  Two dimensional compressible magneto-hydrodynamical (MHD) simulations run for
20 crossing times on a 800x640 grid with two stable thermal states show
persistent hierarchical density structures and Kolmogorov turbulent motions in
the interaction zone between incoming non-linear Alfven waves. These structures
and motions are similar to what are commonly observed in weakly
self-gravitating interstellar clouds, suggesting that these clouds get their
fractal structures from non-linear magnetic waves generated in the intercloud
medium; no internal source of turbulent energy is necessary. The clumps in the
simulated clouds are slightly warmer than the interclump medium as a result of
magnetic dissipational and compressive heating. Thus the interclump medium has
a lower pressure than the clumps, demonstrating that the clumps owe their
existence to transient compressive motions, not pressure confinement. Clump
lifetimes increase with size, and are about one sound crossing time. Two
specific problems are addressed. We find that low ionization fractions in
molecular clouds do not necessarily lead to increased cloud smoothing, but that
enhanced density in a self-gravitating cloud does, because of external wave
self-shielding. Bulk cloud self-gravity may therefore promote star formation in
the relatively quiescent gas pools that contain a thermal Jeans mass or more.
