Plasma Density and Magnetic Field Fluctuations in the Ion Gyro‐Frequency Range Near the Diamagnetic Cavity of Comet 67P. Issue 12 (2nd December 2020)

• Record Type: Journal Article
• Authors: Odelstad, E.; Eriksson, A. I.; André, M.; Graham, D. B.; Karlsson, T.; Vaivads, A.; Vigren, E.; Goetz, C.; Nilsson, H.; Henri, P.; Stenberg‐Wieser, G.
• Is Part Of: Journal of geophysical research. Volume 125:Issue 12(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-12-02
• Subjects: comets -- plasma -- ion Bernstein waves -- diamagnetic cavity -- Langmuir probe -- Rosetta; Magnetospheric physics -- Periodicals; Space environment -- Periodicals; Cosmic physics -- Periodicals; Planets -- Atmospheres -- Periodicals; Heliosphere (Astrophysics) -- Periodicals; Geophysics -- Periodicals; 523.01
• DOI: 10.1029/2020JA028592
• Languages: English
• ISSNs: 2169-9380

Abstract

Abstract: We 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 ≲ 0 . 5 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. Key Points: Large‐amplitude low‐frequency (around 0.1 Hz) plasma density fluctuations are observed at comet 67P They coincide with the plasma density and magnetic field enhancements surrounding the diamagnetic cavity This is a new type of waves at comets, probably ion Bernstein waves, possibly driven by velocity space anisotropies arising near the cavity