[Paper Review] Anisotropic electron heating in turbulence-driven magnetic reconnection in the near-Sun solar wind

Luca Franci, Emanuele Papini+6 more|arXiv (Cornell University)|May 18, 2022

Solar and Space Plasma Dynamics118 references32 citations

TL;DR

This study uses a high-resolution 2D fully-kinetic simulation to investigate turbulence-driven magnetic reconnection in the near-Sun solar wind, revealing that electron-scale coherent structures—particularly reconnection events—drive strong anisotropic electron heating. The key finding is that electron temperature increases sharply during reconnection, with significant parallel temperature anisotropy developing in reconnection outflows, indicating that localized, impulsive reconnection events are more efficient at heating electrons than wave-like damping mechanisms.

ABSTRACT

We perform a high-resolution two-dimensional fully-kinetic numerical simulation of a turbulent plasma system with observation-driven conditions, in order to investigate the interplay between turbulence, magnetic reconnection, and particle heating from ion to sub-electron scales in the near-Sun solar wind. We find that the power spectra of the turbulent plasma and electromagnetic fluctuations show multiple power-law intervals down to scales smaller than the electron gyroradius. Magnetic reconnection is observed to occur in correspondence of current sheets with a thickness of the order of the electron inertial length, which form and shrink due to interacting ion-scale vortexes. In some cases, both ion and electron outflows are observed (the classic reconnection scenario), while in others -- typically for the shortest current sheets -- only electron jets are presents ("electron-only reconnection"). At the onset of reconnection, the electron temperature starts to increase and a strong parallel temperature anisotropy develops. This suggests that in strong turbulence electron-scale coherent structures may play a significant role for electron heating, as impulsive and localized phenomena such as magnetic reconnection may transfer energy from the electromagnetic fields to particles more efficiently than damping mechanisms related to interactions with wave-like fluctuations.

Motivation & Objective

• To investigate the interplay between turbulence, magnetic reconnection, and particle heating from ion to sub-electron scales in the near-Sun solar wind.
• To determine the role of electron-scale coherent structures, such as thin current sheets and reconnection events, in heating electrons in a turbulent, collisionless plasma.
• To examine whether magnetic reconnection—especially 'electron-only' reconnection—acts as a dominant heating mechanism at sub-electron scales.
• To assess the impact of turbulence correlation length and current sheet thickness on the nature of reconnection (standard vs. electron-only).

Proposed method

• Performs a 2D fully-kinetic simulation with plasma parameters representative of the near-Sun solar wind (mi/me = 100, c/vAi = 200, c/vAe = 20).
• Uses initial conditions driven by observational constraints to simulate turbulence from ion to sub-electron scales.
• Applies a reconnection detection algorithm based on current density thresholds, velocity and temperature jumps, and J·E energy transfer to identify reconnection events.
• Analyzes power spectra of electromagnetic and plasma fluctuations to identify multiple power-law intervals down to scales below the electron gyroradius.
• Tracks electron temperature anisotropy (T∥e / T⊥e) and its spatial correlation with reconnection outflows and X-points.
• Compares standard reconnection (with ion and electron jets) to 'electron-only' reconnection (only electron jets) to assess their heating efficiency.

Experimental results

Research questions

• RQ1 How does turbulence-driven magnetic reconnection influence electron heating at sub-electron scales in the near-Sun solar wind?
• RQ2 What is the role of electron-scale coherent structures—particularly thin current sheets—in generating anisotropic electron heating?
• RQ3 Under what conditions does 'electron-only' reconnection occur, and how does it differ in heating efficiency from standard reconnection?
• RQ4 How do the spectral properties of turbulent fluctuations evolve across ion and electron scales, and what do they reveal about the energy cascade?
• RQ5 To what extent do localized reconnection events contribute more to electron heating than wave-like damping mechanisms?

Key findings

• The power spectra of turbulent fluctuations exhibit multiple power-law intervals, with spectral indices decreasing from α ≈ −5/3 in the inertial range to α ≈ −4 at sub-ion scales and further to α ≈ −5 at electron scales.
• Magnetic reconnection occurs at current sheets with thickness on the order of the electron inertial length, formed and shrunk by interacting ion-scale vortexes.
• Both standard reconnection (with ion and electron outflows) and 'electron-only' reconnection (only electron jets) are observed, with the latter occurring predominantly in the shortest current sheets.
• Electron temperature increases sharply at the onset of reconnection, with a strong parallel temperature anisotropy (T∥e / T⊥e > 1) developing in reconnection outflows.
• The regions of maximum electron temperature increase are spatially localized in reconnection outflows, confirming that reconnection is a primary driver of anisotropic electron heating.
• The transition from standard to electron-only reconnection is linked to current sheet length and turbulence correlation length, with shorter sheets favoring electron-only events.