Recent observations of the low column density parts of molecular clouds have revealed the presence of elongated structures, usually referred to as striations (see Figure). Wherever striations are observed they appear to be well aligned with the magnetic field as this is probed from polarimetric measurements.

Ideal magnetohydrodynamic (MHD) simulations strongly suggest that models of streamers, which were the most popular scenario for the formation of these structures, fail to reproduce the observed properties of striations. In an entirely different scenario, in which striations are formed from the excitation of compressional magnetosonic waves, the observed contrast between adjacent striations and the power spectrum of cuts perpendicular to the long axis of striations were successfully reproduced. In fact, the qualitative agreement between observations and simulations of MHD waves is remarkable. In this model, fluctuations of magnetic pressure create striations. Magnetosonic waves are naturally excited from Alfven waves due to density inhomogeneities.