Rosetta observations of solar wind interaction with the comet 67P/Churyumov-Gerasimenko

¹ Space Science and Engineering Division, Southwest Research Institute (SwRI), 6220 Culebra Rd San Antonio, TX 78238, USA
e-mail: tbroiles@swri.edu
² Heliophysics Division, Goddard Space Flight Center, 8800 Greenbelt Road Greenbelt, MD 20771, USA
³ Department of Physics and Astronomy, The Catholic University of America, 620 Michigan Ave NE, Washington, DC 20064, USA
⁴ Institut für Geophysik und extraterrestrische Physik, Technische Universität Braunschweig, Mendelssohnstraße 3, 38106 Braunschweig, Germany
⁵ Swedish Institute for Space Physics, PO Box 812, 981 28 Kiruna, Sweden
⁶ Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Rymdcampus 1, 981 28 Kiruna, Sweden
⁷ The University of Texas at San Antonio, Department of Physics and Astronomy, 1 UTSA Circle, San Antonio, TX 78249, USA

Received: 6 March 2015
Accepted: 20 May 2015

Abstract

Context. The Rosetta spacecraft arrived at the comet 67P/Churyumov-Gerasimenko on August 6, 2014, which has made it possible to perform the first study of the solar wind interacting with the coma of a weakly outgassing comet.

Aims. It is shown that the solar wind experiences large deflections (>45°) in the weak coma. The average ion velocity slows from the mass loading of newborn cometary ions, which also slows the interplanetary magnetic field (IMF) relative to the solar wind ions and subsequently creates a Lorentz force in the frame of the solar wind. The Lorentz force in the solar wind frame accelerates ions in the opposite direction of cometary pickup ion flow, and is necessary to conserve momentum.

Methods. Data from the Ion and Electron Sensor are studied over several intervals of interest when significant solar wind deflection was observed. The deflections for protons and for He⁺⁺ were compared with the flow of cometary pickup ions using the instrument’s frame of reference. We then fit the data with a three-dimensional Maxwellian, and rotated the flow vectors into the Comet Sun Equatorial coordinate system, and compared the flow to the spacecraft’s position and to the local IMF conditions.

Results. Our observations show that the solar wind may be deflected in excess of 45° from the anti-sunward direction. Furthermore, the deflections change direction on a variable timescale. Solar wind protons are consistently more deflected than the He⁺⁺. The deflections are not ordered by the spacecraft’s position relative to the comet, but large changes in deflection are related to changes in the orthogonal IMF components.