The Evolution of Gas Clouds Falling in the Magnetized Galactic Halo:
  High Velocity Clouds (HVCs) Originated in the Galactic Fountain
  In the Galactic fountain scenario, supernovae and/or stellar winds propel
material into the Galactic halo. As the material cools, it condenses into
clouds. By using FLASH three-dimensional magnetohydrodynamic simulations, we
model and study the dynamical evolution of these gas clouds after they form and
begin to fall toward the Galactic plane. In our simulations, we assume that the
gas clouds form at a height of z=5 kpc above the Galactic midplane, then begin
to fall from rest. We investigate how the cloud's evolution, dynamics, and
interaction with the interstellar medium (ISM) are affected by the initial mass
of the cloud. We find that clouds with sufficiently large initial densities (>
0.1 hydrogen atoms per cc) accelerate sufficiently and maintain sufficiently
large 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 also
investigate the effects of various possible magnetic field configurations. As
expected, the ISM's resistance is greatest when the magnetic field is strong
and perpendicular to the motion of the cloud. The trajectory of the cloud is
guided by the magnetic field lines in cases where the magnetic field is
oriented diagonal to the Galactic plane. The model cloud simulations show that
the interactions between the cloud and the ISM can be understood via analogy to
the shock tube problem which involves shock and rarefaction waves. We also
discuss accelerated ambient gas, streamers of material ablated from the clouds,
and the cloud's evolution from a sphere-shaped to a disk- or cigar-shaped
object.
