Interpreting ~1 Hz magnetic compressional waves in Mercury's inner magnetosphere in terms of propagating ion‐Bernstein waves

Journal Article · Mon Jun 01 00:00:00 EDT 2015 · Journal of Geophysical Research. Space Physics

DOI:https://doi.org/10.1002/2014JA020910· OSTI ID:2279216

Boardsen, S. A. ^[1]; Kim, E. ‐H. ^[2]; Raines, J. M. ^[3]; Slavin, J. A. ^[3]; Gershman, D. J. ^[4]; Anderson, B. J. ^[5]; Korth, H. ^[5]; Sundberg, T. ^[6]; Schriver, D. ^[7]; Travnicek, P. ^[8]

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1. Goddard Planetary Heliophysics Institute University of Maryland, Baltimore County Baltimore Maryland USA, Heliophysics Science Division NASA Goddard Space Flight Center Greenbelt Maryland USA
2. Princeton Center for Heliophysics and Princeton Plasma Physics Laboratory Princeton University Princeton New Jersey USA
3. Department of Atmospheric, Oceanic and Space Sciences University of Michigan Ann Arbor Michigan USA
4. Department of Atmospheric, Oceanic and Space Sciences University of Michigan Ann Arbor Michigan USA, Geospace Physics Laboratory NASA Goddard Space Flight Center Greenbelt Maryland USA
5. Johns Hopkins University Applied Physics Laboratory Laurel Maryland USA
6. School of Physics and Astronomy Queen Mary University of London London UK
7. Department of Physics University of California Los Angeles California USA
8. Space Science Laboratory University of California Berkeley California USA

Abstract We show that ~1 Hz magnetic compressional waves observed in Mercury's inner magnetosphere could be interpreted as ion‐Bernstein waves in a moderate proton beta ~0.1 plasma. An observation of a proton distribution with a large planetary loss cone is presented, and we show that this type of distribution is highly unstable to the generation of ion‐Bernstein waves with low magnetic compression. Ray tracing shows that as these waves propagate back and forth about the magnetic equator; they cycle between a state of low and high magnetic compression. The group velocity decreases during the high‐compression state leading to a pileup of compressional wave energy, which could explain the observed dominance of the highly compressional waves. This bimodal nature is due to the complexity of the index of refraction surface in a warm plasma whose upper branch has high growth rate with low compression, and its lower branch has low growth/damping rate with strong compression. Two different cycles are found: one where the compression maximum occurs at the magnetic equator and one where the compression maximum straddles the magnetic equator. The later cycle could explain observations where the maximum in compression straddles the equator. Ray tracing shows that this mode is confined within ±12° magnetic latitude which can account for the bulk of the observations. We show that the Doppler shift can account for the difference between the observed and model wave frequency, if the wave vector direction is in opposition to the plasma flow direction. We note that the Wentzel‐Kramers‐Brillouin approximation breaks down during the pileup of compressional energy and that a study involving full wave solutions is required.

• Sponsoring Organization: USDOE
• OSTI ID: 2279216
• Journal Information: Journal of Geophysical Research. Space Physics, Journal Name: Journal of Geophysical Research. Space Physics Vol. 120 Journal Issue: 6; ISSN 2169-9380
• Publisher: American Geophysical Union (AGU)
• Country of Publication: United States
• Language: English