The cometary plasma environment at low gas production rates is dominated by highly compressional, large-amplitude magnetic field waves in the 10–100 mHz range. They are thought to be caused by an ion-Weibel instability due to a cross-field current, which is caused by the cometary ions that are accelerated along the solar wind convective electric field. We devise a new method to determine the location of the wave source, the wave power, frequency, and bandwidth. It is found that the wave occurs everywhere in the coma, regardless of electric field direction. There is no correlation between the wave frequency and the measured plasma density. This is not in agreement with previous studies. A dependence of the frequency on the position of the spacecraft in a comet-fixed frame is in agreement with the prediction from the ion-Weibel instability. We infer a wave generation region much larger than the cometocentric distances covered by Rosetta.