Faraday rotation measures for extragalactic sources were determined in a ∼ 12° by 10° area of the sky. The Hα emission from this region of the sky was also measured. These measurements allowed the unambiguous detection of turbulent magnetic field fluctuations in the diffuse interstellar medium. We compare these observations with the predictions of several ISM turbulence models. We find that the observed turbulence cannot be explained by an ensemble of magnetosonic waves propagating at large angles with respect to the mean magnetic field lines. The measurement of the turbulent magnetic field fluctuations allows us to quantify the energy contained in the turbulence which gives us an estimate of the turbulent dissipation rate. The effects of this turbulent dissipation on the heating of the diffuse ISM are investigated. It is found that the turbulent heating can explain the differences in observed line intensity ratios (such as [S II]/Hα and [N II]/Hα) between H II regions and the diffuse ionized gas (DIG) in our galaxy.

Observations

The Faraday rotation measures of 38 extragalactic sources, many of which are double lobed radio sources, were measured in a ∼ 12° by 10° region of the sky (RA 2h–3h, DEC 33°–43°) (Minter & Spangler 1996). This region of the sky was chosen due to the Hα emission from the diffuse ionized gas (DIG ≡ WIM ≡ Reynolds layer) in our galaxy having been previously mapped by Reynolds (1980).