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We report a rare regime of Earth's magnetosphere interaction with sub‐Alfvénic solar wind in which the windsock‐like magnetosphere transforms into one with Alfvén wings. In the magnetic cloud of a Coronal Mass Ejection (CME) on 24 April 2023, NASA's Magnetospheric Multiscale mission distinguishes the following features: (a) unshocked and accelerated low‐beta CME plasma coming directly against Earth's dayside magnetosphere; (b) dynamical wing filaments representing new channels of magnetic connection between the magnetosphere and foot points of the Sun's erupted flux rope; (c) cold CME ions observed with energized counter‐streaming electrons, evidence of CME plasma captured due to by reconnection between magnetic‐cloud and Alfvén‐wing field lines. The reported measurements advance our knowledge of CME interaction with planetary magnetospheres, and open new opportunities to understand how sub‐Alfvénic plasma flows impact astrophysical bodies such as Mercury, moons of Jupiter, and exoplanets close to their host stars. Like supersonically fast fighter jets creating sonic shocks in the air, planet Earth typically moves in the magnetized solar wind at super‐Alfvénic speeds and generates a bow shock. Here we report unprecedented observations of Earth's magnetosphere interacting with a sub‐Alfvénic solar wind brought by an erupted magnetic flux rope from the Sun, called a coronal mass ejection (CME). The terrestrial bow shock disappears, leaving the magnetosphere exposed directly to the cold CME plasma and the strong magnetic field from the Sun's corona. Our results show that the magnetosphere transforms from its typical windsock‐like configuration to having wings that magnetically connect our planet to the Sun. The wings are highways for Earth's plasma to be lost to the Sun, and for the plasma from the foot points of the Sun's erupted flux rope to access Earth's ionosphere. Our work indicates highly dynamic generation and interaction of the wing filaments, shedding new light on how sub‐Alfvénic plasma wind may impact astrophysical bodies in our solar and other stellar systems. MMS observed a rare regime of magnetosphere interaction with unshocked low‐beta CME plasmaWing filaments represent dynamical channels of magnetic connection between the magnetosphere and foot points of the Sun's erupted flux ropeCold CME ions observed on closed field lines, likely generated by dual‐wing reconnection MMS observed a rare regime of magnetosphere interaction with unshocked low‐beta CME plasma Wing filaments represent dynamical channels of magnetic connection between the magnetosphere and foot points of the Sun's erupted flux rope Cold CME ions observed on closed field lines, likely generated by dual‐wing reconnection

An LMNcoordinate system for magnetic reconnection events is sometimes determined by defining Nas the direction of the gradient across the current sheet and Las the direction of maximum variance of the magnetic field. The third direction, M, is often assumed to be the direction of zero gradient, and thus the orientation of the X line. But when there is a guide field, the X line direction may have a significant component in the L direction defined in this way. For a 2D description, a coordinate system describing such an event would preferably be defined using a different coordinate direction M′ oriented along the X line. Here we use a 3D particle‐in‐cell simulation to show that the X line is oriented approximately along the direction bisecting the asymptotic magnetic field directions on the two sides of the current sheet. We describe two possible ways to determine the orientation of the X line from spacecraft data, one using the minimum gradient direction from Minimum Directional Derivative analysis at distances of the order of the current sheet thickness from the X line, and another using the bisection direction based on the asymptotic magnetic fields outside the current sheet. We discuss conditions for validity of these estimates, and we illustrate these conditions using several Magnetospheric Multiscale (MMS) events. We also show that intersection of a flux rope due to secondary reconnection with the primary X line can destroy invariance along the X line and negate the validity of a two‐dimensional description. At an interface between two regions with magnetic field pointing in different directions, the magnetic fields can reconnect across the interface. While real magnetic reconnection events are three‐dimensional, there can sometimes be a direction of approximate invariance, so that a two‐dimensional description can be valid. In such cases, it can be beneficial to define a coordinate system with one coordinate along the direction of the smallest gradient in the magnetic field. Using a simulation of magnetic reconnection, we show how the direction of smallest gradient, eM′, is determined, and also discuss how spacecraft observations could be used to find that direction. We also illustrate how the invariant direction can be determined using several events observed by the Magnetospheric Multiscale (MMS) spacecraft. When there is a guide field, the orientation of the X line may be tilted toward the direction of maximum magnetic field varianceUnder certain circumstances Minimum Directional Derivative analysis can be used to determine the orientation of the X lineIntersection of a flux rope with the primary X line due to secondary reconnection can destroy two dimensionality When there is a guide field, the orientation of the X line may be tilted toward the direction of maximum magnetic field variance Under certain circumstances Minimum Directional Derivative analysis can be used to determine the orientation of the X line Intersection of a flux rope with the primary X line due to secondary reconnection can destroy two dimensionality

