A simulation study of Kelvin-Helmholtz waves at Saturn's magnetopause

In global simulations vortices form along Saturn's dawnside magnetopause when there is high-velocity shear and the condition for the generation of Kelvin-Helmholtz (K-H) waves is met. The wave vectors (k) are related to the thickness of the magnetopause layer (d) by kd similar to 1. K-H waves form on both the magnetopause and the inner edge of the boundary layer. In the simulation waves are similar to those found in simulations of the Earth's magnetosphere. The main difference is that at Saturn rotating flows are important for making the boundary unstable. The waves form for northward interplanetary magnetic field (IMF) and not for southward IMF. (Note that Saturn's magnetic field is opposite to that at Earth so northward IMF can lead to reconnection at the dayside magnetopause.) For northward IMF the effect of reconnection is to enhance the electric field on the dawnside so that rotating flows within the magnetosphere are enhanced. This leads to instability. About 20 h after a northward IMF first interacts with the dayside magnetopause, reconnection occurs in the tail. Flows from the tail to the dayside reduce the plasma density in the dayside magnetosphere, causing the boundary to become stable. Within the magnetosphere the K-H waves lead to oscillations in the magnetic field. Similar oscillations are found in Cassini observations. K-H waves also can form at the afternoon magnetopause for northward IMF. They form later and tend to have smaller amplitude than those found in the morning.