  We analyze Faraday rotation and depolarization of extragalactic radio point
sources in the direction of the inner Galactic plane to determine the outer
scale and amplitude of the rotation measure power spectrum. Structure functions
of rotation measure show lower amplitudes than expected when extrapolating
electron density fluctuations to large scales assuming a Kolmogorov spectral
index. This implies an outer scale of those fluctuations on the order of a
parsec, much smaller than commonly assumed. Analysis of partial depolarization
of point sources independently indicates a small outer scale of a Kolmogorov
power spectrum. In the Galaxy's spiral arms, no rotation measure fluctuations
on scales above a few parsecs are measured. In the interarm regions
fluctuations on larger scales than in spiral arms are present, and show power
law behavior with a shallow spectrum. These results suggest that in the spiral
arms stellar sources such as stellar winds or protostellar outflows dominate
the energy injection for the turbulent energy cascade on parsec scales, while
in the interarm regions supernova and super bubble explosions are the main
sources of energy on scales on the order of 100 parsecs.
