Radio scintillation due to discontinuities in the interstellar plasma density

We develop the theory of interstellar scintillation as caused by an irregul ar plasma having a power-law spatial-density spectrum with a spectral expon ent of beta = 4 corresponding to a medium with abrupt changes in its densit y. An "outer scale" is included in the model that represents the typical sc ale over which the density of the medium remains uniform. Such a spectrum c ould be used to model plasma shock fronts in supernova remnants or other pl asma discontinuities. We investigate and develop equations for the decorrel ation bandwidth of diffractive scintillations and the refractive scintillat ion index and compare our results with pulsar measurements. We consider bot h a medium concentrated in a thin layer and an extended irregular medium. W e conclude that the beta = 4 model gives satisfactory agreement for many di ffractive measurements, in particular the VLBI measurements of the structur e function exponent between 5/3 and 2. However, it gives less satisfactory agreement for the refractive scintillation index than does the Kolmogorov t urbulence spectrum. The comparison suggests that the medium consists of a p ervasive background distribution of turbulence embedded with randomly place d discrete plasma structures such as shocks or H II regions. This can be mo deled by a composite spectrum following the Kolmogorov form at high wavenum bers and steepening at lower wavenumbers corresponding to the typical (inve rse) size of the discrete structures. Such a model can also explain the ext reme scattering events. However, lines of sight through the enhanced scatte ring prevalent at low Galactic latitudes are accurately described by the Ko lmogorov spectrum in an extended medium and do not appear to have a similar low-wavenumber steepening.