Evolution of Turbulence in the Kelvin–Helmholtz Instability mediated by the Magnetopause and its Boundary Layer

The turbulence at the interface between the solar wind and the Earth’s magnetosphere, mediated by the magnetopause and its boundary layer are investigated by using Geotail and THEMIS spacecraft data during ongoing Kelvin-Helmholtz instability (KHI). The efficient transfer of energy across scales for which the turbulence is responsible, achieves the connection between the macroscopic flow and the microscopic dissipation of this energy. This boundary layer is thought to be the result of the observed plasma transfer, driven by the development of the KHI, originating from the velocity shear between the solar wind and the almost static near-Earth plasma. A collection of 20 events spatially located on the tail-flank magnetopause, selected from previously studied by Hasegawa et al. 2006 and Lin et al. 2014, have been tested against standard diagnostics for intermittent turbulence. In light of the results obtained, we have investigated the behaviour of several parameters as a function of the progressive departure along the Geocentric Solar Magnetosphere coordinates, which roughly represent the direction in which we expect the KHI vortices to evolve towards fully developed turbulence. It appears that a fluctuating behaviour of the parameters exist, visible as a decreasing, quasi-periodic modulation with an associated periodicity, estimated to correspond to approximately 6.4 Earth Radii. Such observed wavelength is consistent with the estimated vortices roll-up wavelength reported in the literature for these events. If the turbulence is pre-existent, it is possible that the KHI modulates its properties along the magnetosheath, as we observed. On the other hand, if we assume that the KHI has been initiated near the magnetospheric nose and develops along the flanks, then the different intervals we study may be sampling the plasma at different stages of evolution of the KH-generated turbulence, after the instability has injected energy in a cascading process as large-scale structures.