Magnetospheric models of pulsars–outstanding questions

Abstract

The difficulties of the pulsar magnetosphere problem are illustrated by two models for the axisymmetric magnetic rotator: (a) a classical model, in which the return current problem is linked with angular momentum dissipation’ through incoherent gamma radiation beyond the light-cylinder; and (b) a quantum model, in which electron positron pairs are produced near the star, and spin-down is primarily through the transport of angular momentum by an e+-e-wind. The first model has some pedagogic value, but it makes the embarrassing prediction that all the spin-down energy is emitted as gammaradiation. In the second model a predicted small but significant emission in gamma-and X-rays is again linked with the return current problem, but the bulk of the energy emission is through the wind. The presence of both a high-γ primary electron beam and a moderate-γ pair plasma is as required by most models of coherent microwave emission. Problems persist in ensuring that the macroscopic conditions on the electric field are locally consistent with the microphysics. The most promising picture is of a spontaneous local hydromagnetic instability that yields a large effective resistivity. Some remarks are made on future extension of the theory to oblique geometry. For agreement with observations of the Crab nebula, the predicted dominance in the wind of the Poynting flux must be reversed in the far magnetosphere.