Earth's Magnetopause Research Articles

Impact of the Out‐Of‐Plane Flow Shear on Magnetic Reconnection at the Flanks of Earth's Magnetopause

AbstractMagnetic reconnection changes the magnetic field topology and facilitates the energy and particle exchange at magnetospheric boundaries such as the Earth's magnetopause. The flow shear perpendicular to the reconnecting plane prevails at the flank magnetopause under southward interplanetary magnetic field conditions. However, the effect of the out‐of‐plane flow shear on asymmetric reconnection is an open question. In this study, we utilize kinetic simulations to investigate the impact of the out‐of‐plane flow shear on asymmetric reconnection. By systematically varying the flow shear strength, we analyze the flow shear effects on the reconnection rate, the diffusion region structure, and the energy conversion rate. We find that the reconnection rate increases with the upstream out‐of‐plane flow shear, and for the same upstream conditions, it is higher at the dusk side than at the dawn side. The diffusion region is squeezed in the outflow direction due to magnetic pressure which is proportional to the square of the Alfvén Mach number of the shear flow. The out‐of‐plane flow shear increases the energy conversion rate , and for the same upstream conditions, the magnitude of is larger at the dusk side than at the dawn side. This study reveals that out‐of‐plane flow shear not only enhances the reconnection rate but also significantly boosts energy conversion, with more pronounced effects on the dusk‐side flank than on the dawn‐side flank. These insights pave the way for better understanding the solar wind‐magnetosphere interactions.

Design of Three-Dimensional Magnetic Probe System for Space Plasma Environment Research Facility (SPERF).

A three-dimensional magnetic probe system has been designed and implemented at the Space Plasma Environment Research Facility (SPERF). This system has been developed to measure the magnetic field with high spatial and temporal resolution, enabling studies of fundamental processes in space physics, such as magnetic reconnection at the Earth's magnetopause, on the basis of SPERF. The system utilizes inductive components as sensors, arranged in an array and soldered onto a printed circuit board (PCB), achieving a spatial resolution of 2.5 mm. The system's electrical parameters have been measured, and its amplitude-frequency response characteristics have been simulated. The system has demonstrated good performance with response capabilities below 50 kHz. The experimental setup and results are discussed, highlighting the system's effectiveness in accurately measuring weak magnetic signals and its suitability for magnetic reconnection experiments.

Plasma Mixing During Active Kelvin‐Helmholtz Instability Under Different IMF Orientations

AbstractWhen the velocity shear between the two plasmas separated by Earth's magnetopause is locally super‐Alfvénic, the Kelvin‐Helmholtz (KH) instability can develop. A crucial role is played by the interplanetary magnetic field (IMF) orientation, which can stabilize the velocity shear. Although, in a linear regime, the instability threshold is equally satisfied during both northward and southward IMF orientations, in situ measurements show that KH instability is preferentially excited during the northward IMF orientation. We investigate this different behavior by means of a mixing parameter which we apply to two KH events to identify both boundaries and the center of waves/vortices. During the northward orientation, the waves/vortex boundaries have stronger electrons than ions mixing, while the opposite is observed at their center. During the southward orientation, instead, particle mixing is observed predominantly at the boundaries. In addition, stronger local ion and electron non‐thermal features are observed during the northward than the southward IMF orientation. Specifically, ion distribution functions are more distorted, due to field‐aligned beams, and electrons have a larger temperature anisotropy during the northward than the southward IMF orientation. The observed kinetic features provide an insight into both local and remote processes that affect the evolution of KH structures.

Quantitative Analysis of Electron Heating and Acceleration in Coalescing Magnetic Flux Ropes at Earth's Magnetopause

AbstractCoalescence of magnetic flux ropes (MFRs) is suggested as a crucial mechanism for electron acceleration in various astrophysical plasma systems. However, how electrons are being accelerated/heated via MFR coalescence is not fully understood. In this paper, we quantitatively analyze electron heating and acceleration during the coalescence of three MFRs at Earth's magnetopause using in‐situ Magnetospheric Multiscale (MMS) observations. We find that suprathermal electrons are enhanced in the coalescing MFRs than those in the ambient magnetosheath and non‐coalescing MFRs. Both first‐order Fermi and E|| acceleration were responsible for this electron acceleration, while the overall effect of betatron process cooled the electrons. The most intense Fermi acceleration was observed in the trailing part of the middle MFR, while E|| acceleration occurred primarily at the reconnection sites between the coalescing MFRs. For non‐coalescing MFRs, the dominant mechanism for energizing electrons is the E|| acceleration. Our results further consolidate the important role of MFR coalescence in electron heating/acceleration in space plasma.

