  The classical vacuum gap model of Ruderman & Sutherland, in which
spark-associated subbeams of subpulse emission circulate around the magnetic
axis due to the EB drift, provides a natural and plausible physical mechanism
of the subpulse drift phenomenon. Recent progress in the analysis of drifting
subpulses in pulsars has provided a strong support to this model by revealing a
number of subbeams circulating around the magnetic axis in a manner compatible
with theoretical predictions. However, a more detailed analysis revealed that
the circulation speed in a pure vacuum gap is too high when compared with
observations. Moreover, some pulsars demonstrate significant time variations of
the drift rate, including a change of the apparent drift direction, which is
obviously inconsistent with the EB drift scenario in a pure vacuum gap. We
resolved these discrepancies by considering a partial flow of iron ions from
the positively charged polar cap, coexisting with the production of outflowing
electron-positron plasmas. By fitting the observationally deduced drift-rates
to the theoretical values, we managed to estimate polar cap surface
temperatures in a number of pulsars. The estimated surface temperatures
correspond to a small charge depletion of the order of a few percent of the
corotational charge density. We also argue that if the thermionic electron
outflow from the surface of a negatively charged polar cap is slightly below
the Goldreich-Julian density, then the resulting small charge depletion will
have similar consequences as in the case of the ions outflow. We thus believe
that the sparking discharge of a partially shielded acceleration potential drop
occurs in all pulsars, with both positively (``pulsars'') and negatively
(``anti-pulsars'') charged polar caps.
