It has been well demonstrated that the nonlinear Kelvin‐Helmholtz (KH) instability plays a critical role for the solar wind interaction with the Earth's magnetosphere. Although the two‐dimensional KH instability has been fully explored during the past decades, more and more studies show the fundamental difference between the two‐ and three‐dimensional KH instability. For northward interplanetary magnetic field (IMF) conditions, the nonlinear KH wave that is localized in the vicinity of the equatorial plane can dramatically bend the magnetic field line, generating strong antiparallel magnetic field components at high latitudes in both North and South Hemispheres, which satisfy the onset condition for magnetic reconnection. This high‐latitude double reconnection process can exchange the portion of magnetosheath and magnetospheric flux tubes between those two reconnection sites. This study used a high‐resolution 3‐D magnetohydrodynamic simulation to demonstrate that nonlinear KH waves can generate a large amount of double‐reconnected flux during the northward IMF condition, which can efficiently transport the plasma with a high diffusion coefficient of 1 × 1010 m2 s−1 for typical magnetopause conditions at the Earth. The presence of the magnetic field component along the shear flow direction not only decreases the KH growth rate but also causes north‐south asymmetry, which generates more open flux and reduces the efficiency of the plasma transport process.

The multifluid Lyon‐Fedder‐Mobarry (MFLFM) global magnetosphere model is used to study the interactions between solar wind and rapidly rotating, internally driven Jupiter magnetosphere. The MFLFM model is the first global simulation of Jupiter magnetosphere that captures the Kelvin‐Helmholtz instability (KHI) in the critically important subsolar region. Observations indicate that Kelvin‐Helmholtz vortices are found predominantly in the dusk sector. Our simulations explain that this distribution is driven by the growth of KHI modes in the prenoon and subsolar region (e.g., >10 local time) that are advected by magnetospheric flows to the dusk sector. The period of density fluctuations at the dusk terminator flank (18 magnetic local time, MLT) is roughly 1.4 h compared with 7.2 h at the dawn flank (6 MLT). Although the simulations are only performed using parameters of the Jupiter's magnetosphere, the results may also have implications for solar wind‐magnetosphere interactions at other corotation‐dominated systems such as Saturn. For instance, the simulated average azimuthal speed of magnetosheath flows exhibit significant dawn‐dusk asymmetry, consistent with recent observations at Saturn. The results are particularly relevant for the ongoing Juno mission and the analysis of dawnside magnetopause boundary crossings for other planetary missions.

Using gross averages of the azimuthal component of flow in Saturn's magnetosheath, we find that flows in the prenoon sector reach a maximum value of roughly half that of the postnoon side. Corotational magnetodisc plasma creates a much larger flow shear with solar wind plasma prenoon than postnoon. Maxwell stress tensor analysis shows that momentum can be transferred out of the magnetosphere along tangential field lines if a normal component to the boundary is present, i.e., field lines which pierce the magnetopause. A Kelvin‐Helmholtz unstable flow gives rise to precisely this situation, as intermittent reconnection allows the magnetic field to thread the boundary. We interpret the Kelvin‐Helmholtz instability acting along the magnetopause as a tangetial drag, facilitating two‐way transport of momentum through the boundary. We use reduced magnetosheath flows in the dawn sector as evidence of the importance of this interaction in Saturn's magnetosphere.

Based on magnetic field fluctuations, Saturn's magnetosphere can be divided into quiet (little or not much fluctuation) and disturbed periods (large fluctuation of the magnetic field). Kaminker et al. (2017, https://doi.org/10.1002/2016JA023834) showed that the average heating rate density of entire magnetosphere of Saturn is ∼10−17 W m−3 based on magnetic field fluctuations. Here, we categorize the magnetosphere of Saturn on the basis of magnetic field fluctuation. Using the eigenvalues of the variance analysis of the magnetic field, it has been found that the magnetodisc is in a disturbed state almost 6%–7% of the time. Dwell time normalization of the disturbed events suggests that most of the events happen at all latitudes between −5° and +30° and mostly on the dayside. Kinetic turbulent heating due to magnetic field fluctuation (δB) could potentially provide a significant amount of the required power.

For ∼2,000 Cassini magnetopause encounters, we analyse plasma and magnetic fields. The boundary can be unstable to the Kelvin–Helmholtz instability (KHI), which can drive large‐scale flows identifiable in plasma measurements. Bulk flow reversed from the expected direction near the magnetopause can indicate vorticity associated with active KH, and events are found dominantly in the dawn–subsolar region. KHIs are also responsible for magnetic field fluctuations, and hybrid simulations indicate heating, and transport is significant in an actively growing vortex. Cassini observations are filtered for disturbed magnetic fields near the magnetopause, similar to the signatures from hybrid simulations, with significant fluctuation and current sheet crossings. We also find that these occur most frequently in the dawn–subsolar region. A turbulent heating rate density and mass diffusion coefficient are calculated for these disturbed events and compared with the hybrid simulation to test whether enhanced values for these quantities can identify active KH events.

