We examine observational characteristics of multi-phase turbulent flows in the diffuse interstellar medium ( ISM ) using a synthetic radiation field of atomic and molecular lines .
We consider the multi-phase ISM which is formed by thermal instability under the irradiation of UV photons with moderate visual extinction A _ { V } \sim 1 .
Radiation field maps of C ^ { + } , C ^ { 0 } , and CO line emissions were generated by calculating the non-local thermodynamic equilibrium ( nonLTE ) level populations from the results of high resolution hydrodynamic simulations of diffuse ISM models .
By analyzing synthetic radiation field of carbon lines of [ C ii ] 158 \mu m , [ C i ] ^ { 3 } P _ { 2 } - ^ { 3 } P _ { 1 } ( 809 GHz ) , ^ { 3 } P _ { 1 } - ^ { 3 } P _ { 0 } ( 492 GHz ) , and CO rotational transitions , we found a high ratio between the lines of high- and low-excitation energies in the diffuse multi-phase interstellar medium .
This shows that simultaneous observations of the lines of warm- and cold-gas tracers will be useful in examining the thermal structure , and hence the origin of diffuse interstellar clouds .