The Effect of Thermal Pressure on Collisionless Magnetic Reconnection Rate

TL;DR

This paper investigates how thermal pressure influences collisionless magnetic reconnection rates, revealing that higher plasma beta reduces the reconnection rate and deriving a scaling law for this effect through simulations and analytical modeling.

Contribution

It introduces a new analytical framework that accounts for thermal pressure effects on reconnection rates at higher plasma beta, extending beyond previous low-beta models.

Findings

Reconnection rate decreases with increasing plasma beta.

Derived scaling law: reconnection rate ~ 0.1 / sqrt(beta_i0).

Simulation results confirm the analytical predictions.

Abstract

Modeling collisionless magnetic reconnection rate is an outstanding challenge in basic plasma physics research. While the seemingly universal rate of an order $\mathcal{O}(0.1)$ is often reported in the low-$\beta$ regime, it is not clear how reconnection rate scales with a higher plasma $\beta$. Due to the complexity of the pressure tensor, the available reconnection rate model is limited to the low plasma-$\beta$ regime, where the thermal pressure is arguably negligible. However, the thermal pressure effect becomes important when $\beta \gtrsim \mathcal{O}(1)$. Using first-principle kinetic simulations, we show that both the reconnection rate and outflow speed drop as $\beta$ gets larger. A simple analytical framework is derived to take account of the self-generated pressure anisotropy and pressure gradient in the force-balance around the diffusion region, explaining the varying trend of key quantities and reconnection rates in these simulations with different $\beta$. The predicted scaling of the normalized reconnection rate is $\simeq \mathcal{O}(0.1/\sqrt{\beta_{i0}})$ in the high $\beta$ limit, where $\beta_{i0}$ is the ion $\beta$ of the inflow plasma.