MAGNETOPAUSE

The shape of the dayside magnetopause has been studied from both a the oretical and an empirical perspective for several decades. Early theor etical studies of the magnetopause shape assumed an inviscid interacti on and normal pressure balance along the entire boundary, with the int erior magnetic field and magnetopause currents being solved self-consi stently and iteratively, using the Biot-Savart Law. The derived shapes are complicated, due to asymmetries caused by the nature of the dipol e field and the direction of flow of the solar wind. These models cont ain a weak field region or cusp through which the solar wind has direc t access to the ionosphere. More recent MHD model results have indicat ed that the closed magnetic field lines of the dayside magnetosphere c an be dragged tailward of the terminator plane, so that there is no di rect access of the magnetosheath to the ionosphere. Most empirical stu dies have assumed that the magnetopause can be approximated by a simpl e conic section with a specified number of co-efficients, which are de termined by least squares fits to spacecraft crossing positions. Thus most empirical models resemble more the MHD models than the more compl ex shape of the Biot-Savart models. In this work, we examine empirical ly the effect of the cusp regions on the shape of the dayside magnetop ause, and we test the accuracy of these models. We find that during pe riods of northward IMF, crossings of the magnetopause that are close t o one of the cusp regions are observed at distances closer to Earth th an crossings in the equatorial plane. This result is consistent with t he results of the inviscid Biot-Savart models and suggests that the ma gnetopause is less viscous than is assumed in many MHD models.