Concurrent Particle Acceleration and Pitch-angle Anisotropy Driven by Magnetic Reconnection: Ion-electron Plasmas
Luca Comisso
Abstract
Abstract Particle acceleration and pitch-angle anisotropy resulting from magnetic reconnection are investigated in highly magnetized ion-electron plasmas. By means of fully kinetic particle-in-cell simulations, we demonstrate that magnetic reconnection generates anisotropic particle distributions <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>f</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>s</mml:mi> </mml:mrow> </mml:msub> <mml:mfenced close=")" open="("> <mml:mrow> <mml:mo stretchy="false">∣</mml:mo> <mml:mi>cos</mml:mi> <mml:mi>α</mml:mi> <mml:mo stretchy="false">∣</mml:mo> <mml:mo>,</mml:mo> <mml:mi>ε</mml:mi> </mml:mrow> </mml:mfenced> </mml:math> , characterized by broken power laws in the particle energy spectrum f s ( ε ) ∝ ε − p and pitch angle <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo stretchy="false">〈</mml:mo> <mml:msup> <mml:mrow> <mml:mi>sin</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> <mml:mi>α</mml:mi> <mml:mo stretchy="false">〉</mml:mo> <mml:mo>∝</mml:mo> <mml:msup> <mml:mrow> <mml:mi>ε</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>m</mml:mi> </mml:mrow> </mml:msup> </mml:math> . The characteristics of these distributions are determined by the relative strengths of the magnetic field’s guide and reconnecting components ( B g / B 0 ) and the plasma magnetization ( σ 0 ). Below the injection break energy ε 0 , ion and electron energy spectra are extremely hard ( p < ≲ 1) for any B g / B 0 and σ 0 ≳ 1, while above ε 0 the spectral index steepens ( p > ≳ 2), displaying high sensitivity to both B g / B 0 and σ 0 . The pitch angle displays power-law ranges with negative slopes ( m < ) below and positive slopes ( m > ) above <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>ε</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>min</mml:mi> <mml:mi>α</mml:mi> </mml:mrow> </mml:msub> </mml:math> , steepening with increasing B g / B 0 and σ 0 . The ratio B g / B 0 regulates the redistribution of magnetic energy between ions (Δ E i ) and electrons (Δ E e ), with Δ E i ≫ Δ E e for B g / B 0 ≪ 1, Δ E i ∼ Δ E e for B g / B 0 ∼ 1, and Δ E i ≪ Δ E e for B g / B 0 ≫ 1, with Δ E i /Δ E e approaching unity when σ 0 ≫ 1. The anisotropic distribution of accelerated particles results in an optically thin synchrotron power spectrum F ν ( ν ) ∝ ν (2−2 p + m )/(4+ m ) and a linear polarization degree Π lin = ( p + 1)/( p + 7/3 + m /3) for a uniform magnetic field. Pitch-angle anisotropy also induces temperature anisotropy and eases synchrotron cooling, along with producing beamed radiation aligned with the magnetic field, which is potentially responsible for rapid frequency-dependent variability.