The subpulse modulation properties of pulsars at 92 cm and the frequency dependence of subpulse modulation

Context. A large sample of pulsars has been observed to study their subpulse modulation at an observing wavelength (when achievable) of both 21 and 92 cm using the Westerbork Synthesis Radio Telescope. In this paper we present the 92-cm data and a comparison is made with the already published 21-cm results. Aims. The main goals are to determine what fraction of the pulsars have drifting subpulses, whether those pulsars share some physical properties and to find out if subpulse modulation properties are frequency dependent. Methods. We analysed 191 pulsars at 92 cm searching for subpulse modulation using fluctuation spectra. The sample of pulsars is as unbiased as possible towards any particular pulsar characteristics. Results. For 15 pulsars drifting subpulses are discovered for the first time and 26 of the new drifters found in the 21-cm data are confirmed. We discovered nulling for 8 sources and 8 pulsars are found to intermittently emit single pulses that have pulse energies similar to giant pulses. Another pulsar was shown to exhibit a subpulse phase step. It is estimated that at least half of the total population of pulsars have drifting subpulses when observations with a high enough signal-to-noise ratio would be available. It could well be that the drifting subpulse mechanism is an intrinsic property of the emission mechanism itself, although for some pulsars it is difficult or impossible to detect. Drifting subpulses are in general found at both frequencies, although the chance of detecting drifting subpulses is possibly slightly higher at 92 cm. It appears that the youngest pulsars have the most disordered subpulses and the subpulses become more and more organized into drifting subpulses as the pulsar ages. The modulation indices measured at the two frequencies are clearly correlated, although at 92 cm they are on average possibly higher. At 92 cm the modulation index appears to be correlated with the characteristic age of the pulsar and the complexity parameters as predicted by three different emission models. The correlations with the modulation indices are argued to be consistent with the picture in which the radio emission can be divided in a drifting subpulse signal plus a quasi-steady signal which becomes, on average, stronger at high observing frequencies. The measured values of P3 at the two frequencies are highly correlated, but there is no evidence for a correlation with other pulsar parameters.