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1. Ultra-High-Energy Cosmic Rays Accelerated by Magnetically Dominated Turbulence

   https://doi.org/10.3847/2041-8213/ad955f  

   Comisso, Luca; Farrar, Glennys R; Muzio, Marco S (December 2024, The Astrophysical Journal Letters)

   Abstract Ultra-high-energy cosmic rays (UHECRs), particles characterized by energies exceeding 10¹⁸eV, are generally believed to be accelerated electromagnetically in high-energy astrophysical sources. One promising mechanism of UHECR acceleration is magnetized turbulence. We demonstrate from first principles, using fully kinetic particle-in-cell simulations, that magnetically dominated turbulence accelerates particles on a short timescale, producing a power-law energy distribution with a rigidity-dependent, sharply defined cutoff well approximated by the form $f_{\mathrm{cut}}\left(E,E_{\mathrm{cut}}\right)=\mathrm{sech}\left({\left(E/E_{\mathrm{cut}}\right)}^2\right)$. Particle escape from the turbulent accelerating region is energy dependent, witht_esc∝E^−δandδ∼ 1/3. The resulting particle flux from the accelerator follows $\mathit{dN}/\mathit{dEdt}\propto E^{-s}\mathrm{sech}\left({\left(E/E_{\mathrm{cut}}\right)}^2\right)$, withs∼ 2.1. We fit the Pierre Auger Observatory’s spectrum and composition measurements, taking into account particle interactions between acceleration and detection, and show that the turbulence-associated energy cutoff is well supported by the data, with the best-fitting spectral index being $s={2.1}_{-0.13}^{+0.06}$. Our first-principles results indicate that particle acceleration by magnetically dominated turbulence may constitute the physical mechanism responsible for UHECR acceleration. 

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2. Intermittency and Lower Dimensional Dissipation in Incompressible Fluids

   https://doi.org/10.1007/s00205-023-01954-w  

   De_Rosa, Luigi; Isett, Philip (February 2024, Archive for Rational Mechanics and Analysis)

   Abstract In the context of incompressible fluids, the observation that turbulent singular structures fail to be space filling is known as “intermittency”, and it has strong experimental foundations. Consequently, as first pointed out by Landau, real turbulent flows do not satisfy the central assumptions of homogeneity and self-similarity in the K41 theory, and the K41 prediction of structure function exponents$$\zeta _p={p}/{3}$$ ${\zeta }_p=p/3$might be inaccurate. In this work we prove that, in the inviscid case, energy dissipation that is lower-dimensional in an appropriate sense implies deviations from the K41 prediction in everyp-th order structure function for$$p>3$$ $p>3$. By exploiting a Lagrangian-type Minkowski dimension that is very reminiscent of the Taylor’sfrozen turbulencehypothesis, our strongest upper bound on$$\zeta _p$$ ${\zeta }_p$coincides with the$$\beta $$ $\beta$-model proposed by Frisch, Sulem and Nelkin in the late 70s, adding some rigorous analytical foundations to the model. More generally, we explore the relationship between dimensionality assumptions on the dissipation support and restrictions on thep-th order absolute structure functions. This approach differs from the current mathematical works on intermittency by its focus on geometrical rather than purely analytical assumptions. The proof is based on a new local variant of the celebrated Constantin-E-Titi argument that features the use of a third order commutator estimate, the special double regularity of the pressure, and mollification along the flow of a vector field. 

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3. Kinetic Simulations of the Kruskal–Schwarzschild Instability in Accelerating Striped Outflows: Dynamics and Energy Dissipation

   https://doi.org/10.3847/1538-4357/adc446  

   Groger, William; Hakobyan, Hayk; Sironi, Lorenzo (April 2025, The Astrophysical Journal)

