Collimation mechanism of magnetohydrodynamic jets based on helicity and cross-helicity dynamos, with reference to astronomical jets

Abstract

A collimation mechanism of magnetohydrodynamic jets is sought on the basis of the turbulent-dynamo theory within the framework of fluid incompressibility. The momentum diffusion giving rise to the broadening of the jets is suppressed through the cross-helicity effect, whereas the diffusion of the magnetic field is hampered owing to the helicity effect. These two effects lead to the collimation of electrically conducting jets in the presence of turbulent fluctuations of the velocity and magnetic field. The relevance to collimated astronomical jets is discussed with the reservation about effects of fluid compressibility.