Magnetic Field and Faraday Rotation Measure in the Turbulent Warm
  Ionized Medium
  Wu et al. (2009) suggested an empirical relation between the magnetic field
strength along the line of sight (LOS) and the dispersion of Faraday rotation
measure (RM) distribution in turbulent media with root-mean-square sonic Mach
number $M_s \simeq 1$. In this paper, we extend the work by incorporating the
Mach number dependence. Media with $\sim 0.5 < M_s < \sim 2$ are considered to
cover the Mach number range of the warm ionized medium (WIM) of our Galaxy.
Three-dimensional, magnetohydrodynamic isothermal turbulence simulations with
solenoidal forcing are used. We suggest a new relation among the LOS magnetic
field strength, the dispersion of RM distribution, and the Mach number, which
approximately represents the relation for Alfv\'enic perturbations. In
addition, we suggest a relation between the Mach number and the dispersion of
log-normal distribution of emission measure (EM), which is basically the
relation for the Mach number and the density dispersion. The relations could be
used for a quick and rough estimation of the LOS magnetic field strength in the
turbulent WIM.