The structure of the current sheet along the Magnetospheric Multiscale (MMS) orbit is examined during the 11 July 2017 Electron Diffusion Region (EDR) event. The location of MMS relative to the X‐line is deduced and used to obtain the spatial changes in the electron parameters. The electron velocity gradient values are used to estimate the reconnection electric field sustained by nongyrotropic pressure. It is shown that the observations are consistent with theoretical expectations for an inner EDR in 2‐D reconnection. That is, the magnetic field gradient scale, where the electric field due to electron nongyrotropic pressure dominates, is comparable to the gyroscale of the thermal electrons at the edge of the inner EDR. Our approximation of the MMS observations using a steady state, quasi‐2‐D, tailward retreating X‐line was valid only for about 1.4 s. This suggests that the inner EDR is localized; that is, electron outflow jet braking takes place within an ion inertia scale from the X‐line. The existence of multiple events or current sheet processes outside the EDR may play an important role in the geometry of reconnection in the near‐Earth magnetotail. Magnetic reconnection is the process by which magnetic field lines coming from one region are broken and reconnected with magnetic field lines coming from another region. The simplest descriptions of magnetic reconnection are two dimensional, and a number of theoretical predictions have been made using the two‐dimensional assumption. We study a magnetic reconnection event observed by the Magnetospheric Multiscale spacecraft on 11 July 2017 and find approximate agreement between the observations and the predictions of a two‐dimensional model. The agreement includes the scale size of the reconnection region, details of the particle orbits, and the rate of reconnection. Current sheet structure in electron diffusion region is deduced using multipoint measurements from MMS to infer reconnection parametersRate of electron acceleration along the out‐of‐plane electric field is consistent with theory of meandering electrons in 2‐D reconnectionConsistency with 2‐D reconnection theory is found in reconnection electric field from electron pressure gradient and spatial extent of EDR

We analyze the local dynamics of magnetotail reconnection onset using Magnetospheric Multiscale (MMS) data. In conjunction with MMS, the macroscopic dynamics of this event were captured by a number of other ground and space‐based observatories, as is reported in a companion paper. We find that the local dynamics of the onset were characterized by the rapid thinning of the cross‐tail current sheet below the ion inertial scale, accompanied by the growth of flapping waves and the subsequent onset of electron tearing. Multiple kinetic‐scale magnetic islands were detected coincident with the growth of an initially sub‐Alfvénic, demagnetized tailward ion exhaust. The onset and rapid enhancement of parallel electron inflow at the exhaust boundary was a remote signature of the intensification of reconnection Earthward of the spacecraft. Two secondary reconnection sites are found embedded within the exhaust from a primary X‐line. The primary X‐line was designated as such on the basis that (a) while multiple jet reversals were observed in the current sheet, only one reversal of the electron inflow was observed at the high‐latitude exhaust boundary, (b) the reconnection electric field was roughly five times larger at the primary X‐line than the secondary X‐lines, and (c) energetic electron fluxes increased and transitioned from anti‐field‐aligned to isotropic during the primary X‐line crossing, indicating a change in magnetic topology. The results are consistent with the idea that a primary X‐line mediates the reconnection of lobe magnetic field lines and accelerates electrons more efficiently than its secondary X‐line counterparts. Magnetotail reconnection onset was triggered by electron tearing during a solar wind pressure pulseOnset was characterized by the rapid collapse of the current sheet thickness and kinetic‐scale flux rope formationA primary X‐line was established within minutes of the onset Magnetotail reconnection onset was triggered by electron tearing during a solar wind pressure pulse Onset was characterized by the rapid collapse of the current sheet thickness and kinetic‐scale flux rope formation A primary X‐line was established within minutes of the onset

