TOWARD A THEORY OF ASTROPHYSICAL PLASMA TURBULENCE AT SUBPROTON SCALES

We present an analytical study of subproton electromagnetic fluctuations in a collisionless plasma with a plasma beta of the order of unity. In the linear limit, a rigorous derivation from the kinetic equation is conducted focusing on the role and physical properties of kinetic-Alfven and whistler waves. Then, nonlinear fluid-like equations for kinetic-Alfven waves and whistler modes are derived, with special emphasis on the similarities and differences in the corresponding plasma dynamics. The kinetic-Alfven modes exist in the lower-frequency region of phase space, omega << k(perpendicular to)v(Ti), where they are described by the kinetic-Alfven system. These modes exist both below and above the ion-cyclotron frequency. The whistler modes, which are qualitatively different from the kinetic-Alfven modes, occupy a different region of phase space, k(perpendicular to)v(Ti) << omega << k(z)v(Te), and they are described by the electron magnetohydrodynamics (MHD) system or the reduced electron MHD system if the propagation is oblique. Here, k(z) and k(perpendicular to) are the wavenumbers along and transverse to the background magnetic field, respectively, and v(Ti) and v(Te) are the ion and electron thermal velocities, respectively. The models of subproton plasma turbulence are discussed and the results of numerical simulations are presented. We also point out possible implications for solar-wind observations.