Electron-only reconnection and ion heating in 3D3V hybrid-Vlasov plasma turbulence

TL;DR

This study uses 3D hybrid-Vlasov simulations to explore electron-only reconnection and ion heating in turbulence near ion scales, revealing how plasma beta influences ion-electron decoupling and energy distribution in collisionless plasmas.

Contribution

It demonstrates the role of ion beta in electron-only reconnection and ion heating, highlighting the importance of inertial scales and turbulence injection in collisionless plasma dynamics.

Findings

Ion-electron decoupling increases with plasma beta.

Significant ion heating occurs across all beta values studied.

Ion heating is anisotropic at low beta and marginally isotropic at high beta.

Abstract

We perform 3D3V hybrid-Vlasov simulations of turbulence with quasi-isotropic, compressible injection near ion scales to mimic the Earth's magnetosheath plasma, and investigate the novel electron-only reconnection, recently observed by the NASA's MMS mission, and its impact on ion heating. Retaining electron inertia in the generalized Ohm s law enables collisionless magnetic reconnection. Spectral analysis shows a shift from kinetic Alfv\'en waves (KAW) to inertial kinetic Alfv\'en (IKAW) and inertial whistler waves (IWW) near electron scales. To distinguish the roles of inertial scale and gyroradius ($d_{\rm{i}}$ and $\rho_{\rm{i}}$), three ion beta ($\beta_{\rm{i}} = 0.25, 1, 4$) values are studied. Ion-electron decoupling increases with $\beta_{\rm{i}}$, as ions become less mobile when the injection scale is closer to $\rho_{\rm{i}}$ than $d_{\rm{i}}$, highlighting the role of $\rho_{\rm{i}}$ in achieving an electron magnetohydrodynamic (EMHD) regime at sub-ion scales. This regime promotes electron-only reconnection in turbulence with small-scale injection at $\beta_{\rm{i}} \gtrsim 1$. We observe significant ion heating even at large $\beta_{\rm{i}}$, with $Q_{\rm{i}}/\epsilon \approx 69\%, 91\%, 96\%$ at $\beta_{\rm{i}} = 0.25, 1, 4$ respectively. While ion heating is anisotropic at $\beta_{\rm{i}} \leq 1$ ($T_{\rm i,\perp} > T_{\rm i,\parallel}$), it is marginally anisotropic at $\beta_{\rm{i}} > 1$ ($T_{\rm i,\perp} \gtrsim T_{\rm i,\parallel}$). These findings have implications for other collisionless astrophysical environments, like high-$\beta$ plasmas in intracluster medium, where processes such as micro-instabilities or shocks may inject energy near ion-kinetic scales.