  We try to explain the subpulse drift phenomena adopting the space-charge
limited flow (SCLF) model and comparing the plasma drift velocity in the inner
region of pulsar magnetospheres with the observed velocity of drifting
subpulses. We apply the approach described in a recent paper of van Leeuwen &
Timokhin (2012), where it was shown that the standard estimation of the
subpulse drift velocity through the total value of the scalar potential drop in
the inner gap gives inaccurate results, while the exact expression relating the
drift velocity to the gradient of the scalar potential should be used instead.
After considering a selected sample of sources taken from the catalog of
Weltevrede, Edwards & Stappers (2006) with coherently drifting subpulses and
reasonably known observing geometry, we show that their subpulse drift
velocities would correspond to the drift of the plasma located very close or
above the pair formation front. Moreover, a detailed analysis of PSR B0826-34
and PSR B0818-41 reveals that the variation of the subpulse separation with the
pulse longitude can be successfully explained by the dependence of the plasma
drift velocity on the angular coordinates.
