ar X iv : a st ro - p h / 03 05 28 8 v 1 1 5 M ay 2 00 3 On the Axisymmetric Force - Free Pulsar Magnetosphere

We investigate the axisymmetric magnetosphere of an aligned rotating magnetic dipole surrounded by an ideal force-free plasma. We concentrate on the magnetic field structure around the point of intersection of the separatrix between the open and closed field-line regions and the equatorial plane. We first study the case where this intersection point is located at the Light Cylinder. We find that in this case the separatrix equilibrium condition implies that all the poloidal current must return to the pulsar in the open-field region, i.e., that there should be no finite current carried by the separatrix/equator current sheet. We then perform an asymptotic analysis of the pulsar equation near the intersection point and find a unique self-similar solution; however, a Light Surface inevitably emerges right outside the Light Cylinder. We then perform a similar analysis for the situation where the intersection point lies somewhere inside the Light Cylinder, in which case a finite current flowing along the separatrix and the equator is allowed. We find a very simple behavior in this case, characterized by a 90-degree angle between the separatrix and the equator and by finite vertical field in the closed-field region. Finally, we discuss the implications of our results for global numerical studies of pulsar magnetospheres. Subject headings: MHD — pulsars:general — magnetic fields preprint NSF-KITP-03-36