Dayside Pc2 Waves Associated With Flux Transfer Events in a 3D Hybrid‐Vlasov Simulation

AbstractFlux transfer events (FTEs) are transient magnetic flux ropes at Earth's dayside magnetopause formed due to magnetic reconnection. As they move across the magnetopause surface, they can generate disturbances in the ultralow frequency (ULF) range, which then propagate into the magnetosphere. This study provides evidence of ULF waves in the Pc2 wave frequency range (>0.1 Hz) caused by FTEs during dayside reconnection using a global 3D hybrid‐Vlasov simulation (Vlasiator). These waves resulted from FTE formation and propagation at the magnetopause are particularly associated with large, rapidly moving FTEs. The wave power is stronger in the morning than afternoon, showing local time asymmetry. In the pre and postnoon equatorial regions, significant poloidal and toroidal components are present alongside the compressional component. The noon sector, with fewer FTEs, has lower wave power and limited magnetospheric propagation.

Design and construction of the near-earth space plasma simulation system of the Space Plasma Environment Research Facility

Our earth is immersed in the near-earth space plasma environment, which plays a vital role in protecting our planet against the solar-wind impact and influencing space activities. It is significant to investigate the physical processes dominating the environment, for deepening our scientific understanding of it and improving the ability to forecast the space weather. As a crucial part of the National Major Scientific and Technological Infrastructure–Space Environment Simulation Research Infrastructure (SESRI) in Harbin, the Space Plasma Environment Research Facility (SPERF) builds a system to replicate the near-earth space plasma environment in the laboratory. The system aims to simulate the three-dimensional (3-D) structure and processes of the terrestrial magnetosphere for the first time in the world, providing a unique platform to reveal the physics of the 3-D asymmetric magnetic reconnection relevant to the earth's magnetopause, wave–particle interaction in the earth's radiation belt, particles’ dynamics during the geomagnetic storm, etc. The paper will present the engineering design and construction of the near-earth space plasma simulation system of the SPERF, with a focus on the critical technologies that have been resolved to achieve the scientific goals. Meanwhile, the possible physical issues that can be studied based on the apparatus are sketched briefly. The earth-based system is of great value in understanding the space plasma environment and supporting space exploration.

Energy Conversion by a Twisted Magnetic Structure at Magnetopause Boundary Layer: MMS Observations

AbstractThe Earth's magnetopause is an ion‐scale boundary that separates the magnetosphere from the shocked solar wind. At such boundary, energy‐conversion processes frequently occur. Previous studies suggested that such energy conversions are related to the magnetic reconnection process. Here, we report a new mechanism that the coaction between the twisted magnetic structure and lower‐hybrid waves can also drive energy conversion (J⋅ E, J is current density and E is electric field) at the magnetopause boundary layer, by using high‐resolution measurements of the four Magnetospheric Multiscale spacecraft. We find that such energy conversion is efficient (with magnitude up to 20 nW/m3) and is attributed to an intense current filament (j ≈ 3,800 nA/m2) and a fluctuating electric field driven by lower‐hybrid waves. With the help of the First‐Order Taylor Expansion method, we find that the intense current filament is driven by a twisted magnetic structure at electron scale. Our study improves the understanding of energy‐conversion processes at the Earth's magnetopause.

Kinetic and Shear Alfvén Waves in a Large Guide-field Reconnection at Earth's Magnetopause

A large guide-field (∼1.1B o) reconnection X-line, observed by the Magnetospheric Multiscale spacecraft during an outbound magnetopause crossing, is studied for Alfvén waves. Here B o is the reconnecting field magnitude. The current sheet thickness of the magnetopause was ∼2.6 ion inertial lengths (∼269 km), where field-aligned counter-streaming electrons were observed and Hall electromagnetic fields were identified. A remarkable finding was that a kinetic Alfvén wave (KAW) was seen in the magnetopause upstream region after a shear Alfvén wave (SAW) was encountered in the magnetopause layer. The presence of both the SAW and KAW near the reconnection X-line is for the first time reported. In the spacecraft frame of reference, the SAW has a dominant frequency at ∼0.74 Hz, while the KAW has two dominant frequencies at ∼0.38 and ∼0.64 Hz. The wave energy for KAW and SAW was mostly carried away from the reconnection site by the Poynting flux parallel to the magnetic field. The parallel temperatures for ions and electrons were increased at KAW. The peaks of T ∥/T ⊥ for ions were located near the wave peaks, while the ratio peaks for electrons were near the wave troughs. Our findings suggest that KAWS and SAWs can be generated by asymmetric reconnection with a large guide field.

