Magnetic signatures of Kelvin-Helmholtz vortices on Saturn's magnetopause: Global survey

摘要

Saturn's rapid rotation combined with relatively weak magnetic fields in the outer magnetosphere and sheath lead to conditions that are favorable for the Kelvin-Helmholtz (KH) instability. A Kelvin-Helmholtz unstable magnetopause boundary has important consequences for Saturn's interaction with the solar wind due to mass, momentum, and energy transport that can occur at the magnetopause boundary. Previous attempts to identify vortices have been hampered by limited plasma data to unambiguously reveal vortical flow. The magnetic field data, on the other hand, may be able to identify the KH instability due to intense magnetic fluctuations that are associated with KH vortices. We have conducted two-dimensional hybrid code simulations of Saturn's magnetopause boundary to illustrate the expected magnetic field signatures of KH. Specifically, our simulations show strong field-aligned current sheet filaments or strong bipolar fluctuations of the in-plane magnetic field components, bounding the KH vortices. A global search for these characteristic magnetic field signatures near the magnetopause boundary was made of the Cassini mission data from 2004 to 2009. We find that most of the potential KH activity is found on the dusk flank, contrary to expectations. We suggest that KH growth is supported in the prenoon and subsolar regions and that these vortices are transported through coupling to the rotating planet, past noon and tailward on the dusk flank. In addition, we find many instances in the subsolar magnetosphere of possible plasmoid formation (B_z northward) in conjunction with these intense magnetic field fluctuations.