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In the Galactic fountain scenario, supernovae and/or stellar winds propelmaterial into the Galactic halo. As the material cools, it condenses intoclouds. By using FLASH three-dimensional magnetohydrodynamic simulations, wemodel and study the dynamical evolution of these gas clouds after they form andbegin to fall toward the Galactic plane. In our simulations, we assume that thegas clouds form at a height of z=5 kpc above the Galactic midplane, then beginto fall from rest. We investigate how the cloud's evolution, dynamics, andinteraction with the interstellar medium (ISM) are affected by the initial massof the cloud. We find that clouds with sufficiently large initial densities (>0.1 hydrogen atoms per cc) accelerate sufficiently and maintain sufficientlylarge column densities as to be observed and identified as high-velocity clouds(HVCs) even if the ISM is weakly magnetized (1.3 micro Gauss). We alsoinvestigate the effects of various possible magnetic field configurations. Asexpected, the ISM's resistance is greatest when the magnetic field is strongand perpendicular to the motion of the cloud. The trajectory of the cloud isguided by the magnetic field lines in cases where the magnetic field isoriented diagonal to the Galactic plane. The model cloud simulations show thatthe interactions between the cloud and the ISM can be understood via analogy tothe shock tube problem which involves shock and rarefaction waves. We alsodiscuss accelerated ambient gas, streamers of material ablated from the clouds,and the cloud's evolution from a sphere-shaped to a disk- or cigar-shapedobject.

The Evolution of Gas Clouds Falling in the Magnetized Galactic Halo: High Velocity Clouds (HVCs) Originated in the Galactic Fountain