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1. The Enhancement of Ion Heating in Kinetic, Antiparallel Reconnection in the Presence of a Flow Shear

   https://doi.org/10.3847/1538-4357/ae2014  

   Haggerty, Colby C; Sikorski, Derek; Shay, Michael A; Phan, Tai D; Cassak, Paul A; Murtas, Giulia; Pyakurel, Prayash S (December 2025, The Astrophysical Journal)

   Abstract We investigate the kinetic effects of upstream, magnetic field-aligned flow shear on antiparallel magnetic reconnection using 2.5D particle-in-cell simulations. Our results demonstrate that flow shear significantly alters the reconnection process, leading to enhanced ion heating, reduced outflow speeds, and a modified reconnection geometry. In contrast to previous Hall magnetohydrodynamic studies, we find that reconnection becomes a more efficient plasma heating mechanism in the presence of sub-Alfvénic flow shear, with ion heating increasing by as much as 300%. This enhanced heating is achieved by efficiently converting the incoming flow shear energy into thermal energy through isotropization in the exhaust. The enhanced heating leads to a pressure gradient away form thex-line exerting a force that reduces the outflow jet speed and slows down the reconnection process. This conversion is due to beam selection effects, mixing, and scattering in the exhaust. A theoretical model is developed that predicts well the exhaust heating and outflow speed reduction. These results offer a potential explanation for recent Parker Solar Probe observations of suppressed reconnection in the presence of flow shear and carry significant implications for energy dissipation in turbulent plasmas. 

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2. Characterizing velocity–space signatures of electron energization in large-guide-field collisionless magnetic reconnection

   https://doi.org/10.1063/5.0082213  

   McCubbin, Andrew J.; Howes, Gregory G.; TenBarge, Jason M. (May 2022, Physics of Plasmas)

   Magnetic reconnection plays an important role in the release of magnetic energy and consequent energization of particles in collisionless plasmas. Energy transfer in collisionless magnetic reconnection is inherently a two-step process: reversible, collisionless energization of particles by the electric field, followed by collisional thermalization of that energy, leading to irreversible plasma heating. Gyrokinetic numerical simulations are used to explore the first step of electron energization, and we generate the first examples of field–particle correlation signatures of electron energization in 2D strong-guide-field collisionless magnetic reconnection. We determine these velocity space signatures at the x-point and in the exhaust, the regions of the reconnection geometry in which the electron energization primarily occurs. Modeling of these velocity–space signatures shows that, in the strong-guide-field limit, the energization of electrons occurs through bulk acceleration of the out-of-plane electron flow by the parallel electric field that drives the reconnection, a non-resonant mechanism of energization. We explore the variation of these velocity–space signatures over the plasma beta range 0.01≤βi≤1. Our analysis goes beyond the fluid picture of the plasma dynamics and exploits the kinetic features of electron energization in the exhaust region to propose a single-point diagnostic, which can potentially identify a reconnection exhaust region using spacecraft observations. 

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3. Characterizing Velocity-Space Signatures of Electron Energization in Large-Guide-Field Collisionless Magnetic Reconnection

   McCubbin, Andrew J.; Howes, Gregory G.; TenBarge, Jason M. (December 2021, ArXivorg)

   Magnetic reconnection plays an important role in the release of magnetic energy and consequent energization of particles in collisionless plasmas. Energy transfer in collisionless magnetic reconnection is inherently a two-step process: reversible, collisionless energization of particles by the electric field, followed by collisional thermalization of that energy, leading to irreversible plasma heating. Gyrokinetic numerical simulations are used to explore the first step of electron energization, and we generate the first examples of field-particle correlation (FPC) signatures of electron energization in 2D strong-guide-field collisionless magnetic reconnection. We determine these velocity space signatures at the x-point and in the exhaust, the regions of the reconnection geometry in which the electron energization primarily occurs. Modeling of these velocity-space signatures shows that, in the strong-guide-field limit, the energization of electrons occurs through bulk acceleration of the out-of-plane electron flow by parallel electric field that drives the reconnection, a non-resonant mechanism of energization. We explore the variation of these velocity-space signatures over the plasma beta range 0.01 < beta_i < 1. Our analysis goes beyond the fluid picture of the plasma dynamics and exploits the kinetic features of electron energization in the exhaust region to propose a single-point diagnostic which can potentially identify a reconnection exhaust region using spacecraft observations. 

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4. Ohm’s Law, the Reconnection Rate, and Energy Conversion in Collisionless Magnetic Reconnection

   https://doi.org/10.1007/s11214-025-01142-0  

   Liu, Yi-Hsin; Hesse, Michael; Genestreti, Kevin; Nakamura, Rumi; Burch, James L; Cassak, Paul A; Bessho, Naoki; Eastwood, Jonathan P; Phan, Tai; Swisdak, Marc; et al (February 2025, Space Science Reviews)

   Magnetic reconnection is a ubiquitous plasma process that transforms magnetic energy into particle energy during eruptive events throughout the universe. Reconnection not only converts energy during solar flares and geomagnetic substorms that drive space weather near Earth, but it may also play critical roles in the high energy emissions from the magnetospheres of neutron stars and black holes. In this review article, we focus on collisionless plasmas that are most relevant to reconnection in many space and astrophysical plasmas. Guided by first-principles kinetic simulations and spaceborne in-situ observations, we highlight the most recent progress in understanding this fundamental plasma process. We start by discussing the non-ideal electric field in the generalized Ohm’s law that breaks the frozen-in flux condition in ideal magnetohydrodynamics and allows magnetic reconnection to occur. We point out that this same reconnection electric field also plays an important role in sustaining the current and pressure in the current sheet and then discuss the determination of its magnitude (i.e., the reconnection rate), based on force balance and energy conservation. This approach to determining the reconnection rate is applied to kinetic current sheets with a wide variety of magnetic geometries, parameters, and background conditions. We also briefly review the key diagnostics and modeling of energy conversion around the reconnection diffusion region, seeking insights from recently developed theories. Finally, future prospects and open questions are discussed. 

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5. Transition from ion-coupled to electron-only reconnection: Basic physics and implications for plasma turbulence

   Prayash Sharma Pyakurel, Michael Shay (January 2019, Physics of plasmas)

   Using 2.5 dimensional kinetic particle-in-cell (PIC) simulations, we simulate reconnection conditions appropriate for the magnetosheath and solar wind, i.e., plasma beta (ratio of gas pressure to magnetic pressure) greater than 1 and low magnetic shear (strong guide field). Changing the simulation domain size, we find that the ion response varies greatly. For reconnecting regions with scales comparable to the ion inertial length, the ions do not respond to the reconnection dynamics leading to “electron-only” reconnection with very large quasi-steady reconnection rates. Note that in these simulations the ion Larmor radius is comparable to the ion inertial length. The transition to more traditional “ion-coupled” reconnection is gradual as the reconnection domain size increases, with the ions becoming frozen-in in the exhaust when the magnetic island width in the normal direction reaches many ion inertial lengths. During this transition, the quasi-steady reconnection rate decreases until the ions are fully coupled, ultimately reaching an asymptotic value. The scaling of the ion outflow velocity with exhaust width during this electron-only to ion-coupled transition is found to be consistent with a theoretical model of a newly reconnected field line. In order to have a fully frozen-in ion exhaust with ion flows comparable to the reconnection Alfven speed, an exhaust width of at least several ion inertial lengths is needed. In turbulent systems with reconnection occurring between magnetic bubbles associated with fluctuations, using geometric arguments we estimate that fully ion-coupled reconnection requires magnetic bubble length scales of at least several tens of ion inertial lengths. 

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