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

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 Alfvén waves (KAW) to inertial kinetic Alfvén (IKAW) and inertial whistler waves (IWW) near electron scales. To distinguish the roles of inertial scale and gyroradius ($d_{\mathrm{i}}$ and $\ddot{I}{}_{\mathrm{i}}$), three ion beta (${\hat{I}}_{\mathrm{i}}^2=0.25,1,4$) values are studied. Ion-electron decoupling increases with ${\hat{I}}_{\mathrm{i}}^2$, as ions become less mobile when the injection scale is closer to $\ddot{I}{}_{\mathrm{i}}$ than $d_{\mathrm{i}}$, highlighting the role of $\ddot{I}{}_{\mathrm{i}}$ in achieving an electron magnetohydrodynamic (EMHD) regime at sub-ion scales. This regime promotes electron-only reconnection in turbulence with small-scale injection at ${\hat{I}}_{\mathrm{i}}^2\gtrsim 1$. We observe significant ion heating even at large ${\hat{I}}_{\mathrm{i}}^2$, with $Q_{\mathrm{i}}/\hat{I}\mu \approx 69\%,91\%,96\%$ at ${\hat{I}}_{\mathrm{i}}^2=0.25,1,4$ respectively. While ion heating is anisotropic at ${\hat{I}}_{\mathrm{i}}^2\le 1$ ($T_{\mathrm{i},\perp }>T_{\mathrm{i},\parallel }$), it is marginally anisotropic at ${\hat{I}}_{\mathrm{i}}^2>1$ ($T_{\mathrm{i},\perp }\gtrsim T_{\mathrm{i},\parallel }$). These findings have implications for other collisionless astrophysical environments, like high-${\hat{I}}^2$ plasmas in intracluster medium, where processes such as micro-instabilities or shocks may inject energy near ion-kinetic scales.

Accepted