Turbulent Fluctuations in the Interstellar Plasma Density and Magnetic Field

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Session 36 -- ISM: Structure, Molecular Clouds
Display presentation, Wednesday, June 14, 1995, 9:20am - 6:30pm

[36.05] Turbulent Fluctuations in the Interstellar Plasma Density and Magnetic Field

Anthony Minter and Steve Spangler (University of Iowa)

We have measured Faraday rotation measures (RM) of 37 extragalactic sources in the region from $1^{\rm h}~50^{\rm m}$ to $3^{\rm h}~10^{\rm m}$ Right Ascension and $32\deg$ to $44\deg$ Declination in order to provide new information on the properties of plasma turbulence in the Interstellar Medium (ISM). Of particular interest is the possibility that such Faraday Rotation measurements can provide information on the magnetic field component of interstellar turbulence. This area of the sky is unique in that it is the only region away from the galactic plane where the $\rm{H} \alpha$ intensity, and thus emission measure (EM), has been mapped completely, both spatially and in velocity (\cite{rey80}). The rotation measures were found to vary in a relatively smooth fashion across the entire region. The structure functions of rotation measure and emission measure were analyzed in order to extract information on the nature of the plasma turbulence in the interstellar medium. The rotation measure and emission measure structure functions are determined by the spatial power spectra of the magnetic field and the plasma density in the interstellar medium, and by the outer scale of those spectra. These measured structure functions for angular lags ${\rm \delta\theta \leq 0.5 \deg}$ are consistent with a Kolmogorov spectrum for both the magnetic field and plasma density. For larger angular lags, the measure structured functions are of a different functional form. We interpret this change in the form of the observed structure functions as due to an outer scale to the turbulence of about 7 parsecs. Perhaps the most interesting result emergent from this study is that for spatial scales smaller than the outer scale, the level of the rotation measure structure function is larger than would be expected from only plasma density fluctuations in an otherwise uniform galactic magnetic field. Our measurement of the rotation measure structure function requires, in addition to the plasma density fluctuations, a turbulent magnetic field with an rms amplitude of ${\rm \sim 2~\mu G}$. Thus the fluctuating component of the interstellar magnetic filed is comparable to the mean value, even on scales of a few parsecs. To our knowledge, this represents the first measurement of magnetic field fluctuations in the interstellar plasma on spatial scales which appropriately may be described as turbulence.

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