We demonstrate using linear theory and particleincell (PIC) simulations that a synchrotroncooling collisionless plasma acquires pressure anisotropy and, if the plasma beta is sufficiently high, becomes unstable to the firehose instability, in a process that we dub the synchrotron firehose instability (SFHI). The SFHI channels free energy from the pressure anisotropy of the radiating, relativistic electrons (and/or positrons) into smallamplitude, kineticscale, magneticfield fluctuations, which pitchangle scatter the particles and bring the plasma to a nearthermal state of marginal instability. The PIC simulations reveal a nonlinear cyclic evolution of firehose bursts interspersed by periods of stable cooling. We compare the SFHI for electron–positron and electron–ion plasmas. As a byproduct of the growing electronfirehose magneticfield fluctuations, magnetized ions gain a pressure anisotropy opposite to that of the electrons. If these ions are relativistically hot, we find that they also experience cooling due to collisionless thermal coupling with the electrons, which we argue is mediated by a secondary ioncyclotron instability. We suggest that the SFHI may be activated in a number of astrophysical scenarios, such as within ejecta from black hole accretion flows and relativistic jets, where the redistribution of energetic electrons from low to high pitch angles may cause transient burstsmore »
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Abstract 
We study within a fully kinetic framework the generation of “seed” magnetic fields through the Weibel instability, driven in an initially unmagnetized plasma by a largescale shear force. We develop an analytical model that describes the development of thermal pressure anisotropy via phase mixing, the ensuing exponential growth of magnetic fields in the linear Weibel stage, and the saturation of the Weibel instability when the seed magnetic fields become strong enough to instigate gyromotion of particles and thereby inhibit their freestreaming. The predicted scaling dependencies of the saturated fields on key parameters (e.g., ratio of system scale to electron skin depth and forcing amplitude) are confirmed by twodimensional and threedimensional particleincell simulations of an electron–positron plasma. This work demonstrates the spontaneous magnetization of a collisionless plasma through largescale motions as simple as a shear flow and therefore has important implications for magnetogenesis in dilute astrophysical systems.Free, publiclyaccessible full text available May 10, 2023

Magnetic reconnection, especially in the relativistic regime, provides an efficient mechanism for accelerating relativistic particles and thus offers an attractive physical explanation for nonthermal highenergy emission from various astrophysical sources. I present a simple analytical model that elucidates key physical processes responsible for reconnectiondriven relativistic nonthermal particle acceleration in the largesystem, plasmoiddominated regime in two dimensions. The model aims to explain the numerically observed dependencies of the powerlaw index $p$ and highenergy cutoff $\gamma _c$ of the resulting nonthermal particle energy spectrum $f(\gamma )$ on the ambient plasma magnetization $\sigma$ , and (for $\gamma _c$ ) on the system size $L$ . In this selfsimilar model, energetic particles are continuously accelerated by the outofplane reconnection electric field $E_{\rm rec}$ until they become magnetized by the reconnected magnetic field and eventually trapped in plasmoids large enough to confine them. The model also includes diffusive Fermi acceleration by particle bouncing off rapidly moving plasmoids. I argue that the balance between electric acceleration and magnetization controls the powerlaw index, while trapping in plasmoids governs the cutoff, thus tying the particle energy spectrum to the plasmoid distribution.

Magnetic reconnection, a plasma process converting magnetic energy to particle kinetic energy, is often invoked to explain magnetic energy releases powering highenergy flares in astrophysical sources including pulsar wind nebulae and black hole jets. Reconnection is usually seen as the (essentially twodimensional) nonlinear evolution of the tearing instability disrupting a thin current sheet. To test how this process operates in three dimensions, we conduct a comprehensive particleincell simulation study comparing two and threedimensional evolution of long, thin current sheets in moderately magnetized, collisionless, relativistically hot electron–positron plasma, and find dramatic differences. We first systematically characterize this process in two dimensions, where classic, hierarchical plasmoidchain reconnection determines energy release, and explore a wide range of initial configurations, guide magnetic field strengths and system sizes. We then show that threedimensional (3D) simulations of similar configurations exhibit a diversity of behaviours, including some where energy release is determined by the nonlinear relativistic driftkink instability. Thus, 3D current sheet evolution is not always fundamentally classical reconnection with perturbing 3D effects but, rather, a complex interplay of multiple linear and nonlinear instabilities whose relative importance depends sensitively on the ambient plasma, minor configuration details and even stochastic events. It often yields slower but longerlasting andmore »

