Momentum and pressure balance of a comet ionosphere

We calculate the momentum flux and pressure of ions measured by the Ion Composition Analyzer (ICA) on the Rosetta mission at comet 67P/Churyumov-Gerasimenko. The total momentum flux stays roughly constant over the mission, but the contributions of different ion populations change depending on heliocentric distance. The magnetic pressure, calculated from Rosetta magnetometer measurements, roughly corresponds with the cometary ion momentum flux. When the spacecraft enters the solar wind ion cavity, the solar wind fluxes drop drastically, while the cometary momentum flux becomes roughly ten times the solar wind fluxes outside of the ion cavity, indicating that pickup ions behave similarly to the solar wind ions in this region. We use electron density from the Langmuir probe to calculate the electron pressure, which is particularly important close to the comet nucleus where flow changes from antisunward to radially outward.
Comment: 14 pages, 2 figures, accepted to Geophysical Review Letters