Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Plasma Physics
Volume 67, Number 15
Monday–Friday, October 17–21, 2022; Spokane, Washington
Session GM09: Mini-Conference: Heating and Non-Thermal Particle Acceleration during Magnetic Reconnection in Laboratory, Heliophysical and Astrophysical Plasmas IIILive Streamed
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Chair: Fan Guo, Los Alamos National Laboratory; Lorenzo Sironi, Columbia University Room: 206 AB |
Tuesday, October 18, 2022 9:30AM - 9:55AM |
GM09.00001: Nonthermal Particle Acceleration in Radiative Magnetic Reconnection in Relativistic Astrophysical Environments Dmitri A Uzdensky In this talk I will review observational evidence and theoretical arguments for nonthermal particle acceleration in various astrophysical scenarios, with a focus on relativistic plasmas around compact objects such as neutron stars and black holes. I will place a particular emphasis on the role of radiation reaction in controlling particle acceleration to very high energies and its interplay with complex, multi-scale collective plasma dynamics in the context of 2D and 3D plasmoid-dominated magnetic reconnection in the astrophysically-relevant large-system regime. I will also identify the key theoretical and computational challenges to our understanding of this important problem and will outline the prospects for overcoming them in future first-principles simulation studies. |
Tuesday, October 18, 2022 9:55AM - 10:15AM |
GM09.00002: Radiative magnetic reconnection in black hole and neutron star magnetospheres. Alexander A Philippov In this talk I will review recent progress in modeling magnetic reconnection in the regime of dynamically important radiative losses. I will highlight applications to modeling multi-wavelength non-thrermal emission from neutron stars and flares from accreting supermassive black holes. |
Tuesday, October 18, 2022 10:15AM - 10:35AM |
GM09.00003: Particle Injection and Nonthermal Particle Acceleration in Relativistic Magnetic Reconnection Omar J French, Fan Guo, Qile Zhang, Dmitri A Uzdensky Magnetic reconnection in the relativistic regime has been proposed as an important process for efficiently accelerating particles and producing high-energy emissions. Using fully kinetic PIC simulations, we investigate how guide field strength and domain size affect two stages of particle energization: (1) acceleration from the upstream thermal energy to the injection energy and (2) the further acceleration responsible for high-energy power-law spectra. In the first stage, we evaluate the contributions of parallel electric fields, Fermi reflections, and pickup acceleration. For weak guide fields, Fermi reflections and pickup acceleration dominate both stages. For a strong guide field, however, parallel electric fields decisively dominate particle injection but contribute comparably to perpendicular electric fields to total acceleration. We also find that the power-law index and injection energy increase with guide field strength and converge with increasing domain size. These findings will help explain the nonthermal acceleration and emissions in black hole jets and pulsar wind nebulae. |
Tuesday, October 18, 2022 10:35AM - 10:55AM |
GM09.00004: Particle Acceleration in 'Reconnecting' Turbulence Luca Comisso Turbulence and magnetic reconnection are ubiquitous in astrophysical environments, and they are often invoked to explain the origin of non-thermal particles inferred to occur in a variety of astrophysical sources. Yet, the mechanisms responsible for accelerating particles to ultra-relativistic energies are still poorly understood. Recent fully-kinetic particle-in-cell simulations suggest that turbulence and magnetic reconnection operate in synergy, with reconnection being responsible for particle injection from the thermal pool and stochastic scattering off turbulent fluctuations leading to extended non-thermal power-law tails. The acceleration mechanisms, as well as the resulting particle distributions, depend on particle species. The ion energy spectrum is harder than the electron one, and both distributions get harder for higher plasma magnetization. The energization of electrons is accompanied by a significant energy-dependent pitch-angle anisotropy, with most electrons moving parallel to the local magnetic field, while ions stay roughly isotropic. Parallel electric fields associated with magnetic reconnection are responsible for the initial energy gain of electrons, whereas perpendicular electric fields control the overall energization of ions. These findings have important implications for the origin of non-thermal particles in space and astrophysical plasmas. |
Tuesday, October 18, 2022 10:55AM - 11:15AM |
GM09.00005: A Model for Nonthermal Particle Acceleration in Relativistic Magnetic Reconnection Xiaocan Li, Fan Guo, Yi-Hsin Liu The past decade has seen an outstanding development of nonthermal particle acceleration in magnetic reconnection in magnetically-dominated systems, with clear signatures of power-law energy distributions as a common outcome of first principle kinetic simulations. Here we propose a model for investigating nonthermal particle acceleration in reconnection in a systematic approach. We show that particle energy distributions are well determined by particle injection, acceleration, and escape processes. Using a series of ab initio kinetic simulations, we accurately evaluate the energy- and time-dependent transport coefficients. The resulting spectral characteristics, including the spectral index, lower and upper bound of the power-law distribution, agree well with the simulation results. |
Tuesday, October 18, 2022 11:15AM - 11:30AM |
