Ubiquity of Kelvin-Helmholtz Waves at Earth’s Magnetopause

Abstract:

Kelvin-Helmholtz waves (KHW) at Earth’s magnetopause are thought to be important for plasma entry and for the excitation of ultra low frequency (ULF) waves.However, the relative importance of KHW remained controversial because no statistical data on their occurrence frequency existed. Here, we survey seven years of data from the NASA THEMIS mission and find that KHW occurs at the magnetopause ~21% of the time, regardless of the solar wind (SW) and interplanetary magnetic field (IMF) conditions. The KHW occurrence rate increases with SW speed and SW Alfven Mach number, but is mostly independent of SW number density and IMF magnitude. The occurrence rate increases with IMF cone angle and maximizes at zero IMF clock angle. However, KHW occurs at a significantly higher rate than expected for southward IMF. KHW may thus be more important for plasma transport across the magnetopause than previously thought, and frequently drive magnetospheric ULF waves.