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This study presents observations of magnetopause reconnection and erosion at geosynchronous orbit, utilizing in situ satellite measurements and remote sensing ground‐based instruments. During the main phase of a geomagnetic storm, Geostationary Operational Environmental Satellites (GOES) 15 was on the dawnside of the dayside magnetopause (10.6 MLT) and observed significant magnetopause erosion, while GOES 13, observing duskside (14.6 MLT), remained within the magnetosphere. Combined observations from the THEMIS satellites and Super Dual Auroral Radar Network radars verified that magnetopause erosion was primarily caused by reconnection. While various factors may contribute to asymmetric erosion, the observations suggest that the weak reconnection rate on the duskside can play a role in the formation of asymmetric magnetopause shape. This discrepancy in reconnection rate is associated with the presence of cold dense plasma on the duskside of the magnetosphere, which limits the reconnection rate by mass loading, resulting in more efficient magnetopause erosion on the dawnside.

Plasma waves can initiate, regulate, or reflect magnetic reconnection efficiently converting magnetic energy into plasma energy. While waves ranging from below the ion cyclotron frequency to above the electron plasma frequency are commonly observed near reconnection sites, electromagnetic ion cyclotron (EMIC) waves—frequent in other plasma environments—have been rarely observed in the reconnection region. Here, we report the first detection of EMIC waves in a magnetic reconnection exhaust at Earth's magnetopause. The free energy required for EMIC wave growth was supplied by the strong perpendicular‐to‐parallel temperature anisotropy of hot proton beams. This proton temperature anisotropy was generated by magnetopause reconnection, rather than inherited from the magnetosheath. Our findings differ from previous reports of parallel‐preferential proton heating during magnetopause reconnection, calling for revised theoretical frameworks to reconcile observed perpendicular‐preferential heating with established reconnection paradigms.

On 24 April 2023, a Coronal Mass Ejection event caused the solar wind to become sub‐Alfvénic, leading to the development of an Alfvén Wing configuration in the Earth's magnetosphere. Alfvén Wings have previously been observed as cavities of low flow around moons in Jupiter's and Saturn's magnetospheres, but the observing spacecraft did not have the ability to directly measure the Alfvén Wings' current structures. Through in situ measurements made by the Magnetospheric Multiscale spacecraft, the 24 April event provides us with the first direct measurements of current structures during an Alfvén Wing configuration. These structures are observed to be significantly more anti‐field‐aligned and electron‐driven than the typical diamagnetic magnetopause current, indicating the disruption caused to the magnetosphere current system by the Alfvén Wing formation. The magnetopause current is then observed to recover more of its typical, perpendicular structure during the magnetosphere's recovery from the Alfvén Wing formation. Plain Language Summary The solar wind applies pressure on the Earth's magnetic field, distorting it from a dipole into its compressed dayside and stretched tail configuration. However, this typical structure can be disrupted by eruptive solar events such as Coronal Mass Ejections (CMEs), which may cause the solar wind's pressure to drop low enough that it is no longer able to push the magnetosphere back to form a single unified tail. When this occurs, the tail splits into two separate structures, called Alfvén Wings. While this configuration is rare at Earth, it is common from interactions of the outer planets' magnetosphere's with their moons, where Alfvén Wing configurations have been studied and modeled. However, because the observing spacecraft lacked the necessary instrumentation, we have not yet directly observed the Alfvén Wing current structures. On 24 April 2023, a CME event led to the creation of an Alfvén Wing formation in the Earth's magnetosphere. We observed this event using the Magnetospheric Multiscale spacecraft, which enabled us to make the first direct observations of Alfvén Wing current structures. These currents were found to be mainly parallel to the local magnetic field, in contrast to typical magnetopause currents. Key Points On 24 April 2023, the Magnetospheric Multiscale (MMS) spacecraft observed an Alfvén Wing formation along the dawn‐flank of Earth's magnetosphere MMS's observations represent the first in situ measurements of Alfvén Wing current structures The current structures are found to be primarily anti‐field‐aligned, electron‐driven, and filamentary

One of the major questions about magnetic reconnection is how specific solar wind and interplanetary magnetic field conditions influence where reconnection occurs at the Earth’s magnetopause. There are two reconnection scenarios discussed in the literature: a) anti-parallel reconnection and b) component reconnection. Early spacecraft observations were limited to the detection of accelerated ion beams in the magnetopause boundary layer to determine the general direction of the reconnection X-line location with respect to the spacecraft. An improved view of the reconnection location at the magnetopause evolved from ionospheric emissions observed by polar-orbiting imagers. These observations and the observations of accelerated ion beams revealed that both scenarios occur at the magnetopause. Improved methodology using the time-of-flight effect of precipitating ions in the cusp regions and the cutoff velocity of the precipitating and mirroring ion populations was used to pinpoint magnetopause reconnection locations for a wide range of solar wind conditions. The results from these methodologies have been used to construct an empirical reconnection X-line model known as the Maximum Magnetic Shear model. Since this model’s inception, several tests have confirmed its validity and have resulted in modifications to the model for certain solar wind conditions. This review article summarizes the observational evidence for the location of magnetic reconnection at the Earth’s magnetopause, emphasizing the properties and efficacy of the Maximum Magnetic Shear Model.

