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Magnetic reconnection is a key fundamental process in collisionless plasmas that explosively converts magnetic energy to plasma kinetic and thermal energies through a change of magnetic field topology in a central electron‐scale region called the electron diffusion region (EDR). Past simulations and observations demonstrated that this process causes efficient energy conversion through the formation of multiple macro‐scale or micro‐scale magnetic islands/flux ropes. However, the coupling of these phenomena on different spatiotemporal scales is still poorly understood. Here, based on a new large‐scale fully kinetic simulation with a realistic, initially fluctuating magnetic field, we demonstrate that macro‐scale evolution of turbulent reconnection involving merging of macro‐scale islands induces repeated, quick formation of new electron‐scale islands within the EDR which soon grow to larger scales. This process causes an efficient cross‐scale energy transfer from electron‐ to larger‐scales, and leads to strong electron energization within the growing islands. Plain Language Summary: Space above the Earth's atmosphere is broadly filled with ionized gas, called plasma. Since the density of the space plasma is mostly small enough to neglect the viscosity, the behavior of it is essentially different from neutral viscous fluids. In such a collisionless plasma system, the boundary layer between regions with different electromagnetic field and plasma properties plays a central role in transferring energy. One of the representative energy transfer processes in collisionless plasmas is magnetic reconnection that explosively converts magnetic energy to plasma kinetic energy through the topology change of magnetic field lines across the boundary layer with a large magnetic shear. On the other hand, understanding how the energy transfer between different spatiotemporal scales in turbulence, which has been commonly observed in space, is also a key for understanding the energy transfer physics in collisionless plasmas. In this study, based on a new plasma kinetic simulation of magnetic reconnection newly considering realistic, turbulent magnetic field fluctuations, it is found that during macro‐scale evolution of the background fluctuations, the topology change of the reconnecting field lines occurs at multiple points within the micro‐scale central region of reconnection. This process causes an efficient cross‐scale energy transfer from micro‐ to larger‐scales. Key Points: 2‐1/2 dimensional fully kinetic simulation of turbulent reconnection with a realistic, initially fluctuating magnetic field is performedTurbulent reconnection involving merging of macro‐scale islands induces repeated micro‐scale island formation in electron diffusion regionDuring the macro‐scale island merging, the micro‐scale islands grow to larger scales, leading to an efficient cross‐scale energy transfer [ABSTRACT FROM AUTHOR]

At the Earth's low‐latitude magnetopause, clear signatures of the Kelvin‐Helmholtz (KH) waves have been frequently observed during periods of the northward interplanetary magnetic field (IMF), whereas these signatures have been much less frequently observed during the southward IMF. Here, we performed the first 3‐D fully kinetic simulation of the magnetopause KH instability under the southward IMF condition. The simulation demonstrates that fast magnetic reconnection is induced at multiple locations along the vortex edge in an early nonlinear growth phase of the instability. The reconnection outflow jets significantly disrupt the flow of the nonlinear KH vortex, while the disrupted turbulent flow strongly bends and twists the reconnected field lines. The resulting coupling of the complex field and flow patterns within the magnetopause boundary layer leads to a quick decay of the vortex structure, which may explain the difference in the observation probability of KH waves between northward and southward IMF conditions. Plain Language Summary: Space between planets is filled with ionized gas released from the upper atmosphere of the Sun, called the solar wind. Although the Earth's magnetic field basically acts as a barrier to prevent energetic solar wind from penetrating into the region filled with the Earth's magnetic field, called the magnetosphere, it is known that the solar wind frequently leaks into the magnetosphere. The Kelvin‐Helmholtz (KH) instability, which is a flow‐driven instability and can be unstable by the antisunward flowing solar wind, has been considered as an important candidate process for the solar wind leaks. However, past spacecraft observations have revealed that the observation probability of KH waves is very low when the magnetic field in the solar wind, called the interplanetary magnetic field (IMF), is oriented southward, that is, opposite from the Earth's magnetic field. In this study, based on a plasma kinetic simulation, it is found that when the IMF is southward, magnetic reconnection, which is an explosive plasma process that rearranges the magnetic topology across the boundary, occurs at multiple locations in the KH waves and rapidly destroys the wave structures. This may explain the low observation probability of the KH waves under the southward IMF. Key Points: Three‐dimensional fully kinetic simulation of Kelvin‐Helmholtz instability at the Earth's magnetopause under the southward IMF condition is performedFast reconnection causes a rapid decay of the nonlinear vortex structure in the early nonlinear growth phase of the instabilityThe vortex decay can lead to a lower probability of observing magnetopause Kelvin‐Helmholtz waves/vortices during southward IMF periods [ABSTRACT FROM AUTHOR]

At the Earth's magnetopause, the Kelvin‐Helmholtz (KH) instability, driven by the persistent velocity shear between the magnetosheath and the magnetosphere, has been frequently observed during northward interplanetary magnetic field periods and considered as one of the most important candidates for transporting and mixing plasmas across the magnetopause. However, how this process interacts with magnetic field fluctuations, which persistently exist near the magnetopause, has been less discussed. Here we perform a series of 2‐D fully kinetic simulations of the KH instability at the magnetopause considering a power law spectrum of initial fluctuations in the magnetic field. The simulations demonstrate that when the amplitude level of the initial fluctuations is sufficiently large, the KH instability evolves faster, leading to a more efficient plasma mixing within the vortex layer. In addition, when the spectral index of the initial fluctuations is sufficiently small, the modes whose wavelength is longer than the theoretical fastest growing mode grow dominantly. The fluctuating magnetic field also results in the formation of the well‐matured turbulent spectrum with a −5/3 index within the vortex layer even in the early nonlinear growth phase of the KH instability. The obtained spectral features in the simulations are in reasonable agreement with the features in KH waves events at the magnetopause observed by the Magnetospheric Multiscale mission and conjunctively by the Geotail and Cluster spacecraft. These results indicate that the magnetic field fluctuations may really contribute to enhancing the wave activities especially for longer wavelength modes and the associated mixing at the magnetopause. Key Points: The 2‐D fully kinetic simulations of magnetopause Kelvin‐Helmholtz instability initially imposing power law field fluctuations are performedThe growth of the instability especially for long wavelength modes is enhanced by the fluctuating field, leading to more efficient mixingSpectral features obtained from the simulations are in reasonable agreement with past spacecraft observations at the Earth's magnetopause [ABSTRACT FROM AUTHOR]

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]

An unsolved problem in plasma turbulence is how energy is dissipated at small scales. Particle collisions are too infrequent in hot plasmas to provide the necessary dissipation. Simulations either treat the fluid scales and impose an ad hoc form of dissipation (e.g., resistivity) or consider dissipation arising from resonant damping of small amplitude disturbances where damping rates are found to be comparable to that predicted from linear theory. Here, we report kinetic simulations that span the macroscopic fluid scales down to the motion of electrons. We find that turbulent cascade leads to generation of coherent structures in the form of current sheets that steepen to electron scales, triggering strong localized heating of the plasma. The dominant heating mechanism is due to parallel electric fields associated with the current sheets, leading to anisotropic electron and ion distributions which can be measured with NASA's upcoming Magnetospheric Multiscale mission. The motion of coherent structures also generates waves that are emitted into the ambient plasma in form of highly oblique compressional and shear Alfven modes. In 3D, modes propagating at other angles can also be generated. This indicates that intermittent plasma turbulence will in general consist of both coherent structures and waves. However, the current sheet heating is found to be locally several orders of magnitude more efficient than wave damping and is sufficient to explain the observed heating rates in the solar wind. [ABSTRACT FROM AUTHOR]