Energy transfer in biphasic magnetised interstellar medium

The transfer of energy from large scales to smaller scales is investigated within the context of the neutral atomic phases of the interstellar medium threaded by a magnetic field. More precisely, we derive the reflection and transmission coefficients for linear transverse hydromagnetic waves incident normally on a cloud in slab geometry. Then, we obtain various interesting physical quantities such as the energy density transferred into the cloud. The results obtained differ from those given previously in the literature. We show in particular that the transfer of energy is efficient enough so that (i) the induced internal motions are of the same order of magnitude than the bulk motion of the clouds and (ii) the internal energy density provides significative additional non-thermal support for cloud stability, reaching the same order of magnitude than support by the static magnetic field. Finally, the case of molecular clouds is also briefly considered for which similar results are obtained.