Kelvin‐Helmholtz Waves and Magnetic Reconnection at the Earth's Magnetopause Under Southward Interplanetary Magnetic Field

AbstractWe present Magnetospheric Multiscale (MMS) observations of a K‐H wave event under southward IMF conditions, accompanied by ongoing magnetic reconnection. The nonlinear K‐H waves are characterized by quasi‐periodic fluctuations, the presence of low‐density and high‐speed ions, and variations in the boundary normal vectors at both the leading and trailing edges. Our observations reveal clear evidence of on‐going magnetic reconnection through the identification of Alfvénic ion jets and the escape of energetic magnetospheric electrons. Among the 36 magnetopause current‐sheet crossings in this event, 19 exhibit unambiguous signatures of reconnection at both the leading (7) and trailing (12) edges. Notably, the estimated current‐sheet thicknesses at both edges are comparable to the ion‐inertial scale, confirming the compression effect resulting from the large‐scale evolution of the K‐H waves. The reconnection jets potentially contribute to the suppression of K‐H growth through boundary‐layer broadening and the development of complex flow and magnetic field patterns.

Statistical Comparison of Various Dayside Magnetopause Reconnection X‐Line Prediction Models

AbstractReconnection at Earth's magnetopause drives magnetospheric convection and provides mass and energy input into the magnetosphere/ionosphere system thereby driving the coupling between solar wind and terrestrial magnetosphere. Despite its importance, the factors governing the location of dayside magnetopause reconnection are not well understood. Though a few models can predict X‐line locations reasonably well, the underlying physics is still unresolved. In this study we present results from a comparative analysis of 274 magnetic reconnection events as observed by the Magnetospheric Multiscale (MMS) mission to determine what quantities affect the accuracy of such models and are most strongly associated with the occurrence of dayside magnetopause reconnection. We also attempt to determine under what upstream solar wind conditions each global X‐line model becomes least reliable.

Large‐Amplitude Shocklets With Trapped Hot‐Electrons in Space Plasmas

AbstractLarge‐amplitude stationary electron‐acoustic (EA) waves are studied, which are developed into the shocklets with the passage of time in an unmagnetized non‐isothermal plasma. The latter comprises two groups of electrons; namely the hot and cool electrons that are distinctly characterized by different electron densities and temperatures. On an EA timescale, the cool electrons behave as fluid, while non‐isothermal hot electrons obey the vortex‐like trapped distribution in the background of immobile ions. The nonlinear fluid equations are solved together both analytically and numerically within the framework of diagonalization matrix technique. Various parameters such as the free hot‐to‐trapped hot electron temperature ratio (β), the cool electron density ratio (α), and the cool‐to‐hot electron temperature ratio (σ) are numerically analyzed for dayside auroral zone plasma, showing a significant modification of solitary and shocklet structures. The plasma model is also applied to the electron diffusion region at the earth magnetopause, revealing the potential excitations depending significantly on the trapping parameter. These findings are important for understanding the nonlinear properties and steepening effects of the EA waves, especially in auroral zone and electron diffusion regions of earth's magnetosphere, where different types of electron populations are observed.

Reconnection Rates at the Earth's Magnetopause and in the Magnetosheath

AbstractReconnection electric fields and normalized reconnection rates (RRs) were determined with the four Magnetospheric Multiscale (MMS) spacecraft for 14 reconnection events on the dayside of the Earth's magnetosphere. Half of the events occurred at the magnetopause (MP) and the other half in the magnetosheath (MS). The RRs were determined by measurements of the reconnection electric field within electron diffusion regions. Since the reconnection electric field component is much smaller than the other components, special care had to be taken to eliminate contamination by the larger components. Normalized RRs between 0.02 and 0.48 were determined with average values in the range of theoretical predictions for steady‐state reconnection (0.1–0.2). Using measurements from individual MMS spacecraft, 20 normalized RRs were determined for the seven MP events with a mean of 0.14 and a standard deviation of 0.09. For the 27 RRs that were determined for the seven MS events, the normalized RRs had a mean of 0.16 and a standard deviation of 0.12. These results show variations of over an order of magnitude between the fastest and slowest RRs. However, when summed over all spacecraft, calculations suggest that the RR per event is relatively constant, as predicted theoretically. Searches for dependence of the normalized RR on local or solar‐wind parameters were inconclusive. However, we do find that for the MS events, the reconnection electric field showed a direct dependence on solar‐wind dynamic pressure.

