Modelling the size of the very dynamic diamagnetic cavity of comet 67P/Churyumov–Gerasimenko

Abstract After the first detection of the diamagnetic cavity of comet 67P/Churyumov–Gerasimenko, it became apparent that the boundary of this plasma region is very dynamic. To date hundreds of short cavity crossing events were detected, none lasting longer than an hour. This intermittent set of short events is very different from the classical cavity observation near 1P/Halley, where Giotto remained continuously inside the cavity. The distance of the cavity boundary at 67P is larger than that predicted by recent models, so it was not clear whether these short cavity-like regions are connected to a global diamagnetic cavity, or they are due to some local effects. Here, we revisit the neutral-drag model of Cravens (1986) and we provide a very good phenomenological approximation for the highly variable size of this dynamic region. The model uses the cometary neutral production rate and the solar wind dynamic pressure as inputs. For the production rate, we use averaged and detrended data derived from Rosetta Orbiter Spectrometer for Ion and Neutral Analysis neutral density measurements. We show that instead of the local neutral pressure, the global production rate drives the size of the cavity. The solar wind pressure is derived from space weather models and independently from the magnetic field measurements of Rosetta Magnetometer (MAG). We accurately estimate the highly variable size of the cavity using this data. Our results suggest that at the time of the measurements a global diamagnetic cavity existed around comet 67P, the size of which varied dynamically following the changing cometary gas production and solar wind pressure.