A TECHNIQUE FOR CONSTRAINING THE DRIVING SCALE OF TURBULENCE AND A MODIFIED CHANDRASEKHAR–FERMI METHOD

Abstract

ABSTRACT The Chandrasekhar–Fermi (CF) method is a powerful technique for estimating the strength of the mean magnetic field projected on the plane of the sky. In this paper, we present a technique for improving the CF method in which we take into account the averaging effect arising from independent eddies along the line of sight (LOS). In the conventional CF method, the strength of fluctuating magnetic field divided by 4 π ρ ¯ , where ρ ¯ is average density, is assumed to be comparable to the LOS velocity dispersion. However, this is not true when the driving scale of turbulence L f , i.e., the outer scale of turbulence, is smaller than the size of the system along the LOS L los. In fact, the conventional CF method overestimates the strength of the mean plane-of-the-sky magnetic field by a factor of ∼ L los / L f . We show that the standard deviation of centroid velocities divided by the average LOS velocity dispersion is a good measure of L los / L f , which enables us to propose a modified CF method.