The secular variation of the Earth's internal magnetic field changes on time scales of the order of a year. Such impulses occurred in 1969 and probably in 1913; they provide constraints on deep mantle conductivity, which appears to be rather low, and on the motions of the top of the fluid core, which are probably dominated by westward drift. Present data favour the existence of upwelling of core material at the core–mantle boundary. The impulses can be correlated with extrema in the Earth's rotation rate and interpreted in terms of electromagnetic core–mantle coupling.