Slow scintillation in the interstellar medium.

The slow fading of pulsars, on time scales of days to months, is shown to be due to interstellar scintillation (ISS). The fluctuations in electron density which cause the slow fading are part of the same power law spectrum which produces the "normal" (minutes to hours) ISS. The spectrum appears to be power law from scales of less than 10⁸ m to at least 10¹³ m. The component of ISS which produces slow fading is relatively broad band and is less sensitive to source size than the normal component. It is speculated that other slow variations in source intensity, particularly, those of "low frequency variables", may also be caused by the interstellar medium.