Abstract Relativistic magnetized jets, such as those from AGN, GRBs, and XRBs, are susceptible to current and pressuredriven MHD instabilities that can lead to particle acceleration and nonthermal radiation. Here, we investigate the development of these instabilities through 3D kinetic simulations of cylindrically symmetric equilibria involving toroidal magnetic fields with electron–positron pair plasma. Generalizing recent treatments by Alves et al. and Davelaar et al., we consider a range of initial structures in which the force due to toroidal magnetic field is balanced by a combination of forces due to axial magnetic field and gas pressure. We argue that the particle energy limit identified by Alves et al. is due to the finite duration of the fast magnetic dissipation phase. We find a rather minor role of electric fields parallel to the local magnetic fields in particle acceleration. In all investigated cases, a kink mode arises in the central core region with a growth timescale consistent with the predictions of linearized MHD models. In the case of a gaspressurebalanced (Zpinch) profile, we identify a weak local pinch mode well outside the jet core. We argue that pressuredriven modes are important for relativistic jets, in regions where sufficient gas pressure is producedmore »

The physical picture of interacting magnetic islands provides a useful paradigm for certain plasma dynamics in a variety of physical environments, such as the solar corona, the heliosheath and the Earth's magnetosphere. In this work, we derive an island kinetic equation to describe the evolution of the island distribution function (in area and in flux of islands) subject to a collisional integral designed to account for the role of magnetic reconnection during island mergers. This equation is used to study the inverse transfer of magnetic energy through the coalescence of magnetic islands in two dimensions. We solve our island kinetic equation numerically for three different types of initial distribution: Dirac delta, Gaussian and powerlaw distributions. The time evolution of several key quantities is found to agree well with our analytical predictions: magnetic energy decays as $\tilde {t}^{1}$ , the number of islands decreases as $\tilde {t}^{1}$ and the averaged area of islands grows as $\tilde {t}$ , where $\tilde {t}$ is the time normalised to the characteristic reconnection time scale of islands. General properties of the distribution function and the magnetic energy spectrum are also studied. Finally, we discuss the underlying connection of our islandmerger models to the (selfsimilar) decaymore »

ABSTRACT Turbulent highenergy astrophysical systems often feature asymmetric energy injection: for instance, Alfvén waves propagating from an accretion disc into its corona. Such systems are ‘imbalanced’: the energy fluxes parallel and antiparallel to the largescale magnetic field are unequal. In the past, numerical studies of imbalanced turbulence have focused on the magnetohydrodynamic regime. In this study, we investigate externally driven imbalanced turbulence in a collisionless, ultrarelativistically hot, magnetized pair plasma using 3D particleincell (PIC) simulations. We find that the injected electromagnetic momentum efficiently converts into plasma momentum, resulting in net motion along the background magnetic field with speeds up to a significant fraction of lightspeed. This discovery has important implications for the launching of accretion disc winds. We also find that although particle acceleration in imbalanced turbulence operates on a slower timescale than in balanced turbulence, it ultimately produces a powerlaw energy distribution similar to balanced turbulence. Our results have ramifications for black hole accretion disc coronae, winds, and jets.

We report on an analytical and numerical study of the dynamics of a threedimensional array of identical magnetic flux tubes in the reducedmagnetohydrodynamic description of the plasma. We propose that the longtime evolution of this system is dictated by fluxtube mergers, and that such mergers are dynamically constrained by the conservation of the pertinent (ideal) invariants, viz. the magnetic potential and axial fluxes of each tube. We also propose that in the direction perpendicular to the merging plane, flux tubes evolve in a critically balanced fashion. These notions allow us to construct an analytical model for how quantities such as the magnetic energy and the energycontaining scale evolve as functions of time. Of particular importance is the conclusion that, like its twodimensional counterpart, this system exhibits an inverse transfer of magnetic energy that terminates only at the system scale. We perform direct numerical simulations that confirm these predictions and reveal other interesting aspects of the evolution of the system. We find, for example, that the early time evolution is characterized by a sharp decay of the initial magnetic energy, which we attribute to the ubiquitous formation of current sheets. We also show that a quantitatively similar inverse transfer of magnetic energy is observed when the initial conditionmore »