Solar Wind Protons in the Diamagnetic Cavity at Comet 67P/Churyumov‐Gerasimenko

• Published in: Journal of geophysical research. Space physics 2023-04, Vol.128 (4), p.e2022JA031249-n/a
• Main Authors: Goetz, Charlotte, Scharré, Lucie, Wedlund, Cyril Simon, Moeslinger, Anja, Nilsson, Hans, Odelstad, Elias, Taylor, Matthew G. G. T., Volwerk, Martin
• Format: Article
• Language: English
• Subjects: Charge transfer; Charged particles; Diamagnetism; Dynamic pressure; Interplanetary magnetic field; Magnetic fields; Magnetospheric Physics; Neutral gases; Oil and gas production; Protons; Saturn; Solar and Heliospheric Physics; Solar wind; Solar Wind Interactions with Unmagnetized Bodies; Solar wind ions; Solar wind protons; Solar wind velocity; Wind speed; Wind velocities
• ISSN: 2169-9380; 2169-9402; 2169-9402
• DOI: 10.1029/2022JA031249

The plasma environment at a comet can be divided into different regions with distinct plasma characteristics. Two such regions are the solar wind ion cavity, which refers to the part of the outer coma that does not contain any solar wind ions anymore; and the diamagnetic cavity, which is the region of unmagnetized plasma in the innermost coma. From theory and previous observations, it was thought that under usual circumstances no solar wind ion should be observable near or inside of the diamagnetic cavity. For the first time, we report on five observations that show that protons near solar wind energies can also be found inside the diamagnetic cavity. We characterize these proton signatures, where and when they occur, and discuss possible mechanisms that could lead to protons penetrating the inner coma and traversing the diamagnetic cavity boundary. By understanding these observations, we hope to better understand the interaction region of the comet with the solar wind under nonstandard conditions. The protons detected inside the diamagnetic cavity have directions and energies consistent with protons of solar wind origin. The five events occur only at intermediate gas production rates and low cometocentric distances. Charge transfer reactions, high solar wind dynamic pressure and a neutral gas outburst can be ruled out as causes. We suggest that the anomalous appearance of protons in the diamagnetic cavity is due to a specific solar wind configuration where the solar wind velocity is parallel to the interplanetary magnetic field, thus inhibiting mass‐loading and deflection. Key Points Unexpectedly, protons of solar wind origin were observed in the diamagnetic cavity at comet 67P We have ruled out internal and external transients as well as charge‐exchange as triggers for this unusual observation We theorize protons are able to penetrate the diamagnetic cavity due to solar wind conditions with the velocity and magnetic field parallel