| 1. |
- Odelstad, Elias, et al.
(author)
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Ion Velocity and Electron Temperature Inside and Around the Diamagnetic Cavity of Comet 67P
- 2018
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In: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 123:7, s. 5870-5893
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Journal article (peer-reviewed)abstract
- Abstract A major point of interest in cometary plasma physics has been the diamagnetic cavity, an unmagnetized region in the innermost part of the coma. Here we combine Langmuir and Mutual Impedance Probe measurements to investigate ion velocities and electron temperatures in the diamagnetic cavity of comet 67P, probed by the Rosetta spacecraft. We find ion velocities generally in the range 2?4 km/s, significantly above the expected neutral velocity 1 km/s, showing that the ions are (partially) decoupled from the neutrals, indicating that ion-neutral drag was not responsible for balancing the outside magnetic pressure. Observations of clear wake effects on one of the Langmuir probes showed that the ion flow was close to radial and supersonic, at least with respect to the perpendicular temperature, inside the cavity and possibly in the surrounding region as well. We observed spacecraft potentials V throughout the cavity, showing that a population of warm (?5 eV) electrons was present throughout the parts of the cavity reached by Rosetta. Also, a population of cold ( ) electrons was consistently observed throughout the cavity, but less consistently in the surrounding region, suggesting that while Rosetta never entered a region of collisionally coupled electrons, such a region was possibly not far away during the cavity crossings.
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| 2. |
- Odelstad, Elias, et al.
(author)
-
Plasma density and magnetic field fluctuations in the ion gyro-frequency range near the diamagnetic cavity of comet 67P
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Other publication (other academic/artistic)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 to 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 lag w.r.t. the density fluctuations. The fluctuations are observed in association with asymmetric plasma and magnetic field enhancements previously found in the region surrounding the diamagnetic cavity, occurring predominantly on their descending slopes. We speculate that they are Ion Bernstein waves (IBWs) excited by the drift-cyclotron instability resulting from strong plasma inhomogeneities in the region surrounding the diamagnetic cavity.
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| 3. |
- Odelstad, Elias, et al.
(author)
-
Plasma density and magnetic field fluctuations in the ion gyro-frequency range near the diamagnetic cavity of comet 67P
-
Other publication (other academic/artistic)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 to 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 lag w.r.t. the density fluctuations. The fluctuations are observed in association with asymmetric plasma and magnetic field enhancements previously found in the region surrounding the diamagnetic cavity, occurring predominantly on their descending slopes. We speculate that they are Ion Bernstein waves (IBWs) excited by the drift-cyclotron instability resulting from strong plasma inhomogeneities in the region surrounding the diamagnetic cavity.
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