Magnetic Fields in Molecular Clouds -- Observation and Interpretation
  The Zeeman effect and dust grain alignment are two major methods for probing
magnetic fields (B-fields) in molecular clouds, largely motivated by the study
of star formation, as the B-field may regulate gravitational contraction and
channel turbulence velocity. This review summarizes our observations of
B-fields over the past decade, along with our interpretation. Galactic B-fields
anchor molecular clouds down to cloud cores with scales around 0.1 pc and
densities of 10^4-5 H_2/cc. Within the cores, turbulence can be slightly
super-Alfvenic, while the bulk volumes of pa-rental clouds are sub-Alfvenic.
The consequences of these largely ordered cloud B-fields on fragmentation and
star formation are observed. The above paradigm is very different from the
generally accepted theory during the first decade of the century, when cloud
turbulence was assumed to be highly super-Alfvenic. Thus, turbulence anisotropy
and turbulence-induced ambipolar diffusion are also revisited.
