Abstract

The spin-down power of a pulsar is thought to be carried away in anMHDwind in which, at least close to the star, the energy transport is dominated by Poynting flux. The pulsar drives a low frequency wave in this wind, consisting of stripes of toroidal magnetic field of alternating polarity, propagating in a region around the equatorial plane. The current implied by this configuration falls off more slowly with radius than the number of charged particles available to carry it, so that the MHD picture must, at some point, fail. Recently, magnetic reconnection in such a structure has been shown to accelerate the wind significantly. This reduces the magnetic field in the comoving frame and, consequently, the required current, enabling the solution to extend to much larger radius. This scenario is discussed and, for the Crab Nebula, the range of validity of the MHD solution is compared with the radius at which the flow appears to terminate. For sufficiently high particle densities, it is shown that a low frequency entropy wave can propagate out to the termination point. In this case, the ‘termination shock’ itself must be responsible for dissipating the wave.

This paper is dedicated to Don Melrose on his 60th birthday.