LOFAR observations of PSR B0943+10: profile evolution and discovery of a
  systematically changing profile delay in Bright mode
  We present broadband, low-frequency (25-80 MHz and 110-190 MHz) LOFAR
observations of PSR B0943+10, with the goal of better illuminating the nature
of its enigmatic mode-switching behaviour. This pulsar shows two relatively
stable states: a Bright (B) and Quiet (Q) mode, each with different
characteristic brightness, profile morphology, and single-pulse properties. We
model the average profile evolution both in frequency and time from the onset
of each mode, and highlight the differences between the two modes. In both
modes, the profile evolution can be well explained by radius-to-frequency
mapping at altitudes within a few hundred kilometres of the stellar surface. If
both B and Q-mode emission originate at the same magnetic latitude, then we
find that the change of emission height between the modes is less than 6%. We
also find that, during B-mode, the average profile is gradually shifting
towards later spin phase and then resets its position at the next Q-to-B
transition. The observed B-mode profile delay is frequency-independent (at
least from 25-80 MHz) and asymptotically changes towards a stable value of
about 0.004 in spin phase by the end of mode instance, much too large to be due
to changing spin-down rate. Such a delay can be interpreted as a gradual
movement of the emission cone against the pulsar's direction of rotation, with
different field lines being illuminated over time. Another interesting
explanation is a possible variation of accelerating potential inside the polar
gap. This explanation connects the observed profile delay to the gradually
evolving subpulse drift rate, which depends on the gradient of the potential
across the field lines.
