Intermittency and electron heating in kinetic-Alfvén-wave turbulence

Recent high-resolution, in situ measurements of the electromagnetic fluctuations and plasma distribution functions have provided unprecedented opportunities to study the rich plasma dynamics in the kinetic range of the turbulence. We report analytical and numerical investigations of sub-ion scale turbulence in weakly collisional, low beta plasmas using a hybrid fluid-kinetic model, focusing on the spectral properties of the fluctuations and electron heating. In the isothermal limit, the numerical results strongly support a description of the turbulence as a critically-balanced Kolmogorov-like cascade of kinetic Alfvén wave fluctuations, as amended by Boldyrev & Perez (Astrophys. J. Lett. 758, L44 (2012)) to include intermittent effects. With the inclusion of electron kinetic physics, the energy spectrum is found to steepen due to electron Landau damping, which is enabled by the local weakening of nonlinearities in current sheets, and yields significant energy dissipation in the velocity space. The use of a Hermite formalism to express the velocity space dependence of the electron distribution function allows us to obtain an analytical, zeroth-order solution for the Hermite moments of the distribution, which is borne out by numerical simulations.