Low-frequency waves at comet 67P/Churyumov-Gerasimenko^⋆

Observations compared to numerical simulations

¹ Institut für Geophysik und extraterrestrische Physik, Technische Universität Braunschweig, Mendelssohnstraße 3, 38106 Braunschweig, Germany
e-mail: c.koenders@tu-braunschweig.de
² Institut für theoretische Physik, Technische Universität Braunschweig, Mendelssohnstraße 3, 38106 Braunschweig, Germany
³ DLR-Institut für Planetenforschung, Rutherfordstr. 2, 12489 Berlin, Germany
⁴ Max-Planck Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany

Received: 27 April 2016
Accepted: 23 June 2016

Abstract

Context. A new type of low-frequency wave was detected by the magnetometer of the Rosetta Plasma Consortium at the comet during the initial months after the arrival of the Rosetta spacecraft at comet 67P/Churyumov-Gerasimenko. This large-amplitude, nearly continuous wave activity is observed in the frequency range from 30 mHz to 80 mHz where 40 mHz to 50 mHz is the dominant frequency. This type of low frequency is not closely related to the gyrofrequency of newborn cometary ions, which differs from previous wave activity observed in the interaction region of comets with the solar wind.

Aims. This work aims to reveal a global view on the wave activity region using simulations of the comet-solar wind interaction region. Parameters, such as wavelength, propagation direction, and propagation patterns, are within the focus of this study. While the Rosetta observations only provide local information, numerical simulations provide further information on the global wave properties.

Methods. Standard hybrid simulations were applied to the comet-solar wind interaction scenario. In the model, the ions were described as particles, which allows us to describe kinetic processes of the ions. The electrons were described as a fluid.

Results. The simulations exhibit a threefold wave structure of the interaction region. A Mach cone and a Whistler wing are observed downstream of the comet. The third kind of wave activity found are low-frequency waves at 97 mHz, which corresponds to the waves observed by Richter et al. (2015, Ann. Geophys., 33, 1031). These waves are caused by the initial pick-up of the cometary ions that are perpendicular to the solar wind flow and in the interplanetary magnetic field direction. The associated electric current becomes unstable. The simulations show that wave activity is only detectable in the + E hemisphere and that the Mach cone and whistler wings need to be distinguished from the newly found instability driven wave activity.