Dynamic field line draping at comet 67P/Churyumov-Gerasimenko during the Rosetta dayside excursion

Abstract

Context. The Rosetta dayside excursion took place in September–October 2015 when comet 67P/Churyumov-Gerasimenko (67P/CG) was located at ~1.36 AU from the Sun after it had passed perihelion on 13 August 2015 at ~1.25 AU. At this time, the comet was near its most active period, and its interaction with the solar wind was expected to be at its most intense, with ion pickup and magnetic field line draping. The dayside excursion was planned to move through different regions that were expected upstream of the cometary nucleus, and to possibly detect the location of the bow shock. Aims. The goal of this study is to describe the dynamic field line draping that takes place around the comet and the plasma processes that are connected to this. Methods. The data from the full Rosetta Plasma Consortium (RPC) were used to investigate the interaction of solar wind and comet, starting from boxcar-averaged magnetic field data in order to suppress high-frequency noise in the data. Through calculating the cone and clock angle of the magnetic field, we determined the draping pattern of the magnetic field around the nucleus of the comet. Then we studied the particle data in relation to the variations that are observed in the magnetic field. Results. During the dayside excursion, the magnetic field cone angle changed several times, which means that the magnetic field direction changes from pointing sunward to anti-sunward. This is caused by the changing directions of the interplanetary magnetic field that is transported toward the comet. The cone-angle direction shows that mass-loading of the interplanetary magnetic field of the solar wind leads to dynamic draping. The ion velocity and the magnetic field strength are correlated because the unmagnetized ions are accelerated more (less) strongly by the increasing (decreasing) magnetic field strength. There is an indication of an anticorrelation between the electron density and the magnetic field strength, which might be caused by the magnetized electrons being mirrored out of the strong field regions. The Rosetta RPC has shown that (dynamic) draping also occurs as mildly active comets, as was found at highly active comets such as 1P/Halley and 21P/Giacobini-Zinner, but also that determining both dynamic and nested draping will require a combination of fast flybys and slow excursions for future missions.