Momentum and Pressure Balance of a Comet Ionosphere

• Published in: Geophysical research letters 2020-08, Vol.47 (15), p.n/a
• Main Authors: Williamson, H. N., Nilsson, H., Stenberg Wieser, G., Eriksson, A. I., Richter, I., Goetz, C.
• Format: Article
• Language: English
• Subjects: Atmosphere; Charged particles; Comet nuclei; Comet nucleus; Cometary atmospheres; Cometary ions; Electron density; Electron pressure; Fluctuations; Fluxes; Ionosphere; Ions; Magnetic field; Magnetic fields; Magnetometers; Mathematical analysis; Momentum; Momentum flux; Momentum transfer; Pressure; Rosetta mission; Solar wind; Solar wind ions; Spacecraft; Wind shear
• ISSN: 0094-8276; 1944-8007; 1944-8007
• DOI: 10.1029/2020GL088666

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 10 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. Plain Language Summary To understand the atmosphere of a comet, we must understand how its charged particles interact with the solar wind. Here we look at momentum flux, the amount of momentum carried by particles flowing through a certain area, measured at the comet 67P/Churyumov‐Gerasimenko by the Rosetta spacecraft. We find that the total momentum flux changes the amount we would expect from the comet getting closer to the Sun. When in the part of the atmosphere with no solar wind, the ions coming from the comet have more momentum flux than the solar wind ions elsewhere in the comet atmosphere. The increase matches what we expect from the comet getting closer to the Sun, which increases the total density of the comet atmosphere. The cometary ions replace the solar wind ions in the atmosphere, so the total stays the same. The magnetic field varies with the cometary ions. We find electron pressure with measured electron data. It is higher than the momentum flux, especially in the part of the atmosphere close to the comet nucleus. This means that the atmosphere here is primarily moving outward from the comet, while farther away from the nucleus it is mainly flowing away from the Sun. Key Points Momentum flux of Rosetta ion data shows the evolution of a cometary ionosphere, including boundary regions Magnetic pressure is on the order of the total ion momentum flux and roughly corresponds with cometary ion momentum flux The cometary pickup ions in the solar wind ion cavity take over the role of the solar wind ions outside the cavity