Abstract
AbstractWe report the detection of large‐amplitude, quasi‐harmonic density fluctuations with associated magnetic field oscillations in the region surrounding the diamagnetic cavity of comet 67P. Typical frequencies are ∼0.1 Hz, corresponding to ∼10 times the water and times the proton gyro‐frequencies, respectively. Magnetic field oscillations are not always clearly observed in association with these density fluctuations, but when they are, they consistently have wave vectors perpendicular to the background magnetic field, with the principal axis of polarization close to field‐aligned and with a ∼90° phase shift with respect to the density fluctuations. The fluctuations are observed in association with asymmetric plasma density and magnetic field enhancements previously found in the region surrounding the diamagnetic cavity, occurring predominantly on their descending slopes. This is a new type of waves not previously observed at comets. They are likely ion Bernstein waves, and we propose that they are excited by unstable ring, ring‐beam, or spherical shell distributions of cometary ions just outside the cavity boundary. These waves may play an important role in redistributing energy between different particle populations and reshape the plasma environment of the comet.
Highlights
The plasma environments of active comets are dominated by the interaction of the solar wind with newly born cometary heavy ions
Before Rosetta, four comets had been visited by spacecraft carrying instruments capable of observing plasma waves: 21P/Giacobini-Zinner, 1P/Halley, 26P/Grigg-Skjellerup and 19P/Borrelly
We have not found any clear distinguishing circumstances between occurrences of the oscillations with and without clear magnetic signatures, this should be further investigated in future works
Summary
The plasma environments of active comets are dominated by the interaction of the solar wind (hereafter SW) with newly born cometary heavy ions. These are mainly water group ions H2O+ and H3O+, produced by ionization (predominantly by solar EUV radiation, and charge exchange and electron impact reactions with the SW and high-energy electrons) of cometary neutral volatiles (mostly H2O) over large distances (∼105–106 km) in the extensive and diffuse cometary coma. Waves are important in determining many of the properties of the cometary plasma environment. They can, for example, heat or cool plasma populations, produce supra-thermal electrons, reduce plasma anisotropies and gradients, couple different plasma species, and provide anomalous resistivity
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