Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

Nonthermal electron acceleration in turbulent post-flare coronal loops

Clarissa Mora, Fabio Bacchini, Rony Keppens

TL;DR

This study investigates how Kelvin-Helmholtz-instability–induced turbulence in post-flare coronal looptops accelerates and traps electrons, using test-particle electrons embedded in a time-evolving 2.5D MHD looptop model. A key methodological advance is an energy-conserving guiding-centre approximation (GCA) that explicitly evolves the perpendicular energy, resolving spurious energy changes that plagued standard GCA implementations, and is validated with a 2D Alfvén-wave test. In turbulence, electrons develop suprathermal tails with a net energization of about $ abla E \\sim 0.8\%$ of their initial energy over one MHD time, dominated by perpendicular-gradient effects (second-order Fermi-like acceleration), while curvature effects contribute for long trajectories; strong energization is associated with particles trapped in bouncing motions within magnetic structures. The results support the view that KHI-driven looptop turbulence can sustain nonthermal electron populations responsible for looptop X-ray emission, while acknowledging 2.5D limitations and outlining a path toward fully 3D, kinetic-accurate extensions and proton acceleration studies.

Abstract

The generation of nonthermal electrons during solar flares plays a critical role in energy transport from the corona to the chromosphere, producing regions of observed intense X-ray emission. Turbulence in post-flare loops, particularly from Kelvin-Helmholtz instabilities (KHI), has been suggested and investigated as a mechanism for trapping and accelerating electrons in such scenarios. Starting from past results, we aim to characterize the energization process of electrons trapped in a turbulent post-flare looptop, quantifying the contributions of different acceleration mechanisms, and establishing a coherent numerical framework for describing particle energetics. We perform test-particle simulations with the guiding-centre approximation on top of a 2.5D magnetohydrodynamic model of a time-evolving post-flare coronal looptop. We implement an improved formulation of the guiding-centre equations that explicitly conserves energy, enabling a consistent analysis of electron acceleration in the turbulent plasma. We find that, in the plasma turbulence inside the looptop, electrons develop suprathermal energy distributions with tails compatible with hard X-ray emission. The dominant energization channel arises from perpendicular gradient effects in the form of second-order Fermi-like stochastic acceleration, while curvature effects are dominant for particles on long trajectories. Statistical correlations with the measured particle pitch angle confirm that the strongest acceleration occurs for electrons trapped in bouncing motions within turbulent magnetic structures. Our results provide an understanding of how KHI-induced turbulence in coronal looptops produces and sustains populations of trapped nonthermal electrons. We dissect and clarify the relative roles of different magnetic effects and the emergence of stochastic Fermi-like energization.

Nonthermal electron acceleration in turbulent post-flare coronal loops

TL;DR

This study investigates how Kelvin-Helmholtz-instability–induced turbulence in post-flare coronal looptops accelerates and traps electrons, using test-particle electrons embedded in a time-evolving 2.5D MHD looptop model. A key methodological advance is an energy-conserving guiding-centre approximation (GCA) that explicitly evolves the perpendicular energy, resolving spurious energy changes that plagued standard GCA implementations, and is validated with a 2D Alfvén-wave test. In turbulence, electrons develop suprathermal tails with a net energization of about of their initial energy over one MHD time, dominated by perpendicular-gradient effects (second-order Fermi-like acceleration), while curvature effects contribute for long trajectories; strong energization is associated with particles trapped in bouncing motions within magnetic structures. The results support the view that KHI-driven looptop turbulence can sustain nonthermal electron populations responsible for looptop X-ray emission, while acknowledging 2.5D limitations and outlining a path toward fully 3D, kinetic-accurate extensions and proton acceleration studies.

Abstract

The generation of nonthermal electrons during solar flares plays a critical role in energy transport from the corona to the chromosphere, producing regions of observed intense X-ray emission. Turbulence in post-flare loops, particularly from Kelvin-Helmholtz instabilities (KHI), has been suggested and investigated as a mechanism for trapping and accelerating electrons in such scenarios. Starting from past results, we aim to characterize the energization process of electrons trapped in a turbulent post-flare looptop, quantifying the contributions of different acceleration mechanisms, and establishing a coherent numerical framework for describing particle energetics. We perform test-particle simulations with the guiding-centre approximation on top of a 2.5D magnetohydrodynamic model of a time-evolving post-flare coronal looptop. We implement an improved formulation of the guiding-centre equations that explicitly conserves energy, enabling a consistent analysis of electron acceleration in the turbulent plasma. We find that, in the plasma turbulence inside the looptop, electrons develop suprathermal energy distributions with tails compatible with hard X-ray emission. The dominant energization channel arises from perpendicular gradient effects in the form of second-order Fermi-like stochastic acceleration, while curvature effects are dominant for particles on long trajectories. Statistical correlations with the measured particle pitch angle confirm that the strongest acceleration occurs for electrons trapped in bouncing motions within turbulent magnetic structures. Our results provide an understanding of how KHI-induced turbulence in coronal looptops produces and sustains populations of trapped nonthermal electrons. We dissect and clarify the relative roles of different magnetic effects and the emergence of stochastic Fermi-like energization.
Paper Structure (8 sections, 6 equations, 5 figures)

This paper contains 8 sections, 6 equations, 5 figures.

Table of Contents

  1. Introduction
  2. Numerical method
  3. Ideal MHD setup
  4. Particle motion: Guiding Centre Approximation
  5. Energy-conserving GCA particle integration
  6. Test case: Alfvén wave propagation
  7. Electron energization in a turbulent coronal looptop
  8. Conclusions

Figures (5)

  • Figure 1: Distribution functions for individual "i" type contributions to particle energization at the initial test-particle simulation time $t = 2.5t_0$ inside the turbulent looptop.
  • Figure 2: Left column: failing perpendicular (a) and fulfilled parallel (c) energy conservation for a the test particle evolving in the Alfvén wave test case. Right column: (b) fixed perpendicular energy after the new GCA implementation and (d) effect of the new equation on the magnetic moment evolution in time. $\mu_{const}$ is the constant value of the magnetic moment in the standard GCA evolution, while $\mu$ is the varying value in the current new energy-conserving GCA implementation.
  • Figure 3: Evolution of electron distributions for total energy (top-left panel), parallel energy (top-right panel), and perpendicular energy (bottom-left panel) for $t \in [2.5, 3.5]t_0$. Distributions are evenly spaced in time with a cadence $\delta t = 0.05t_0$. The evolution of the mean energy relative to the initial energy, $\gamma/\gamma_0$ with $\gamma \leq 2$, is shown in the bottom-right panel for the same time period.
  • Figure 4: Top panel: trajectories of 9 representative particles during the time interval $[2.65-2.75 ] \ t_0$ on top of the 2D coronal loop magnetic field structure at $t = 2.65t_0$, colored by the in-plane field intensity (code units) and threaded by field lines. Bottom: plots of the total energy evolution (integral form of Eqs.[\ref{['Wtot']}], in code units) decomposed into single-term contributions.
  • Figure 5: Two-dimensional distributions of particle energy gain $\Delta \gamma$ as a function of pitch angle $\theta$ for total, parallel, and perpendicular energies.