2006A&A...455..195S

Multi-frequency observations of interstellar scintillation toward the pulsar PSR B1642-03 were analyzed to estimate the spectrum of interstellar plasma inhomogeneities in the direction of this pulsar. Using data over the frequency range from 103 MHz to 5 GHz, we constructed the composite structure function (SF) of phase fluctuations, which covers a corresponding wide range of turbulence scales. The structure function shows that the interstellar plasma spectrum in the direction to this pulsar follows a piecewise power law. The power law is well described by two exponents: n=3.7 for scales from 10⁹ to 10¹⁵cm (Kolmogorov spectrum) and n=3.35 for scales less than 10⁹cm. We interpret the unusual behaviour of the spectrum to be caused by the line of sight passing through the North Polar Spur, which may have plasma properties similar to the anisotropic plasma of the solar wind, although at a very different density.