Numerical simulations of a shock interacting with multiple magnetized clouds

We present 2D adiabatic magnetohydrodynamic (MHD) simulations of a shock inter-acting with groups of two or three cylindrical clouds. We study how the presence of a nearby cloud influences the dynamics of this interaction, and explore the resulting differences and similarities in the evolution of each cloud. The understanding gained from this small-scale study will help to interpret the behaviour of systems with many 10’s or 100’s of clouds. We observe a wide variety of behaviour in the interactions studied, which is de-pendent on the initial positions of the clouds and the orientation and strength of the magnetic field. We find: i) some clouds are stretched along their field-lines, whereas others are confined by their field-lines; ii) upstream clouds may accelerate past down-stream clouds (though magnetic tension can prevent this); iii) clouds may also change their relative positions transverse to the direction of shock propagation as they “sling-shot ” past each other; iv) downstream clouds may be offered some protection from the oncoming flow as a result of being in the lee of an upstream cloud; v) the cycle of cloud compression and re-expansion is generally weaker when there are nearby neighbouring clouds; vi) the plasma β in cloud material can vary rapidly as clouds collide with one another, but low values of β are always transitory. This work is relevant to studies of multi-phase regions, where fast, low-density gas interacts with dense clouds, such as in circumstellar bubbles, supernova remnants, superbubbles and galactic winds. Key words: hydrodynamics – ISM: clouds – ISM: kinematics and dynamics – shock waves – supernova remnants – turbulence