  Coherent radio emission in pulsars is excited due to instabilities in a
relativistically streaming non-stationary plasma flow, which is generated from
sparking discharges in the inner acceleration region (IAR) near the stellar
surface. A number of detailed works have shown the IAR to be a partially
screened gap (PSG) dominated by non-dipolar magnetic fields with continuous
outflow of ions from the surface. The phenomenon of subpulse drifting is
expected to originate due to variable $\mathbf{E}\times\mathbf{B}$ drift of the
sparks in PSG, where the sparks lag behind corotation velocity of the pulsar.
Detailed observations show a wide variety of subpulse drifting behaviour where
subpulses in different components of the profile have different phase
trajectories. But the drifting periodicity is seen to be constant, within
measurement errors, across all components of the profile. Using the concept of
sparks lagging behind corotation speed in PSG as well as the different
orientations of the surface non-dipolar magnetic fields we have simulated the
expected single pulse behaviour in a representative sample of pulsars. Our
results show that the different types of drifting phase behaviour can be
reproduced using these simple assumptions of spark dynamics in a non-dipolar
IAR.
