On stochastic heating and its phase-space signatures in low-$\beta$ kinetic turbulence

Abstract

We revisit the theory of stochastic heating of ions and investigate its phase-space signatures in kinetic turbulence of relevance to low-$\beta$ portions of the solar wind. We retain a full scale-dependent approach in our treatment, and consider the case in which electric-field fluctuations can be described by a generalized Ohm's law that includes Hall and thermo-electric effects. These two electric-field terms provide the dominant contributions to stochastic ion heating when the ion-Larmor scale is much smaller than the ion skin depth, $\rho_{\mathrm{i}}\ll d_{\mathrm{i}}$, which is the case at $\beta{\ll}1$. Employing well-known spectral scaling laws for Alfv\'en-wave and kinetic-Alfv\'en-wave turbulent fluctuations, we obtain scaling relations characterizing the field-perpendicular particle-energization rate and energy diffusion coefficient associated with stochastic heating in these two regimes. Phase-space signatures of ion heating are then investigated using 3D hybrid-kinetic simulations of continuously driven Alfv\'enic turbulence at low $\beta$. In these simulations, energization of ions parallel to the magnetic field is sub-dominant compared to its perpendicular counterpart ($Q_{\parallel,\mathrm{i}}\ll Q_{\perp,\mathrm{i}}$), and the fraction of turbulent energy that goes into ion heating is ${\approx}75$\% at $\beta_{\mathrm{i}}=0.3$ and ${\approx}40$\% at $\beta_{\mathrm{i}}{\simeq}0.1$. The phase-space signatures of ion energization are consistent with Landau-resonant collisionless damping and a ($\beta$-dependent) combination of ion-cyclotron and stochastic heating. We demonstrate good agreement between our theory and various signatures associated with the stochastic portion of the heating. We discuss the effect of intermittency on stochastic heating and the implications of our work for the interpretation of stochastic heating in solar-wind spacecraft data.