Search

Starting in the evening of 10 May 2024 the Earth's magnetosphere was hit by the coronal mass ejections (CMEs) creating the largest geomagnetic storm in ∼ ${\sim} $20 years. The CME encounter was characterized by variations of plasma number density and magnetic field. Here, I present the ARTEMIS observations at the lunar orbit during this event. The IMF bz ${b}_{z}$ ranged from −60 to +40 nT both with ∼ ${\sim} $hour to minutes periodicity with plasma jets propagating in ±zGSE $\pm {z}_{\mathit{GSE}}$‐direction within multi‐scale wave structures. Similar signature has been recently reported at the magnetopause by MMS spacecraft (Li et al., 2023, https://doi.org/10.1029/2023GL105539; Nykyri, 2024, https://doi.org/10.1029/2024GL108605) during a strongly southward IMF. Here, I show that the CME boundaries were KH unstable leading to multi‐scale density and magnetic field fluctuations including reconnection jets. The wavelengths varied from ∼ ${\sim} $60 to 270 RE ${R}_{E}$, suggesting that the magnetosphere was periodically exposed to successive intervals of strongly northward and southward IMF leading to enhanced mass and magnetic flux loading. Plain Language Summary: Coronal mass ejections (CMEs) are giant explosions of plasma and magnetic field from the Sun which can produce beautiful Auroras, but also destroy our satellites, power delivery networks, and avionics systems. The seriousness of the storm depends on the pre‐existing conditions of the magnetosphere, the strength and orientation of the magnetic field within the CME structure, its size and speed which drives its duration, as well as the properties of the plasma within it. In this paper, I discuss and analyze the CME observations during the recent Mother's day storm (started on 10 May 2024) using the ARTEMIS spacecraft data at the lunar orbit. During the CME encounter, the SC were exposed to periodic north‐south‐variations of the magnetic field orientation and plasma density which allowed the Earth's magnetosphere to be successively filled with plasma, and magnetic flux likely enabling its effective acceleration in large‐quantities. I show that these periodic variations were produced by giant ∼ ${\sim} $ 60–270 RE ${R}_{E}$ waves at the boundary of the CMEs, such that the entire Earth's magnetosphere was surfing the crests and troughs of these waves, and absorbing the kinetic and magnetic energy from the solar wind. Key Points: Velocity shear at the boundary of a coronal mass ejection (CME) ejecta created Kelvin‐Helmholtz Instability (KHI) ∼ ${\sim} $7 million km upstream the Earth‐Sun L‐1 pointKHI generated multi‐scale density and IMF bz ${b}_{z}$ fluctuations from −60 to +40 nT at the boundary layers of the CME ejecta encountersKH wavelengths varied from ∼ ${\sim} $60 to ∼ ${\sim} $270 RE ${R}_{E}$ implying periodic and successive mass and magnetic flux loading of the magnetosphere system [ABSTRACT FROM AUTHOR]

The secondary processes driven by the velocity shear driven Kelvin‐Helmholtz Instability (KHI) in the magnetized plasmas have been shown to be important in producing plasma transport and heating from the shocked solar wind into the Earth's magnetosphere (MSP). The plasma transport into the MSP due to KHI has been shown to be strongest during northward interplanetary magnetic field (IMF) via KHI driven low‐ and mid‐latitude reconnection process. In a recent article, Li et al. (2023), https://doi.org/10.1029/2023gl105539 show Magnetosphere Multi‐Scale (MMS) spacecraft observations of multiple, Alfvénic reconnection jets during southward IMF at the dawn‐side MSP flank. The quasi‐periodic oscillations in plasma parameters and compressed, ion‐scale current sheets were strongly indicative of the MMS crossing regions of MSP‐like and magnetosheath‐like plasma within Kelvin‐Helmholtz waves. In this brief commentary, the importance of this new discovery for magnetospheric and solar wind dynamics is discussed. Plain Language Summary: Earth's magnetic field serves as a protective barrier, forming a magnetic bubble known as the magnetosphere in the solar wind. Acting like a shield, the magnetopause is a crucial surface that delineates the separation between the solar wind and the plasma enveloped within Earth's magnetosphere. However, this shield is not impervious; a phenomenon called magnetic reconnection can compromise it, permitting direct access for solar wind plasma. This process is particularly effective when the interplanetary magnetic field (IMF) carried by the solar wind opposes Earth's magnetic field direction, a configuration known as "southward" IMF. Under these conditions, the Earth's magnetic field opens at the dayside, facilitating the accumulation of mass, momentum, and energy into the night‐side magnetotail. In a recent discovery, NASA's four Magnetosphere Multiscale spacecraft revealed that magnetic reconnection can also occur during southward IMF at the tail flanks of our magnetic bubble. This reconnection, strongly driven by colossal waves with wavelengths of 56,000 km (Kelvin‐Helmholtz waves) creates portals in our magnetic shield, enabling both the escape and entry of particles. In this brief commentary, the importance of this finding is discussed, both for magnetospheric and solar wind dynamics. Key Points: Kelvin‐Helmholtz Instability (KHI) can generate compressed current sheets, effectively driving magnetic reconnection at flanks during southward interplanetary magnetic fieldKHI driven reconnection can aid energetic particle escape from the magnetosphereKHI driven reconnection may also play role for energetic particle dynamics within solar wind structures [ABSTRACT FROM AUTHOR]

