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Session 17 - The Galactic Interstellar Gas.
Display session, Monday, January 13
We examine the refractive scintillation hypothesis of low frequency variability via numerical simulation in order to account for the spectral characteristics of the observed fluctuations. Plane waves from extragalactic radio sources propagating through the interstellar medium, a medium with fluctuations of the refractive index due to electron density irregularities, emerge from that medium corrugated. Fluctuations present on scales greater than the Fresnel scale act refractively, the emerging wavefront has a curvature, which produces a refractive amplification or deamplification in the flux density. We developed a numerical algorithm to characterize the phase fluctuations in the wavefront and we simulated the resulting intensity distribution in the limit of geometrical optics. We then produce light curves by taking trajectories in the simulated observer's plane and we compare our results with the statistical properties of existing data from a 5-year monitoring program of 32 extragalactic sources at frequencies 0.318, 0.430, 0.606, 0.880 and 1.4 GHz. We find that the refractive scintillation hypothesis reproduces crucial properties of the observed lightcurves, most importantly the octave frequency correlation scale observed in much low frequency variability.
Program listing for Monday