Gyrokinetic simulation of turbulent cascade of slow waves in sub-ion-gyroradius scale in low-beta plasma

At spatial scales larger than the ion gyroradius, solar wind turbulence is understood to be dominated by an active Alfvenic cascade accompanied by a passive cascade of compressible fluctuations of both density and magnetic field strength (Schekochihin et al. 2009). Though the compressible fluctuations only contain approximately 10% of the turbulent energy, characterization of these fluctuations is necessary to account for the damping and dissipation of solar wind turbulence. Recent observations (Howes et al. 2012, Klein et al. 2012) show that compressible fluctuations in the solar wind are largely consistent with the linear characteristics of the large-scale slow wave. The passive cascade of slow waves is also consistent with the observations of Kolmogorov scaling of power spectral density (PSD) of plasma density fluctuations. The decoupling of the slow-wave and Alfven-wave cascade was studied in detail and confirmed theoretically (Lithwick & Goldreich 2001, Schekochihin et al. 2009) and numerically (Maron & Goldreich 2001, Cho & Lazarian 2002 and 2003). However, a study of this compressible cascade as it crosses kinetic scales has not been performed. In this work, we report results from a nonlinear gyro kinetic simulation of the passive slow-wave cascade from the inertial range to sub-ion-gyroradius scales in a low beta magnetized plasma. The behavior of this cascade, including features such as the PSD scaling and associated particle heating, is compared to pure Alfvenic turbulence, and the interaction of the compressible and Alfvenic cascades is compared to established theory.