Analytical Study of Ambipolar Diffusion

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Session 84 -- Star Forming Clouds
Display presentation, Wednesday, 11, 1995, 9:20am - 6:30pm

[84.05] Analytical Study of Ambipolar Diffusion

Dinshaw S. Balsara and Michael L. Norman (N.C.S.A., Univ. of Illinois at Urbana-Champaign)

Several detailed studies of velocity dispersions in molecular clouds have already been carried out. The line width studies show that the velocities are supersonic and subalfvenic. It has long been anticipated that these line widths are indicative of turbulent motions in the clouds. Thus the issue of wave propagation in such environments becomes very important. The allied question has to do with sources for replenishing the wave energy. The above issue is actually made doubly important by the fact that torsional alfven waves have been conjectured as a mechanism for removing angular momentum from a protostellar cloud. Theoretical work has shown that ambipolar diffusion plays a significant role in such environments. Thus the present study focusses on analytically studying the propagation and damping of all families of MHD waves at all angles to the field in a self-gravitating magnetized plasma with ambipolar diffusion. Both one and two-fluid studies have been done.

The one-fluid study shows that on length scales on which ambipolar diffusion becomes important the alfven and fast magnetosonic waves are strongly damped. The slow waves propagate mostly unimpeded and in directions parallel to the magnetic field they propagate virtually unimpeded. A physical reason for this comes from examining the completeness of the MHD eigensystem. This finding makes direct contact with the fact that the line widths are supersonic and subalfvenic.

On larger scales, corresponding to the Jean's length, the pair of slow modes bifurcate to become the ones with exponential growth and decay. Since a gravitationally collapsing slow mode on these larger scales would most easily excite a propagating slow mode on smaller scales a source for sustaining the turbulence is found.

The two-fluid study is also used to trace out the behavior on the shortest length scales where the viscous coupling between the ions and neutrals becomes very weak.

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