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

¹ Institut für Geophysik und extraterrestrische Physik, TU Braunschweig, Mendelssohnstr. 3, 38106 Braunschweig, Germany
e-mail: c.goetz@tu-bs.de
² Physikalisches Institut, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
³ Southwest Research Institute, PO Drawer 28510, San Antonio, TX 78228-0510, USA
⁴ Space and Atmospheric Physics Group, Imperial College London, Exhibition Road, London SW7 2AZ, UK
⁵ Swedish Institute of Space Physics, Angström Laboratory, Lägerhyddsvägen 1, 75105 Uppsala, Sweden
⁶ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
⁷ Laboratoire de Physique et Chimie de l’Environnement et de l’Espace, UMR 7328 CNRS, Université d’Orléans, 45100 Orléans, France
⁸ Wigner Research Centre for Physics, Konkoly Thege Miklós út 29-33, 1121 Budapest, Hungary
⁹ Swedish Institute of Space Physics, PO Box 812, 981 28 Kiruna, Sweden
¹⁰ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
¹¹ European Space Astronomy Centre, 28691 Villanueva de la Canada, Madrid, Spain
¹² Space Research Institute, Austrian Academy of Sciences, Schmiedlstr. 6, 8042 Graz, Austria

Received: 10 November 2015
Accepted: 4 February 2016

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.