Shocks and Molecules in Diffuse Interstellar Cloud Pairs

The diffuse interstellar medium is dynamic, and its chemistry evolution determined by shock fronts as well photodissociation. Shocks are implied the supersonic motions velocity dispersion often statistically called "turbulence". We compare models of magnetohydrodynamic (MHD) shocks, with speeds typical cloud through ISM (3-25 km/s) densities cold neutral gas (~ 100 cm-3), to archival observations atomic hydrogen 21-cm line for kinematics, far-infrared emission dust mass, mid-infrared high-resolution morphology, identify in three high-latitude clouds pairs masses order 50 suns. have `heads' extended `tails', images show arcs on leading edges "heads" that could be individual shocks. HI shows higher-velocity at due shock-accelerated material. For two pairs, one has an active indicated broad offset HI, while other already been shocked predominantly `CO-dark' molecular hydrogen. Two-dimensional MHD simulations shocks parallel magnetic field a remarkable similarity observed features, including merged "tails" aligned flow field, which leads lateral confinement downstream. A alignment between may lead formation small clouds.