Multi-Scale Kelvin-Helmholtz Vortices Along Mercury’s Magnetopause

Tuesday, 16 December 2014: 1:58 PM
Daniel J Gershman1, Jim M Raines2, James A Slavin2, Thomas Zurbuchen3, Torbjorn Sundberg4, Scott A Boardsen5, Brian J Anderson6, Haje Korth7 and Sean C Solomon8, (1)NASA Goddard Space Flight Center, Heliophysics Science Division, Greenbelt, MD, United States, (2)University of Michigan Ann Arbor, Ann Arbor, MI, United States, (3)Univ Michigan, Ann Arbor, MI, United States, (4)Queen Mary University of London, School of Physics and Astronomy, London, United Kingdom, (5)NASA Goddard SFC, Greenbelt, MD, United States, (6)Johns Hopkins University, Baltimore, MD, United States, (7)The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, United States, (8)Lamont-Doherty Earth Observatory, Palisades, NY, United States
Data from the Fast Imaging Plasma Spectrometer (FIPS) and Magnetometer (MAG) sensors on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft have revealed stark differences in the Kelvin-Helmholtz (K-H) instability at Mercury compared with that at Earth. Although K-H vortices have been documented in planetary magnetospheres at the interface of magnetosheath and magnetospheric plasmas, such features at Mercury have been observed exclusively on the dusk side. From a survey of 58 K-H events, we find that these vortices have two distinct sets of behavior separated by the dusk terminator. On the dayside, the wave frequencies measured by MAG are nearly constant at ~0.025 Hz (~40 s period) under a variety of magnetosheath conditions, whereas the wave frequency measured on the nightside is correlated with the strength of the magnetic field near the magnetopause and matches the local Na+ gyrofrequency. The polarization of these waves inside the magnetosphere is distinctly right-handed, consistent with non-linear roll-up of K-H vortices as opposed to the left-handed ion-cyclotron wave mode. During these events, measurements from FIPS reveal strong (~30%) concentrations of Na+ in the nightside plasma sheet adjacent to the magnetopause. The keV energies of these planetary ions provide them with gyroradii that are ~500-1000 km, a scale at which a kinetic description of the K-H instability may be appropriate at Mercury. These data suggest a transition from fluid-scale to kinetic scale K-H vortices from day to night along Mercury’s duskside magnetopause.