Long-Term Scintillation Studies of Pulsars. I. Observations and Basic Results

We report long-term scintillation observations of 18 pulsars in the dispersion measure range 3-35 pc cm^-3 carried out from 1993 January to 1995 August using the Ooty Radio Telescope at 327 MHz. These observations were made with the aim of studying refractive effects in pulsar scintillation and obtaining reliable estimates of diffractive and refractive scintillation properties. Dynamic scintillation spectra of pulsars were regularly monitored at 10-90 epochs spanning 100-1000 days. Significant changes are observed in the dynamic spectra over timescales as short as a few days. Large-amplitude fluctuations are observed in quantities such as decorrelation bandwidth, scintillation timescale, drift rate, and flux density. Several pulsars show organized features, such as drifting bands in a highly pronounced manner. For some pulsars, gradual and systematic variations are seen in the drift rate of patterns that undergo several sign reversals during the observing time spans. Anomalous behavior, such as persistent drifts lasting over many months, is seen for PSRs B0834+06 and B1919+21. Four pulsars were studied for 2-4 well-separated observing sessions, each lasting over ~100 days. In some cases, significant variations are seen in the average scintillation properties and/or flux densities between successive observing sessions. From our data, we have been able to obtain more accurate and reliable estimates of scintillation properties and flux densities than those from the earlier observations by averaging out the fluctuations due to refractive scintillation effects. These measurements are used to derive parameters such as the strength of scattering and scintillation speeds. The scintillation speed estimates are found to be reasonably good indicators of proper-motion speeds of pulsars. The present measurements are compared with earlier measurements and the long-term stability of scintillation properties and flux densities is discussed.