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We aim to understand the formation of dense cores by magnetosonic waves inregions where the thermal to magnetic pressure ratio is small. Because of thelow-ionisation fraction in molecular clouds, neutral and charged particles areweakly coupled. Ambipolar diffusion then plays an important role in theformation process. A quiescent, uniform plasma is perturbed by a fast-modewave. Using 2D numerical simulations, we follow the evolution of the fast-modewave. The simulations are done with a multifluid, adaptive mesh refinement MHDcode. Initial perturbations with wavelengths that are 2 orders of magnitudelarger than the dissipation length are strongly affected by the ion-neutraldrift. Only in situations where there are large variations in the magneticfield corresponding to a highly turbulent gas can fast-mode waves generatedense cores. This means that, in most cores, no substructure can be produced.However, Core D of TMC-1 is an exception to this case. Due to its atypicallyhigh ionisation fraction, waves with wavelengths up to 3 orders of magnitudegreater than the dissipation length can be present. Such waves are only weaklyaffected by ambipolar diffusion and can produce dense substructure withoutlarge wave-amplitudes. Our results also explain the observed transition fromAlfvenic turbulent motion at large scales to subsonic motions at the level ofdense cores.

The effect of ambipolar resistivity on the formation of dense cores