Kinetic-Alfvén-wave turbulence in the low beta limit: role of current sheets and electron Landau damping

We report analytical and numerical investigations of sub-ion scale turbulence in weakly collisional, low beta plasmas using a hybrid fluid-kinetic model. In the isothermal limit, the same scalings for the energy spectrum and for the eddy anisotropy can be obtained from two distinct approaches: (i) tearing-mediated energy cascade (Loureiro & Boldyrev 2017), and (ii) intermittency corrections, arising from magnetic and density fluctuations concentrated mostly in two-dimensional structures (Boldyrev & Perez 2012). Our numerical results indicate that the latter case is the more plausible in this regime. 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.