Context. Relativistic magnetic reconnection studies have so far focused on symmetric configurations where the upstream plasma has identical properties on the two sides of the layer. Yet just like nonrelativistic reconnection on the dayside of the Earths magnetosphere relativistic reconnection can also operate at the interface between highly asymmetric environments. The boundary layer between a relativistic jet and an accretion flow forming around a supermassive black hole can present asymmetric configurations in terms of plasma composition bulk velocity temperature and magnetization. Aims. We conducted the first study of relativistic magnetic reconnection where the upstream plasma is composed of electronpositron pairs on one side and electrons and ions on the other. We also investigated the impact of a relativistic symmetric shear flow applied along the reconnecting field lines. Methods. We simulated magnetic reconnection using 2D particleincell simulations. The initial setup was adapted from a classic Harris layer without a guide field modified to accommodate plasmacomposition and shear asymmetries in the upstream medium. Results. For a compositionasymmetric setup we find that the reconnection dynamics is driven by the electronion side which is the plasma with the lowest magnetization. The energy partition favors accelerating ions at the expense of electrons even more than in a corresponding symmetric setup. With respect to shear a superAlfvnic upstream decreases the laboratoryframe reconnection rate but unlike in nonrelativistic studies does not shut off reconnection completely. Conclusions. The asymmetries examined in this work lower the overall efficiency of electron acceleration relative to corresponding symmetric configurations. In the context of a black hole jetdisk boundary asymmetric reconnection alone is probably not efficient at accelerating electrons to very high energies but it might facilitate plasma mixing and particle injection for other acceleration channels at the interface.
['acceleration of particles', 'magnetic reconnection', 'plasmas', 'methods: numerical', 'Astrophysics - High Energy Astrophysical Phenomena', 'Physics - Plasma Physics']
Compositionasymmetric and sheared relativistic magnetic reconnection
{'Composition-asymmetric and sheared relativistic magnetic reconnection': 'Enzo Figueiredo 1 , Benoît Cerutti 1 , John Mehlha ff 1 , and Nicolas Scepi 1  \nUniv. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France e-mail: enzo.figueiredo@univ-grenoble-alpes.fr  \nReceived September 23, 2024; accepted ...', 'ABSTRACT': "Context. Relativistic magnetic reconnection studies have focused on symmetric configurations so far, where the upstream plasma has identical properties on each side of the layer. Yet, just like nonrelativistic reconnection on the day side of the Earth's magnetosphere, relativistic reconnection may also operate at the interface between highly asymmetric environments. The boundary layer between a relativistic jet and an accretion flow forming around a supermassive black hole may present such asymmetric configurations in terms of plasma composition, bulk velocity, temperature and magnetization.  \nAims. In this work, we aim to conduct the first study of relativistic magnetic reconnection where the upstream plasma is composed of electron-positron pairs on one side, and electrons and ions on the other. We also investigate the role of a relativistic symmetric shear flow applied along the reconnecting field lines.  \nMethods. We simulate magnetic reconnection using two-dimensional particle-in-cell simulations. The initial setup is adapted from a classic Harris layer without guide field, modified to accommodate plasma-composition and shear asymmetries in the upstream medium.  \nResults. For a composition-asymmetric setup, we find that the reconnection dynamics is driven by the electron-ion side, which is the plasma with the lowest magnetization. The energy partition favors accelerating ions at the expense of electrons even more than in a corresponding symmetric setup. With respect to shear, a super-Alfvénic upstream decreases the laboratory-frame reconnection rate, but, unlike in non-relativistic studies, does not shut o ff reconnection completely.  \nConclusions. The asymmetries examined in this work diminish the overall e ffi ciency of electron acceleration relative to corresponding symmetric configurations. In the context of a black hole jet-disk boundary, asymmetric reconnection alone is probably not e ffi cient at accelerating electrons to very high energies, but it might facilitate plasma mixing and particle injection for other acceleration channels at the interface.  \nKey words. acceleration of particles - plasmas - magnetic reconnection - methods: numerical", '1. Introduction': "Magnetic reconnection is a widely studied plasma phenomenon, heating and accelerating particles through a change in the global magnetic topology (Zweibel & Yamada 2009). This process typically occurs when two antiparallel magnetic field lines snap and recombine across a thin current layer. The relativistic regime of reconnection has encountered wider interest in the last decade, as it provides e ffi cient nonthermal particle acceleration (e.g., Hoshino et al. 2001; Hoshino & Lyubarsky 2012; Cerutti et al. 2012b; Sironi & Spitkovsky 2014; Guo et al. 2014; Kagan et al. 2015; Werner et al. 2016). Here, in contrast to non-relativistic reconnection, the energy associated with the magnetic field dominates over the plasma rest mass energy, providing a significant free-energy reservoir for particle energization. Several regimes of relativistic magnetic reconnection have now been studied, including: with di ff erent plasma compositions (i.e., electronpositron pairs, and electron-ion plasmas, Melzani et al. 2014; Werner et al. 2018); involving strong radiative cooling and pair production (Jaroschek & Hoshino 2009; Cerutti et al. 2013; Hakobyan et al. 2019; Schoe ffl er et al. 2019; Mehlha ff et al. 2024); and driven by other plasma processes such as turbulence (Zhdankin et al. 2017; Comisso & Sironi 2018; Meringolo et al. 2023) and Kelvin-Helmholtz or Rayleigh-Taylor instabili-  \nties (Cerutti & Giacinti 2020; Sironi et al. 2021; Zhdankin et al. 2023).  \nRelativistic magnetic reconnection is a key driver of highenergy emission in various astrophysical environments. In pulsar magnetospheres, the equatorial current sheet forming beyond the light cylinder undergoes reconnection in a strongly radiative and pair-producing regime, emitting gamma-ray emission through synchrotron radiation (Lyubarskii 1996; Uzdensky & Spitkovsky 2014; Cerutti et al. 2016). Similar processes may also power gamma-ray flares in the Crab nebula (Uzdensky et al. 2011; Cerutti et al. 2012a), magnetar eruptions and fast radio bursts (Lyutikov 2003; Yuan et al. 2020; Mahlmann et al. 2022). In active galactic nuclei, magnetic reconnection is considered a plausible mechanism for explaining gamma-ray flares within blazar jets (Giannios et al. 2009; Nalewajko et al. 2011; Petropoulou et al. 2016; Mehlha ff et al. 2020) and also bursting activity closer to the central supermassive black hole, for instance X-ray flares in SgrA* (Ball et al. 2016; Scepi et al. 2022) and gamma-ray flares in M87* (Crinquand et al. 2021; Ripperda et al. 2022; Hakobyan et al. 2023; Stathopoulos et al. 2024).  \nRecent VLBI observations of M87* show a clear edgebrightened jet connected to the innermost parts of the accretion flow (Lu et al. 2023), suggesting that particles are accelerated at the interface between these two entities. In the framework of  \na Poynting-flux dominated jet (Blandford & Znajek 1977), this boundary layer may separate a relativistic ultra-magnetized jet loaded with pairs produced near the ergosphere on one side from a hot mildly magnetized accretion flow composed of electrons and ions on the other side. Such abrupt discontinuities in plasma composition, temperature, bulk velocity and magnetization may lead to qualitatively di ff erent particle acceleration compared to a symmetric environment.  \nRecent global modeling e ff orts suggest magnetic reconnection as a prominent particle acceleration mechanism at such highly asymmetric jet-disk boundaries. The accretion of magnetic loops onto a spinning black hole leads to the formation of reconnecting current sheets between the jet and the disk (Parfrey et al. 2015; Yuan et al. 2019; El Mellah et al. 2022, 2023). Magnetic flux eruptions reported in general relativistic magnetohydrodynamic (GRMHD) simulations also provoke large-scale reconnection events that push magnetized flux tubes through the accretion flow (e.g., see Igumenshchev 2008; Tchekhovskoy et al. 2011). This phenomenon can be accompanied with additional current sheet formation at the interface between the jet and the disk (Ripperda et al. 2020, 2022; Chashkina et al. 2021; Vos et al. 2023). However, it is unclear how magnetic reconnection and particle acceleration proceed within such asymmetric boundary layers.  \nAsymmetric and sheared magnetic reconnection have been previously studied in the non-relativistic regime, in particular in the context of the interaction between the Earth's magnetosphere and the solar wind (i.e., the magnetopause). An important takeaway message from these studies is that a strong velocity shear applied along the field lines decreases the reconnection rate. It can even halt the process entirely for super-Alfvénic flows (La Belle-Hamer et al. 1995; Cassak & Otto 2011; Cassak 2011). An asymmetry in the upstream plasma density and magnetic field strength leads to a global reorganization of the current layer and a change in the plasmoids' shape (Murphy et al. 2012; Eastwood et al. 2013). However, from these studies alone it is unclear how such asymmetries would a ff ect particle acceleration in a relativistic context.  \nThe first study of relativistic asymmetric magnetic reconnection was recently carried out by Mbarek et al. (2022). They simulate a pair plasma with an upstream density and magnetic field strength contrast on each side of the reconnection layer, hence creating a magnetization asymmetry in the reconnection process. They find that the lowest magnetization side dictates the reconnection rate, and that particle acceleration remains in an intermediate regime between the higher and lower magnetizations. This result suggests that particle acceleration via reconnection could be quenched at a black hole jet-disk interface.  \nIn this study, our first objective is to investigate the role of plasma asymmetries on particle acceleration and mixing within relativistic reconnection layers, using two-dimensional (2D) particle-in-cell (PIC) simulations. More specifically, and in contrast to Mbarek et al. (2022), we investigate the role of a plasma-composition asymmetry where one side is filled with an ultra-relativistic ultra-magnetized plasma of pairs, while the other side is composed of a mildly relativistic and magnetized electron-ion plasma. This configuration is reminiscent of a black hole jet-disk boundary layer. Our second objective is to evaluate the impact of a relativistic shear flow directed along the field lines - another property reminiscent of the jet-disk boundary - on the reconnection rate and on the particle acceleration e ffi ciency.  \nThis article is organized as follows. In Section 2, we detail the modifications made to the classic relativistic Harris setup to  \nimplement a velocity shear and a composition asymmetry in the upstream medium. In sections 3 and 4, we respectively present our findings on composition asymmetry and shear. We summarize and discuss the implications of our results in Section 5.", '2.1. Asymmetric Harris equilibrium': 'Fig. 1. Sketch of the simulation setup. The red dashed line separates the pair plasma on the jet side (upper half) from the electron-ion plasma on the disk side (bottom half). Shear flow is parallel to the magnetic field on each side. \'BC\' stands for \'boundary conditions\'.  \n<!-- image -->  \nIn this work, we employ Zeltron , an electromagnetic PIC code developed by Cerutti et al. (2013); Cerutti & Werner (2019). Our aim is to simulate a current layer that would have been locally formed by instabilities on a global jet-disk interface using a 2D local Cartesian box. In our simulations, the initial plasma is composed of pairs above the reconnection layer, representing the \'jet side\', while the plasma below is composed of electrons and ions, representing the \'disk side\'. As shown by GRMHD simulations (e.g., Vos et al. 2023), the magnetic field strength is expected to be similar on each side of the interface. The magnetization contrast should then be only imposed by the composition and / or density asymmetry. We therefore impose the same initial magnetic field strength on both sides of the reconnection layer. This is another di ff erence (in addition to the plasma composition and shear) from the setup used by Mbarek et al. (2022).  \nOur boundary conditions are reflective on the upper and lower sides of the box and periodic on the right and left sides. These conditions apply for both the fields and the particles. For the initial conditions, we adapt the classic relativistic Harris setup with no guide (out-of-plane) field component, which involves an initial antiparallel magnetic field supported by an equatorial current layer (e.g., Kirk & Skjæraasen 2003). This initial current sheet is razor thin (on the plasma skin depth scale, defined below): perhaps much thinner than in a real jet-disk interface. This merely helps kickstart reconnection, allowing us to study the nonlinear evolution of the current sheet in its more developed and e ff ectively thicker stage.  \nThe initial Harris magnetic field profile is given by  \nB ( y ) = B 0 tanh y -Ly / 2 δ ! e x , (1)  \nwhere δ is the initial current layer thickness, Ly ( Lx ) is the height (width) of the box, and e x is the unit vector along the x -direction. This magnetic field is supported by a layer of positively and negatively charged particles counterstreaming, or drifting , in the out-of-plane (i.e., z -) direction. Their bulk drift velocity is set to VH / c = 0 . 6, giving the current density,  \nJ = -2 eVHnH ( y ) e z , (2)  \nwhere e is the positron charge. The drifting particle number density reads  \nnH ( y ) = nH , 0 cosh -2 y -Ly / 2 δ ! , (3)  \nwhere nH , 0 = cB 0 / 8 π eVH δ . In symmetric reconnection studies, the box is filled with a uniform background plasma. However, in our scenario, the characteristics of the background plasma vary depending on the vertical position in the box. In the upper half of the box ( y > Ly / 2, i.e., the jet side), the background plasma is composed exclusively of pairs of total mass density 2 menj , while in the bottom half ( y < Ly / 2, i.e., the disk side), it is composed of electrons and ions of total mass density ( me + mi ) nd = (1 + µ ) mend , where µ = mi / me is the ion-to-electron mass ratio. On the jet (disk) side, the plasma moves along field lines with bulk velocity V s = Vs e x ( -Vs e x ) with a corresponding Lorentz factor Γ s = (1 -V 2 s / c 2 ) -1 / 2 : a symmetric shear with an abrupt transition at the midplane.  \nIn order for the simulation to start in equilibrium, we need to ensure pressure balance both between the current sheet and the upstream magnetic field, and between the jet and the disk. This requires  \nB 2 0 8 π = 2 nH , 0 kBT ∗ H Γ H and (4)  \nnjkBTj = ndkBTd , (5)  \nwhere Tj and Td represent the respective initial temperatures of the jet and disk plasmas in the laboratory frame, kB is the Boltzmann constant and T ∗ H is the drifting particles\' comoving temperature (considering a Lorentz factor Γ H = (1 -V 2 H / c 2 ) -1 / 2 ). We note that the plasmaβ parameter, defined as β = 16 π nkBT / B 2 0 , has an identical value for both the jet and disk sides (see Mbarek et al. (2022) for a β asymmetric setup), but the magnetization in the laboratory frame, defined as σ j = B 2 0 / 8 π Γ snjmec 2 for the jet and σ d = B 2 0 / 4 π Γ snd ( me + mi ) c 2 for the disk, changes because of the composition contrast. A global sketch of the initial setup is provided in Fig. 1.  \nWe need the initial particle energy distribution functions in order to impose the previously defined macroscopic quantities. They follow relativistic drifting Maxwellians as defined by Swisdak (2013). Therefore, we have to initialize two distribution functions for the current layer (pairs and ions), two for the disk side (electrons and ions) and one for the jet side (pairs). We define four species of particles: the positrons (in the jet region), the ions (of charge + e in the disk region) and the electrons in each region. Their respective density profiles n e , p , i d , j are  \nn e j ( y ) = njH ( y -Ly / 2) + f nH ( y ) , n e d ( y ) = ndH ( Ly / 2 -y ) + (1 -f ) nH ( y ) , n p j ( y ) = njH ( y -Ly / 2) + f nH ( y ) , n i d ( y ) = ndH ( Ly / 2 -y ) + (1 -f ) nH ( y ) ,  \nwhere H is the Heaviside step function, and the parameter f represents the mixing fraction in the Harris sheet. For f = 0 ( f = 1), the initial current layer is exclusively composed of disk (jet) plasma. This parameter only sets the initial state of our setup, and we have checked that it does not a ff ect the simulation behavior. Thus, we set its value to f = 1 / 2. When defining the particles\' momenta, we need to decide whether we consider them to be part of the Harris sheet, in which case we assign them a drift velocity ± VH e z , or in the disk / jet, in which case we assign them a shear bulk motion ± Vs e x . For a particle at a given coordinate y , the following number is computed:  \nP ( y ) = nd , j H ( ± y ∓ Ly / 2) n e , p , i d , j ( y ) . (6)  \nSubsequently, a random number between 0 and 1 is drawn. If it exceeds P ( y ), then the particle is considered to follow the Harris layer energy distribution function; otherwise, it will have a background energy distribution function. It can be easily verified that, far from the current sheet, P ( y ) → 1 so that the particle will almost always follow a background distribution. Conversely, in the current sheet it will likely follow a Harris distribution.  \nFollowing Werner et al. (2018), a small (one percent) tearinglike perturbation is applied to the initial magnetic field (Eq. 1). This procedure serves two purposes: (i) it speeds up the onset of reconnection, and (ii) it predefines the location of the main X- and O-points during the active phase of the simulation and facilitates the measurement of the reconnection rate as shown in Sect. 2.3 hereafter. With this perturbation, the initial out-of-plane component of the magnetic vector potential reads  \nAz = B 0 δ ( ln " cosh y -Ly / 2 δ !# -ln " cosh Ly 2 δ !#) × ( 1 + 0 . 01 cos 2 π x -3 Lx / 4 Lx ! cos 2 π y -Ly / 2 Ly !) . (7)  \nThe dimensions of the cells in each direction, ∆ x and ∆ y , are set to be equal and the time step, ∆ t , is set at 0 . 99 times the critical Courant-Friedrichs-Lewy time step. We initialize the simulations with 16 particles per species per cell.  \nIn summary, the setup allows for the manipulation of several free parameters, including plasma β , the disk magnetization σ d , the jet-disk density ratio nj / nd , and the shear flow velocity Vs . For the remainder of this study, we set nj = nd for numerical convenience, thus only imposing a magnetization asymmetry through the mass ratio between pairs and ions. This assumption may not be very realistic in the context of a jet-disk boundary layer where a strong number density contrast is expected, but it still captures a mass density contrast, 2 njme ≪ nd ( mi + me ).', '2.2. Sets of simulations': "Given our setup, this work consists of two series of simulations. In the first, we study the e ff ect of composition asymmetry on magnetic reconnection without velocity shear (i.e., setting Vs = 0). This asymmetry leads to a magnetization contrast since, for µ ≫ 1, the jet magnetization is σ j ≃ µσ d / 2. We set µ = 100 so that the magnetization ratio is σ j /σ d ≃ 50, and we explore the parameter space ( β, σ d ) in the 9 simulations listed in Table 1. We adopt plasma conditions previously studied by Ball et al. (2018) and similar to those observed at jet-disk interfaces in GRMHD simulations, where the ion magnetization is transrelativistic ( σ d ≲ 1) while that for pairs is ultrarelativistic ( σ j ≫ 1). In addition to our 9 asymmetric simulations,  \n√  \nTable 1. Full list of simulations performed in this study. Top: Asymmetric setup exploring the ( β, σ d ) parameter space. Middle: Corresponding symmetric electron-ion reconnection simulations included for comparison with the asymmetric runs. Bottom: Shear simulation parameters.  \nwe present 2 symmetric electron-ion simulations for comparison where the jet's plasma has the same properties as the disk's. The resolution is fixed in terms of the nominal electron Larmor radius ρ 0 = mec 2 / eB 0 = 8 ∆ x / 3. Thus, the skin-depth ranges from de = c /ω pe ∼ 8 ∆ x to de ∼ 27 ∆ x , where ω pe = p 4 π nde 2 / me is the electron plasma frequency. The initial current sheet width is set to δ ≃ 5 ∆ x .  \nThe second set of simulations focuses on the e ff ect of a velocity shear along the reconnection layer with a symmetric pair plasma ( µ = 1). Here, we fix β = 5 × 10 -2 and the magnetization, σ d , is set to either 1 or 10. For both values of σ d , we scan the dimensionless shear four-velocity, Us = Γ sVs / c , from 0 to 2 UA , where UA = √ σ d is the relativistic Alfvén four-velocity. The resolution is set such that Γ s ρ 0 = 16 ∆ x for σ d = 1, and Γ s ρ 0 = 2 ∆ x for σ d = 10. This ensures that the hot skin depth de , h = √ Γ sde is conserved for all simulations sharing the same σ d value: de , h ∼ 6 ∆ x ( σ d = 10), de , h ∼ 16 ∆ x ( σ d = 1). The current sheet width is set to δ = 4 ∆ x . A summary of the simulation parameters is provided in Table 1.", '2.3. Measurement of the reconnection rate': "We measure the reconnection rate based on the evolution of the reconnected magnetic flux, denoted as Φ rec, and defined by the equation,  \nΦ rec = Az ( X ) -Az ( O ) . (8)  \nHere, 'X' represents the location of the major X-point, which corresponds to the maximum of Az in the midplane. On the other hand, 'O' denotes the location of the major