Ideal Magnetohydrodynamic Solution to the σ Problem in Crab‐like Pulsar Winds and General Asymptotic Analysis of Magnetized Outflows

Using relativistic, steady, axisymmetric, ideal magnetohydrodynamics (MHD) we analyze the super-Alfvenic regime of a pulsar wind by means of solving the momentum equation along the flow as well as in the transfield direction. Employing a self-similar model, we demonstrate that ideal MHD can account for the full acceleration from high (>>1) to low (<<1) values of $\sigma$, the Poynting-to-matter energy flux ratio. The solutions also show a transition from a current-carrying to a return-current regime, partly satisfying the current-closure condition. We discuss the kind of the boundary conditions near the base of the ideal MHD regime that are necessary in order to have the required transition from high to low $\sigma$ in realistic distances, and argue that this is a likely case for an equatorial wind. Examining the MHD asymptotics in general, we extend the analysis of Heyvaerts & Norman and Chiueh, Li, & Begelman by including two new elements: classes of quasi-conical and parabolic field line shapes that do not preclude an efficient and much faster than logarithmic acceleration, and the transition $\sigma=\sigma_c$ after which the centrifugal forces (poloidal and azimuthal) are the dominant terms in the transfield force-balance equation.Comment: 14 pages, 7 figures, ApJ accepte