Can WNM survive inside Molecular Clouds ?
  Recent high resolution numerical simulations have suggested that the interstellar atomic hydrogen clouds have a complex two-phase structure. Since molecular clouds form through the contraction of HI gas, the question arises as to whether this structure is maintained in the molecular phase or not. Here we investigate whether the warm neutral atomic hydrogen (WNM) can exist in molecular clouds. We calculate how far a piece of WNM which is not heated by the UV photons could penetrate into the cloud, and find that in the absence of any heating it is unlikely that large fraction of WNM survives inside high pressure molecular clouds. We then consider two possible heating mechanisms, namely dissipation of turbulent energy and dissipation of MHD waves propagating in the WNM inside the cloud. We find that the second one is sufficient to allow the existence of WNM inside a molecular cloud of size $\simeq$ 1 pc having pressure equal to $\simeq 10 \times P_{\rm ISM}$. This result suggests the possibility that channels of magnetised WMN may provide efficient energy injection for sustaining internal turbulence which otherwise decays in a crossing time. 
P. Hennebelle and S. Inutsuka