宇宙線の巨大なエネルギーの源に関する新発見(A New Discovery About the Source of the Vast Energy in Cosmic Rays)

2024-12-10 コロンビア大学

＜関連情報＞

• https://news.columbia.edu/news/new-discovery-about-source-vast-energy-cosmic-rays
• https://iopscience.iop.org/article/10.3847/2041-8213/ad955f
• https://iopscience.iop.org/article/10.3847/2041-8213/ac8422

超高エネルギー宇宙線は磁気的乱流によって加速される Ultra-High-Energy Cosmic Rays Accelerated by Magnetically Dominated Turbulence

Luca Comisso, Glennys R. Farrar, and Marco S. Muzio
The Astrophysical Journal Letters  Published: 2024 December 4
DOI:10.3847/2041-8213/ad955f

Abstract

Ultra-high-energy cosmic rays (UHECRs), particles characterized by energies exceeding 10¹⁸ eV, 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_cut(E,E_cut)=sech[(E/E_cut)²]. Particle escape from the turbulent accelerating region is energy dependent, with t_esc ∝ E^−δ and δ ∼ 1/3. The resulting particle flux from the accelerator follows dN/dEdt∝E−^ssech[(E/E_cut)²], with s ∼ 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^+0.06_−0.13. Our first-principles results indicate that particle acceleration by magnetically dominated turbulence may constitute the physical mechanism responsible for UHECR acceleration.

完全運動論的プラズマ乱流におけるイオンと電子の加速 Ion and Electron Acceleration in Fully Kinetic Plasma Turbulence

Luca Comisso and Lorenzo Sironi
The Astrophysical Journal Letters  Published: 2022 September 13
DOI:10.3847/2041-8213/ac8422

Abstract

Turbulence is often invoked to explain the origin of nonthermal particles in space and astrophysical plasmas. By means of 3D fully kinetic particle-in-cell simulations, we demonstrate that turbulence in low-β plasmas (β is the ratio of plasma pressure to magnetic pressure) accelerates ions and electrons into a nonthermal energy distribution with a power-law energy range. The ion spectrum is harder than the electron one, and both distributions get steeper for higher β. We show that 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. We demonstrate that particle injection from the thermal pool occurs in regions of high current density. 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. Our findings have important implications for the origin of nonthermal particles in space and astrophysical plasmas.