Thermal instability is one of the most important processes in the formation of clumpy substructure in magnetic molecular clouds. On the other hand, ambipolar diffusion, or ion–neutral friction, has long been thought to be an important energy dissipation mechanism in these clouds. Thus, it would interesting to investigate the effect of ambipolar diffusion on the thermal instability and formation of clumps in the magnetic molecular clouds. For this purpose, as a first step, we turn our attention to the linear perturbation stage. In this way, we obtain a non‐dimensional characteristic equation that reduces to the prior characteristic equation in the absence of the magnetic field and ambipolar diffusion. With numerical manipulation of this characteristic equation, we conclude that there are solutions where the thermal instability allows compression along the magnetic field but not perpendicular to it. We infer that this aspect might be in evidence in the formation of observed disc‐like (oblate) clumps in magnetic molecular clouds.

Frictional heating by the ion‐neutral drift is calculated and its effect on the isobaric thermal instability is studied. Ambipolar drift heating of a one‐dimensional self‐gravitating magnetized molecular slab is used under the assumptions of quasi‐magnetohydrostatic and local ionization equilibrium. We see that ambipolar drift heating is inversely proportional to density and its value in some regions of the slab can be significantly larger than the average heating rates of cosmic rays and turbulent motions. The results show that isobaric thermal instability can occur in some regions of the slab, and thus it may produce slab fragmentation and formation of astronomical unit scale condensations.

We study magnetothermal instability in the ionized plasmas including the effects of Ohmic, ambipolar and Hall diffusion. Magnetic field in the single fluid approximation does not allow transverse thermal condensations, however, non-ideal effects highly diminish the stabilizing role of the magnetic field in thermally unstable plasmas. Therefore, enhanced growth rate of thermal condensation modes in the presence of the diffusion mechanisms speed up the rate of structure formation.

Low‐mass condensations (LMCs) have been observed within molecular cloud cores. In this study, we investigate the effect of the application of isobaric thermal instability (TI) in forming these LMCs. For this purpose, we first investigate the occurrence of TI in molecular clouds. Then, to study the significance of linear isobaric TI, we use a contracting axisymmetric cylindrical core with an axial magnetic field. Consideration of cooling and heating mechanisms in molecular clouds shows that including the heating due to ambipolar diffusion can lead to the occurrence of TI on a time‐scale smaller than the dynamical time‐scale. Application of linear perturbation analysis shows that isobaric TI can take place in the outer regions of molecular cloud cores. Furthermore, the results show that perturbations with wavelengths greater than few astronomical units are protected from the destabilization property of thermal conduction, so that they can grow to form LMCs. Thus, the results show that the mechanism of TI can be used to explain the formation of LMCs as the progenitors of collapsing protostellar entities, brown dwarfs or protoplanets.