Small structures via thermal instability of partially ionized plasma. I. condensation mode

Thermal instability of partially ionized plasma is investigated by means of a linear perturbation analysis. The previous studies under the one-fluid approach did not precisely consider the effect of the ion-neutral friction, since they did not treat the flow as two fluids which are composed of ions and neutrals. Then, we revisit the effect of the ion-neutral friction of the two fluid on the growth of the thermal instability. According to our study: ( 1) The instability which is characterized by the mean molecular weight of neutrals is suppressed via the friction only when the magnetic field and the friction are sufficiently strong. The suppression owing to the friction occurs even along the field line. If the magnetic field and the friction are not so strong, the instability is not stabilized. ( 2) The effect of the friction and the magnetic field is mainly reduction of the growth rate of the instability. ( 3) The effect of friction does not affect the critical wavelength lambda(F) for the instability. This yields that lambda(F) is not enlarged even when the magnetic field exists. We insist that the thermal instability of the weakly ionized plasma in the magnetic field can grow up even at the small length scale where the instability under the assumption of the one-fluid plasma cannot grow owing to the stabilization by the magnetic field. ( 4) The wavelength of the maximum growth rate shifts shortward according to the decrement of the growth rate. Therefore, smaller structures than those without the friction are expected to appear. Our results indicate that the friction with the magnetic field affects the morphology and evolution of the interstellar matter.