The E ect of Small Scale Magnetic Fields on the GalacticDynamoBenjamin

If the galaxy is born with a very weak seed eld, magnetic energy starts growing rapidly on scales smaller than the smallest turbulent eddies in the interstellar medium (ISM). The small scale eld acts like a tangled network of rubber bands, making the ISM elastic to large scale motions. The hydrodynamic shear modes of the interstellar turbulence, which start out with zero frequency, become Alfven-like elastic waves with frequency kv A , where v A is an eeective Alfven speed calculated from the small scale magnetic energy. As the small scale eld grows, the linear elastic restoring force acting on the turbulent velocity modes eventually dominates over nonlinear mode coupling, rst at small scales but eventually at even the largest turbulent scales, and the interstellar motions make a transition from strong turbulence to weak turbulence. In weak turbulence, the turbulent power spectrum J(k; !) becomes narrowly peaked around the linear regime wave frequency (which is kv A in the current problem), and J(k; 0) drops dramatically. This reduces the growth rate of the large scale mean eld which depends only on the zero frequency component of the power spectrum. The calculation of the extent of this reduction will require future investigations of the nonlinear mode coupling of the Alfven-like elastic waves and the growth of the small scale eld as it approaches equipartition with the turbulence.