Electric potential difference between the upstream solar wind and the observation point (V) as a function of time before the solar wind ion cavity (top) and after the solar wind ion cavity (bottom). Green dots show individual data points. The orange line shows the same data filtered using a 400-point running median filter. Arrows marked A, B, and C indicate the cases discussed in Sect. 4.6. The grey line shows the comet-Sun distance in AU.