In the dense clouds, the ionization fraction is low.
Since the uv/optical radiations from stars can not reach the cloud center,
potential ionization comes from the cosmic ray particles.
In this case the rate of ionization per volume is given as ,
In Figure 4.6, the ionization fraction for various density is shown
(Nakano, Nishi, & Umebayashi 2002).
This clearly shows that the fraction of ions decrease approximately in proportion to
This is understood as follows:
Equilibrium balance between one kind of ion and its neutral
Kohji Tomisaka 2012-10-03