We report on magnetic field measurements made
in the innermost coma of 67P/Churyumov-Gerasimenko
in its low activity state. Quasi-coherent, large-amplitude
(δB/B ∼ 1), compressional magnetic field oscillations at
5 ∼ 40 mHz dominate the immediate plasma environment
of the nucleus. This differs from previously studied cometinteraction
regions where waves at the cometary ion gyrofrequencies
are the main feature. Thus classical pick-up ion
driven instabilities are unable to explain the observations. We
10 propose a cross-field current instability associated with newborn
cometary ion currents as a possible source mechanism