We analyze a magnetotail reconnection onset event on 3 July 2017 that was observed under otherwise quiescent magnetospheric conditions by a fortuitous conjunction of six space and ground‐based observatories. The study investigates the large‐scale coupling of the solar wind–magnetosphere system that precipitated the onset of the magnetotail reconnection, focusing on the processes that thinned and stretched the cross‐tail current layer in the absence of significant flux loading during a 2‐hr‐long preconditioning phase. It is demonstrated with data in the (a) upstream solar wind, (b) at the low‐latitude magnetopause, (c) in the high‐latitude polar cap, and (d) in the magnetotail that the typical picture of solar wind‐driven current sheet thinning via flux loading does not appear relevant for this particular event. We find that the current sheet thinning was, instead, initiated by a transient solar wind pressure pulse and that the current sheet thinning continued even as the magnetotail and solar wind pressures decreased. We suggest that field line curvature‐induced scattering (observed by magnetospheric multiscale) and precipitation (observed by Defense Meteorological Satellite Program) of high‐energy thermal protons may have evacuated plasma sheet thermal energy, which may require a thinning of the plasma sheet to preserve pressure equilibrium with the solar wind. Magnetotail reconnection onset was observed during a fortuitous multiscale conjunction of the heliophysics observatoriesA transient solar wind pressure pulse triggered thinning and stretching of the cross‐tail current sheet without significant flux loadingA second solar wind pressure pulse caused the thinned current sheet to rapidly collapse and reconnect Magnetotail reconnection onset was observed during a fortuitous multiscale conjunction of the heliophysics observatories A transient solar wind pressure pulse triggered thinning and stretching of the cross‐tail current sheet without significant flux loading A second solar wind pressure pulse caused the thinned current sheet to rapidly collapse and reconnect

We present characteristics of current layers in the off‐equatorial near‐Earth plasma sheet boundary observed with high time‐resolution measurements from the Magnetospheric Multiscale mission during an intense substorm associated with multiple dipolarizations. The four Magnetospheric Multiscale spacecraft, separated by distances of about 50 km, were located in the southern hemisphere in the dusk portion of a substorm current wedge. They observed fast flow disturbances (up to about 500 km/s), most intense in the dawn‐dusk direction. Field‐aligned currents were observed initially within the expanding plasma sheet, where the flow and field disturbances showed the distinct pattern expected in the braking region of localized flows. Subsequently, intense thin field‐aligned current layers were detected at the inner boundary of equatorward moving flux tubes together with Earthward streaming hot ions. Intense Hall current layers were found adjacent to the field‐aligned currents. In particular, we found a Hall current structure in the vicinity of the Earthward streaming ion jet that consisted of mixed ion components, that is, hot unmagnetized ions, cold E× Bdrifting ions, and magnetized electrons. Our observations show that both the near‐Earth plasma jet diversion and the thin Hall current layers formed around the reconnection jet boundary are the sites where diversion of the perpendicular currents take place that contribute to the observed field‐aligned current pattern as predicted by simulations of reconnection jets. Hence, multiscale structure of flow braking is preserved in the field‐aligned currents in the off‐equatorial plasma sheet and is also translated to ionosphere to become a part of the substorm field‐aligned current system. Multiscale field‐aligned currents in the boundary of the expanding plasma sheet during plasma jet braking intervals are resolvedIntense Hall current layers are found at the inner boundary of the hot Earthward streaming ion jets and flow shear regionsBoth plasma jet diversion and Hall effects from reconnection region contribute to the structure of the substorm wedge currents