O-point at the center of the largest plasmoid; it coincides with the minimum of Az in the midplane. Using Faraday's law, the instantaneous dimensionless reconnection rate, denoted as β rec, can then be expressed  \nas,  \nβ rec = 1 cB 0 d Φ rec d t . (9)  \nFollowing the methodology outlined by Werner et al. (2018), we calculate a mean reconnection rate, denoted as ⟨ β rec ⟩ , by averaging the previously defined instantaneous rate over the time interval between t 20% and t 40%. Here, t 20% and t 40% correspond to the instants when 20% and 40% of the initial magnetic flux in half of the box, Φ 0 = B 0 Ly / 2, has reconnected. By measuring in this time interval, we aim to isolate the intrinsic reconnection rate, avoiding artificial contributions from both the initial phase, when reconnection is still ramping up, and from the late-time phase, when reconnection starts to slow down due to the finite flux available in the box. Because of the intermittent nature of magnetic reconnection, there is a scatter of about 10% to 15% in our measurements of the reconnection rate.", '3.1. Kinematics and global behavior': 'We show two representative snapshots of the plasma density and field lines from our composition-asymmetric runs in Fig. 2. At first glance, there are no noticeable di ff erences with previous symmetric studies: the current sheet becomes quickly unstable to the tearing mode, creating a chain of merging plasmoids separated by X-points. However, a closer look at the system - and in particular its time evolution - reveals that fast magnetosonic waves launched during plasmoid mergers propagate faster and on shorter lengthscales on the pair side than on the electron-ion side, an e ff ect resulting from the higher σ of the pair plasma. This wave-speed asymmetry leads to di ff ering plasma response  \nFig. 2. Plasma number density in the composition asymmetric setup (no shear) shown in the middle of the reconnection process (top left panel, Φ rec ∼ 0 . 15 Φ 0) and towards the saturation stage (bottom left panel, Φ rec ∼ 0 . 45 Φ 0) for σ d = 0 . 1, β = 10 -3 and σ j = 5 (run SD01B0001 ). White contours represent magnetic field lines. We provide on the top and bottom right panels a plot of di ff erent quantities for a slice shown by the green dashed line on the density maps. The blue curve represents the x-component of the magnetic field and the red curve represents the plasma mass density ρ (in logarithmic scale). The box midplane is represented by the black dashed lines. For the time evolution, see the animation on the journal website.  \n<!-- image -->  \ntimes on each side, which provokes an oscillatory vertical motion of the current layer that is absent in symmetric simulations (see the full time evolution of the simulation in the animation provided on the journal website).  \nAs opposed to previous studies of both relativistic and nonrelativistic asymmetric magnetic reconnection (Murphy et al. 2012; Mbarek et al. 2022), the shape of the plasmoids seems rather symmetric. This behavior can be explained by the fact that the magnetic field strength is symmetric in our setup. The magnetic pressure is thus the same on both sides of the reconnection layer, resulting in the absence of strong plasmoid deformation.  \nWe measure reconnection rates in our asymmetric configurations that are slightly higher ( ∼ 30% more) than in our corresponding symmetric electron-ion plasma simulations. For instance, we obtain ⟨ β rec ⟩ ≃ 0 . 029 for the asymmetric run with σ d = 0 . 1 and β = 10 -3 while the corresponding symmetric simulation showed ⟨ β rec ⟩ ≃ 0 . 022 (see Table 1). Even so, the rate stays much smaller than what would be expected for symmetric pair-plasma reconnection (i.e., β rec ∼ 0 . 1). This suggests that the side with the lowest magnetization primarily dictates the overall reconnection rate, as previously demonstrated in the context of density and magnetic field asymmetries by Mbarek et al. (2022).', '3.2. Energy partition and mixing': "We investigate the particle and energy mixing between the disk and the jet shown in the left panels of Fig. 3. The density mixing between the jet and the disk is very e ffi cient at the X-points and  \nin the plasmoids. This behavior may explain the previously observed dynamics of the reconnection process: because reconnection e ffi ciently mixes particles, the magnetization in the reconnection layer is locally the one imposed by the disk ions, since, after mixing, these particles control the local mass density. Looking at the energy mixing in Fig. 3 (right panels), we observe that the energy density of the disk significantly prevails within the plasmoids. This dominance of the disk energy, coupled with the homogeneous mixing of particles, implies that the disk undergoes more substantial energization compared to the jet plasma during the reconnection process. Additionally, we remark that a few highly energetic protons manage to get to the upper edge of plasmoids. We speculate that, in 3D, where plasmoids have been shown to be less e ffi cient at confining high-energy particles (Zhang et al. 2021; Chernoglazov et al. 2023), this tendency would allow particles to escape plasmoids even more e ff ectively and contaminate the jet.  \nWe plot the ratio of the jet-to-disk particles' total kinetic energies in Fig. 4 for our entire composition-asymmetric simulation campaign. The total kinetic energies for disk and jet particles are defined as  \nEd = Z ( γ -1)       d N d e d γ + µ d N d i d γ       d γ and (10)  \nEj = Z ( γ -1)        d N j e d γ + d N j p d γ        d γ. (11)  \nFig. 3. Plasma number density mixing (left panels) defined as ( nj -nd ) / ( nd + nj ), and plasma kinetic energy density mixing (right panels) defined as ( ϵ j -ϵ d ) / ( ϵ d + ϵ j ), where ϵ j / d = γ nj / dmc 2 , in the early (top panels, Φ rec ∼ 0 . 