Angular Independence of Break Position for Magnetic Power Spectral Density in Solar Wind Turbulence

The break in power spectral density (PSD) around the ion scales indicates the onset of dissipation and/or dispersion of kinetic turbulence. For Alfven waves in the kinetic regime, the dissipation and dispersion are individually dependent on the propagation angle, theta(kB), which has theta(RB) (the angle between radial direction and local mean magnetic field direction) as a proxy in solar wind measurements. The relation between theta(RB) and the break position helps us find the role of dissipation and/or dispersion for deforming the PSD profile. In order to locate the spectral break position automatically and quantitatively, we develop a dual-power-law fitting method to fit the PSD profiles in both MHD and kinetic ranges simultaneously. The break position f(b) is found to change little with theta(RB), suggesting an angular independence of the spectral break. Furthermore, fb in our statistical study of fast solar wind near 1 au is consistent with a wavenumber k satisfying k(rho(p) + d(p)) similar to 1 (rho(p) is the thermal proton gyroradius and d(p) is the proton inertial length), independently of theta(RB). To interpret this independence, we incorporate the effects of both dissipation and dispersion in a unified description, which is the breakdown of the magnetic frozen-in condition in wavenumber space (k(vertical bar vertical bar), k(perpendicular to)). The breakdown of the frozen-in condition is relatively isotropic compared to the strong anisotropy of dispersion and dissipation. Furthermore, the spatial scale for the onset of the breakdown frozen-in condition is estimated to be the sum of rho(p) and d(p).