We analyzed MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) magnetic field and plasma measurements taken during 319 crossings of Mercury's cross‐tail current sheet. We found that the measured BZin the current sheet is higher on the dawnside than the duskside by a factor of ≈3 and the asymmetry decreases with downtail distance. This result is consistent with expectations based upon MHD stress balance. The magnetic fields threading the more stretched current sheet in the duskside have a higher plasma beta than those on the dawnside, where they are less stretched. This asymmetric behavior is confirmed by mean current sheet thickness being greatest on the dawnside. We propose that heavy planetary ion (e.g., Na+) enhancements in the duskside current sheet provides the most likely explanation for the dawn‐dusk current sheet asymmetries. We also report the direct measurement of Mercury's substorm current wedge (SCW) formation and estimate the total current due to pileup of magnetic flux to be ≈11 kA. The conductance at the foot of the field lines required to close the SCW current is found to be ≈1.2 S, which is similar to earlier results derived from modeling of Mercury's Region 1 field‐aligned currents. Hence, Mercury's regolith is sufficiently conductive for the current to flow radially then across the surface of Mercury's highly conductive iron core. Mercury appears to be closely coupled to its nightside magnetosphere by mass loading of upward flowing heavy planetary ions and electrodynamically by field‐aligned currents that transfer momentum and energy to the nightside auroral oval crust and interior. Heavy planetary ion enhancements in Mercury's duskside current sheet provide explanation for cross‐tail asymmetries found in this study. The total current due to the pileup of magnetic flux and conductance required to close the SCW current is found to be ≈11 kA and 1.2 S. Mercury is coupled to magnetotail by mass loading of heavy ions and field‐aligned currents driven by reconnection‐related fast plasma flow. Heavy planetary ion enhancements in Mercury's duskside current sheet provide explanation for cross‐tail asymmetries found in this studyThe total current due to the pileup of magnetic flux and conductance required to close the SCW current is found to be almost equal to 11 kA and 1.2 SMercury is coupled to magnetotail by mass loading of heavy ions and field‐aligned currents driven by reconnection‐related fast plasma flow

The structure, X‐line location, and magnetohydrodynamic (MHD) stress balance of Mercury's magnetotail were examined between −2.6 < XMSM< −1.4 RMusing MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) measurements from 319 central plasma sheet (CPS) crossings. The mean plasma βin the CPS calculated from MESSENGER data is ~ 6. The CPS magnetic field was southward (i.e., tailward of X‐line) ~ 2–18% of the time. Extrapolation of downtail variations in BZindicates an average X‐line location at −3 RM. Modeling of magnetic field measurements produced a cross‐tail current sheet (CS) thickness, current density, and inner CS edge location of 0.39 RM, 92 nA/m2and −1.22 RM, respectively. Application of MHD stress balance suggests that heavy planetary ions may be important in maintaining stress balance within Mercury's CPS. Qualitative similarities between Mercury's and Earth's magnetotail are remarkable given the differences in upstream conditions, internal plasma composition, finite gyro‐radius scaling, and Mercury's lack of ionosphere. Physical properties of Mercury's cross‐tail current sheet are more intense but similar to Earth'sInner edge of cross‐tail current sheet and mean NMNL location are found to be ~1.2 and −3 RMStress balance seems to require significant contributions from heavy planetary ions and/or H+anisotropy

The properties and acceleration mechanisms of electrons (<200 keV) associated with a pair of tailward traveling flux ropes and accompanied reconnection X‐lines in Earth's plasma sheet are investigated with MMS measurements. Energetic electrons are enhanced on both boundaries and core of the flux ropes. The power‐law spectra of energetic electrons near the X‐lines and in flux ropes are harder than those on flux rope boundaries. Theoretical calculations show that the highest energy of adiabatic electrons is a few keV around the X‐lines, tens of keV immediately downstream of the X‐lines, hundreds of keV on the flux rope boundaries, and a few MeV in the flux rope cores. The X‐lines cause strong energy dissipation, which may generate the energetic electron beams around them. The enhanced electron parallel temperature can be caused by the curvature‐driven Fermi acceleration and the parallel electric potential. Betatron acceleration due to the magnetic field compression is strong on flux rope boundaries, which enhances energetic electrons in the perpendicular direction. Electrons can be trapped between the flux rope pair due to mirror force and parallel electric potential. Electrostatic structures in the flux rope cores correspond to potential drops up to half of the electron temperature. The energetic electrons and the electron distribution functions in the flux rope cores are suggested to be transported from other dawn‐dusk directions, which is a 3‐dimensional effect. The acceleration and deceleration of the Betatron and Fermi processes appear alternately indicating that the magnetic field and plasma are turbulent around the flux ropes. Theoretical physicists have been using simulations to explain the origin of energetic electrons in astrophysics and space plasma physics. The simulations and test particle calculations showed that flux ropes, an important magnetic structure generated by magnetic reconnection, can accelerate energetic electrons. Our study provides a comprehensive investigation of the properties and acceleration mechanisms associated with a pair of flux ropes and reconnection X‐lines. The three fundamental acceleration mechanisms, that is, the Betatron process, the Fermi process, and the parallel electric field, have been directly assessed and are shown to be important in different regions. The results of our investigations should advance our understanding of the role of magnetic flux ropes in charged particle acceleration of all cosmic plasmas. Fermi and parallel potential is strong near X‐lines, Betatron is strong on flux rope boundaries, and electrostatic structures in flux ropeAdiabatic electrons are to a few keV around X‐line, 10's keV downstream of X‐line, 100's keV on flux rope boundary and a few MeV in flux ropeEnergetic electrons and electron distribution function suggest that field lines in flux rope core connect to X‐line in dawn‐dusk direction Fermi and parallel potential is strong near X‐lines, Betatron is strong on flux rope boundaries, and electrostatic structures in flux rope Adiabatic electrons are to a few keV around X‐line, 10's keV downstream of X‐line, 100's keV on flux rope boundary and a few MeV in flux rope Energetic electrons and electron distribution function suggest that field lines in flux rope core connect to X‐line in dawn‐dusk direction

