On Stochastic Heating and Its Phase-space Signatures in Low-beta Kinetic Turbulence

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. In particular, we retain a full scale-dependent approach in our treatment, and we explicitly consider the case in which electric-field fluctuations can be described by a generalized Ohm's law that includes Hall and thermoelectric 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 (i) MUCH LESS-THAN d (i), which is the case at beta MUCH LESS-THAN 1. Employing well-known spectral scaling laws for Alfven-wave and kinetic-Alfven-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 three-dimensional hybrid-kinetic simulations of continuously driven Alfvenic turbulence at low beta (namely, beta (i) = beta (e) = 0.3 and beta (i) = beta (e) = 1/9). In these simulations, energization of ions parallel to the magnetic field is subdominant compared to its perpendicular counterpart (Q (parallel to,i) MUCH LESS-THAN Q (perpendicular to,i)), and the fraction of turbulent energy that goes into ion heating is approximate to 75% at beta (i) = 0.3 and approximate to 40% at beta (i) similar or equal to 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 scale-dependent theory and various signatures associated with the stochastic portion of the heating. We discuss briefly the effect of intermittency on stochastic heating and the implications of our work for the interpretation of stochastic heating in solar-wind spacecraft data.

Automated summary

The study revisited the theory of stochastic ion heating in kinetic turbulence of the low-beta solar wind, emphasizing the role of Hall and thermoelectric effects in electric-field fluctuations. Analyzing phase-space signatures using hybrid-kinetic simulations, it was found that energization of ions perpendicular to the magnetic field dominates, with approximately 75% and 40% of turbulent energy going into ion heating at beta(i) = 0.3 and beta(i) ≈ 0.1 respectively. The findings are consistent with Landau-resonant collisionless damping and a combination of ion-cyclotron and stochastic heating, demonstrating good agreement with the scale-dependent theory.