Particle Acceleration in Strong Turbulence in the Earth’s Magnetotail

In many space, astrophysical, and laboratory plasmas the energy contained in the magnetic field or plasma flow exceeds the thermal energy. Magnetic field annihilation, often enabled by magnetic reconnection, transfers magnetic energy to particles. Shocks transfer bulk flow energy to particles. If there is a sufficiently large energy transfer, strong turbulence develops, which, in turn, can result in nonthermal acceleration. In this article, we investigate acceleration in a finite-sized region of strong turbulence driven by magnetic reconnection with analytical modeling and test-particle simulations. This research is based on detailed observations in the Earth's magnetotail. We find that the primary transfer of magnetic energy to particle energy is advanced by large-amplitude electric field structures generated by the strong turbulence. To no surprise, ion energization is dominated by intense DC, near the ion cyclotron frequency (fci), and/or variations at scales near the ion gyroradius. Electron energization comes from higher-frequency. The turbulent cascade continuously regenerates near fci and higher frequencies. Importantly, the turbulence also creates magnetic depletions that can trap particles and considerably increase their dwell time in regions of strong energization, which substantially enhances nonthermal acceleration. Moreover, energization is primarily perpendicular to, so particles have difficulty escaping regions of depleted, which can lead to near runaway acceleration. We discuss how this process may be active in large-scale settings such as supernova shells and may contribute, at least in in part, to the development of the cosmic ray spectrum.