MESSENGER orbital observations of large‐amplitude Kelvin‐Helmholtz waves at Mercury's magnetopause

Abstract

[1] We present a survey of Kelvin-Helmholtz (KH) waves at Mercury's magnetopause during MESSENGER's first Mercury year in orbit. The waves were identified on the basis of the well-established sawtooth wave signatures that are associated with nonlinear KH vortices at the magnetopause. MESSENGER frequently observed such KH waves in the dayside region of the magnetosphere where the magnetosheath flow velocity is still subsonic, which implies that instability growth rates at Mercury's magnetopause are much larger than at Earth. We attribute these greater rates to the limited wave energy dissipation in Mercury's highly resistive regolith. The wave amplitude was often on the order of 100 nT or more, and the wave periods were $10-20 s. A clear dawn-dusk asymmetry is present in the data, in that all of the observed wave events occurred in the postnoon and duskside sectors of the magnetopause. This asymmetry is likely related to finite Larmor-radius effects and is in agreement with results from particle-in-cell simulations of the instability. The waves were observed almost exclusively during periods when the north-south component of the magnetosheath magnetic field was northward, a pattern similar to that for most terrestrial KH wave events. Accompanying plasma measurements show that the waves were associated with the transport of magnetosheath plasma into the magnetosphere.

Dawn–dusk asymmetry in the Kelvin–Helmholtz instability at Mercury

“…This configuration where the IMF is parallel to the planetary magnetic field in the MP is favourable for the KHI along the equatorial magnetospheric flanks 16 . The 29-September and subsequent observations show a clear asymmetry in the KHI in favour of dusk 11 , which will be explained through consideration of asymmetries in the forces acting on the ions at dusk and dawn.…”

“…The IMF orientation in the magnetosheath before these sawtooth oscillations varied from southward to northward 8 , but in the majority of cases where the KHI is observed the IMF is strongly northward 11 . This configuration where the IMF is parallel to the planetary magnetic field in the MP is favourable for the KHI along the equatorial magnetospheric flanks 16 .…”

“…The KHI is of interest, because it has a strong role in controlling mass and energy transport throughout Mercury's as well as the Earth's 9 magnetosphere. The observations 10,11 reveal that the KH waves are routinely present with a source region that is close to the subsolar point along the post-noon MP. The KHI also produces disturbances over a large portion of the dayside magnetosphere 11 .…”

“…The observations 10,11 reveal that the KH waves are routinely present with a source region that is close to the subsolar point along the post-noon MP. The KHI also produces disturbances over a large portion of the dayside magnetosphere 11 . It is these observations that we attempt to explain here.…”

Multipoint observations of plasma phenomena made in space by Cluster

“…Recently, in-situ observations from Cluster, THEMIS, Geotail, Double Star, MESSENGER, and Cassini have indicated that the KHI appears capable of generating surface waves (e.g., Fairfield et al 1990;Hasegawa et al 2004;Nykyri et al 2006;Fairfield et al 2007;Volwerk et al 2007;Taylor and Lavraud 2008;Boardsen et al 2010;Masters et al 2010;Cutler et al 2011;Hwang et al 2011bHwang et al , 2012Sundberg et al 2012). KHW have also been identified in the magnetosheath (Walker et al 2011b), the solar corona (Ofman and Thompson 2011), and the ionospheres of Mars and Venus (Pope et al 2009).…”