  We present the results of a systematic, unbiased search for subpulse
modulation of 187 pulsars performed with the Westerbork Synthesis Radio
Telescope (WSRT) in the Netherlands at an observing wavelength of 21 cm. Using
new observations and archival WSRT data we have expanded the list of pulsars
which show the drifting subpulse phenomenon by 42, indicating that at least one
in three pulsars exhibits this phenomenon. The real fraction of pulsars which
show the drifting phenomenon is likely to be larger than some 55%. The majority
of the analysed pulsars show subpulse modulation (170), of which the majority
were not previously known to show subpulse modulation and 30 show clear
systematic drifting. The large number of new drifters we have found allows us,
for the first time, to do meaningful statistics on the drifting phenomenon. We
find that the drifting phenomenon is correlated with the pulsar age such that
drifting is more likely to occur in older pulsars. Pulsars which drift more
coherently seem to be older and have a lower modulation index. There is no
significant correlation found between P3 and other pulsar parameters (such as
the pulsar age), as has been reported in the past. There is no significant
preference of drift direction and the drift direction is not found to be
correlated with pulsar parameters. None of the four complexity parameters
predicted by different emission models (Jenet & Gil 2003) are shown to be
inconsistent with the set of modulation indices of our sample of pulsars.
Therefore none of the models can be ruled out based on our observations. We
also present results on some interesting new individual sources like a pulsar
which shows similar subpulse modulation in both the main- and interpulse and
six pulsars with opposite drift senses in different components.

Astrophysics