Evidence of surface wave on the dusk flank of Saturn’s magnetopause possibly caused by the Kelvin‐Helmholtz instability

摘要

Boundary waves on Saturn’s dusk side magnetopause were identified from magnetopause crossings made by the Cassini spacecraft. These crossings occurred over a 48 h period from 14 July to 15 July 2007 when Cassini was on the inbound leg of revolution (Rev.) 48, and were located at around 17:00 SLT at low latitudes above the equatorial plane. The boundary normal for each magnetopause crossing was determined from minimum variance analysis (MVA) on the magnetic field within the current sheet interval. MVA was then performed on the normals themselves to identify a maximum variance direction, in an attempt to identify a preferred plane in which the normals varied. The first few crossings of the group exhibited no wave activity. The following 17 crossings showed clear oscillation of consecutive boundary normals about a mean direction in a well defined plane, with a clear periodicity of the order 50 min, suggesting the presence of surface waves with similar characteristics to those observed by Masters et al. (2009). Subsequent normals also exhibited clear consecutive oscillation in a preferred plane, but with no apparent periodicity. The direction along which maximum variance of the normals occurred was assumed to be the direction of wave propagation, and was found to be almost perpendicular to the average magnetospheric field for the duration of the crossings. Both the wave periodicity and propagation direction support the Kelvin‐Helmholtz instability as a strong candidate mechanism for generating these waves.