Small Structures via Thermal Instability of Partially Ionized Plasma I. Condensation Mode Tsubasa Fukue and Hideyuki Kamaya

Thermal instability of partially ionized plasma is investigated by means of a linear perturbation analysis. According to the previous studies under the one fluid approach, the thermal instability is suppressed due to the magnetic pressure. However, the previous studies did not precisely consider the effect of the ion-neutral friction, since they did not treat the flow as two fluid which is composed of ions and neutrals. Then, we revisit the effect of the ion-neutral friction of the two fluid to the growth of the thermal instability. According to our study, the characteristic features of the instability are the following four points: (1) The instability which is characterized by the mean molecular weight of neutrals is suppressed via the ion-neutral 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 thermal instability of weakly ionized plasma. (3) The effect of friction does not affect the critical wavelength λF for the thermal instability. This yields that λF of the weakly ionized plasma 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 can not grow owing to the stabilization by the magnetic field. (4) The wavelength of the maximum growth rate of the instability shifts shortward Current address: Department of Earth and Ocean Sciences, School of Applied Sciences, National Defense Academy of Japan, Yokosuka 239-8686, Japan – 2 – according to the decrement of the growth rate. This is because the friction is effective at rather larger scale. Therefore, smaller structures are expected to appear than those without the ion-neutral friction. Our results indicate the friction with the magnetic field affects the morphology and evolution of the interstellar matter. In summary, the ion-neutral friction is important for the evaluation of the thermal instability in weakly ionized plasma along and perpendicular to the magnetic field. Subject headings: instabilities — ISM: evolution