Chandrasekhar & Fermi (1953) obtained the strength of interstellar
Applying the same method to the molecular cloud,
Crutcher et al. (2004) obtained the magnetic field strength in the plane of the sky
in three prestellar clouds as
The method is as follows:
Using the phase velocity of transverse Alfvén wave,
the wave is described as
where is a displacement in the plane of the sky and perpendicular
to the mean magnetic field.
where represents the time average.
The rhs of the equation, which is the one-dimensional velocity dispersion
in direction, seems to be equal to
the velocity dispersion in the line-of-sight
The lhs of the equation is given with a dispersion
of polarization direction (rad)
in which the lhs is measured by the wave-pattern of the Alfvén wave.
Therefore, the magnetic field strength is given as