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Abstract. Interstellar scintillation multi-frequency observations of PSR 0329+54 in the frequency range from 102 MHz to 5 GHz were analysed to estimate the spectrum of interstellar plasma inhomogeneities in the direction of this pulsar. Based on the theory of diffractive scintillation, the composite structure function of phase fluctuations covering a large range of turbulence scales was constructed. We found that the spectrum is well described by a power law with n = 3.5 for scales from 10 6 to 10 9 m, which differs from the value known for a Kolmogorov spectrum. We can, however, within the accuracy of our data not exclude a Kolmogorov spectrum. It became also clear that angular refraction of emission must be taken into account to fit the data points at all observing frequencies. The size of the irregularities responsible for the angular refraction is estimated to be about 3 × 10 13 m. They can be identified with clouds of neutral hydrogen that can be considered as holes of electron density.

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The aim of our work was to study the spatial structure of inhomogeneities of interstellar plasma in the directions of five pulsars: B0823+26, B0834+06, B1237+25, B1929+10, and B2016+28. Observations of these pulsars were made with RadioAstron space-ground radio interferometer at 324 MHz. We measured the angular size of the scattering disks to be in range between 0.63 and 3.2 mas. We determined the position of scattering screens on the line of sight. Independent estimates of the distances to the screens were made from the curvature of parabolic arcs revealed in the secondary spectra of four pulsars. The model of uniform distribution of inhomogeneities on the line of sight is not suitable. According to the results, we came to the conclusion that scattering is mainly produced by compact plasma layers and the uniform model of inhomogeneties distribution on the line of sight in not applicable.

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We compare signal and noise for observations of the scintillating pulsar B0834+06, using verylong baseline interferometry and a single-dish spectrometer. Comparisons between instruments and with models suggest that amplitude variations of the pulsar strongly affect the amount and distribution of self-noise. We show that noise follows a quadratic polynomial with flux density, in spectral observations. Constant coefficients, indicative of background noise, agree well with expectation; whereas second-order coefficients, indicative of self-noise, are ≈ 3 times values expected for a pulsar with constant on-pulse flux density. We show that variations in flux density during the 10-sec integration accounts for the discrepancy. In the secondary spectrum, ≈ 97% of spectral power lies within the pulsar's typical scintillation bandwidth and timescale; an extended scintillation arc contains ≈ 3%. For a pulsar with constant on-pulse flux density, noise in the dynamic spectrum will appear as a uniformly-distributed background in the secondary spectrum. We find that this uniform noise background contains 95% of noise in the dynamic spectrum for interferometric observations; but only 35% of noise in the dynamic spectrum for single-dish observations. Receiver and sky dominate noise for our interferometric observations, whereas self-noise dominates for single-dish. We suggest that intermittent emission by the pulsar, on timescales < 300 µsec, concentrates self-noise near the origin in the secondary spectrum, by correlating noise over the dynamic spectrum. We suggest that intermittency sets fundamental limits on pulsar astrometry or timing. Accounting of noise may provide means for detection of intermittent sources, when effects of propagation are unknown or impractical to invert.