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 KH 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), 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.

Magnetopause Kelvin-Helmholtz (KH) waves are believed to mediate solar wind plasma transport via small-scale mechanisms. Vortex-induced reconnection (VIR) was predicted in simulations and recently observed using NASA’s Magnetospheric Multiscale (MMS) mission data. Flux Transfer Events (FTEs) produced by VIR at multiple locations along the periphery of KH waves were also predicted in simulations but detailed observations were still lacking. Here we report MMS observations of an FTE-type structure in a KH wave trailing edge during KH activity on 5 May 2017 on the dawnside flank magnetopause. The structure is characterised by (1) bipolar magnetic BY variation with enhanced core field BZ and (2) enhanced total pressure with dominant magnetic pressure. The cross-section size of the FTE is found to be consistent with vortex-induced flux ropes predicted in the simulations. Unexpectedly, we observe an ion jet (VY), electron parallel heating, ion and electron density enhancements, and other signatures that can be interpreted as a reconnection exhaust at the FTE central current sheet. Moreover, pitch angle distributions of suprathermal electrons on either side of the current sheet show different properties, indicating different magnetic connectivities. This FTE-type structure may thus alternatively be interpreted as two interlaced flux tubes with reconnection at the interface as reported by Kacem et al. (2018) and Øieroset et al. (2019). The structure may be the result of interaction between two flux tubes, likely produced by multiple VIR at the KH wave trailing edge, and constitutes a new class of phenomenon induced by KH waves.

This White Paper outlines the importance of addressing the fundamental science theme “How are charged particles energized in space plasmas” through a future ESA mission. The White Paper presents five compelling science questions related to particle energization by shocks, reconnection, waves and turbulence, jets and their combinations. Answering these questions requires resolving scale coupling, nonlinearity, and nonstationarity, which cannot be done with existing multi-point observations. In situ measurements from a multi-point, multi-scale L-class Plasma Observatory consisting of at least seven spacecraft covering fluid, ion, and electron scales are needed. The Plasma Observatory will enable a paradigm shift in our comprehension of particle energization and space plasma physics in general, with a very important impact on solar and astrophysical plasmas. It will be the next logical step following Cluster, THEMIS, and MMS for the very large and active European space plasmas community. Being one of the cornerstone missions of the future ESA Voyage 2050 science programme, it would further strengthen the European scientific and technical leadership in this important field.