Whistler waves are often observed in magnetopause reconnection associated with electron beams. We analyze seven MMS crossings surrounding the electron diffusion region (EDR) to study the role of electron beams in whistler excitation. Waves have two major types: (a) Narrow‐band waves with high ellipticities and (b) broad‐band waves that are more electrostatic with significant variations in ellipticities and wave normal angles. While both types of waves are associated with electron beams, the key difference is the anisotropy of the background population, with perpendicular and parallel anisotropies, respectively. The linear instability analysis suggests that the first type of wave is mainly due to the background anisotropy, with the beam contributing additional cyclotron resonance to enhance the wave growth. The second type of broadband waves are excited via Landau resonance, and as seen in one event, the beam anisotropy induces an additional cyclotron mode. The results are supported by particle‐in‐cell simulations. We infer that the first type occurs downstream of the central EDR, where background electrons experience Betatron acceleration to form the perpendicular anisotropy; the second type occurs in the central EDR of guide field reconnection. A parametric study is conducted with linear instability analysis. A beam anisotropy alone of above ∼3 likely excites the cyclotron mode waves. Large beam drifts cause Doppler shifts and may lead to left‐hand polarizations in the ion frame. Future studies are needed to determine whether the observation covers a broader parameter regime and to understand the competition between whistler and other instabilities. In EDRs observed by MMS, electron distributions of background plus beams excite whistler by beam drift and anisotropy of both populationsDifferent types of distributions and waves are inferred to depend on the distance from the X‐lineA parametric study with the linear instability analysis is used to discuss the competition between different whistler modes In EDRs observed by MMS, electron distributions of background plus beams excite whistler by beam drift and anisotropy of both populations Different types of distributions and waves are inferred to depend on the distance from the X‐line A parametric study with the linear instability analysis is used to discuss the competition between different whistler modes

In order to determine particle velocities and electric field in the frame of the magnetic structure, one first needs to determine the velocity of the magnetic structure in the frame of the spacecraft observations. Here, we demonstrate two methods to determine a two‐dimensional magnetic structure velocity for the magnetic reconnection event observed in the magnetotail by the Magnetospheric Multiscale (MMS) spacecraft on July 11, 2017, Spatio‐Temporal Difference (STD) and the recently developed polynomial reconstruction method. Both of these methods use the magnetic field measurements; the reconstruction technique also uses the current density measured by the particle instrument. We find rough agreement between the results of our methods and with other velocity determinations previously published. We also explain a number of features of STD and show that the polynomial reconstruction technique is most likely to be valid within a distance of 2 spacecraft spacings from the centroid of the MMS spacecraft. Both of these methods are susceptible to contamination by magnetometer calibration errors. We demonstrate use of Spatio‐Temporal Difference (STD) and polynomial reconstruction to determine the 2‐D velocity of a magnetic structureVelocities from STD and reconstruction roughly agree with each other and with estimates from other references for the July 11, 2017 eventPolynomial reconstruction is most likely to be accurate within a distance of 2 spacecraft spacings from the centroid of the Magnetospheric Multiscale spacecraft We demonstrate use of Spatio‐Temporal Difference (STD) and polynomial reconstruction to determine the 2‐D velocity of a magnetic structure Velocities from STD and reconstruction roughly agree with each other and with estimates from other references for the July 11, 2017 event Polynomial reconstruction is most likely to be accurate within a distance of 2 spacecraft spacings from the centroid of the Magnetospheric Multiscale spacecraft