New Insights on the High Reconnection Rate and the Diminishment of Ion Outflow

TL;DR

This paper investigates the physical mechanisms behind electron-only reconnection, high reconnection rates, and ion outflow diminishment using particle-in-cell simulations, revealing the roles of field line bending, system size, and plasma beta.

Contribution

It introduces a new understanding of high reconnection rates and ion outflow diminishment, emphasizing the importance of ion diffusion region development and system parameters.

Findings

High normalized reconnection rate ($R_e$) remains ~0.1.

High $R_i$ is due to insufficient field line bending outside the electron diffusion region.

Ion outflow diminishes at high $eta_i$ when $ ho_i$ exceeds system size.

Abstract

The recently discovered electron-only reconnection has drawn great interests due to abnormal features like lack of ion outflows and high reconnection rates. Using particle-in-cell simulations, we investigate their physical mechanisms. The reconnection rate, when normalized by ion parameters ($R_i$), may appear anomalously high, whereas that normalized by electron parameters ($R_e$) remains ~0.1. We propose that the essence of high $R_i$ is insufficient field line bending outside the electron diffusion region, indicating an incomplete development of the ion diffusion region. It may result from bursty reconnection in thin current sheets, or small system sizes. The ion outflow diminishes at high $\beta_i$ when the gyroradius ($\rho_i$) exceeds the system size. Low-velocity ions still experience notable acceleration from Hall fields. However, a local distribution includes many high-velocity ions that experience random accelerations from different electric fields across $\rho_i$, resulting in near-zero bulk velocities. Our study helps understand reconnection structures and the underlying physics for transitions between different regimes.