The tropopause region is crucial for the stratosphere‐troposphere exchange (STE) and acts as an indicator of climate change. Double tropopauses (DTs) act to increase the STE process but their driving mechanisms remain an open question. The present assessment offers for the first time the linkage between tropospheric gravity waves (GWs) and DT events by exploring a global data set of multi‐year radiosonde measurements. In the extratropics, the occurrence frequency of DT events keeps a remarkably consistent spatial‐temporal structure with GW total energy. Under the DT scenario, GW total energy has increased by 37.67% compared to single tropopause events. Based on a statistical assessment, the upward propagating GWs throughout the second tropopause region can probably raise Kelvin‐Helmholtz instability or turbulence, leading permanent irregularities in thermodynamic structure, and consequently, increasing the likelihood of DT. Plain Language Summary: Atmospheric disturbances above the troposphere are predominantly associated with waves generated in the troposphere. Among others, gravity waves (GWs) play major roles in transport of energy and momentum from the lower to the upper atmosphere. The double tropopause (DT) caused by irregular temperature changes is a common phenomenon in the lowermost stratosphere, which has an indicative significance for climate change. The upward propagating GWs sourced from the troposphere can probably alter the temperature variability in the tropopause region, and therefore, they are likely associated with the occurrence of DTs. By analyzing a large amount of global radiosonde data, simultaneous measurements of tropospheric GWs and irregularities in the tropopause enable us to argue that GWs sourced from troposphere could increase the likelihood of DTs. Key Points: A good level of agreement in spatial‐temporal variability has been identified between Double tropopause (DT) events and tropospheric gravity wave (GW) total energySubstantial tropospheric GW and Kelvin‐Helmholtz instability frequency enhancements in the tropopause region can be observed with DT phenomenaThe tropospheric GWs may increase the likelihood of DT events by introducing permanent localized irregularities or instabilities [ABSTRACT FROM AUTHOR]

We present Magnetospheric Multiscale observations of an electron‐scale reconnecting current sheet in the mixing region along the trailing edge of a Kelvin‐Helmholtz vortex during southward interplanetary magnetic field conditions. Within this region, we observe intense electrostatic wave activity, consistent with lower‐hybrid waves. These waves lead to the transport of high‐density magnetosheath plasma across the boundary layer into the magnetosphere and generate a mixing region with highly compressed magnetic field lines, leading to the formation of a thin current sheet associated with electron‐scale reconnection signatures. Consistencies between these reconnection signatures and a realistic, local, fully‐kinetic simulation modeling this current sheet indicate a temporal evolution of the observed electron‐scale reconnection current sheet. The multi‐scale and inter‐process character of this event can help us understand plasma mixing connected to the Kelvin‐Helmholtz instability and the temporal evolution of electron‐scale reconnection. Plain Language Summary: Like wind blowing over water, the stream of ionized gas released from the Sun, called the solar wind, can lead to waves and rolled‐up vortex structures at the boundary of Earth's magnetosphere, called the magnetopause. These so‐called Kelvin‐Helmholtz waves have been shown to be connected to various different plasma processes on different scales. This multi‐scale and multi‐process character makes them an ideal candidate to study the relation between these processes from both spacecraft observations and simulations. By using spacecraft data from the Magnetospheric Multiscale mission, which was designed for the study of small‐scale plasma processes in Earth's magnetosphere, we show observations of electron‐scale magnetic reconnection, an explosive energy conversion process in plasmas, in a region along the trailing edge of these waves. These observations shed new light on the multi‐scale and multi‐process character of the Kelvin‐Helmholtz instability and the energy conversion processes along its boundary. Key Points: A reconnecting electron‐scale current sheet is observed by Magnetospheric Multiscale (MMS) in mixing plasma along the trailing edge of a Kelvin‐Helmholtz vortexRealistic 2.5D fully‐kinetic simulation shows reasonable agreement with MMS dataConsistencies between the simulation and MMS indicate a temporal evolution of the reconnecting current sheet [ABSTRACT FROM AUTHOR]

The generation mechanism of stratospheric gravity waves (SGWs) in tropical cyclones (TCs) is investigated with an idealized experiment using numerical model. The results show that there are two peaks of SGW amplitude during the mature period of TC, one above the eyewall and the other above the rainband. The SGWs can only exist in unbalanced flow, so a simplified balance equation suitable for the special structure of the TC is proposed to investigate the SGWs. Diagnosis of this equation shows there is significant unbalanced flow around the eyewall and rainband caused by the convection and the vertical shear between outflow and compensating inflow. Diagnosis of the source function indicates that diabatic heating and mechanical oscillations are the main mechanisms generating the SGWs above the eyewall and rainband. The Kelvin–Helmholtz instability induced by vertical shear also makes a non‐negligible contribution to the generation of SGWs above the rainband. Plain Language Summary: As intense convective systems, tropical cyclones (TCs) can generate intense stratospheric gravity waves (SGWs). However, TCs contain many unique structures such as the eyewall, spiral rainband, outflow layer, and inflow layer. How are the SGWs distributed in TCs and what effect does TC structure have on the generation of SGWs? Here we find there are two peaks of SGW amplitude during the mature period of TCs, one above the eyewall and one above the rainband. They appear mainly in the unbalanced flow regions, and the unbalanced flow in TCs is generated by convection around the eyewall and rainband and the vertical shear of the radial wind caused by outflow above the rainband. Thus, the convection and vertical shear play key roles in the generation of SGWs in TCs. Diabatic heating is the main mechanism generating SGWs above the eyewall and rainband by convection. Meanwhile, mechanical oscillation also generates convective SGWs above the eyewall. The Kelvin–Helmholtz instability induced by the vertical shear also makes a non‐negligible contribution to the generation of SGWs above the rainband. Key Points: There is significant unbalanced flow that can generate stratospheric gravity waves around the eyewall and rainband of tropical cyclonesThe unbalanced flow is generated by convection and vertical shear of the radial wind around the eyewall and outflow above the rainbandDiabatic heating, mechanical oscillation, and Kelvin–Helmholtz instability contribute to the generation of stratospheric gravity waves [ABSTRACT FROM AUTHOR]

