Abstract
Abstract. We analytically discuss wave excitation in a homogeneous three component plasma consisting of solar wind protons, electrons and a beam of cometary water ions applied to the plasma environment of comet 67P/Churyumov-Gerasimenko. The resulting dispersion relations are studied in a solar wind rest frame, where a cometary current is solely generated by the water ion beam, and a cometary rest frame representing the rest frame of the Rosetta spacecraft. A modified ion-Weibel instability is excited by the cometary current and predominantly grows perpendicular to this current. The corresponding water ion mode is connected to a frequency of about 40 mHz in agreement with wave measurements of Rosetta's magnetometer in the cometary rest frame. Furthermore, the superposition of the strongest growing waves result in a fan-like phase structure close to the comet.
Highlights
The study of waves in plasma environments is an extensive field in plasma physics
We analytically discuss wave excitation in a homogeneous three component plasma consisting of solar wind protons, electrons and a beam of cometary water ions applied to the plasma environment of comet 67P/ChuryumovGerasimenko
The resulting dispersion relations are studied in a solar wind rest frame, where a cometary current is solely generated by the water ion beam, and a cometary rest frame representing the rest frame of the Rosetta spacecraft
Summary
The study of waves in plasma environments is an extensive field in plasma physics. Special attention is paid to cometary magnetospheres, where plasma waves and turbulence are one of the most remarkable observations at comets like 1P/Halley, 21P/Giacobini-Zinner and 26P/GriggSkjellerup (Tsurutani and Smith, 1986; Yumoto et al, 1986; Neubauer et al, 1986; Glassmeier et al, 1989; Glassmeier and Neubauer, 1993; Volwerk et al, 2014). At that time the comet was still beyond 2.7 AU from the sun, the cometary activity just started and a strong magnetosphere was neither expected nor observed (Koenders et al, 2013; Rubin et al, 2014; Nilsson et al, 2015) In this environment the outgassing of the comet combined with the ionization of the water molecules generates a beam of cometary water ions possibly able to cause a type of beam instability. We discuss this scenario with a very simple ansatz of a cold, homogeneous three-component plasma, which is composed of magnetized solar wind protons, magnetized electrons and an unmagnetized beam of cometary water ions, in two different frames of reference.
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