Refractive scintillation effects in pulsars are powerful techniques for discriminating between different models proposed for the electron density fluctuation spectrum in the interstellar medium .
Data from our long-term scintillation study of eighteen pulsars in the dispersion measure range 3 - 35 { pc~ { } cm ^ { -3 } } ( Paper I ) are used to investigate two important observable effects of refractive scintillation , viz . ( i ) modulations of diffractive scintillation observables and flux density , and ( ii ) drifting bands in dynamic scintillation spectra .
Our data provide simultaneous measurements of decorrelation bandwidth , scintillation time scale , flux density and drift rate of patterns .
The observed modulations of the first three are compared with the available theoretical predictions , and constraints are placed on the power spectrum of plasma density fluctuations .
The measured modulation indices are found to be larger than predicted by a Kolmogorov form of density spectrum .
The properties of the drift rate of patterns along with the diffractive scintillation parameters have been used to independently estimate the slope of the density power spectrum , which is found to be consistent with a Kolmogorov form for several pulsars .
The contradictory results from these two independent methods of constraining the electron density spectrum are not reconcilable with the simple theoretical models based on power-law forms of density spectrum .
Our observations show anomalous scintillation behaviour like persistent drifting bands for some pulsars .
This can be interpreted as an excess power in the low wavenumber range ( \sim 10 ^ { -12 } -10 ^ { -13 } { m ^ { -1 } } ) compared to the Kolmogorov expectations , or the existence of localized density structures .
The results from our observations are discussed in combination with those from earlier studies in an attempt to understand the overall nature of the density spectrum .
The emerging picture is a Kolmogorov-like spectrum ( \alpha \approx \mbox { $ { 11 \over 3 } $ } ) in the wavenumber range \sim 10 ^ { -6 } { m ^ { -1 } } to \sim 10 ^ { -11 } { m ^ { -1 } } , which either steepens or has a bump near \sim 10 ^ { -12 } -10 ^ { -13 } { m ^ { -1 } } .
The accumulated data also suggest the existence of discrete density structures along some lines of sight .
We also discuss the possible implications of our results for the theoretical models .