  PSR B0943+10 is known to show remarkably stable drifting subpulses, which can
be interpreted in terms of a circumferential motion of 20 sparks, each
completing one circulation around the periphery of the polar cap in 37 pulsar
periods. We use this observational constraint and argue that the vacuum gap
model can adequately describe the observed drift patterns. Further we
demonstrate that {\em only} the presence of strong non-dipolar surface magnetic
field can favor such vacuum gap formation. Subsequently, for the first time we
are able to constrain the parameters of the surface magnetic field, and model
the expected magnetic structure on the polar cap of PSR B0943+10 considering
the inverse Compton scattering photon dominated vacuum gap.
