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When the solar wind speed falls below the local Alfvén speed, the magnetotail transforms into an Alfvén wing configuration. A Grid Agnostic Magnetohydrodynamics for Extended Research Applications (GAMERA) simulation of Earth's magnetosphere using solar wind parameters from the 24 April 2023 sub‐Alfvénic interval is examined to reveal modifications of Dungey‐type magnetotail reconnection during sustained sub‐Alfvénic solar wind. The simulation shows new magnetospheric flux is generated via reconnection between polar cap field lines from the northern and southern hemisphere, similar to Dungey‐type magnetotail reconnection between lobe field lines mapping to opposite hemispheres. The key feature setting the Alfvén wing reconnection apart from the typical Dungey‐type is that the majority of new magnetospheric flux is added to the polar cap at local times 1–3 (21‐23) in the northern (southern) hemisphere. During most of the sub‐Alfvénic interval, reconnection mapping to midnight in the polar cap generates relatively little new magnetospheric flux. Plain Language Summary: Similar to how a shock wave forms around a supersonic plane, the supersonic plasma emanating from the sun forms a shock wave around Earth. However, the speed of sound through the plasma depends on different parameters that vary substantially based on the origin and evolution of solar material flowing into interplanetary space. In some coronal mass ejections, the characteristics of the plasma are such that the flow is sub‐sonic, leaving the magnetosphere in a unique state. Determining whether there are any space weather impacts associated with the sub‐sonic flow has been difficult due to lack of observations, but a recent event has ignited interest. This study examines the global structure and dynamics of the magnetosphere in a simulation representative of the sub‐sonic flow interval of the April 2023 geomagnetic storm. Key Points: On 24 April 2023, Earth's magnetosphere experienced an interval of sustained sub‐Alfvénic solar wind drivingSub‐Alfvénic driving suppresses typical Dungey‐type magnetotail reconnection but polar cap expansion is still limitedGlobal simulations have strong Earthward flows localized ∼10 RE tailward of theterminator, where most new magnetospheric flux is generated [ABSTRACT FROM AUTHOR]

This article presents a statistical analysis of overlapping double ion-energy dispersion events in the northern cusp ("double dispersion"). Double dispersion in either cusp is a phenomenon associated with multiple reconnections occurring on the dayside magnetosphere as a result of its constant interaction with the variable solar wind. Using observations from a low Earth orbiting (LEO) Defense Meteorological Satellite Program (DMSP) satellite, we analyze 138 dayside events selected by the automatic algorithm extended from our previous work. We conducted a correlation study between the number of detected double dispersion events and 1) the month of the year to analyze the seasonal response of the cusp, and 2) solar wind interplanetary magnetic field (IMF) components and clock/cone angles to investigate its relationship with magnetic reconnection. We found that dispersion events occur more frequently during the northern summer months (i.e., when the dipole is tilted Sunward) and when the By component of IMF is positive. In addition, we provide a machine-readable list of the events and the code used to automatically detect the events. [ABSTRACT FROM AUTHOR]

Magnetic reconnection has been observed in the transition region of quasi‐parallel shocks. In this work, the particle‐in‐cell method is used to simulate three‐dimensional reconnection in a quasi‐parallel shock. The shock transition region is turbulent, leading to the formation of reconnecting current sheets with various orientations. Two reconnection sites with weak and strong guide fields are studied, and it is shown that reconnection is fast and transient. Reconnection sites are characterized using diagnostics including electron flows and magnetic flux transport. In contrast to two‐dimensional simulations, weak guide field reconnection is realized. Furthermore, the current sheets in these events form in a direction almost perpendicular to those found in two‐dimensional simulations, where the reconnection geometry is constrained. Plain Language Summary: Quasi‐parallel shocks are regions where there is a large angle between the shock surface and the upstream magnetic field. Particles reflected from the shock move upstream and excite waves, creating a turbulent environment. This is favorable for the generation of current sheets and a process called magnetic reconnection, in which magnetic energy is converted to kinetic energy with a change in field topology. We use simulations to study reconnection, characterizing reconnection sites and providing comparisons to other simulations and observations. Key Points: Turbulent reconnection events in a 3D quasi‐parallel shock simulation are characterizedBoth strong and weak guide field events are found, consistent with the range of values seen in observationsReconnection sites have different 3D orientations not captured by 2D simulations [ABSTRACT FROM AUTHOR]

