Density power spectrum in the local interstellar medium

Abstract

Since the identification of interstellar scintillation, the interstellar medium (ISM) has been known to contain electron density fluctuations having scales ∼10⁹ m. The scattering properties of these irregularities have been used in pulsar and continuum source studies and to investigate the small scale turbulence itself^1–11. (We use ‘turbulence’ here to imply a disordered, irregular flow.) If the ISM turbulence extends over a broad enough range of fluctuation scales, it could have an important role in galactic cosmic ray transport and confinement^12–14. Here we present observational evidence that electron density irregularities exist over a very wide range of scale sizes. Radio scattering observations associated with the wavenumber range ∼10⁻¹¹–10⁻⁶m⁻¹ are consistent with a density spectrum of the form (wavenumber)^−3.7±0.6. At lower spatial wavenumbers (10⁻¹⁶–10⁻¹⁸m⁻¹) the power spectrum can be estimated by computation of the density fluctuations near the ‘outer’ scale of the spectrum¹⁵, comparison with density irregularities predicted by theoretical models of the ISM¹⁶, and comparison with observations of the velocity structure function^13,17,18. When combined with the high-wavenumber (radio) data, these low-wavenumber spectrum estimates are most simply interpreted in terms of a density spectrum behaving as (wavenumber)^−3.6±0.2 over a 12 decade scale size range (∼100 pc to ∼10⁷m). There may be theoretical difficulties^12,14,19 with such a spectrum, however. Alternatively, the low-wavenumber data could be associated with ISM clouds which are physically distinct from the small scale turbulence.