Global asymptotic solutions for non-relativistic MHD jets and winds

Abstract

We present general and global analytical solutions, valid from pole to equator, for the asymptotic structure of non-relativistic, rotating, stationary, axisymmetric, polytropic, unconfined, perfect MHD winds. The asymptotic structure of such winds consists of field-regions virtually devoid of poloidal current. We show that an Hamilton-Jacobi equation, or equivalently a Grad-Shafranov equation, gives the asymptotic structure in the field regions.These field regions are bordered by current-carrying boundary layers around the polar axis and near null magnetic surfaces. The solution is given in the form of matched asymptotics separately valid outside and inside these boundary layers. The polar boundary layer is pressure-supported against the pinching force exerted by the axial poloidal current and has the structure of a current pinch, while the null-surface boundary layers have the structure of current sheet pinches.For winds which are kinetic-energy dominated at infinity we derive WKBJ analytic solutions whose magnetic surfaces focus into paraboloids. The current slowly weakens as the inverse of the logarithm of the distance to the wind source while the axial plasma density falls-off as a negative power of this logarithm.For winds carrying Poynting flux at large distances the solutions asymptotically approach to nested cylindrical and conical magnetic surfaces.