Comparison of Kelvin–Helmholtz waves observed simultaneously at the dawn and dusk flanks of the Earth’s magnetopause

Across the Earth’s magnetopause, unless the magnetic fields stabilise the boundary, the velocity shear between the magnetospheric plasma and the shocked plasma of the solar wind can lead to the Kelvin–Helmholtz instability. This instability can develop into large-scale surface waves and vortices at the magnetopause, causing the different plasma regions to mix, which plays an important role in the transfer of energy across the magnetopause. We know from spacecraft observations and simulations that the way Kelvin–Helmholtz waves grow and evolve can be different at dawn and dusk. However, very few studies have directly observed this phenomenon on both flanks of the magnetopause simultaneously, nor have they provided a consistent explanation for the question of symmetrical or asymmetrical dawn–dusk growth of the waves. By combining measurements from the THEMIS and Cluster missions, we can report here on an event where such a simultaneous observation of the Kelvin–Helmholtz waves is possible.

For this event, we investigate and compare the typical wave parameters and the plasma mixing on the two flanks. Our results suggest an asymmetric evolution of the Kelvin–Helmholtz waves at dawn and dusk. Comparing these results with previous studies of simultaneously observed events and linking them to solar wind conditions further shows that this asymmetric growth seems to occur during the Parker spiral IMF, but probably only if the magnetic fields are strong enough to effectively stabilise the boundary at the dusk flank due to field line draping.