Gravity waves (GWs) and their associated multi‐scale dynamics are known to play fundamental roles in energy and momentum transport and deposition processes throughout the atmosphere. We describe an initial machine learning model—the Compressible Atmosphere Model Network (CAM‐Net). CAM‐Net is trained on high‐resolution simulations by the state‐of‐the‐art model Complex Geometry Compressible Atmosphere Model (CGCAM). Two initial applications to a Kelvin‐Helmholtz instability source and mountain wave generation, propagation, breaking, and Secondary GW (SGW) generation in two wind environments are described here. Results show that CAM‐Net can capture the key 2‐D dynamics modeled by CGCAM with high precision. Spectral characteristics of primary and SGWs estimated by CAM‐Net agree well with those from CGCAM. Our results show that CAM‐Net can achieve a several order‐of‐magnitude acceleration relative to CGCAM without sacrificing accuracy and suggests a potential for machine learning to enable efficient and accurate descriptions of primary and secondary GWs in global atmospheric models. Plain Language Summary: Atmospheric gravity waves (GWs) are well described by the Navier‐Stokes equations, but solving these equations including small scale remains daunting, limited by the very high computational cost of resolving the smallest spatial‐temporal features in a global context. To address this challenge, we developed a machine learning model called CAM‐Net. Our model demonstrates that neural networks can be trained on high‐resolution compressible atmospheric model data and then used to simulate GW evolution. Importantly, initial results show that using such trained model can achieve computational savings of >1,000 times compared to a physics‐based simulation while still achieve highly accurate results. These findings are exciting, as they suggest that CAM‐Net can overcome the limitations of current GW parameterizations and provide a promising avenue for studying the effects of sub‐grid‐scale processes in atmospheric science and properly incorporating them in global models. The development of CAM‐Net opens up major new opportunities for improving effective model resolution, accuracy, and efficiency. Key Points: A machine learning model (CAM‐Net) tailored for nonlinear Gravity wave (GW) simulations is developedCAM‐Net can achieve a several order‐of‐magnitude acceleration relative to physics‐based model without sacrificing accuracyCAM‐Net opens a new window to improve the parameterization of primary and secondary GWs in the global atmospheric models [ABSTRACT FROM AUTHOR]

We use the Kelvin‐Helmholtz instability (KHI) condition with particle and magnetic field observations from Jovian Auroral Distributions Experiment and MAG on Juno along the dawn flank of Jupiter's magnetosphere. We identify the occurrence of magnetopause crossings that show evidence of being KH (Kelvin‐Helmholtz) unstable. When estimating the k vector to be parallel to the velocity shear, we find that 25 of 62 (40%) magnetopause crossings satisfy the KHI condition. When considering the k vector of the maximum growth rate through a solid angle approach, we find that 60 of 62 (97%) events are KH unstable. This study shows evidence of KH waves at Jupiter's dawn flank, including primary drivers such as high velocity shears and changes in plasma pressure. Signatures of magnetic reconnection were also observed in ∼25% of the KH unstable crossings. We discuss these results and their implication for the prevalence of KHI at Juno's dawn magnetopause as measured by Juno. Plain Language Summary: The Kelvin‐Helmholtz instability occurs when a boundary separating two fluids of different densities is perturbed and these fluids are moving at different speeds, directions, or both. The difference in speeds across the perturbed boundary that separates the fluids creates wave structures as these fluids diffuse into each other. The Kelvin‐Helmholtz instability may be observed at the boundary that separates a planetary magnetic field (magnetosphere) from the stream of charged particles emitted by the Sun (solar wind); this boundary is known as the magnetopause. This instability is confirmed to occur at Earth and Saturn, but is not confirmed at Jupiter. This study analyzes the properties of the plasma and magnetic field in Jupiter's magnetosphere and the surrounding solar wind to identify signatures of the Kelvin‐Helmholtz instability. We find that out of 62 occurrences where the Juno spacecraft crosses the magnetopause, 25 events signify that the Kelvin‐Helmholtz instability is possible—primarily due to large differences in velocities—and 37 events do not. Key Points: There is evidence of Kelvin‐Helmholtz instability (KHI)‐driven waves along Jupiter's dawn flank magnetopause during the Juno prime mission24 (38.7%) crossings satisfied the KHI condition and 38 (61.3%) crossings did not satisfy the KHI conditionMagnetopause crossings that satisfied the KHI condition had, in general, larger velocity shears than those that did not [ABSTRACT FROM AUTHOR]