By analyzing continuous Magnetospheric Multiscale observations at the magnetopause boundary layer, combining both magnetohydrodynamic and kinetic signatures, we have successfully captured dynamic magnetic reconnection processes in exceptional detail. Our results demonstrate that magnetic reconnection exhibits rapid transitions between distinct operational modes, characterized by: (a) primary single X‐line reconnection punctuated by intermittent secondary reconnection, leading to large‐scale multiple X‐line formations; (b) stable single X‐line reconnection with oscillating X‐line positions; (c) rapid switching of reconnection X‐lines between opposite sides of the spacecraft; and (d) transient suppression occurring during otherwise steady reconnection periods. These observations provide definitive evidence for the inherently dynamic and intermittent behavior of magnetopause reconnection, revealing its capacity for swift configuration changes under varying conditions.

Magnetic reconnection is believed to be the main driver to transport solar wind into the Earth’s magnetosphere when the magnetopause features a large magnetic shear. However, even when the magnetic shear is too small for spontaneous reconnection, the Kelvin–Helmholtz instability driven by a super-Alfvénic velocity shear is expected to facilitate the transport. Although previous kinetic simulations have demonstrated that the non-linear vortex flows from the Kelvin–Helmholtz instability gives rise to vortex-induced reconnection and resulting plasma transport, the system sizes of these simulations were too small to allow the reconnection to evolve much beyond the electron scale as recently observed by the Magnetospheric Multiscale (MMS) spacecraft. Here, based on a large-scale kinetic simulation and its comparison with MMS observations, we show for the first time that ion-scale jets from vortex-induced reconnection rapidly decay through self-generated turbulence, leading to a mass transfer rate nearly one order higher than previous expectations for the Kelvin–Helmholtz instability. Vortex-induced reconnection originates from non-linear vortex flows due to Kelvin-Helmholtz instability in the Earth’s magnetosphere. Here, the authors perform a large-scale kinetic simulation to unveil dynamics of the vortex-induced reconnection and resulting turbulent mixing process.

Magnetic reconnection is a fundamental process known to play a crucial role in electron acceleration and heating, however, the mechanism of electron energization during reconnection is still not fully understood. This study introduces a novel electron acceleration mechanism in which electrons can be accelerated by secondary reconnection in the separatrix region. The secondary reconnection occurs in a thin current sheet resulted from the shear of the out‐of‐plane Hall magnetic fields of the primary magnetopause reconnection. It results in the intense electron energy fluxes toward the primary X‐line. This mechanism will likely be an important piece in the puzzle of particle acceleration by reconnection. Plain Language Summary Magnetic reconnection is a fundamental process that plays a critical role in electron acceleration and heating. The separatrix region of magnetic reconnection, which distinguishes the inflow and outflow regions, is a crucial area in the particle acceleration of the reconnection process. However, previous studies mainly focused on the electron acceleration mechanism in the separatrix region under two‐dimensional reconnection, without considering the influence of the three‐dimensional structures. This paper reports, for the first time, the secondary reconnection occurring in the opposite Hall magnetic field in the three‐dimensional separatrix region of magnetopause primary reconnection. This secondary reconnection can accelerate and heat the inflow electrons of the primary reconnection, which provides important clues on how the particles are accelerated and heated by reconnection. Key Points Secondary reconnection occurs between Hall magnetic fields in the separatrix region of the primary magnetopause reconnection Intense electron enthalpy fluxes are injected toward the primary X‐line along the separatrix Electrons are accelerated by the secondary reconnection before being injected into the primary X‐line for further energization

Uranus' magnetosphere presents a unique system to examine global magnetic reconnection processes due to its location far from the Sun. We assess reconnection effectiveness during Voyager 2's Uranus flyby, using a physics‐based analytical model to calculate reconnection voltages applied to the magnetopause. This assessment of reconnection conditions at Uranus uses real‐time varying solar wind conditions as opposed to steady conditions or parameter studies. Here we show that reconnection effectiveness varied considerably over the 2 weeks before Voyager 2 passed into the magnetosphere, with voltages fluctuating between ∼0 and 76 kV; peaking and then sharply dropping off shortly before Voyager 2's entry. Variability in the voltages was found to be strongly influenced by Uranus' extreme magnetic obliquity driving diurnal modulation. Furthermore, changes in solar wind conditions, in particular density and interplanetary magnetic field strength were found to drive additional variations which disrupts the expected ∼17‐hr periodicity of the voltages.