Kinetics of Cold Ions in Asymmetric Reconnection: Particle‐In‐Cell Simulation

AbstractCold ions from Earth's ionosphere and plasmasphere have frequently been observed at the magnetopause, where they impact reconnection and get energized in the process. The behavior of cold ions in different regions of magnetopause reconnection is not well understood. This study investigates their kinetics in asymmetric reconnection through a 2.5‐D fully kinetic simulation. The simulation starts with cold ions being present only in the magnetosphere. We find that the velocity distribution functions (VDFs) of cold ions in different reconnection regions are composed of two types of particles, distinguished by their ability to penetrate the magnetosheath. One type produces ring‐shaped distributions in the magnetosheath due to meandering motion, while the other generates ring‐shaped distributions in the magnetospheric separatrix as a result of gyromotion. The reconnection electric field Ey has a negative effect on one type and a positive effect on the other. Moreover, the Hall electric field Ez can stepwise accelerate the meandering particles. The cold ion temperature increases significantly in the magnetosheath as compared to its original temperature, the enhancement is a kinetic effect as a result of the ring‐shaped velocity distribution. On the other hand, in the remaining regions, cold ions experience bulk heating, resulting in noticeable temperature elevation. These findings further emphasize the necessity to examine the VDF for an all‐round comprehension of the particle heating mechanism and contribute to a better understanding of cold ion dynamics at the Earth's magnetopause.

Magnetospheric Response to a Pressure Pulse in a Three‐Dimensional Hybrid‐Vlasov Simulation

AbstractVlasiator is a high‐performance ion‐kinetic code that is now conducting 3D hybrid‐Vlasov simulations of the global magnetosphere. We use Vlasiator to investigate the impact of a pressure pulse with southward‐oriented magnetic field on the Earth's magnetosphere. The simulation driving parameters are comparable to conditions that have led to geomagnetic storms. Our pressure pulse simulation reproduces many physical effects, namely the expansion of the auroral oval, the development of field‐aligned currents, enhanced particle precipitation near the open/closed field line boundary, and compression of Earth's magnetopause. This demonstrates the effectiveness of the hybrid‐Vlasov approach for moderate driving conditions. Our investigation of the time‐dependent magnetopause compression motivates a generalization of the existing theory. Specifically, we find that accounting for the finite transition time of the solar wind dynamic pressure improves the model's description of the magnetopause oscillations.

Turbulence, Particle Acceleration, and Heating Due To the Lower Hybrid Mode Near Magnetic Reconnection Regions in Magnetopause

AbstractMMS mission by NASA has observed lower hybrid waves (LHWs) and associated turbulence near the reconnection regions at Earth's magnetopause. The presented work studies LHWs (electromagnetic mode) and turbulence in the presence of magnetic islands (at the reconnection sites) in magnetopause. A nonlinear model representing electromagnetic 3D LHW has been developed. We have considered that nonlinear effects are due to the ponderomotive force. The dynamical equation, thus obtained, is solved using numerical methods and computer simulation. For spatial integration, we have used the pseudo‐spectral method and the finite difference method is used to study temporal evolution. The simulation results show the spatiotemporal development of the LHW localized structures and current sheets and confirm the presence of turbulence. Using a semi‐analytic model, we have determined the scale size of current sheets and localized structures and shown that these scale sizes depend upon the lower hybrid power. The turbulent power spectra have also been studied. The power scaling of k−2.8 (approximately) and k−2.1 (approximately) are observed along perpendicular and parallel directions, respectively. The power law scaling of turbulence generation has been used to study the formation of the thermal tail of energetic electrons. The fractional diffusion method has also been exploited to determine electron acceleration and plasma heating. We anticipate LHW may be responsible for the acceleration of the energetic electrons and anomalous heating in magnetopause.

Kinetic Simulation of Cold Ion Heating in Asymmetric Reconnection

AbstractCold ions from the plasmasphere can reach the Earth's magnetopause through the plasma wind and plume, affecting the reconnection at the magnetopause, such as changing the reconnection rate and influencing the energy budget. Recent observations found that cold ions were substantially heated by magnetopause reconnection. Although a few mechanisms have been proposed to account for the cold ion acceleration at the magnetopause, the dominant mechanism remains unclear. In this paper, we perform 2.5‐dimensional fully kinetic simulations to study cold ion heating in asymmetric reconnection at Earth's magnetopause. We find that cold ions absorb about 10%–25% of the total released magnetic energy, which is predominantly converted into the thermal energy of cold ions. The proportion of the energy gained by cold ions increases with the increment of the initial density ratio of the cold ions to hot ions in the magnetosphere. Cold ions are primarily accelerated by the Hall electric field at the magnetospheric separatrix region, while the out‐of‐plane electric field does negative work. The increase in thermal energy of cold ions is mainly caused by stochastic heating, that is, the viscous heating associated with the non‐gyrotropic pressure tensor, , contributes the most. The work done by pressure is most significant around the magnetosheath separatrix region. These results can significantly deepen our understanding of the cold ion dynamics at Earth's magnetopause.