A clear air turbulence (CAT) occurred on 30 December 2020 over Tokyo, Japan. The CAT was largely generated by breaking Kelvin‐Helmholtz (KH) instability waves in the free atmosphere. A regional numerical weather prediction model simulated the event with fine resolution (35 m). Onboard‐recorded flight data and a flight simulation were utilized to validate the meteorological simulation. The locations of the reproduced strong turbulence agree well with the regions where flights encountered turbulence on that day. In a simulation with the finest resolution, the KH waves and their breaking were resolved. When the resolution was finer, the turbulent eddies were stronger, causing meteorological effects on the airplanes. The response of a virtual airplane to the simulated turbulence was estimated using a flight simulation. By comparing onboard‐recorded data with virtual flight data, we confirm that turbulent eddies are reasonably reproduced. Plain Language Summary: Clear air turbulence (CAT) is a threat to aviation safety. CAT occurs outside clouds, making it difficult to detect by pilots and onboard radars. When an airplane encounters strong CAT, changes in aerodynamic forces and moments cause significant shaking of the airplane. Although CAT generation in the free atmosphere has been studied by high‐resolution numerical simulations, few studies simulated aircraft‐scale turbulence eddies and validated them with high‐frequency airborne observation. We investigate a past CAT event in Japan using a very high resolution (35 m) simulation based on a regional weather prediction model of daily weather forecasts. The simulated turbulence was verified by comparing the virtual flight in simulated wind fields with realistic flight data recorded by airplanes during the event. Simulated aircraft responses in finer weather simulations match flight records better. Key Points: Clear air turbulence in a real case is reproduced by the use of a numerical weather prediction modelIf the resolution is fine enough for a large eddy simulation (LES), turbulent eddies associated with Kelvin‐Helmholtz instability are reproducedTurbulence eddies are shown to be realistic because virtual flight data in the LES data agree well with real flights [ABSTRACT FROM AUTHOR]

Fritts, Wang, Lund, and Thorpe (2022, https://doi.org/10.1017/jfm.2021.1085) and Fritts, Wang, Thorpe, and Lund (2022, https://doi.org/10.1017/jfm.2021.1086) described a 3‐dimensional direct numerical simulation of interacting Kelvin‐Helmholtz instability (KHI) billows and resulting tube and knot (T&K) dynamics that arise at a stratified shear layer defined by an idealized, large‐amplitude inertia‐gravity wave. Using similar initial conditions, we performed a high‐resolution compressible simulation to explore the emission of GWs by these dynamics. The simulation confirms that such shear can induce strong KHI with large horizontal scales and billow depths that readily emit GWs having high frequencies, small horizontal wavelengths, and large vertical group velocities. The density‐weighted amplitudes of GWs reveal "fishbone" structures in vertical cross sections above and below the KHI source. Our results reveal that KHI, and their associated T&K dynamics, may be an important additional source of high‐frequency, small‐scale GWs at higher altitudes. Plain Language Summary: A high‐resolution compressible atmosphere model is applied to explore gravity wave emissions from a shear with Kelvin‐Helmholtz Instability initiated by a three‐dimensional, small‐amplitude initial noise field in velocity, such as must always occur in the atmosphere. Simulations reveal that a wind shear with an amplitude of 65 m/s and a half‐width of 0.8 km can induce strong Kelvin‐Helmholtz Instability dynamics, which can further emit gravity waves having periods of ∼10–20 min and horizontal wavelengths of ∼20 km. These gravity waves have high frequencies and small horizontal scales. The density‐weighted amplitudes of gravity waves created a "fishbone" structure in z‐t plots due to upward‐ and downward‐propagating gravity waves arising at the layer of Kelvin‐Helmholtz Instability. Our results demonstrate that Kelvin‐Helmholtz Instability and the resulting instability dynamics may be a prevalent source of gravity waves impacting higher altitudes. Key Points: Kelvin‐Helmholtz instabilities (KHI) generated by a stratified shear layer induce gravity waves (GWs) that penetrate to high altitudesKHI‐radiated GWs may be a major influence of near‐stationary shears at high altitudes to which they cannot readily propagate directlyGWs generated by KHI can account for "fishbone" structures seen in vertical profiling [ABSTRACT FROM AUTHOR]

A feature of Earth's storm‐time magnetosphere outside the plasmapause is the occurrence of broad‐spectrum Alfvénic fluctuations. In this letter observations from the Van Allen Probes are compared with 3‐D fluid‐kinetic simulations of an evolving convective flow channel to investigate the mechanisms generating the observed spectrum. It is shown how narrow channels of fast convection are unstable to the Kelvin‐Helmholtz instability which on closed field‐lines initiates a cascade to small scales. Sustained driving of the flow combined with reflection from the topside ionosphere leads to the generation of an intensified spectrum of electromagnetic structures having similar spectral and morphological characteristics to those observed. This process couples enhanced magnetospheric convection to kinetic scale electromagnetic fluctuations that drive particle transport, scattering and energization through the outer radiation belt and ring current during geomagnetic storms. Plain Language Summary: A persistent challenge in advancing understanding of near‐Earth space is the development of models that accurately describe energy transport and conversion. It is known that energy is transported in flows of charged particles through near‐Earth space and converted into energized particle populations; but for this conversion to occur intermediary processes are required to transform the flow's energy into a form capable of driving particle acceleration. This research explores one such intermediary, showing how gradients in energy containing flows are unstable to structuring via a process similar to the action of "wind over water" known as the Kelvin‐Helmholtz instability. It is demonstrated how this instability in the inhomogeneous environment of near‐Earth space drives the generation of structures trapped within Earth's geomagnetic field that deliver energy to fluctuating electromagnetic fields on spatial scales of the order of particle orbits. Electromagnetic fields on these scales accelerate and scatter particles. This process provides a means for sustained particle acceleration and scattering in electromagnetic field variations powered by energy flows prevalent during intervals of disturbed space weather known as geomagnetic storms. Key Points: Fast flow channels in the storm‐time inner‐magnetosphere may be unstable to the Kelvin‐Helmholtz instabilityThe Kelvin‐Helmholtz instability and wave reflection from the ionosphere generate a broad spectrum of turbulent Alfvénic field variationsThe properties of the turbulent fields generated from sheared flows and reflection replicate those observed from spacecraft [ABSTRACT FROM AUTHOR]

