First detection of a diamagnetic cavity at comet 67P/Churyumov-Gerasimenko

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.
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C Goetz, C Koenders, I Richter, K Altwegg, J Burch, et al.. First detection of a diamagnetic cavity at comet 67P/Churyumov-Gerasimenko. Astronomy and Astrophysics - A&A, EDP Sciences, 2016, 588, pp.A24. ⟨10.1051/0004-6361/201527728⟩. ⟨insu-01318108⟩

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