Temporal, Spatial, and Velocity‐Space Variations of Electron Phase Space Density Measurements at the Magnetopause

AbstractTemporal, spatial, and velocity‐space variations of electron phase space density are measured observationally and compared for the first time using the four magnetospheric multiscale (MMS) spacecraft at Earth's magnetopause. Equipped with these unprecedented spatiotemporal measurements offered by the MMS tetrahedron, we compute each term of the electron Vlasov equation that governs the evolution of collisionless plasmas found throughout the universe. We demonstrate how to use single spacecraft measurements to improve the resolution of the electron pressure gradient that supports nonideal parallel electric fields, and we develop a model to intuit the types of kinetic velocity‐space signatures that are observed in the Vlasov equation terms. Furthermore, we discuss how the gradient in velocity‐space sheds light on plasma energy conversion mechanisms and wave‐particle interactions that occur in fundamental physical processes such as magnetic reconnection and turbulence.

Design and construction of the magnetopause shape control coils of the Space Plasma Environment Research Facility to regulate the field configuration of the simulated Earth's magnetopause magnetic reconnection

Two groups of magnetopause shape control coils are used on the SPERF (Space Plasma Environment Research Facility) facility to regulate the magnetic field configuration of the simulated Earth's magnetopause magnetic reconnection. Here, design and construction of the magnetopause shape control coils is presented to resolve the issues regarding the strong pulsed current, high voltage, and vacuum sealing, etc. At first, the physical requirement imposed on the coils is explained, followed by clarification of the key technological issues facing the coils' design. Afterwards, the fundamental design of the coils is shown, with emphasis on the segmented structure and reliable vacuum sealing. Then, the supporting and motion mechanism of the coils are designed, and the layout of the leading wires in the supporting tubes is optimized to alleviate the impact of the magnetic forces between them. Further, the connection structure is designed to connect the leading wires of the coils with the pulsed power supply. At last, the critical measurements done for the magnetopause shape control coils are given to demonstrate the design. Presently, installation of the coils on the SPERF has completed, and they will be used in the physical experiments in the future.

Observations of Dynamical Flux Ropes and Active Multiple X‐Line Reconnection at Earth's Magnetopause

AbstractThis paper presents the magnetospheric multiscale (MMS) observations of three flux ropes (FRs) sequentially in a reconnection exhaust at Earth's magnetopause. Three active X‐lines, which are responsible for the formation of these FRs, were also detected by MMS, providing strong evidence that the FRs were generated by multiple X‐line reconnection. We find that the core field inside the FRs considerably affects the Hall current in the adjacent X‐lines. The intense core field of the FRs deflected the electron outflow jet, forming the electron‐scale Hall magnetic field on the magnetosheath side. Additionally, an active secondary reconnection was observed at the center of one FR. This secondary reconnection split the FR into two smaller FRs and produced sharp density gradients and intense fluctuations of electric field and current within the FR. These observations advance the understanding of the formation and evolution of FRs during multiple X‐line reconnection at magnetopause.

An Interpretable Machine Learning Procedure Which Unravels Hidden Interplanetary Drivers of the Low Latitude Dayside Magnetopause

AbstractIn this study, we propose an interpretable machine learning procedure to unravel the importance of multiple interplanetary parameters to the Earth's magnetopause standoff distance (MSD). We construct the interpretable procedure based on SHapley Additive exPlanations. A magnetopause crossings database from the Time History of Events and Macroscale Interactions during Substorms satellites and the multiple interplanetary parameters from OMNI during the period of 2007–2016 are utilized. The solar wind dynamic pressure and the interplanetary magnetic field (IMF) BZ are widely suggested as the important two interplanetary parameters that drive the MSD. However, the examination of the interpretable procedure suggests that the magnitude of the IMF is the second significant parameter after the dynamic pressure. Although the magnetic pressure, which is the function of the IMF magnitude was considered in previous studies, the importance of the IMF magnitude was underestimated. The interpretable procedure also reveals that the IMF magnitude and the BZ have different effects on the MSD. Their joint effect is the formation of the MSD sag near BZ = 5 nT. This is for the first time the interpretable concept is being applied to construct a machine‐learning magnetopause model.