Surface waves on Saturnʹs magnetopause

Waves on the surface of a planetary magnetopause promote energy transport into the magnetosphere, representing an important aspect of solar wind–magnetosphere coupling. At Saturnʹs magnetopause it has been proposed that growth of the Kelvin–Helmholtz (K–H) instability produces greater wave activity on the dawn side of the surface than on the dusk side. We test this hypothesis using data taken by the Cassini spacecraft during crossings of Saturnʹs magnetopause. Surface orientation perturbations are primarily controlled by the local magnetospheric magnetic field orientation, and are generally greater at dusk than at dawn. 53% of all crossings were part of a sequence of regular oscillations arising in consecutive surface normals that is strong evidence for tailward propagating surface waves, with no detectable local time asymmetry in this phenomenon. We estimate the dominant wave period to be ∼5 h at dawn and ∼3 h at dusk. The role played by the magnetospheric magnetic field, tailward wave propagation, and the dawn–dusk difference in wave period suggests that K–H instability is a major wave driving mechanism. Using linear K–H theory we estimate the dominant wavelength to be ∼10 Saturn radii (RS) and amplitude to be ∼1 RS at both dawn and dusk, giving propagation speeds of ∼30 and ∼50 km s−1 at dawn and dusk, respectively. The lack of the hypothesized dawn–dusk asymmetry in wave activity demonstrates that we need to revise our understanding of the growth of the K–H instability at Saturnʹs magnetopause, which will have implications for the study of other planetary magnetospheres.