Evidence of Hydrodynamic Cascade Process in the Solar Wind Turbulence

The solar wind turbulence is usually described by magnetohydrodynamic (MHD) turbulence theories. Elsässer variables z^±= dV±dV_A are widely considered as counter-propagating Alfvén waves. Here, dV and dV_A = dB/(μ₀ρ)^1/2 are velocity and magnetic field fluctuations, respectively. It is generally believed that nonlinear interactions between z^± can produce energy cascade and that the turbulence is anisotropic. In this work, we present observational evidence on the noise nature of z^- and on the isotropic feature of autocorrelation level contours. We find that (1) in highly Alfvénic fluctuations, z^- fluctuations consist of high-frequency white noise and low-frequency pseudo structures. It indicates that in the studied cases, z^- may not contribute importantly to the interactions with z⁺ to produce energy cascade; (2) the autocorrelation level contours of velocity and magnetic field become isotropic for the scales from about 10 hours to 1 hour, although they elongate along magnetic field direction at the durations of 2 days and 1 day in the slow solar wind. The isotropic nature of the solar wind fluctuations shown by this result is not consistent with the existed MHD cascade theory. Accordingly, we suggest that the solar wind turbulence may be described by hydrodynamic (HD) theories. A scenario of the solar wind cascading process controlled by HD mechanism will open a new way for studying the solar wind turbulence.