We report analysis of an 8 hr observation of PSR B0943+10 at 325 MHz performed at the Giant Metrewave Radio Telescope (GMRT) in India. B0943+10 is well known for displaying regular sub-pulse drifting and two emission modes. We investigate the modal behavior of B0943+10. By reconstructing an entire B mode from two consecutive B modes, we estimate that the pulsar spends roughly 7.5 hrs in the B mode and about 2.2 hrs in the Q mode, on average. Although the pulsar can switch modes within one pulse, the sub-pulse drift rate changes with a characteristic time of 1.2 hrs over the course of a B mode. Under the subbeam carousel model we find the drift-rate changes are produced by a 10% increase in the average number of subbeams and a 16% increase in the carousel circulation time. We speculate that under the partially screened gap model the increase in circulation time should be related to a small increase in the neutron star surface temperature.
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