This paper investigates intense energy conversion events (IECEs) at the dayside magnetopause boundary layer (MBL) using high‐resolution data from the Magnetospheric Multiscale (MMS) spacecraft. Most events were detected by MMS in a duration less than 0.5 s, corresponding to a thickness below one ion inertia length. About three‐quarters of the events occurred in local reconnecting current sheets. Interestingly, there are more electron‐only reconnections than standard reconnections. Most of the reconnection IECEs are within the exhaust of the primary reconnection and the Kelvin‐Helmholtz vortices, suggesting that secondary reconnections are important building blocks for the overall energy conversion at the magnetopause. The nonreconnection events are also primarily within the reconnection exhaust and the Kelvin‐Helmholtz vortices, highlighting the importance of reconnection and Kelvin‐Helmholtz instability in regulating the energy conversion at the MBL. Plain Language Summary: Solar wind‐magnetosphere coupling is one of the long‐standing questions in magnetosphere physics. Part of the input energy from the solar wind is converted and dissipated at the magnetopause boundary layer. Identifying where does energy conversion occurs is valuable for understanding the solar wind‐magnetosphere coupling. Here we picked up hundreds of intense energy conversion events using high precision data from Magnetospheric Multiscale at the magnetopause boundary layer to study their locations and fundamental properties. It is found that most events are associated with magnetic reconnection and are more likely located in the reconnection exhaust and Kelvin‐Helmholtz vortices. An interesting finding is that most of these intense energy conversion events are associated with reconnection in which only electrons participate, that is, without ion coupling. Key Points: Hundreds of intense energy conversion events are examined at the dayside magnetopauseThere are more electron‐only reconnections than standard reconnections in these eventsThe exhaust of the primary reconnection and Kelvin‐Helmholtz vortices are two essential regions hosting the intense energy conversion [ABSTRACT FROM AUTHOR]

Using the Mars Atmosphere and Volatile EvolutioN measurements, we observed a wave train of vortices in different developing phases of the Kelvin‐Helmholtz (K‐H) instability at the ionopause above the Martian northern hemisphere during 09:35:00–09:55:00 Universal Time on 24 March 2017. Based on the sawtooth‐like oscillations in the magnetic fields associated with the quasi‐periodic fluctuations in the ion densities, velocities, and total pressure, we identified three fully and two partially developed vortices. The wavelength of the K‐H waves is ∼2 Martian radii and the period is ∼2 min. The magnetic topologies associated with the vortices are observed to change from draped in the magnetosheath to open in the transition region and finally to closed approaching the ionosphere. Significant ion loss caused by the K‐H instability is observed. The total oxygen loss rate is estimated to be ∼1025 s−1, which is comparable to the global ion loss as previously estimated. Plain Language Summary: Shear‐driven Kelvin‐Helmholtz (K‐H) instability can form along the boundary of the solar wind interaction with non‐magnetic planets (such as Venus and Mars). Previous studies reported partially and fully developed K‐H vortices at the Martian induced magnetosheath. In this study, we directly observed both types of the K‐H vortices during the developing process of the K‐H instability at the Martian ionopause. The magnetic topologies associated with the K‐H vortices change from draped in the magnetosheath to open in the transition region and to closed approaching the ionosphere. The K‐H instability can be an effective mechanism that leads to ion escape at Mars. A large number of ions within the K‐H vortices are observed to escape from Mars. The loss rate of oxygen caused by the K‐H instability is estimated to be significantly high. Our results confirm that the K‐H instability is important for the Martian ion loss. Key Points: We report a wave train of Kelvin‐Helmholtz (K‐H) vortices during different developing phases of the K‐H instability at the ionopause of MarsThe wavelength of the K‐H waves is ∼2 Martian radii and the period is ∼2 min; the magnetic topologies within the K‐H vortices changedThe estimated loss rate of oxygen caused by the K‐H instability is on the order of 1025 s−1 [ABSTRACT FROM AUTHOR]

