Steepening of Waves at the Dusk Side Magnetopause

Abstract:

Observations of the dynamic dusk side magnetopause show abundant evidence for the passage of surfaces waves that may have been created or amplified by the Kelvin-Helmholtz instability (KHI). That instability is fed by the flow shear of plasma across the magnetopause boundary. Wave amplification by the KHI involves steepening of the waves’ leading edges, which precedes wave breaking and conversion into KH vortices. Indeed, magnetopause surface waves featuring steeper leading edges have been observed in numerous occasions. By contrast, observations of steeper trailing edges in magnetopause surface waves are rare, and it is unclear how they develop. Chen et al. [J. Geophys. Res., 98, 5727 (1993)] suggested that this inverse wave steepening could originate from the release of magnetic tension in the magnetosheath (plasma depletion layer), based on ISEE measurements. However, more recent findings by Plaschke et al. [J. Geophys. Res., 118, 1483 (2013)] based on THEMIS multi-spacecraft measurements suggest that this explanation may not always be applicable.
New light can be shed on this phenomenon by the newly launched Magnetospheric Multiscale (MMS) mission, which consists of four spacecraft that measure fields and particles with unprecedented time resolution. On July 9th, the MMS spacecraft began to obtain data regularly in a small-scale tetrahedron of about 160 km on a side. This tetrahedron may with time be further reduced to a size of about 10 km on a side. In this tetrahedral configuration, when close to apogee, the spacecraft stay in the (dusk side) magnetopause region over long periods of time, making possible a routine determination of wave steepness by timing analysis. We present a study thereof, relating our findings to local magnetospheric/magnetosheath and upstream conditions.