Fluctuations in the polarization of starlight provide strong support for the concept of a turbulent interstellar medium. We have analyzed theoretically the mean polarization and the variance of polarization about the mean as functions of distance R from the solar system. The mean increases linearly with R. For R less than the correlation length L the variance increases as R$^2$, whereas for R>>L the variance increases linearly with R. The observed mean polarization and the observed variance of polarization are found to be in excellent agreement with these theoretical deductions if the interstellar medium fluctuates irregularly with a correlation length of 150 pc. In view of these results, it is clear that some caution must be exercised in interpretation of observations of individual pulsar signal dispersion and Faraday rotation.
Jokipii, J. R.; Lerche, I.; Schommer, R. A.
1969, The Astrophysical Journal Letters, 157, L119
http://adsabs.harvard.edu/abs/1969ApJ…157L.119J