A technique for constraining the driving scale of turbulence and a
  modified Chandrasekhar-Fermi method
  The Chandrasekhar-Fermi 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 Chandrasekhar-Fermi method, in
which we take into account the averaging effect arising from independent eddies
along the line of sight . In the conventional Chandrasekhar-Fermi method, the
strength of fluctuating magnetic field divided by $\sqrt{4 \pi \bar{\rho}}$,
where $\bar{\rho}$ is average density, is assumed to be comparable to the
line-of-sight velocity dispersion. This however 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 line of sight $L_{los}$. In fact, the
conventional Chandrasekhar-Fermi method over-estimates the strength of the mean
plane-of-the-sky magnetic field by a factor of $\sim \sqrt{ L_{los}/L_f}$. We
show that the standard deviation of centroid velocities divided by the average
line-of-sight velocity dispersion is a good measure of $\sqrt{ L_{los}/L_f}$,
which enables us to propose a modified Chandrasekhar-Fermi method.
