Kinetic Cascade in Solar-wind Turbulence: 3D3V Hybrid-kinetic Simulations with Electron Inertia

Understanding the nature of the turbulent fluctuations below the ion gyroradius in solar-wind (SW) turbulence is a great challenge. Recent studies have been mostly in favor of kinetic Alfvén wave (KAW)-type fluctuations, but other kinds of fluctuations with characteristics typical of magnetosonic, whistler, and ion-Bernstein modes could also play a role depending on the plasma parameters. Here, we investigate the properties of the subproton-scale cascade with high-resolution hybrid-kinetic simulations of freely decaying turbulence in 3D3V phase space, including electron inertia effects. Two proton plasma beta are explored: the “intermediate” β _p = 1 and “low” β _p = 0.2 regimes, both typically observed in the SW and corona. The magnetic energy spectum exhibits {k}_\perp ^-8/3 and {k}_\parallel ^-7/2 power laws at β _p = 1, while they are slightly steeper at β _p = 0.2. Nevertheless, both regimes develop a spectral anisotropy consistent with {k}_\parallel ∼ {k}_\perp ^2/3 at {k}_\perp {ρ }_p> 1 and pronounced small-scale intermittency. In this context, we find that the kinetic-scale cascade is dominated by KAW-like fluctuations at β _p = 1, whereas the low-β case presents a more complex scenario suggesting the simultaneous presence of different types of fluctuations. In both regimes, however, a possible role of the ion-Bernstein-type fluctuations at the smallest scales cannot be excluded.