A Drift-Alfven model for interstellar turbulence

The role of rapid timescale Alfvenic and slow timescale fluid-straining int eractions on spectral transfer in hydromagnetic turbulence is delineated, w ith electron density evolution incorporated as an integral part of the turb ulent response. At issue is the spectral index of electron density fluctuat ions in the diffuse interstellar medium. The popular interpretation of this index as the rational number 5/3 is at odds with hydromagnetic turbulence in which the fluctuation energies are equipartitioned and fluctuations are on the order of mean values. From analytical and numerical analyses of the turbulent response and its role in spectrum balance relations, it is found that transfer of internal energy is mediated by the slow-scale, local (in w avenumber space) fluid-straining decorrelation over the range of observed s cales. The transfer of magnetic energy is always mediated by the fast-scale , nonlocal Alfvenic decorrelation. Kinetic energy transfer is mediated by A lfvenic decorrelation over most of the spectrum, with a one to two decade r ange extending to larger scales from the inner scale in which transfer is m ediated by the fluid-straining decorrelation. The result is a spectrum in w hich magnetic and kinetic energies have spectral indices of 3/2 over all bu t the smallest scales, and internal energy has an index of 7/4. This result , which is consistent with the dynamics of one-fluid MHD and a passively ad vected electron density, is in good agreement with the observed spectrum. A t the smallest scales, electron compressibility effects, complicated cross- field energy transfer involving density, and the dominance of the fluid-str aining decorrelation in kinetic energy transfer must be considered to deter mine spectral indices correctly.