GM09.00006: Onset of Plasmoid Reconnection during Magnetorotational Instability Jarrett Rosenberg, Fatima Ebrahimi The evolution of current sheets in accretion flows undergoing magnetorotational instability (MRI) is examined through two and three dimensional numerical modelling of the resistive MHD equations in global cylindrical geometry. With an initial uniform magnetic field aligned in the vertical (z) direction, MRI produces radially extended toroidal (azimuthal) current sheets. In both 2D and 3D when axisymmetric modes dominate, these current sheets attract each other and merge in the poloidal (rz) plane, driving magnetic reconnection when the Lundquist number S = 3 X 102, making it a possible source of plasmoids (closed magnetic loops) in accretion disks. At high Lundquist numbers in the 2D regime, starting at S = 5 X 103, self-consistent MRI-generated current sheets become thin and subject to plasmoid instability, and therefore spontaneous magnetic reconnection. When non-axisymmetric 3D modes dominate, turbulence makes the azimuthal current sheets further unstable, and stretch vertically. Toroidally extended vertical current sheets in the inner region, as well as larger 3D magnetic islands in the outer regions of the disks are also formed. These findings have strong ramifications for astrophysical disks as potential sources of plasmoids that could cause local heating, particle acceleration, and high energy EM radiation. |
Tuesday, October 18, 2022 11:30AM - 11:45AM |
GM09.00007: Magnetic Reconnection in 3D relativistic and semirelativistic astrophysical plasmas Gregory R Werner, Dmitri A Uzdensky Magnetic reconnection releases magnetic field energy, resulting in plasma heating and nonthermal particle acceleration (NTPA). Reconnection in astrophysical current sheets with relativistic collisionless plasma, e.g., near black holes or neutron stars, may play a key role energizing electrons that emit observable radiation. In this regime, simulations have shown that 2D reconnection yields substantial NTPA, but it remains an outstanding problem whether similar mechanisms operate in 3D. Using large 3D particle-in-cell simulations, we show that there is a regime (in relativistic pair plasma with ambient plasma beta near unity) where current sheet dynamics differ significantly from classic 2D reconnection due to dissolution of flux ropes and competition from the nonlinear drift-kink instability. Despite this, NTPA is seen to be surprisingly robust. We then expand our study to 3D semirelativistic electron-ion plasmas where electrons are relativistic but ions are not, in particular measuring the electrons/ion energy partition, which has important consequences for radiative signatures, e.g., of X-ray and gamma-ray flares from accreting black holes. |
Tuesday, October 18, 2022 11:45AM - 12:00PM |
GM09.00008: Charged Particle Self-Scattering and Acceleration in Reconnection Andrey Beresnyak Unlike X-point small-scale reconnection, which develops strong electric fields and accelerates particles naturally, large-scale reconnection cannot rely on electric fields; rather it has to rely on more classic stochastic mechanisms. Only these stochastic mechanisms allow acceleration up to the Hillas limit, this is usually assumed, but rarely well argued. One of the stochastic mechanisms has to do with straightening magnetic field lines that provide curvature acceleration. The other, having to do with compression in the inflow, is similar to the shock acceleration. All stochastic mechanisms have to do with efficient particle scattering, yet the mechanism for scattering remains elusive. In this presentation I show the results of simulations where particles stream along field lines and scatter by self-generated turbulence. Surprisingly, their dynamics is not diffusive but rather superdiffusive. This superdiffusion depends on particle energy. I will discuss implications of this to the standard stochastic acceleration picture applied to reconnecting current layers. |
Tuesday, October 18, 2022 12:00PM - 12:15PM |
GM09.00009: Comptonization by reconnection plasmoids in black hole coronae Navin Sridhar, Lorenzo Sironi, Andrei M Beloborodov What powers the hard, non-thermal X-rays from accreting black holes is an unsolved mystery. We address this puzzle, and the underlying question of what energizes the electrons of the Comptonizing "corona" against the strong inverse Compton (IC) cooling losses. We perform first principle particle-in-cell simulations of magnetic reconnection in magnetically dominated (σ>>1) electron-positron and mildly-magnetized (σ~1) electron-ion plasmas subject to strong IC cooling. We find that the electron energy spectrum is dominated by a quasi-Maxwellian-shaped peak at trans-relativistic energies (~100 keV), which results primarily from the bulk motions of "plasmoids." In plasmoids, electrons are cooled down to non-relativistic energies, which makes the oft-invoked paradigm of "thermal Comptonization" by hot electrons unfeasible. In electron-ion corona, we find that the radiatively-cooled electrons are not re-heated by the hot ions via collisional or collisionless modes of energy transfer. We complement our particle-in-cell simulations with Monte-Carlo calculations of the transfer of seed soft photons through the reconnection layer, and produce synthetic X-ray spectra. We demonstrate that, regardless of the composition of the corona, Comptonization by the bulk motions of a chain of plasmoids containing IC-cooled electrons can naturally explain the hard-state spectra of accreting black holes. |
Tuesday, October 18, 2022 12:15PM - 12:30PM |
GM09.00010: Particle acceleration by magnetic reconnection from fluid to kinetic regimes Samuel R Totorica, Mami Machida, Amitava Bhattacharjee Magnetic reconnection is widely believed to play a critical role in the production of nonthermal energetic particles in a variety of systems in astrophysics and space physics. Several acceleration mechanisms including direct acceleration by the reconnection electric field at X-points and Fermi acceleration associated with contracting and merging plasmoids are known to contribute to energization. These mechanisms differ greatly in the complexity of the physical model required to model them, and their relative importance remains disputed. Due to the large scales of realistic systems and the computational expense associated with fully kinetic simulations, it is important to determine the simplest physical model that can accurately model the particle acceleration in a given system. Using kinetic particle-in-cell simulations with a relativistically correct Coulomb collision operator, we study the changes in particle energization that occur in the transition from a collisionless plasma to the collisional fluid regime. A detailed analysis of the generalized Ohm's law and self-consistent particle trajectories reveal the roles of ideal and non-ideal electric fields in particle energization. We discuss the implications of these results for modelling large-scale astrophysical systems. |
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