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 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.