Probing Interstellar Turbulence in the Warm Ionized Medium using Emission Lines

Summary

The nature of turbulence in the warm ionized component of the interstellar medium (WIM) can be investigated using Fabry-Perot spectroscopy of optical emission lines. The Hα intensity provides the emission measure (EM) along a line of sight, which is used in conjunction with the scattering measure, rotation measure, and dispersion measure to study interstellar turbulence. Observations at high spectral resolution (∼ 10 km s⁻¹) allow measurements of the bulk radial velocity structure of the emitting gas and investigations of thermal and non-thermal (turbulent) broadening mechanisms through the line widths. By measuring the widths of the Hα line and an emission line from a heavier atom (e.g. the [S II] λ6716 line), one can separate the thermal and non-thermal contributions to the line width. Preliminary studies using this method have shown that the broad range of Hα line widths (typically 15 – 50 km s⁻¹) is mostly due to differences in the non-thermal component of the width and that along many lines of sight this component dominates. The Wisconsin Hα Mapper (WHAM) is in the process of producing a very sensitive kinematic map of the northern sky in Hα at 1° angular resolution and 12 km s⁻¹ spectral resolution. WHAM is also mapping emission lines from heavier atoms such as sulfur and nitrogen for selected regions of the sky. This data set will provide unique new information concerning turbulence in the WIM.