Generation of Turbulence in Kelvin‐Helmholtz Vortices at the Earth's Magnetopause: Magnetospheric Multiscale Observations.

The Kelvin‐Helmholtz instability (KHI) at Earth's magnetopause and associated turbulence are suggested to play a role in the transport of mass and momentum from the solar wind into Earth's magnetosphere. We investigate electromagnetic turbulence observed in Kelvin‐Helmholtz vortices encountered at the dusk flank magnetopause by the Magnetospheric Multiscale (MMS) spacecraft under northward interplanetary magnetic field (IMF) conditions in order to reveal its generation process, mode properties, and role. A comparison with another MMS event at the dayside magnetopause with reconnection but no KHI signatures under a similar IMF condition indicates that while high‐latitude magnetopause reconnection excites a modest level of turbulence in the dayside low‐latitude boundary layer, the KHI further amplifies the turbulence, leading to magnetic energy spectra with a power law index −5/3 at magnetohydrodynamic scales even in its early nonlinear phase. The mode of the electromagnetic turbulence is analyzed with a single‐spacecraft method based on Ampère's law, developed by Bellan (2016, https://doi.org/10.1002/2016JA022827), for estimating wave vectors as a function of spacecraft frame frequency. The results suggest that the turbulence does not consist of propagating normal‐mode waves but is due to interlaced magnetic flux tubes advected by plasma flows in the vortices. The turbulence at sub‐ion scales in the early nonlinear phase of the KHI may not be the cause of the plasma transport across the magnetopause but rather a consequence of three‐dimensional vortex‐induced reconnection, the process that can cause an efficient transport by producing tangled reconnected field lines. Plain Language Summary: Turbulence is ubiquitous in nature and plays an important role in material mixing and energy transport. Turbulence in space plasmas is characterized by fluctuations of flow velocity and/or electromagnetic fields over a broad frequency range and/or length scales and is believed to be the key to efficient plasma transport and heating. However, its generation mechanism is not fully understood because turbulence in space is often fully developed or already relaxed when observed. By analyzing high‐resolution plasma and electromagnetic field data taken by the Magnetospheric Multiscale spacecraft, we study the generation process of electromagnetic turbulence at the outer boundary of Earth's magnetosphere, called the magnetopause, where either a flow shear‐driven Kelvin‐Helmholtz instability or magnetic reconnection or both could drive turbulence. It is shown that while dayside reconnection generates a modest level of turbulence at the magnetopause near noon, the flow shear instability further amplifies the turbulence at the flank magnetopause. Our analysis also suggests that the turbulence may not be the primary cause of plasma transport from solar wind into the magnetosphere but rather a consequence of the flow shear‐induced reconnection that is likely the primary cause of plasma transport at the dayside flank under northward solar wind magnetic field conditions. Key Points: The Kelvin‐Helmholtz instability (KHI) amplifies electromagnetic fluctuations in the magnetopause boundary layerThe turbulent fluctuations in the vortices may not be due to propagating waves but to magnetic structures, that is, interlaced flux tubesThe observed turbulent power law spectra at sub‐ion scales are consistent with those in kinetic simulations of KHI‐driven reconnection [ABSTRACT FROM AUTHOR]