15 Φ 0) and late (bottom panels, Φ rec ∼ 0 . 5 Φ 0) stages of the simulation. Here, σ d = 0 . 3 and β = 10 -2 (run SD03B001 ).  \n<!-- image -->  \nIn order to compare simulations with di ff erent reconnection rates - and thus varying time evolution - we compute this ratio once 50% of the initial magnetic flux has reconnected (the results are unchanged if we consider the instants when 30% and 40% of the initial flux is reconnected). The first conclusion is that in every case, the disk retrieves more energy than the jet from the reconnection event: up to four times more for the lowest plasmaβ simulations, β = 10 -3 . Second, higher magnetization ( σ d = 1) means a more symmetric behavior between the pairs and ions, and thus explains why the energy ratio decreases when σ d = 1. With a higher value of β , the plasma's initial thermal energy is more significant compared with the magnetic energy. Hence the energization during magnetic reconnection is less significant and the energy ratio is thus less a ff ected. However, the fact that σ i = 0 . 3 seems to create slightly more asymmetry than σ i = 0 . 1 at lowβ remains unexplained.  \nWe emphasize the fact that most of the asymmetry in Ed / Ej is due to the ions acquiring much more energy from the reconnection process, as previously observed by Werner et al. (2018); Ball et al. (2018) in electron-ion reconnection studies. Despite  \nthis qualitative similarity with the symmetric case, the preferential energization of ions over electrons is quantitatively more extreme in the presence of asymmetry with respect to findings of earlier symmetric electron-ion reconnection work. We measure, for instance, in our σ = 0 . 1 and β = 10 -3 simulation, that the ion kinetic energy ends up ∼ 5 . 6 times higher than that of the disk electrons (i.e., a higher ratio than Ed / Ej in Fig. 4 because here we compare the disk ions to only the disk electrons). In contrast, for our corresponding symmetric simulation, we measure this ratio as 3.4, consistent with Werner et al. (2018). This extra preferential ion energization tends to vanish for higher magnetizations.", '3.3. Spectra': 'We further investigate the asymmetry in plasma energy gain during the reconnection process by looking at the particle energy distributions (Fig. 5). Looking first at the σ i = 0 . 1, β = 10 -3 run, we notice that the ions reach higher energies than the leptons ( ∼ 3 times more). Given that the magnetization is less than one,  \nFig. 4. Energy partition between the electron-ion (disk) plasma and the pair (jet) plasma at the stage when half of the initial magnetic flux has reconnected, Φ rec = 0 . 5 Φ 0.  \n<!-- image -->  \na distinct nonthermal tail is not apparent. These two observations are in accord with earlier symmetric studies (Melzani et al. 2014; Werner et al. 2018; Ball et al. 2018). Instead of a non thermal tail, the particle spectra are composed of two nearly thermal humps: a low-energy hump representing the initial thermal distribution, and a high-energy component corresponding to the plasma energized by reconnection. The gap of a factor ∼ 10 between the high-energy electron and ion peaks is consistent with the lower e ffi ciency for the lepton heating observed in Sec. 3.2. If the leptons were instead heated as much as the ions, both species should reach a final energy of ( ⟨ γ ⟩-1) m / me ∼ σ imi / me ∼ 10. However, this is only consistent with the energy of the ion peak ( ∼ 10); the lower electron peak ( ∼ 1) can only be explained by a reduced lepton heating e ffi ciency with respect to ions. Examining next the σ d = 1 regime, nonthermal tails with slightly di ff ering slopes start appearing for the leptons. The cuto ff energy seems to be similar for all species (electrons and ions), however the fraction of ions getting to higher energies is significantly higher.  \nWe compare the asymmetric and symmetric runs with σ d = 0 . 1 and β = 10 -3 in Fig. 6. Both runs show similar spectral shapes, but the asymmetric particle distribution appears shifted to slightly higher energies. One reason for this shift could be that there are two times more ions for the same amount of magnetic energy in the symmetric simulation compared to the asymmetric run. Overall, despite subtle di ff erences, the asymmetric and corresponding symmetric runs both show a strong bias, as the magnetization decreases, toward ion acceleration at the expense of electrons. The key insight from this analysis is then that it is this bias - which is evidently independent of the composition asymmetry - that primarily explains the large energization discrepancy between the jet and disk plasmas.', '4.1. Overall picture': 'In our second set of simulations, we return to a a symmetric pairplasma composition. Here, the current layer is antisymmetrically sheared by the upstream pair plasma, which moves with a bulk velocity ± Vs e x . Given that certain simulations involve elevated bulk Lorentz factors ( Γ s ≳ 5), we apply a 9-point digital filter to the current densities at each timestep (Langdon & Birdsall 1970) to avoid the numerical Cherenkov e ff ect.  \nWeshow a plasma density plot for the case of super-Alfvénic shear ( Us = 2 UA ) in Fig. 7. The propagation of fast magne-', 'd = 0.1, =10 3': '<!-- image -->  \nd  \n= 1,  \n=10  \n3  \nFig. 5. Particle energy distributions for all species at the end of the σ d = 0 . 