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In order to investigate the origin of the interstellar turbulence, detailedobservations in the CO J=1--0 and 3--2 lines have been carried out in aninteracting region of a molecular cloud with an HII region. As a result,several 1,000 to 10,000 AU scale cloudlets with small velocity dispersion aredetected, whose systemic velocities have a relatively large scatter of a fewkm/s. It is suggested that the cloud is composed of small-scale dense and coldstructures and their overlapping effect makes it appear to be a turbulententity as a whole. This picture strongly supports the two-phase model ofturbulent medium driven by thermal instability proposed previously. On thesurface of the present cloud, the turbulence is likely to be driven by thermalinstability following ionization shock compression and UV irradiation. Thosesmall scale structures with line width of ~ 0.6 km/s have a relatively high COline ratio of J=3--2 to 1--0, 1 < R(3-2/1-0) < 2. The large velocity gradientanalysis implies that the 0.6 km/s width component cloudlets have an averagedensity of 10^{3-4} cm^{-3}, which is relatively high at cloud edges, but theirmasses are only < 0.05 M_{sun}.

Toward Understanding the Origin of Turbulence in Molecular Clouds: Small Scale Structures as Units of Dynamical Multi-Phase Interstellar Medium