LONG-TERM INTENSITY VARIATIONS OF 20 PULSARS

Intensity fluctuations of 20 pulsars at 0.43 GHz are analyzed over a 4 yr time span. The objects were selected from a long-term timing study of pulsars performed at the Arecibo Observatory covering a range of d ispersion measures between 8 and 403 pc cm(-3). A structure function a nalysis was used to determine the characteristic time scales and modul ation indices of long-term (greater than or equal to few days) intensi ty variations. Variability generally agrees with a model where refract ive scintillations superpose with diffractive interstellar scintillati ons, with the latter quenched to varying degrees from the finite bandw idth. The refractive scintillations, or limits, are consistent with am plitudes expected from a Kolmogorov spectrum for the electron density.