Magnetic Field-Line Curvature and Its Role in Particle Acceleration by Magnetically Dominated Turbulence

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 ratio $\delta B_0/B_0$, we examine curvature $\kappa$ statistics and their connection to particle acceleration. The curvature probability densities display broad power-law wings, scaling linearly in $\kappa$ below the peak and developing hard high-$\kappa$ tails for $\delta B_0/B_0 \gtrsim 1$. As the mean field strengthens, the high-$\kappa$ tails steepen, and large-curvature events are suppressed when $\delta B_0/B_0 \ll 1$. The probability density functions of magnetic field-line contraction, ${\bf v}_E \cdot {\bf \kappa}$, with ${\bf v}_E$ the field-line velocity, develop power-law tails well described by a symmetric Pareto distribution, characteristic of stochastic energy exchanges, with the tails becoming harder as $\delta B_0/B_0$ increases. 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 increasing $\delta B_0/B_0$. For well-magnetized particles, curvature-drift acceleration typically exceeds ${\bf\nabla}B$ drift, 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.