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Both refraction and diffraction from electron inhomogeneities in the interstellar medium modulate radio signals in time and frequency. Diffraction in passing through this random medium gives rise to intensity maxima in pulsar dynamic spectra that are quasi-gaussian in shape and are not tilted with respect to the frequency or time axes. Refraction, on the other hand, can tilt and stretch these intensity maxima or, in extreme cases, give rise to periodic fringe patterns in the dynamic spectrum. At least this is the predicted behavior; this important prediction of refractive scintillation has never been adequately tested. Although multi-epoch dynamic spectrum observations have been reported, they have always been undersampled with respect to the time scale for refractive variability. In order to test these predictions we have initiated a daily dynamic spectrum monitoring program using an NRAO 26m telescope and a high-resolution spectrometer. From the daily dynamic spectrum observations of a few strong pulsars we are able to obtain daily estimates of diffractive scintillation parameters. We will report on the status of this project and preliminary results of the time variations of diffractive scintillation parameters. Implications for the inhomogeneity spectrum of the interstellar medium will also be discussed.
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