First detection of a diamagnetic cavity at comet 67P/Churyumov-Gerasimenko
Altwegg, K.; Glassmeier, K. -H.; Nilsson, H.; Eriksson, A.; Sierks, H.; Güttler, C.; Volwerk, M.; Carr, C.; Goetz, C.; Henri, P.; Mokashi, P.; Richter, I.; Rubin, M.; Tsurutani, B.; Vallat, C.; Burch, J.; Cupido, E.; Koenders, C.; Nemeth, Z.
Germany, Switzerland, United States, United Kingdom, Sweden, France, Hungary, Spain, Austria
Abstract
Context. The Rosetta magnetometer RPC-MAG has been exploring the plasma environment of comet 67P/Churyumov-Gerasimenko since August 2014. The first months were dominated by low-frequency waves which evolved into more complex features. However, at the end of July 2015, close to perihelion, the magnetometer detected a region that did not contain any magnetic field at all.
Aims: These signatures match the appearance of a diamagnetic cavity as was observed at comet 1P/Halley in 1986. The cavity here is more extended than previously predicted by models and features unusual magnetic field configurations, which need to be explained.
Methods: The onboard magnetometer data were analyzed in detail and used to estimate the outgassing rate. A minimum variance analysis was used to determine boundary normals.
Results: Our analysis of the data acquired by the Rosetta Plasma Consortium instrumentation confirms the existence of a diamagnetic cavity. The size is larger than predicted by simulations, however. One possible explanation are instabilities that are propagating along the cavity boundary and possibly a low magnetic pressure in the solar wind. This conclusion is supported by a change in sign of the Sun-pointing component of the magnetic field. Evidence also indicates that the cavity boundary is moving with variable velocities ranging from 230-500 m/s.