Chandrasekhar & Fermi (1953) obtained the strength of interstellar
magnetic field.
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
(L183),
(L1544),
and
(L43).
The method is as follows:
Using the phase velocity of transverse Alfvén wave,
,
the wave is described as

(4.38) 
where is a displacement in the plane of the sky and perpendicular
to the mean magnetic field.
This gives

(4.39) 
where represents the time average.
The rhs of the equation, which is the onedimensional velocity dispersion
in direction, seems to be equal to
the velocity dispersion in the lineofsight
as

(4.40) 
The lhs of the equation is given with a dispersion
of polarization direction (rad)

(4.41) 
in which the lhs is measured by the wavepattern of the Alfvén wave.
Therefore, the magnetic field strength is given as

(4.42) 
Kohji Tomisaka
20121003