How oxygens escape from Venus has long been a fundamental but controversial topic in the planetary research. Among various key mechanisms, the Kelvin‐Helmholtz instability (KHI) has been suggested to play an important role in the oxygen ion escape from Venus. Limited by either scarce in‐situ observations or simplified theoretical estimations, the mystery of oxygen ion escape process associated with KHI is still unsettled. Here we present the first three‐dimensional configuration of KHI at Venus with a global multifluid magnetohydrodynamics model, showing a significantly fine structure and evolution of the KHI. KHI mainly occurred at the low latitude boundary layer if defining the interplanetary magnetic field‐perpendicular plane as the equatorial plane, resulting in escaping oxygen ions through mixing with the solar wind at the Venusian boundary layer, with an escape rate around 4 × 1024 s−1. The results provide new insights into the basic physical process of atmospheric loss at other unmagnetized planet. Plain Language Summary: Kelvin‐Helmholtz instability (KHI) forms where there's a velocity difference across the interface between two fluids: For example, wind blowing over water. The KHI also commonly occurs when solar wind flows past a magnetized body. At Venus, the solar wind directly interacts with the Venusian ionosphere and KHI takes place at the induced magnetosphere. The KHI is often considered as an important source of ion escape and atmospheric loss of Venus, however, limited by either scarce satellite observations or simplified theoretical estimations, the mystery of the oxygen ion escape process associated with the KHI is still unsolved. In this letter, we present the first three‐dimensional (3D) KHI structures at the Venusian boundary layer, with a newly‐developed, global, high‐resolution, magnetohydrodynamic model. The KHI is well resolved and exhibits fine evolutions in the 3D global model. We have also found that the KHI could lead to significant escape of oxygen ions through mixing with the solar wind at the low latitude boundary layer of Venus. The KHI‐induced oxygen ion escape rate of 4 × 1024 s−1 is comparable to the previous observation estimations. The results illustrate new insights into the atmospheric loss at the unmagnetized planets. Key Points: We present the first three‐dimensional (3D) Kelvin‐Helmholtz instability (KHI) structures at the Venusian magnetospheric boundary layer with a high‐resolution multifluid modelThe KHI is well resolved and exhibits dynamic and fine evolutions in the 3D global modelKHI leads to significant escape of oxygen ions through mixing with the solar wind at the low latitude boundary layer of Venus [ABSTRACT FROM AUTHOR]

Using data taken by the ARTEMIS spacecraft near Moon, we statistically study the properties of Kelvin‐Helmholtz (KH) waves and the thicknesses (ΔY) of initial velocity shear layers on the magnetopause at lunar distance. Based on a total of 48 KH wave events and 14 estimates of ΔY, the average KH wavelength (λX) and ΔY are found to be 16.05 RE and 0.53 RE at lunar distance, respectively, which are larger than those at near‐Earth magnetopause. The λX to ΔY ratio ranges from 18 to 41 at lunar distance, way beyond the range (6–13) predicted by the linear theory. This result indicates that the KH waves observed at lunar distance are not of the fastest growing mode according to linear theory. This study poses the first statistical study on λX and ΔY, providing insights into the generation and development of KH waves at lunar distance. Plain Language Summary: Kelvin‐Helmholtz (KH) instability typically occurs when a velocity shear exists between the boundary of two layers in a fluid. Its manifestation is KH surface waves or vortices, depending upon the stage in the development of the instability. The solar wind can create a velocity shear along the magnetopause that possibly satisfies the unstable condition for exciting KH waves. Using data observed from spacecraft on the magnetopause near Earth and at lunar distance, we estimate the thickness of the velocity shear that forms on the magnetopause before an instability occurs and the wavelength of KH waves that reach after the instability develops. The estimation shows that the average wavelength for lunar distance is triple larger than that for near‐Earth, revealing a significant development in spatial scale of KH waves at lunar distance. The ratio of wavelength to thickness, which can be an indicator of the stage of KH waves, demonstrates that the KH waves at lunar distance are not of the fastest growing mode according to linear theory. A statistical study on the combined wavelength and thickness has never been performed before. The insights we have obtained can intrigue KH researchers for further analyses. Key Points: We perform the first statistical study on the wavelength of Kelvin‐Helmholtz (KH) waves on the magnetopause at lunar distanceThe mean ratio of the wavelength to the thickness of initial shear layers at the lunar orbit is twice more than that for near‐EarthThe range of the ratios indicates that the KH waves at lunar distance are not of the fastest growing mode according to linear theory [ABSTRACT FROM AUTHOR]

Mode coupling is an important mechanism to excite Alfvén waves and has been extensively investigated based on numerical simulations. However, no direct observational evidence has been found regarding to this mechanism in the magnetotail lobe. Here, we report an in‐situ observation that an Alfvén wave with a period of several minutes is clearly relevant to a fast mode wave in the magnetotail lobe based on observations of the Cluster spacecraft. These waves occur at the region where the Alfvén speed has a gradient during an enhancement of the solar wind dynamic pressure. This suggests that the Alfvén wave is generated by the mode conversion at the region where the Alfvén speed has a gradient, and the fast mode wave might be driven by the solar wind. Our observations provide an important insight into the wave generation and energy conversion in the magnetosphere. Plain Language Summary: The coupling between the magnetosphere and ionosphere is a fundamental issue in space physics, and the field‐aligned current (FAC) is a key process of this coupling. Alfvén waves are an important form to carry the FAC and thus they play a critical role in the magnetospheric dynamics. Alfvén waves in the magnetosphere can be excited by Kelvin‐Helmholtz instabilities at the magnetopause or the injected particles during substorm activities. Previous numerical simulations also suggested that some Alfvén waves can be generated by the mode coupling with the fast mode waves. Here, we report a direct observational evidence that Alfvén waves are coupled with the fast mode waves in the magnetotail lobe. The fast mode wave occurs during an increase of the solar wind dynamic pressure and the Alfvén wave is generated by the mode conversion at the region where there is an Alfvén speed gradient. Key Points: Direct observational evidence of Alfvén waves coupled with fast mode waves in the magnetotail lobe has been foundThe wave event occurs during an enhancement of the solar wind dynamic pressureThe fast mode wave converts into the Alfvén wave at the region where there is an Alfvén speed gradient [ABSTRACT FROM AUTHOR]

