Electron Acceleration in Magnetic Reconnection-driven Turbulence in the Earth’s Magnetotail

Abstract The Magnetospheric Multiscale mission has identified regions of magnetic reconnection surrounded by strong turbulence associated with particle acceleration in the Earth’s magnetotail. In these turbulent reconnection regions, ion and electron distributions have substantially elevated temperatures and display nonthermal tails indicating acceleration to over ∼100 times their initial thermal energies. We investigate electron energization and acceleration using 3D test-particle simulations based on the measured magnetic and electric fields in a representative magnetic reconnection geometry. We explore stochastic, Fermi, betatron, Speiser, and turbulence-boosted Speiser-like energization and acceleration. We find that stochastic energization and Speiser-like motion (Speiser orbits and turbulence-boosted Speiser-like orbits) dominate energization and acceleration at high turbulence levels and make the largest contribution to the nonthermal tail. Electron energization comes mostly from second-order impulses from electric fields with frequencies above the ion cyclotron frequency. Perpendicular electric fields contribute to the majority of the energization (∼75%). However, the exiting electron fluxes show the reverse; most (∼62%) of the electron energy flux exits as parallel energy. Trapping in the low magnetic field near the magnetic reconnection current sheet plays an important role by increasing dwell time in the turbulence, which enhances stochastic and Speiser-like processes. One of our primary conclusions is that magnetic reconnection and turbulence appear to combine as an efficient accelerator; the turbulent acceleration near a magnetic reconnection current sheet favors high-energy electrons, generating the energetic tail. Given its widespread presence in space and astrophysical plasmas, turbulent magnetic reconnection should be considered an important source of accelerated electrons.