The present paper demonstrates the first observations by the Magnetospheric Multiscale (MMS) mission of the counter‐streaming energetic electrons and trapped energetic protons, localized in the magnetic field depressions between the mirror mode peaks, in the Earth's dusk sector high‐latitude magnetosphere. This region is characterized by high plasma beta, strong ion temperature anisotropy, and intermediate plasma density between magnetospheric and magnetosheath plasma. We show that these plasma conditions are unstable for the drift mirror instability. The counter‐streaming electron feature resembles those of the previously reported energetic electron microinjections, but without the energy‐time dispersion signature. This suggests that MMS is passing through one of the potential microinjection source regions. The energetic ion data in the present study is mainly used to estimate the scale size of the mirror mode structures. Plain Language Summary: Understanding the physical mechanisms that result in energetic electron acceleration and loss within the Van Allen radiation belts has been an active area of research for decades, and due to advances made possible by the Van Allen probe mission are now relatively well understood. However, the origin of the several 10–100s of keV seed population that can be accelerated to relativistic energies has remained more elusive. It is well known that magnetic reconnection and related secondary processes in the Earth's magnetotail during substorms can accelerate particles and inject them inward toward the radiation belts. In this paper we show the first observations of a possible source region of 10–100s of keV electrons and protons at the dayside of the Earth's high‐latitude magnetosphere. Four MMS spacecraft periodically encountered high fluxes of energetic electrons at wide energy range which were streaming both parallel and anti‐parallel to the magnetic field. Enhanced fluxes of counter‐streaming energetic electrons and trapped protons were observed between magnetic field peaks of the ULF waves identified as mirror mode peaks. The source region of these electrons and protons are likely the large diamagnetic cavities created by magnetic reconnection. Key Points: MMS detected dispersionless electron microinjections in ULF magnetic field wave depressions at the high‐latitude magnetosphereULF waves were consistent with mirror mode waves created by the drift mirror instability [ABSTRACT FROM AUTHOR]

Plasma transport in the rapidly rotating giant magnetospheres is thought to involve a centrifugally driven flux tube interchange instability, similar to the Rayleigh‐Taylor (RT) instability. In three dimensions, the convective flow patterns associated with the RT instability can produce strong guide field reconnection, allowing plasma mass to move radially outward while conserving magnetic flux (Ma et al., 2016, https://doi.org/10.1002/2015JA022122). We present a set of hybrid (kinetic ion/fluid electron) plasma simulations of the RT instability using high plasma beta conditions appropriate for the inner and middle magnetosphere at Jupiter and Saturn. A density gradient, combined with a centrifugal force, provide appropriate RT onset conditions. Pressure balance requires only a temperature gradient as the magnetic pressure is constant. Pressure balance is achieved with a temperature gradient in a fixed magnetic field. The three‐dimensional simulation domain represents a local volume of the magnetodisc resonant cavity. Simulated RT growth rates compare favorably with linear theory, where the fundamental mode of the resonant cavity determines the largest (stabilizing) parallel wavelength. We suggest that the perpendicular scale of RT structures is determined by the fundamental mode, which limits growth due to magnetic tension. Finally, we investigated strong guide field magnetic reconnection and diffusive processes as plausible mechanisms to facilitate kinetic‐scale radial transport. Key Points: Hybrid simulations of the Rayleigh‐Taylor instability show magnetic field fluctuations similar to those observed at SaturnThe Rayleigh‐Taylor fingers include diffusive transport, patchy reconnection sites, and kinetic‐scale structures due to an inverse cascadeThe perpendicular scale of the Rayleigh‐Taylor fingers are limited by the parallel wavelengths in the magnetodisc resonant cavity [ABSTRACT FROM AUTHOR]