The Kelvin‐Helmholtz instability at the magnetospheric boundary plays a crucial role in solar wind‐magnetosphere‐ionosphere coupling, particle entry, and energization. The full extent of its impact has remained an open question due, in part, to global models without sufficient resolution to capture waves at higher latitudes. Using global magnetohydrodynamic simulations, we investigate an event when the Magnetospheric Multiscale (MMS) mission observed periodic low‐frequency waves at the dawn‐flank, high‐latitude boundary layer. We show the layer to be unstable, even though the slow solar wind with the draped interplanetary magnetic field is seemingly unfavorable for wave generation. The simulated velocity shear at the boundary is thin (∼0.65RE) and requires commensurately high spatial resolution. These results, together with MMS observations, confirm for the first time in fully three‐dimensional global geometry that KH waves can grow in this region and thus can be an important process for energetic particle acceleration, dynamics, and transport. Plain Language Summary: The boundary separating magnetospheric plasma and the solar wind can become unstable due to the Kelvin‐Helmholtz instability, forming waves that can facilitate mass, momentum, and energy transfer into the magnetosphere. How prevalent Kelvin‐Helmholtz waves are along the magnetopause is therefore a fundamental question to understanding the magnetospheric response to the solar wind. Determining when and where these waves occur on the boundary has remained a challenge. The lower latitudes have been extensively studied while the Cluster mission has provided observations of KH at high‐latitudes. No global models have been capable of resolving the high‐latitude boundary layer, preventing numerical studies of waves within this region. We present a simulation that captures Kelvin‐Helmholtz waves at the high‐latitude boundary for the first time. The instability formed despite the magnetosphere being immersed in pristine slow wind, which reduced the velocity drop across the shear layer at the equator. The simulated period took advantage of an opportunity when the Magnetospheric Multiscale mission was located at the high‐latitude boundary and observed boundary oscillations. Together with the observations, we confirm that Kelvin‐Helmholtz waves can grow at high‐latitudes and thus be able to contribute to particle entry and energization in this region. Key Points: Global magnetohydrodynamic simulation of the magnetosphere captures unstable Kelvin‐Helmholtz waves at the high‐latitude boundary layer for the first timeThe growth of the surface waves occurs despite the stabilizing slow solar wind and draped magnetic field near the high‐latitude cuspThe fastest growing wave mode resolved by the simulation is consistent with Magnetospheric Multiscale mission observations of the same event [ABSTRACT FROM AUTHOR]

On 17 October 2016, Magnetospheric Multiscale (MMS) sampled an electron diffusion region (EDR) embedded in an electron‐scale current sheet within a mixed portion of the high‐latitude magnetosheath adjacent to the magnetopause. We analyze the generalized Ohm's law, dissipation, and electron velocity distributions inside the EDR. The velocities, magnetic fields, and densities observed in the magnetosheath and magnetosphere indicate the magnetopause was unstable to the Kelvin‐Helmholtz instability, which can form electron‐scale current sheets. Alternatively, a current sheet was observed in the solar wind at L1 40 min before the MMS observations, which could have been compressed by the bow shock initiating reconnection. Hundreds of keV particles near the EDR may have been locally accelerated. Plain Language Summary: We present observations of an electron diffusion region (EDR) that was observed well removed from the geomagnetic equator. The EDR is a small volume of the plasma where nonideal physics breaks down and particle acceleration occurs. Magnetic field energy is dissipated as kinetic energy of the plasma, having large‐scale consequences in the plasma even though the conversion occurs on extremely small scales. Access to such small scales has only recently been made possible with the high temporal resolution of instruments on board the Magnetospheric Multiscale spacecraft. Key Points: Observations occur in the dusk sector near the high‐altitude southern geomagnetic cuspParallel electric fields and crescent‐shaped electron distributions indicate MMS crosses through an electron diffusion regionThe observed electron diffusion region occurs at an electron‐scale current sheet near the magnetopause [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]

Global Navigation Satellite Systems (GNSS) are subject to disturbances caused by plasma irregularities in the ionosphere. Studies have suggested that in addition to the gradient drift and Kelvin‐Helmholtz instabilities, electron precipitation may be important for phase scintillations in the dayside auroral region. This study combines in situ Swarm data with ground GNSS observations to investigate the potential role of filamentary field‐aligned currents (FACs) on phase scintillations in the dayside auroral region by analyzing 22 events with phase scintillations exceeding 0.45 radians. We observe colocation between regions of severe phase scintillations and highly filamented FACs with fluctuations measured in the spacecraft frame of the order of 20 Hz. The observations indicate that filamentary FACs are crucial drivers for irregularities responsible for creating severe phase scintillations measured in the dayside auroral region and are thus of significant importance in the context of space weather impact on satellite communication. Plain Language Summary: Satellite‐based navigation systems such as Global Positioning System (GPS) are known to be affected by dynamic phenomena in the upper atmosphere. The signals are subject to disturbances, resulting in reduced position accuracy or even a loss of signal reception. Several processes have been suggested as being responsible for these disturbances in the dayside auroral region, but their relative importance is still unclear. Thus, we used Swarm data as well as ground‐based instrumentation to investigate this issue. We identified 22 events with severe disturbances that were located where we could compare data from the satellite and from ground‐based instruments. We find that the occurrences of severe phase scintillation coincide with structured field‐aligned currents (FACs), making them potentially a primary driver for signal disturbances in the daytime auroral region. Key Points: Severe GNSS phase scintillations in the winter dayside auroral ionosphere coincide with filamentary FACsFilamentary FACs appear to be essential for the creation of severe phase scintillations [ABSTRACT FROM AUTHOR]

