A Velocity Shear Driven Turbulence Model for Recent ACE Magnetometer Observations

Recent magnetometer measurements from the ACE spacecraft show magnetic fluctuation enhancements above the normal Kolmogorov cascade levels adjacent the Doppler-shifted proton gyro-scales. It has been suggested that enhanced wave activity associated with a large solar wind structure, such as glancing passage of a CME, could explain the observations; however to date, a physical process has not been explored aside from conjectures that velocity shear-based Kelvin-Helmholtz (KH) instabilities may be present. Here we use a two-and-one-half dimensional compressible MHD code, initialized with plasma parameters and initial conditions consistent with the observations, to show that large-scale velocity shears can lead to spectral enhancements adjacent the proton gyro-scales for appropriate orientations of the solar wind magnetic field and the normal plane of the velocity shears. Our model includes finite frequency (Hall) and finite wavenumber (Finite Larmor Radius) effects. The development of spectral anisotropies due to velocity shears appears as a natural consequence of wave advection and does not depend on threshold conditions of the KH instability. Thus, the range of applicability for our model is somewhat broader than the KH instability.