The propagation of pulsar signals through the interstellar medium of the Galaxy modifies the properties of the received radiation in several ways. A study of these effects can give useful information about the interstellar medium. One of these effects that has already been looked at is interstellar dispersion. It gives us information about the mean electron density of the interstellar plasma.
Another effect that is significant in pulsar observations is interstellar scintillations. It is caused by scattering of the radiation due to random fluctuations of electron density in the interstellar plasma. It produces the following effects (not all of which are easily observable!): (i) angular broadening of the source, as scattered radiation now arrives from a range of angles around the direction to the pulsar; (ii) temporal pulse broadening due to the delayed arrival of scattered radiation; (iii) random fluctuations of pulsar intensity as a function of time and frequency due to interference effects between radiation arriving from different directions. All these effects increase in strength with decreasing frequency and with increasing length of plasma between source and observer. A detailed study of interstellar scintillation effects in pulsar signals can be used to obtain valuable constraints on the extent and location of scattering plasma in the interstellar medium, as well as on the spatial power spectrum of electron density fluctuations in the medium.
Of the three effects of scintillations described above, the random fluctuations of intensity are the most easily observable and form the best probes of the phenomenon. They are readily seen in pulsar dynamic spectra which are records of the on-pulse intensity as a function of time and frequency. A single time sample in the dynamic spectra is obtained by accumulating the total energy under the pulse window for a given number of pulses, for each of channels. These random intensity fluctuations have typical decorrelation scales in time and frequency, which are estimated by performing a two dimensional autocorrelation of the dynamic spectra data. These decorrelation widths are of the order of a few minutes and hundreds of kHz, respectively, at metre wavelengths. This means that typical observations have to be carried out with time and frequency resolutions of the order of tens of seconds and tens of kHz in order to observe the scintillations. This requirement becomes more stringent at lower frequencies and for more distant pulsars (which are more strongly scattered). Also, the observations need to span enough number of these random scintillations in order to obtain statistically relaible values for the two decorrelation widths. This usually requires observing durations of an hour or so with bandwidths of a few MHz.
Due to the effect of large scale electron density fluctuations in the interstellar medium, the values of the decorrelation widths and the mean pulsar flux, fluctuate with time. A study of this phenomenon (called refractive scintillations) requires regular monitoring of pulsar dynamic spectra at different epochs, typically a few days apart and spanning several weeks to months. Such data can also be used to estimate the mean transverse speeds of pulsars.