Magnetic Field Dissipation in Dense Molecular Clouds

In dense molecular clouds the grains become the main charged particles. When the hydrogen gas density reaches as high as nH = 10¹¹cm^-3 and the grains are assumed to be small polycyclic aromatic hydrocarbons(PAHs), the lower hybrid drift instability develops, leading to the creation of anomalous electric conductivity, which makes the magnetic Reynold's number of the system R_M ~ 1 indicating the dissipation of magnetic fields. When the gas density continues to go up and over10¹¹cm^-3, the collision frequency between PAHs and gas particles becomes larger than the plasma frequency, resulting in the collision process being dominant. In that case the magnetic field also dissipates because RM ~ 1 is still kept.