Determination of the electromagnetic field produced by a magnetic oblique-rotator

Abstract

The structure of the corotating region, which forms an inner portion of a stellar magnetosphere, is reconsidered in a quasi-neutral case by taking into account the inertial effects of electrons as well as that of ions up to the first order in their mass ratio (δ=m₋/m₊). It is emphasized first that the magnetosphere is not globally equipotential even in the frame rotating with a central star (i.e. ϕ#0, where ϕ is the ‘non-Backus’ potential) due at least to the inertial effects of plasma particles. However, it is shown that the condition ϕ=0 is asymptotically recovered in the corotating region owing to the presence of the drift current which can be taken into account only when δ is not entirely neglected. This fact suggests that the deviation of the plasma motion in the outer magnetosphere from the corotation can be attributed to the non-zero ϕ. A globally self-consistent solution is obtained under this condition (ϕ=0). In contrast with the solutions in the ‘force-free’ and the ‘mass-less-electron’ approximations, this solution has a disk structure in the corotation zone in which the plasma and the current density are concentrated to a thin disk near the magnetic equator. Owing to this sheet current in the disk the lines of force of the stellar magnetic field are modified to form a very elongated shape (the magnetodisk) if the plasma β-value is fairly large. Such a disk structure seems to be a common feature in the high β inner magnetospheres of various types of stars.