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We present broadband, low-frequency (25-80 MHz and 110-190 MHz) LOFARobservations of PSR B0943+10, with the goal of better illuminating the natureof its enigmatic mode-switching behaviour. This pulsar shows two relativelystable states: a Bright (B) and Quiet (Q) mode, each with differentcharacteristic brightness, profile morphology, and single-pulse properties. Wemodel the average profile evolution both in frequency and time from the onsetof each mode, and highlight the differences between the two modes. In bothmodes, the profile evolution can be well explained by radius-to-frequencymapping at altitudes within a few hundred kilometres of the stellar surface. Ifboth B and Q-mode emission originate at the same magnetic latitude, then wefind that the change of emission height between the modes is less than 6%. Wealso find that, during B-mode, the average profile is gradually shiftingtowards later spin phase and then resets its position at the next Q-to-Btransition. The observed B-mode profile delay is frequency-independent (atleast from 25-80 MHz) and asymptotically changes towards a stable value ofabout 0.004 in spin phase by the end of mode instance, much too large to be dueto changing spin-down rate. Such a delay can be interpreted as a gradualmovement of the emission cone against the pulsar's direction of rotation, withdifferent field lines being illuminated over time. Another interestingexplanation is a possible variation of accelerating potential inside the polargap. This explanation connects the observed profile delay to the graduallyevolving subpulse drift rate, which depends on the gradient of the potentialacross the field lines.

LOFAR observations of PSR B0943+10: profile evolution and discovery of a systematically changing profile delay in Bright mode