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

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 &amp; G. P. Zank. Through this investigation, an Alfvénic-like Fermi electron acceleration mechanism is discovered for strong guide field MR B g / B 0 &gt; 2.5, where B g is the magnetic guide field. The electrons are accelerated by the intensive electric potential produced through <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>δ</mml:mi> <mml:msub> <mml:mrow> <mml:mi mathvariant="bold-italic">U</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="bold-italic">i</mml:mi> </mml:mrow> </mml:msub> <mml:mo>×</mml:mo> <mml:mrow> <mml:mi mathvariant="bold-italic">B</mml:mi> </mml:mrow> </mml:math> , where the ion velocity fluctuations <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>δ</mml:mi> <mml:msub> <mml:mrow> <mml:mi mathvariant="bold-italic">U</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="bold-italic">i</mml:mi> </mml:mrow> </mml:msub> </mml:math> 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.