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The distribution of the ionized component of the interstellar medium in our Galaxy may be investigated through observations of compact extragalactic sources. Density fluctuations of the ionized material scatter the radio waves from these sources, resulting in the angular broadening of their images. This
broadening is described by the following relation: \begin{displaymath} \theta_{arcsec} = 1.72 \times 10^{-2} \lambda^{2.2} \ \left(\int_{0}^{s} C_{N}^{2}(s^{\prime}) ds^{\prime} \right)^{0.6} \end{displaymath} where $C_{N}^{2}$ is a measure of the scattering strength, $\lambda$ is the observing wavelength in meters, and $s^{\prime}$ the line-of-sight distance to the object in pc. An estimate of the scattering strength along the line of sight to an object may thus be obtained by measuring the angular broadening of its image. Observations of many objects allow the calculation of the scattering strength along a range of lines of sight, and a model of the large-scale distribution of the turbulence in the ionized component of the ISM may be obtained.
Earlier studies have suggested a two-component distribution, with perhaps a spiral pattern at low latitudes.
In an attempt to verify this model, we have made VLBI observations at 0.61, 1.66, and 5 GHz of a sample of 26 flat-spectrum extragalactic sources located at $|b| < 1$ degree, and 9 deg. $< l < 109$ deg. Details of these observations, their data analysis, and the results are presented in this poster.
