 Award ID(s):
 1842561
 NSFPAR ID:
 10323285
 Date Published:
 Journal Name:
 ArXivorg
 ISSN:
 23318422
 Format(s):
 Medium: X
 Sponsoring Org:
 National Science Foundation
More Like this

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 twostep 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 strongguidefield collisionless magnetic reconnection. We determine these velocity space signatures at the xpoint 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 strongguidefield limit, the energization of electrons occurs through bulk acceleration of the outofplane electron flow by the parallel electric field that drives the reconnection, a nonresonant 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 singlepoint diagnostic, which can potentially identify a reconnection exhaust region using spacecraft observations.more » « less

Abstract Using 3D particleincell simulation, we characterize energy conversion, as a function of guide magnetic field, in a thin current sheet in semirelativistic plasma, with relativistic electrons and subrelativistic protons. There, magnetic reconnection, the driftkink instability (DKI), and the fluxrope kink instability all compete and interact in their nonlinear stages to convert magnetic energy to plasma energy. We compare fully 3D simulations with 2D in two different planes to isolate reconnection and DKI effects. In zero guide field, these processes yield distinct energy conversion signatures: ions gain more energy than electrons in 2D
xy (reconnection), while the opposite is true in 2Dyz (DKI), and the 3D result falls in between. The fluxrope instability, which occurs only in 3D, allows more magnetic energy to be released than in 2D, but the rate of energy conversion in 3D tends to be lower. Increasing the guide magnetic field strongly suppresses DKI, and in all cases slows and reduces the overall amount of energy conversion; it also favors electron energization through a process by which energy is first stored in the motional electric field of flux ropes before energizing particles. Understanding the evolution of the energy partition thus provides insight into the role of various plasma processes, and is important for modeling radiation from astrophysical sources such as accreting black holes and their jets. 
ABSTRACT Black hole and neutron star environments often comprise collisionless plasmas immersed in strong magnetic fields and intense baths of lowfrequency radiation. In such conditions, relativistic magnetic reconnection can tap the magnetic field energy, accelerating highenergy particles that rapidly cool by inverse Compton (IC) scattering the dense photon background. At the highest particle energies reached in bright gammaray sources, IC scattering can stray into the Klein–Nishina regime. Here, the Comptonized photons exceed pairproduction threshold with the radiation background and may thus return their energy to the reconnecting plasma as fresh electron–positron pairs. To reliably characterize observable signatures of such Klein–Nishina reconnection, in this work, we present firstprinciples particleincell simulations of pairplasma relativistic reconnection coupled to Klein–Nishina and pairproduction physics. The simulations show substantial differences between the observable signatures of Klein–Nishina reconnection and reconnection coupled only to lowenergy Thomson IC cooling (without pair production). The latter regime exhibits strong harderwhenbrighter behaviour; the former involves a stable spectral shape independent of overall brightness. This spectral stability is reminiscent of flatspectrum radio quasar (FSRQ) GeV high states, furnishing evidence that Klein–Nishina radiative physics operates in FSRQs. The simulated Klein–Nishina reconnection pair yield spans from low to orderunity and follows an exponential scaling law in a single governing parameter. Pushing this parameter beyond its range studied here might give way to a copious paircreation regime. Besides FSRQs, we discuss potential applications to accreting black hole Xray binaries, the M87* magnetosphere, and gammaray binaries.

Abstract In the presence of a strong electric field perpendicular to the magnetic field, the electron crossfield (E × B) flow relative to the unmagnetized ions can cause the socalled electron cyclotron drift instability (ECDI) due to resonances of the ion acoustic mode and the electron cyclotron harmonics. This occurs in, for example, collisionless shock ramps in space, and in E × B discharge devices such as Hall thrusters. A prominent feature of ECDI is its capability to induce an electron flow parallel to the background E field at a speed greatly exceeding predictions by classical collision theory. Such anomalous transport is important due to its role in particle thermalization at space shocks, and in causing plasma flows towards the walls of E × B devices, leading to unfavorable erosion and performance degradation, etc. The development of ECDI and anomalous transport is often considered requiring a fully kinetic treatment. In this work, however, we demonstrate that a reduced variant of this instability, and more importantly, the associated anomalous transport, can be treated selfconsistently in a collisionless twofluid framework without any adjustable collision parameter. By treating both electron and ion species on an equal footing, the free energy due to the interspecies velocity shear allows the growth of an anomalous electron flow parallel to the background E field. We will first present linear analyses of the instability in the twofluid five and tenmoment models, and compare them against the fullykinetic theory. At low temperatures, the twofluid models predict the fastestgrowing mode in good agreement with the kinetic result. Also, by including more ( > = 10 ) moments, secondary (and possibly higher) unstable branches can be recovered. The dependence of the instability on iontoelectron mass ratio, plasma temperature, and background B field strength is also thoroughly explored. We then carry out direct numerical simulations of the crossfield setup using the fivemoment model. The development of the instability, as well as the anomalous transport, is confirmed and in excellent agreement with theoretical predictions. The force balance properties are also studied using the fivemoment simulation data. This work casts new insights into the nature of ECDI and the associated anomalous transport and demonstrates the potential of the twofluid moment model in efficient modeling of E × B plasmas.more » « less

ABSTRACT The time evolution of highenergy synchrotron radiation generated in a relativistic pair plasma energized by reconnection of strong magnetic fields is investigated with 2D and 3D particleincell (PIC) simulations. The simulations in this 2D/3D comparison study are conducted with the radiative PIC code OSIRIS, which selfconsistently accounts for the synchrotron radiation reaction on the emitting particles, and enables us to explore the effects of synchrotron cooling. Magnetic reconnection causes compression of the plasma and magnetic field deep inside magnetic islands (plasmoids), leading to an enhancement of the flaring emission, which may help explain some astrophysical gammaray flare observations. Although radiative cooling weakens the emission from plasmoid cores, it facilitates additional compression there, further amplifying the magnetic field B and plasma density n, and thus partially mitigating this effect. Novel simulation diagnostics utilizing 2D histograms in the nB space are developed and used to visualize and quantify the effects of compression. The nB histograms are observed to be bounded by relatively sharp powerlaw boundaries marking clear limits on compression. Theoretical explanations for some of these compression limits are developed, rooted in radiative resistivity or 3D kinking instabilities. Systematic parameterspace studies with respect to guide magnetic field, system size, and upstream magnetization are conducted and suggest that stronger compression, brighter highenergy radiation, and perhaps significant quantum electrodynamic effects such as pair production, may occur in environments with larger reconnectionregion sizes and higher magnetization, particularly when magnetic field strengths approach the critical (Schwinger) field, as found in magnetar magnetospheres.