A Drift-Alfvén model for interstellar turbulence

The role of rapid timescale Alfvénic and slow timescale fluid-straining interactions on spectral transfer in hydromagnetic turbulence is delineated, with electron density evolution incorporated as an integral part of the turbulent response. At issue is the spectral index of electron density fluctuations 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 wavenumber space) fluid-straining decorrelation over the range of observed scales. The transfer of magnetic energy is always mediated by the fast-scale, nonlocal Alfvénic decorrelation. Kinetic energy transfer is mediated by Alfvénic decorrelation over most of the spectrum, with a one to two decade range extending to larger scales from the inner scale in which transfer is mediated by the fluid-straining decorrelation. The result is a spectrum in which magnetic and kinetic energies have spectral indices of 3/2 over all but 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 advected electron density, is in good agreement with the observed spectrum. At the smallest scales, electron compressibility effects, complicated cross-field energy transfer involving density, and the dominance of the fluid-straining decorrelation in kinetic energy transfer must be considered to determine spectral indices correctly.