1 ( SD01B0001 , top panel) and σ d = 1 ( SD1B0001 , bottom panel) runs, for both β = 10 -3 .  \n<!-- image -->  \nFig. 6. Same as the top panel in Fig. 5 but with the plasma properties identical on both sides (i.e., symmetric reconnection with σ j = σ d = 0 . 1 and β = 10 -3 , run S01B0001SYM ).  \n<!-- image -->  \ntosonic waves is a ff ected by the shear: the wave fronts are inclined with respect to the reconnection layer instead of, as occurs when Vs = 0, nearly parallel to it. Despite the strong bulk motion, X-points and plasmoids are not swept away because the shear is symmetric.  \nSpiral patterns clearly appear in the density mixing map, as shown on the bottom panel of Fig. 7 (e.g., in the rightmost plasmoid with center located near x = 500 de ). In the context of shear, plasma flows into plasmoids from X-points with a preferential velocity direction set by whether the X-point flanks the plasmoid to the right or to the left. Plasma entering from an X-point  \nFig. 7. Simulation of pair-plasma relativistic magnetic reconnection with an antisymmetric shear applied along the current layer. The top panel shows the plasma number density; the bottom panel shows the mixing of both upstream plasmas as in Fig. 3. Here, σ d = 1, Us = 2 UA (run SD1US6 ) and the white contours represent the magnetic field lines. For the time evolution, see the animation on the journal website.  \n<!-- image -->  \nto the right (left) of a given plasmoid comes preferentially from the leftward-moving (rightward-moving) upstream plasma below (above) the layer. This plasma then follows a freshly reconnected magnetic field line, tracing a spiral trajectory around the core of the growing plasmoid. A plasmoid trapped between two active X-points is thus fed by the upper plasma from its left side and by the lower plasma from its right side, producing, respectively, the red- and blue-colored spirals in Fig. 7. Other than such spiral features in the density mixing map, reconnection proceeds in a similar fashion as for an initially static upstream plasma.  \nAccording to the linear analysis performed by Chow et al. (2023), we may expect the appearance of the Kelvin-Helmholtz (KH) instability for simulations where Vs > VA . However, the tearing instability seems to grow more rapidly than the KH instability, and there is no clear sign of KH vortex formation throughout the duration of the simulations. As plasmoids grow during the evolution of the system, the velocity gradient across the midplane decreases, further inhibiting the operation of the KH instability. A sharp velocity gradient is maintained only in the vicinity of X-points. Toward the end of the simulation (e.g. when d Φ rec / d t ∼ 0), we observe that super-Alfvénic shears tend to decelerate to sub-Alfvénic speeds, due in part to the growth of large plasmoids, the periodic boundary conditions, and the finite box size. We believe that an outflowing box setup would mitigate this late-time deceleration of the shear, but it would not strongly impact our measurements (presented below) of the reconnection rate and the particle energy distribution. These are performed much earlier in the simulation: before shear-flow deceleration linked to excessively large (periodic-boundary-induced) plasmoids mature.', '4.2. Reconnection rate': 'We compute the reconnection rates for each shear simulation using the method outlined in Sec. 2.3. Figure 8 presents the reconnection rate, relative to its Vs = 0 reference value, as a  \nfunction of the shear four-velocity, Us = Vs Γ s / c , normalized to the Alfvénic four-velocity, UA = √ σ d . Our primary observation is that reconnection slows down with increasing shear speeds, dropping to about 20% of its reference rate once Us reaches 2 UA . Notably, for super-Alfvénic speeds, the reconnection rate appears to follow a similar trend between the σ = 1 and σ = 10 cases, suggesting a potential universality in this behavior. We point out, however, a local enhancement of the reconnection rate for modest shear speeds ( Us = UA / 3) that occurs only for σ d = 1. This feature is not clearly understood, but it might not be significant given the ∼ 10% uncertainty in the rate measurements reported in Sect. 2.3. This uncertainty is somewhat amplified by the fact that we normalize by the reconnection rate with no shear (which itself is also only accurate to roughly 10%). Similar considerations apply to the slight dip observed at σ d = 10 ( Us = 2 UA / 3). The overall decrease of the reconnection rate with increasing shear-flow velocity may be due to the loss of causal connection between the fast-moving upstream plasma and the nearly static X-points.  \nFig. 8. Reconnection rate as a function of the shear-flow 4-velocity, Us , in units of the Alfvén 4-velocity, UA = √ σ d . The rates are normalized (separately for σ d = 1 and σ d = 10) to the Us = 0 rate denoted as β rec , 0. The error bars represent our estimated relative uncertainty of 10% through occurrences of a given run as described in Sec. 2.3.  \n<!-- image -->  \nThe general slowing of reconnection with increasing shear is in qualitative agreement with previous non-relativistic studies (La Belle-Hamer et al. 1995; Cassak & Otto 2011; Cassak 2011). Quantitatively, however, these earlier works consistently indicate a thorough quenching of reconnection, with a very sharp cuto ff near the Alfvén point. Our relativistic simulations, in contrast, show a much more gradual behavior across this point, and a complete shutdown of magnetic reconnection is not observed. This result is at odds with what has been recently reported by Peery et al. (2024), where relativistic reconnection is already quenched for shear speeds substantially below the Alfvén point. This may be explained by the non-zero guide field or by the small box size used in their simulations.', '4.3. Implications for particle acceleration efficiency': 'To comprehend the impact of shear flow on particle acceleration e ffi ciency, we present, in Fig. 9, the particle energy distributions measured when Φ rec = 0 . 