It is generally believed that variations in the upstream solar wind (SW) and interplanetary magnetic field (IMF) conditions are the main cause of changes of Jupiter's magnetopause (JM) location. However, most previous pressure balance models for the JM are axisymmetric and do not consider internal drivers, for example, the dynamics of the magnetodisc. We use three‐dimensional global magnetosphere simulations to investigate the variation of the JM under constant SW/IMF conditions. These simulations show that even without variations in the upstream driving conditions, the JM can exhibit dynamic variations, suggesting a range as large as 50 Jupiter radii in the subsolar location. Our study shows that the interchange structures in the Jovian magnetodisc will introduce significant radial dynamic pressure, which can drive significant variation in the JM location. The results provide important new context for interpreting the JM location and dynamics, with key implications for other internally mass‐loaded and/or rapidly rotating systems. Plain Language Summary The location of Jupiter's magnetopause is impacted by both external and internal conditions. In observations, the location of Jupiter's magnetopause has been found to change greatly and rapidly. This variation is generally believed to be caused by the significant changes in the external conditions. However, to‐date models and simulations have been mostly axisymmetric and static for the internal environment. Therefore, today, there is no interpretation that the internal mechanisms are sufficient to drive the drastic variation of Jupiter's magnetopause. In fact, because of the rapid rotation of Jupiter, as well as the particle outflow caused by volcanism on Jupiter's moon Io, the internal environment of Jupiter's magnetosphere is quite active. It is possible that the activity of the internal magnetospheric environment has a greater impact on Jupiter's magnetopause. We used three‐dimensional global simulations to investigate the variation of Jupiter's magnetopause under constant external conditions. The results show that even under constant external conditions, Jupiter's magnetopause will be highly impacted by internal conditions. Our study reveals the importance of the internal magnetospheric environment for the variation of Jupiter's magnetopause, even the whole magnetospheric system. Key Points The variation range of Jovian subsolar magnetopause standoff distance is very large even under constant solar wind conditions Interchange structures in the Jovian magnetodisc may affect the location of the magnetopause Significant radial dynamic pressure inside the Jovian magnetosphere may be the root cause of the variations of magnetopause

We analyzed the contribution of electromagnetic ion cyclotron (EMIC) wave driven electron loss to a flux dropout event in September 2017. The evolution of electron phase space density (PSD) through the dropout showed the formation of a radially peaked PSD profile as electrons were lost at high L*, resembling distributions created by magnetopause shadowing. By comparing 2D Fokker Planck simulations of pitch angle diffusion to the observed change in PSD, we found that the μ and K of electron loss aligned with maximum scattering rates at dropout onset. We conclude that, during this dropout event, EMIC waves produced substantial electron loss. Because pitch angle diffusion occurred on closed drift paths near the last closed drift shell, no radial PSD minimum was observed. Therefore, the radial PSD gradients resembled solely magnetopause shadowing loss, even though the local pitch angle scattering produced electron losses of several orders of magnitude of the PSD. Plain Language Summary Extremely energetic charged particles become trapped by Earth's geomagnetic field, forming the Van Allen radiation belts. The total amount of radiation trapped within these belts varies depending on the solar wind conditions, which can disturb the geomagnetic field to produce geomagnetic storms. At the beginning of a geomagnetic storm, there is a relative calm in the radiation belt, produced by the rapid drainage of electrons from the geomagnetic field. It is not fully understood if these electrons are primarily lost into the solar wind, or if they are lost into Earth's atmosphere. In this study, we analyze the remaining trapped electrons to reconstruct the mechanisms of electron escape at the beginning of a geomagnetic storm in September 2017. While previous work found that electrons were primarily lost into the solar wind, we found that loss into the atmosphere also played an important role. Furthermore, we showed that drainage of electrons into the atmosphere can be mistaken for loss into the solar wind if the energy and trajectory of lost electrons are not carefully considered. Key Points Characterizing electron loss through peaks and minima in radial phase space density can misrepresent simultaneous loss mechanisms Analysis of electron loss across all adiabatic invariants μ, K, and L*, is necessary to correctly identify loss mechanisms Observational analysis of phase space density data alone cannot be used to quantify individual contributions of simultaneous loss processes