MESSENGER observations of Kelvin-Helmholtz waves at Mercury's magnetopause

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 non-linear KH vortices at the magnetopause. Remarkably, the results show that MESSENGER frequently observed such KH waves in the dayside region of the magnetosphere where the magnetosheath flow velocity is still sub-sonic, which implies that instability growth rates at Mercury's magnetopause are much larger than at Earth. 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 also present in the data, with all of the observed events taking place in the post-noon and the dusk-side sectors of the magnetopause. This asymmetry is likely related to finite ion-gyroradius effects and is in agreement with the results from particle-in-cell simulations of the instability. Similar to most terrestrial events, the wave observations were made almost exclusively during periods when the north-south component of the magnetosheath magnetic field was northward. Accompanying measurements from the Fast Imaging Plasma Spectrometer (FIPS) show that the waves were associated with a substantial transport of magnetosheath plasma into the magnetosphere.