5 Φ 0, which occurs at di ff ering simulation times due to the change in the laboratory-frame reconnection rate reported above. The initial distributions are shown on the figure with dashed lines for comparison. We observe that the high-energy power-law tail steepens with increasing shear. This is certainly true at low shear and seems to hold at high shear as  \n<!-- image -->  \n<!-- image -->  \nFig. 9. Comparison of the particle energy distributions at a given magnetization for di ff erent shear values and Φ rec = 0 . 5 Φ 0. Dashed lines represent the corresponding initial spectra.  \n<!-- image -->  \nwell, though in the latter case (say, at Us ≥ 4 UA / 3) the initial plasma energy is so high that an unambiguous nonthermal component is less clear.  \nWe also notice, for σ d = 1, an emerging low-energy component in the distributions. Advection of particles from the upstream towards the plasmoids, where they lose their bulk motion energy, could explain this behavior, although only an analysis of particle trajectories could validate such a hypothesis. In summary, the presence of a shear flow decreases the particle acceleration e ffi ciency: we observe a clear trend towards a softening of the particle energy distribution with increasing shear.', '5. Conclusions': "In this work, we analyze separately the impacts of composition asymmetry and shear in the upstream plasma of a relativistic reconnecting current layer. For composition-asymmetric reconnection, the less-magnetized electron-ion upstream side dictates the global reconnection dynamics, in agreement with Mbarek et al. (2022). The measured reconnection rate is very close to, although slightly higher than, that reported in previous symmetric electron-ion reconnection studies (Werner et al. 2018; Ball et al. 2018). Furthermore, we notice that the gap in the energy partition between electrons and ions increases by about 40% with respect to a symmetric electron-ion configuration, meaning that ions are even more preferentially heated in the presence of composition asymmetry. In the second part of our work, focused on a sheared reconnection layer, we show that a relativistic superAlfvénic shear substantially reduces the reconnection rate but,  \nunlike in non-relativistic studies, does not completely halt reconnection. In addition, we observe a steepening of the power-law tail for increasing shear values.  \nThese results have important implications for astrophysical environments. In the context of black hole accretion disks, asymmetric reconnection may occur at a jet-disk interface or during magnetic flux eruptions. One important asymmetry in such reconnection events is with respect to plasma composition. In this case, our results indicate that particle acceleration is dictated by the heavier - and, hence, lower-magnetization - electronion side of the current sheet. Due to the modest ion magnetization ( σ ≲ 1) expected in this environment, reconnection is unlikely to produce prominent power-law tails in the ion energy distribution; a prominent nonthermal component exceeding γ = mi / me σ ≲ mi / me in the electron energy distribution is also not expected.  \nThese properties suggest that asymmetric reconnection at a jet-disk boundary is not a suitable candidate for powering highenergy gamma-ray flares observed in black hole environments. On the other hand, such reconnection sites may contribute to the lower-energy emission observed in limb-brightened jets. The addition of a velocity shear along field lines at the jet-disk boundary does not change these expectations, since, as we show in this work, strong shear slows reconnection down and inhibits nonthermal particle acceleration - e ff ects that would only further hamper e ffi cient acceleration of particles up to gamma-rayemitting energies.  \nAn important limitation of our study is its dimensionality. Once particles are trapped in two-dimensional plasmoids, they are artificially kept from escaping back into the upstream plasma. However, in 3D, as shown by recent studies (Zhang et al. 2021; Chernoglazov et al. 2023), ions can escape plasmoids and get onto Speiser-like trajectories where they undergo intense linear acceleration. In the presence of composition asymmetry, this mechanism may facilitate mixing of ions into the putatively Poynting-flux-dominated and pair-loaded jet. It may also facilitate particle injection into other acceleration processes occurring on larger scales, such as, for instance, shear-flow acceleration along the jet sheath (Ostrowski 1998; Rieger & Du ff y 2004; Sironi et al. 2021). This motivates future studies of 3D composition-asymmetric relativistic magnetic reconnection.  \nAcknowledgements. We thank Jesse Vos for providing us early results from his GRMHD simulations and for fruitful discussions regarding magnetized blackhole accretion. We are thankful to the referee's comments that helped us improving the quality of the manuscript. This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (Grant Agreement No. 863412). 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