An official website of the United States government
Abstract Kinetic simulations of relativistic turbulence have significantly advanced our understanding of turbulent particle acceleration. Recent progress has highlighted the need for an updated acceleration theory that can account for particle acceleration within the plasma’s coherent structures. Here, we investigate how intermittency modeling connects statistical fluctuations in turbulence to regions of high-energy dissipation. This connection is established by employing a generalized She–Leveque model to characterize the exponentsζpfor the structure functions . The fitting of the scaling exponents provides us with a measure of the codimension of the dissipative structures, for which we subsequently determine the filling fraction. We perform our analysis for a range of magnetizationsσand relative fluctuation amplitudesδB0/B0. We find that increasing values ofσandδB0/B0allow the turbulent cascade to break sheetlike structures into smaller regions of dissipation that resemble chains of flux ropes. However, as their dissipation measure increases, the dissipative regions become less volume filling. With this work, we aim to inform future turbulent acceleration theories that incorporate particle energization from interactions with coherent structures within relativistic turbulence.
more »
« less
This content will become publicly available on March 9, 2027
Statistics of Current and Vorticity Structures in Relativistic Turbulence
Abstract Coherent structures created through turbulent cascades play a key role in energy dissipation and particle acceleration. In this work, we investigate both current and vorticity sheets in 3D particle-in-cell simulations of decaying relativistic turbulence in pair plasma by training a self-organizing map to recognize these structures. We subsequently carry out an extensive statistical analysis to reveal their geometric and structural properties. This analysis is systematically applied across a range of magnetizations (σ) and fluctuating-to-mean magnetic field strengths (δB0/B0) to assess how these parameters influence the resulting structures. We find that the structures’ geometric properties form power-law distributions in their probability density functions, with the exception of the structure width, which generally exhibits an exponential distribution peaking around two electron skin depths. The measurements show a weak dependence onσbut a strong dependence onδB0/B0. Finally, we investigate the spatial relationship between current sheets and vorticity sheets. We find that most current sheets are directly associated with at least one vorticity sheet neighbor and are often situated between two vorticity sheets. These findings provide a detailed statistical framework for understanding the formation and organization of coherent structures in relativistic magnetized turbulence, allowing for their incorporation into updated theoretical models for structure-based energy dissipation and particle acceleration processes crucial for interpreting high-energy astrophysical observations.
more »
« less
- Award ID(s):
- 2308944
- PAR ID:
- 10692463
- Publisher / Repository:
- American Astronomical Society
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 999
- Issue:
- 2
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 261
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract We employ first-principles, fully kinetic particle-in-cell simulations to investigate magnetic field-line curvature in magnetically dominated turbulent plasmas and its role in particle acceleration through curvature-drift motion along the motional electric field. By varying the fluctuation-to-mean-magnetic-field ratioδB0/B0, we examine curvatureκstatistics and their connection to particle acceleration. The curvature probability densities display broad power-law wings, scaling linearly inκbelow the peak and developing hard high-κtails forδB0/B0 ≳ 1. As the mean field strengthens, the high-κtails steepen, and large-curvature events are suppressed whenδB0/B0 ≪ 1. The probability density functions of magnetic field-line contraction,vE · κ, withvEthe field-line velocity, develop power-law tails well described by a symmetric Pareto distribution, characteristic of stochastic and intermittent energy exchanges, with the tails becoming harder asδB0/B0increases. Our guiding-center analysis shows that curvature-drift acceleration accounts for a substantial fraction of the energization via the motional electric field, and that it strengthens with increasingδB0/B0. For well-magnetized particles, curvature-drift acceleration typically exceeds∇Bdrift, polarization drift, and betatron contributions. These results identify curvature-drift acceleration as a principal pathway through which magnetized turbulence transfers energy to nonthermal particles in astrophysical plasmas.more » « less
-
Energy-dependent Polarization Angle Variability as a Robust Diagnostic for Blazar Flaring MechanismsAbstract Identifying the physical mechanism driving blazar flares remains a central challenge in high-energy astrophysics. We show that the energy dependence of the standard deviation of the polarization angle variability (σPA) provides a powerful and robust discriminator of blazar flaring mechanisms. Using particle-in-cell-integrated polarized radiative transfer simulations, we perform the most rigorous statistical analyses of polarization variability to date. We demonstrate that magnetic reconnection and magnetized turbulence imprint qualitatively distinct energy dependence ofσPAthat directly reflects their different magnetic field evolution and particle transport. Reconnection predicts higherσPAwith higher photon energy until the synchrotron spectral peak, whereas turbulence produces nearly flatσPAacross the synchrotron spectral component. These trends are resilient to realistic observational limitations. Applying our results to optical and IXPE data of Mrk 421 and 1ES 1959+650, we find strong evidence for reconnection-driven flares embedded in a turbulent blazar zone. Energy-dependentσPAemerges as a decisive new probe of particle acceleration in relativistic jets.more » « less
-
Abstract Strong magnetically dominated Alfvénic turbulence is an efficient engine of nonthermal particle acceleration in a relativistic collisionless plasma. We argue that in the limit of strong magnetization, the type of energy distribution attained by accelerated particles depends on the relative strengths of turbulent fluctuationsδB0and the guide fieldB0. IfδB0≪B0, the particle magnetic moments are conserved, and the acceleration is provided by magnetic curvature drifts. Curvature acceleration energizes particles in the direction parallel to the magnetic field lines, resulting in log-normal tails of particle energy distribution functions. Conversely, ifδB0≳B0, interactions of energetic particles with intense turbulent structures can scatter particles, creating a population with large pitch angles. In this case, magnetic mirror effects become important, and turbulent acceleration leads to power-law tails of the energy distribution functions.more » « less
-
Abstract We study the propagation of cosmic rays (CRs) through a simulation of magnetohydrodynamic (MHD) turbulence at an unprecedented resolution of 10,2403. We drive turbulence that is subsonic and super-Alfvénic, characterized byδBrms/B0 = 2. The high resolution enables an extended inertial range such that the Alfvén scalelA, whereδB(lA) ≈ B0, is well resolved. This allows us to properly capture how the cascade transitions from large amplitudes on large scales to small amplitudes on small scales. We find that sharp bends in the magnetic field are key mediators of particle transport even on small scales via resonant curvature scattering. We further find that particle scattering in the turbulence shows strong hints of self-similarity: (1) the diffusion has a weak energy dependence over almost two decades in particle energy, and (2) the particles’ random walk exhibits a broad power-law distribution of collision times such that the diffusion is dominated by the rarest, long-distance excursions. Our results suggest that large-amplitude MHD turbulence can provide efficient scattering over a wide range of CR energies and may help explain many CR observations above a ∼TeV: the flattening of the B/C spectrum, the hardening of CR primary spectra, and the weak dependence of arrival anisotropy on CR energy.more » « less
