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It is a well established fact that the atomic interstellar gas is a two phase medium. Recent high numerical simulations
have shown that the cold phase is highly fragmented and that the two phases are deeply intervowen. Since molecular
clouds form by the condensation of atomic gas and since molecular clouds are turbulent and surrounded by an halo of atomic gas leading
inevitably to mixing between the atomic and molecular material,
the question arises of wheter this complex structure is preserved within molecular clouds.
In the talk, I will first present high resolution 2D and 3D numerical simulations of the turbulent atomic interstellar gas which show the
high level of thermal fragmentation of the cold phase. Then I will propose a heating mechanism, namely dissipation of Alfven waves by ambipolar
friction between ions and neutrals, which could permit the existence of warm gas deep inside molecular clouds. The consequences of
molecular clouds being two phase objects will then be discussed.
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