The Role of Ambipolar Diffusion in the Formation Process of Moderately Magnetized Diffuse Clouds

We investigate the dynamical condensation process in a magnetized thermally bistable medium. We perform one-dimensional two fluid numerical simulations that describe the neutral and ionized components in the interstellar medium with purely transverse magnetic fields. We find that the clouds that are formed as a consequence of the thermal instability always have a magnetic field strength on the order of a few microgauss irrespective of the initial strength. This shows good agreement with the measurements of the magnetic field strength in diffuse clouds. We show analytically that the final magnetic field strength in the clouds is determined by the balance between ambipolar diffusion and the accumulation of the magnetic field due to condensation.