   Abstract Astrophysical relativistic outflows are launched as Poynting-flux dominated, yet the mechanism governing efficient magnetic dissipation, which powers the observed emission, is still poorly understood. We study magnetic energy dissipation in relativistic “striped” jets, which host current sheets separating magnetically dominated regions with opposite field polarity. The effective gravity forcegin the rest frame of accelerating jets drives the Kruskal–Schwarzschild instability (KSI), a magnetic analog of the Rayleigh–Taylor instability. By means of 2D and 3D particle-in-cell simulations, we study the linear and nonlinear evolution of the KSI. The linear stage is well described by linear stability analysis. The nonlinear stages of the KSI generate thin (skin-depth-thick) current layers, with length comparable to the dominant KSI wavelength. There, the relativistic drift-kink mode and the tearing mode drive efficient magnetic dissipation. The dissipation rate can be cast as an increase in the effective width Δ_effof the dissipative region, which follows $d{\Delta }_{\mathrm{eff}}/dt\simeq 0.05\sqrt{{\Delta }_{\mathrm{eff}}g}$. Our results have important implications for the location of the dissipation region in gamma-ray burst and active galactic nuclei jets. 

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4. Angular Dependence of Third-order Law in Anisotropic MHD Turbulence

   https://doi.org/10.3847/1538-4357/ae4a16  

   Jiang, Bin; Gao, Zhuoran; Yang, Yan; Pecora, Francesco; Gao, Kai; Li, Cheng; Oughton, Sean; Matthaeus, William H; Wan, Minping (March 2026, The Astrophysical Journal)

   Abstract In solar wind turbulence, the energy transfer/dissipation rate is typically estimated using MHD third-order structure functions calculated using spacecraft observations. However, the inherent anisotropy of solar wind turbulence leads to significant variations in structure functions along different observational directions, thereby affecting the accuracy of energy dissipation rate estimation. An unresolved issue is how to optimise the selection of observation angles under limited directional sampling to improve estimation precision. We conduct a series of MHD turbulence simulations with different mean magnetic field strengths,B₀. Our analysis of the third-order structure functions reveals that the global energy dissipation rate estimated around a polar angle ofθ = 60^∘agrees reasonably with the exact one for 0 ≤B₀/b_rms≤ 5, whereb_rmsdenotes the rms magnetic field fluctuation. The speciality of 60^∘polar angle can be understood by the mean value theorem of integrals, since the spherical integral of the polar-angle component ( $\widetilde{T_{\theta }}$) of the divergence of Yaglom flux is zero, and $\widetilde{T_{\theta }}$changes sign around 60^∘. Existing theory on the energy flux vector as a function of the polar angle is assessed, and supports the speciality of the 60^∘polar angle. The angular dependence of the third-order structure functions is further assessed with virtual spacecraft data analysis. The present results can be applied to measure the turbulent dissipation rates of energy in the solar wind, which are of potential importance to other areas in which turbulence takes place, such as laboratory plasmas and astrophysics. 

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5. Transition from Vortical to Alfvénic-like Fermi Electron Acceleration in Magnetic Reconnection with Increasing Guide Field

   https://doi.org/10.3847/1538-4357/adbc60  

   Crawford, C; Che, H; Zank, G P; Benz, A O (April 2025, The Astrophysical Journal)

   Abstract Using particle-in-cell simulations of magnetic reconnection (MR), we investigate how the changing magnetic guide field strength impacts the evolution of electron Kelvin–Helmholtz instability (EKHI) and the associated Fermi electron acceleration proposed by H. Che & G. P. Zank. Through this investigation, an Alfvénic-like Fermi electron acceleration mechanism is discovered for strong guide field MRB_g/B₀ > 2.5, whereB_gis the magnetic guide field. The electrons are accelerated by the intensive electric potential produced through $\delta {\mathit{U}}_{\mathit{i}}\times \mathit{B}$, where the ion velocity fluctuations $\delta {\mathit{U}}_{\mathit{i}}$propagate parallel to the direction of the Alfvén-like waves. Differing from the two-stage second-order Fermi acceleration produced by the stochastic electric field of EKHI, the Alfvén-like wave mechanism is a much more efficient one-stage process that produces a much harder power-law electron energy spectrum, with an index ∼2, than that of the EKHI, with an index ∼4. 

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