摘要：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.
