MESSENGER observations of multiscale Kelvin‐Helmholtz vortices at Mercury

Abstract

Observations by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft in Mercury's magnetotail demonstrate for the first time that Na + ions exert a dynamic influence on Mercury's magnetospheric system. Na + ions are shown to contribute up to~30% of the ion thermal pressure required to achieve pressure balance in the premidnight plasma sheet. High concentrations of planetary ions should lead to Na + dominance of the plasma mass density in these regions. On orbits with northward-oriented interplanetary magnetic field and high (i.e., >1 cm À3) Na + concentrations, MESSENGER has often recorded magnetic field fluctuations near the Na + gyrofrequency associated with the Kelvin-Helmholtz (K-H) instability.These nightside K-H vortices are characteristically different from those observed on Mercury's dayside that have a nearly constant wave frequency of~0.025 Hz. Collectively, these observations suggest that large spatial gradients in the hot planetary ion population at Mercury may result in a transition from a fluid description to a kinetic description of vortex formation across the dusk terminator, providing the first set of truly multiscale observations of the K-H instability at any of the diverse magnetospheric environments explored in the solar system.

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

“…It is the strong compressional nature of this lower branch that makes this wave mode a good candidate for explaining the compressibility of the observed waves, not the upper branch as originally proposed in Boardsen et al []. As mentioned, a proton beta of ~2 would be required for the upper branch to be compressional [ Denton et al , ], such that beta is only observed in the plasma sheet tail at Mercury [ Korth et al , ; Gershman et al , ], where these waves are not observed.…”

“…As proton beta ( β P ) increases, the ion‐Bernstein wave transition from the electrostatic ion‐Bernstein mode for β P ~5 × 10 −4 to a magnetic transverse dominant electromagnetic mode for β P ~0.4 (observed in the Earth's plasma sheet boundary layer) and to a magnetic compressional dominant electromagnetic mode for β P ~2 [ Denton et al , ]. Except in magnetic cavities (plasma sheet proper) at Mercury [ Korth et al , ; Korth et al , ; Gershman et al , , ], such high beta are not observed. In this paper we use warm plasma ray tracing and show that in a smaller beta plasma of ~0.1, waves that are initially transverse dominant can become compressionally dominant in a cyclic manner as they propagate in a trapped magnetic (dipole) geometry.…”

Juno observations of large‐scale compressions of Jupiter's dawnside magnetopause

“…However, Ebert et al [] recently observed Hall magnetic field structures and Alfvénic jets at Jupiter's magnetopause near the dawn terminator, indicating that reconnection may yet play a role in mass transport. Kelvin‐Helmholtz vortices, which are observed at other planetary magnetospheres [ Hasegawa et al , ; Masters et al , ; Delamere et al , ; Sundberg et al , ; Gershman et al , ; Kavosi and Raeder , ; Ma et al , , , ], are expected to be present, but these structures at Jupiter have not yet been reported. Finally, low‐frequency MHD fluctuations with periods up to ~100 s have been observed inside of Jupiter's magnetosphere and in the magnetopause boundary layer (MPBL) [ Tsurutani et al , ].…”

MESSENGER observations of cusp plasma filaments at Mercury

“…The solar wind interaction with Mercury also produces frequent, large flux transfer events (FTEs) observed from the subsolar region to the high‐latitude magnetopause downstream of the cusp [ Slavin et al , , , ; Imber et al , ]. Fully developed Kelvin‐Helmholtz waves are observed along the low‐latitude magnetopause, but only along the dusk flank [ Boardsen et al , ; Sundberg et al , ; Liljeblad et al , ; Gershman et al , ].…”