  A novel scheme has been developed to show that the observed phase behaviour
associated with subpulse drifting from two pulsars, J1034$-$3224 and
J1720$-$2933, can be used to obtain the magnetic field configuration in the
partially screened gap (PSG). The outflowing plasma along the open magnetic
field line region of pulsars is generated due to spark discharges in an inner
acceleration region (IAR) above the polar cap. The IAR has been modelled as a
partially screened gap (PSG) with a steady supply of positively charged ions
emitted from the heated polar cap surface dominated by strong non-dipolar
magnetic fields. In a PSG the sparks are tightly packed and constrained to be
present along the polar cap boundary. The sparks lag behind the rotation of the
star during their lifetimes. As a result the sparking pattern evolves along two
different directions in the clockwise and counter-clockwise manner around a
stationary central spark, and can be associated with the observed phenomenon of
subpulse drifting. PSR J1034$-$3224 has four prominent components and exhibit
bi-drifting where alternate components show opposite sense of drifting, while
PSR J1720$-$2933 has a single component profile and shows systematic coherent
drift bands. We show that the differences in their drifting behaviour can be
directly linked to different natures of the non-dipolar surface magnetic field
configurations.
