  We propose that galactic shocks propagating through interstellar density
fluctuations provide a mechanism for the intermittent replenishment, or
"pumping," of the supersonic motions and internal density enhancements observed
pervasively within cool atomic and molecular interstellar structures, without
necessarily requiring the presence of self-gravity, magnetic fields, or young
stars. The shocks are assumed to be due to a variety of galactic sources on a
range of scales. An analytic result for the kinematic vorticity generated by a
shock passing through a radially-stratified two-dimensional isobaric model
cloud is derived, assuming that the Mach number is not so large that the cloud
is disrupted, and neglecting the shock curvature and cloud distortion.
Two-dimensional lattic gas hydrodynamic simulations at modest Mach numbers were
used to verify the analytic result. The induced internal velocities are
initially a significant fraction of the shock speed divided by the square root
of the density contrast, accounting for both the observed linewidth amplitudes
and the apparent cloud-to-cloud linewidth-density scaling. The linewidth-size
relation could then be interpreted in terms of the well-known power spectrum of
a system of shocks. The induced vortical energy should quickly be converted to
compressible and MHD modes, and so would be difficult to observe directly, even
though it would still be the power source for the other modes. The shockpump
thus produces density structure without the necessity of any sort of
instability. We argue that the shockpump should lead to nested shock-induced
structures, providing a cascade mechanism for supersonic "turbulence" and a
physical explanation for the fractal-like structure of the cool interstellar
medium.