We present a 3‐D two‐fluid simulation using plasma parameters as measured by MMS on 8 September 2015 concerning the nonlinear development of the Kelvin‐Helmholtz instability at the Earth's magnetopause. We observe an extremely rich nonlinear dynamics including the development of a complex magnetic topology, vortex merging, and secondary Kelvin‐Helmholtz instability driven by large‐scale vortices distributed asymmetrically in latitude. Vortex induced and midlatitude magnetic reconnection coexist and produce an asymmetric distribution of magnetic reconnection events. These results are in good agreement with MMS observations on the same day, in particular for the presence of both equatorial and off‐equator reconnection. Regarding the latter only, we note a predominance of reconnection in the Southern Hemisphere during the early nonlinear phase. The estimated effective diffusion coefficient associated with the dynamics is found to be large enough to account for the observed mass transport at the Earth's magnetospheric flanks. Key Points: We present a 3‐D two‐fluid simulation using plasma parameters as measured by MMS on 8 September 2015We observe that (Kelvin‐Helmholtz) vortex induced and midlatitude magnetic reconnection coexist and cooperateWe estimate the effective diffusion associated with reconnection, obtaining Deff  ≃ 1010 m2/s, large enough to explain the observed mixing [ABSTRACT FROM AUTHOR]

Abstract: We investigate the transport of planetary ions in the magnetospheric flanks of Mercury. In situ measurements from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft show evidences of Kelvin‐Helmholtz instability development in this region of space, due to the velocity shear between the downtail streaming flow of solar wind originating protons in the magnetosheath and the magnetospheric populations. Ions that originate from the planet exosphere and that gain access to this region of space may be transported across the magnetopause along meandering orbits. We examine this transport using single‐particle trajectory calculations in model Magnetohydrodynamics simulations of the Kelvin‐Helmholtz instability. We show that heavy ions of planetary origin such as Na+ may experience prominent nonadiabatic energization as they E × B drift across large‐scale rolled up vortices. This energization is controlled by the characteristics of the electric field burst encountered along the particle path, the net energy change realized corresponding to the maximum E × B drift energy. This nonadiabatic energization also is responsible for prominent scattering of the particles toward the direction perpendicular to the magnetic field. [ABSTRACT FROM AUTHOR]

Abstract: 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. [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: Ocean waves occur when even a light breeze blows over the surface of water. Without wind, the air–water interface would be flat, but strong gusts initiate circulating motion, producing the characteristic breaking wave shape. Similar waves occur anywhere two fluids flowing in different directions are in contact. In space plasmas, this situation is common where solar wind flows past a magnetized body. At Saturn, strong internal magnetic fields produce a cavity in the solar wind, which is filled with plasma by Enceladus. This plasma rotates with the planet, leading to asymmetry in the system, where solar wind flows opposite the magnetospheric flow on the dawn side of the planet, and solar wind flows in the same direction as magnetospheric flow on the dusk side. Previous studies have confoundingly concluded that these waves are most prevalent in the dusk sector, but this is due to the difficulty of observing waves on the dawn magnetopause. The difference being waves on the dawn magnetopause quickly roll‐up and "break" similarly to ocean waves, while on the dusk side they keep their structure and move across the magnetopause. We resolve the long‐standing conundrum from a comprehensive analysis of the plasma near Saturn's magnetopause boundary. Key Points: Reversed azimuthal flows, likely driven by Kelvin‐Helmholtz instabilites, occur dominantly in the dawn‐subsolar regionMany identified Kelvin‐Helmholtz events show large turbulent heating, agreeing with ∼10−15 W/m 3 from hybrid simulationsMass diffusion in Kelvin‐Helmholtz simulations can be 1010 m 2/s, suggesting significant transport events [ABSTRACT FROM AUTHOR]

We present and characterize in time and three spatial dimensions a Kelvin‐Helmholtz Instability (KHI) event from polar mesospheric summer echoes (PMSE) observed with the Middle Atmosphere Alomar Radar System. We use a newly developed radar imaging mode, which observed PMSE intensity and line of sight velocity with high temporal and angular resolution. The identified KHI event occurs in a narrow layer of 2.4 km thickness centered at 85 km altitude, is elongated along north‐south direction, presents separation between billows of ∼8 km in the east‐west direction, and its billow width is ∼3 km. The accompanying vertical gradients of the horizontal wind are between 35 and 45 m/s/km and vertical velocities inside the billows are ±12 m/s. Based on the estimated Richardson (<0.25), horizontal Froude (∼0.8), and buoyancy Reynolds (∼2.5 × 10 4) numbers, the observed event is a KHI that occurs under weak stratification and generates strong turbulence. Key Points: Four‐dimensional quantification of a clearly resolved Kelvin‐Helmholtz Instability event is observed with volumetric radar imagingThe Richardson number is estimated to be much less than 0.25, using independent measurementsThe Froude number is estimated to be close to 1 (weak stratification) and a high buoyancy Reynolds number ∼25,000 is estimated [ABSTRACT FROM AUTHOR]