MESSENGER observations of multiscale Kelvin‐Helmholtz vortices at Mercury

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−3) 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.