Saturn's magnetosheath flows exhibit significant dawn/dusk asymmetry. The dawnside flows are reduced from expectation, suggesting significant momentum transport through the magnetopause boundary where the flow shear is maximized. It has been suggested that the solar wind interaction with the giant magnetospheres is, in fact, dominated by a viscous‐like interaction governed by the Kelvin‐Helmholtz instability. In three dimensions, the Kelvin‐Helmholtz instability can generate small‐scale and intermittent magnetic reconnection due, in part, to a twisted magnetic field topology. The net result is a field line threading of the magnetopause boundary and the generation of Maxwell shear stresses. Here we present three‐dimensional hybrid simulations (kinetic ions and massless fluid electrons) of conditions similar to Saturn's dawnside magnetopause boundary to quantify the viscous‐like, tangential drag. Using model‐determined momentum fluxes, we estimate the effect on dawnside sheath flows and find very good agreement with observations.

Understanding the physical mechanisms responsible for the cross-scale energy transport and plasma heating from solar wind into the Earth's magnetosphere is of fundamental importance for magnetospheric physics and for understanding these processes in other places in the universe with comparable plasma parameter ranges. This paper presents observations from the Magnetosphere Multiscale (MMS) mission at the dawn-side high-latitude dayside boundary layer on February 25, 2016 between 18:55 and 20:05 UT. During this interval, MMS encountered both the inner and outer boundary layers with quasiperiodic low frequency fluctuations in all plasma and field parameters. The frequency analysis and growth rate calculations are consistent with the Kelvin-Helmholtz instability (KHI). The intervals within the low frequency wave structures contained several counter-streaming, low-(0-200 eV) and mid-energy (200 eV-2 keV) electrons in the loss cone and trapped energetic (70-600 keV) electrons in alternate intervals. The counter-streaming electron intervals were associated with large-magnitude field-aligned Poynting fluxes. Burst mode data at the large Alfvén velocity gradient revealed a strong correlation between counter streaming electrons, enhanced parallel electron temperatures, strong antifield aligned wave Poynting fluxes, and wave activity from sub-proton cyclotron frequencies extending to electron cyclotron frequency. Waves were identified as Kinetic Alfvén waves but their contribution to parallel electron heating was not sufficient to explain the >100 eV electrons, and rapid nonadiabatic heating of the boundary layer as determined by the characteristic heating frequency, derived here for the first time. Plain Language Summary Electrons, The Riders of the Space Hurricane: Earth's magnetic field forms a barrier in the solar wind, called the magnetosphere, which provides some shielding against solar radiation and galactic cosmic rays. However, this shield can be penetrated by process called magnetic reconnection, and secondary processes created by giant "fluid-scale" space hurricanes (typically 20,000-36,000 km in wave length) aka Kelvin-Helmholtz (KH) waves that are whipped along the magnetic barrier by solar wind flow. One of the puzzling problems of the Earth's magnetosphere is that it is so hot: both electrons and ions are heated to tens of millions of degrees when they get transported from solar wind through the Earth's magnetic barrier. This article shows observations of multiscale wave structures, spanning the fluid-scales, ion scales and electron scales detected by the NASA's magnetosphere multiscale mission consisting of four satellites. We show how these large-scale waves contain ion and electron scale waves that are able to produce some of the observed electron heating and acceleration. We "fingerprint" the exact plasma wave modes (tornadoes) inside the space hurricane that are responsible for resonantly whipping and transferring the wave energy to the electrons surfing the wave. NYKYRI ET AL.

A quantitative investigation of plasma transport rate via the Kelvin‐Helmholtz (KH) instability can improve our understanding of solar‐wind‐magnetosphere coupling processes. Simulation studies provide a broad range of transport rates by using different measurements based on different initial conditions and under different plasma descriptions, which makes cross literature comparison difficult. In this study, the KH instability under similar initial and boundary conditions (i.e., applicable to the Earth's magnetopause environment) is simulated by Hall magnetohydrodynamics with test particles and hybrid simulations. Both simulations give similar particle mixing rates. However, plasma is mainly transported through a few big magnetic islands caused by KH‐driven reconnection in the fluid simulation, while magnetic islands in the hybrid simulation are small and patchy. Anisotropic temperature can be generated in the nonlinear stage of the KH instability, in which specific entropy and magnetic moment are not conserved. This can have an important consequence on the development of secondary processes within the KH instability as temperature asymmetry can provide free energy for wave growth. Thus, the double‐adiabatic theory is not applicable and a more sophisticated equation of state is desired to resolve mesoscale process (e.g., KH instability) for a better understanding of the multi‐scale coupling process.