  The well known drifter PSR B0031-07 is known to exhibit drifting subpulses
where the spacing between the drift bands (P_3) shows three distinct modes A, B
and C corresponding to 12, 6 and 4 seconds respectively. We have investigated
periodicities and polarisation properties of PSR B0031-07 for a sequence of
2700 single pulses taken simultaneously at 328 MHz and 4.85 GHz. We found that
mode A occurs simultaneously at these frequencies, while modes B and C only
occur at 328 MHz. However, when the pulsar is emitting in mode B at the lower
frequency there is still emission at the higher frequency, hinting towards the
presence of mode B emission at a weaker level. Further, we have established
that modes A and B are associated with two orthogonal modes of polarisation,
respectively. Based on these observations, we suggest a geometrical model where
modes A and B at a given frequency are emitted in two concentric rings around
the magnetic axis with mode B being nested inside mode A. Further, it is
evident that this nested configuration is preserved across frequency with the
higher frequency arising closer to the stellar surface compared to the lower
one, consistent with the well known radius-to-frequency mapping operating in
pulsars.
