The role of particle feedback on particle acceleration in magnetic reconnection

TL;DR

Problem: How does particle feedback affect acceleration during magnetic reconnection in large-scale plasmas? Approach: A 2.5D MHD-PIC framework in PLUTO with co-evolving fluid and CR particles, exploring four models with/without feedback and with/without guide field. Findings: Particle feedback amplifies shear flows inside magnetic islands, increasing the convective electric field $c\mathbf E=-\mathbf v_g\times\mathbf B$ and producing higher maximum particle energies and a harder energy spectrum; a guide field suppresses energy transfer to particles and reduces internal energy growth. Implications: Highlights the importance of fluid-particle coupling in reconnection-driven acceleration and informs models of cosmic-ray production in astrophysical reconnection sites.

Abstract

Magnetic reconnection is a ubiquitous process in astrophysical plasmas and an efficient mechanism for particle acceleration. Using 2.5D magnetohydrodynamic (MHD) simulations with a co-evolving fluid-particle framework, we investigate how particle feedback affects reconnection and acceleration. Our simulations demonstrate that particle feedback to the fluid amplifies shear flows within magnetic islands, which strengthens the convective electric field and thereby boosts particle acceleration. This mechanism results in a higher maximum particle energy and a harder non-thermal energy spectrum. The guide field suppresses both the increase in gas internal energy and particle acceleration. These findings highlight the complex interplay between feedback, guide fields, and reconnection dynamics.

Figures (7)

• Figure 1: Plasma density (color bar) snapshots at four simulation times: (a) $t = 1.0 \times 10^5 \omega_{\rm p}^{-1}$; (b) $t = 2.4\times10^5 \omega_{\rm p}^{-1}$; (c) $t = 2.6\times10^5 \omega_{\rm p}^{-1}$ and (d) $t = 6.0\times10^5 \omega_{\rm p}^{-1}$. The data are based on the M1 model.
• Figure 2: Reconnection rate (left axis) and $B_{\rm z,rms}$ (right axis) as functions of time.
• Figure 3: Magnetic energy (panel (a)), kinetic energy (panel (b)) and internal energy (panel (c)) as functions of time. The insert plots in panels (b) and (c) are enlarged versions of the curves after $t\geq 4.0\times10^5\omega_{\rm p}^{-1}$. For convenience, the total magnetic energy is calculated as the sum of the magnetic field energies of the x and y components, i.e., $E_{\rm b} = E_{\rm bx}+E_{\rm by}$.
• Figure 4: Kinetic energy of all particles (panel (a), showing average values) and a selected particle (panel (b)) as functions of time for four models. We select the particle with the highest energy in the final snapshot for all models.
• Figure 5: Panel (a): Temporal evolution of the position of a representative accelerating CR proton, with color encoding kinetic energy. These data points are sampled at uniform time intervals ($\Delta t = 5\times 10^3\omega_{\rm p}^{-1}$) cover the entire evolution period of the system. The background shows the current density distribution at $t=2.6 \times 10^5 \omega_{\rm p}^{-1}$, and the purple line denotes the particle trajectory during the peak acceleration phase ($2.5 \times 10^5 \omega_{\rm p}^{-1}\leq t \leq 2.7 \times 10^5 \omega_{\rm p}^{-1}$). Panel (b): The spatial distribution of particles with different energies at the final snapshot. The data are based on M2. Panels (c) and (d): Current density and the position distribution of the ten highest energy particles at $t = 4.0\times10^5~\omega_{\rm p}^{-1}$. The dashed line represents the centerline of the magnetic island in the y-direction. The data are based on M1 and M2 models.