Generation of turbulence in Kelvin-Helmholtz vortices at the Earth's magnetopause: Magnetospheric Multiscale observations

Abstract

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 SummaryTurbulence 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 wi...

Comparing turbulence in a Kelvin–Helmholtz instability region across the terrestrial magnetopause

“…This shows that, in terms of wavenumbers, the peaks in MS and BL spectra are in good agreement. At lower frequency, both regions have generally irregular but rather flat spectra, a feature that had been observed before in the MS (Breuillard et al 2018;Macek et al 2018;Yordanova et al 2020) and in the BL (Hasegawa et al 2019). This is generally attributed to the decorrelation of the fluctuations occurring at the bow-shock and at KHI, and to the limited time available for nonlinear interactions to form a broad, fully developed Kolmogorov spectrum (Huang et al 2017;Stawarz et al 2019).…”

“…The spectral index is generally closer to the Kolmogorov's −5/3 in the MS intervals, while it is around −3/2 in the BL, for scales between the driving instability and the ion inertial length. Previous observations of magnetic spectra in the BL showed the presence of power-law scaling, with similar exponents, only in the presence of KHI (Treumann 1999;Hasegawa et al 2019). This suggests that while the early-stage turbulence in the BL is mostly being locally driven by the KHI, in the MS there is preexisting turbulence, developed after the bow-shock crossing.…”

“…Previous studies have shown that the turbulent solar wind is heavily processed by the bow shock, so that in the magnetosheath a Kolmogorov turbulent spectrum only develops away from the shock (Breuillard et al 2018;Macek et al 2018;Yordanova et al 2020). On the other hand, in the magnetospheric boundary layer region the plasma is only weakly turbulent, being characterized by smaller fluctuations and reduced bulk motion (Treumann 1999;Hasegawa et al 2019).…”