Origin of Perpendicular Scales in Solar Wind Turbulence

Results of a study of dispersive Alfvén modes propagating outward from the Sun in streaming inhomogeneous plasma are presented for the inner heliosphere (1 AU) region. The results clearly show that a combination of nonlinear wave–particle and wave–wave interactions of outward-only Alfvén modes initially propagating along the local background magnetic field is perfectly capable of explaining the prevalence of turbulent energy in perpendicular (k̂ ) scales over energy contained in scales propagating parallel (k) to the local magnetic field perturbations observed in the solar wind. The currently agreed on explanation for this anisotropy, as well as for the scale dependence of wave energy spectra, involves various nonlinear models of imbalanced incompressible MHD turbulence that require a mixture of inward and outward propagating waves to fuel a nonlinear cascade of energy into the k̂ spectrum. The presented approach, for the first time, bridges a gap between week and strong turbulence theories—the interplay of wave–particle and wave–wave processes allows us to obtain strong turbulence scalings from seemingly week turbulence wave resonances. The reported results have a major implication on the current theories of solar wind turbulence and may require a complete overhaul of the state-of-the-art turbulence paradigm, including reassessment and reevaluation of the magnitude and directions (outward  inward; k k ^  ) of the turbulent cascades that are necessary to explain the observations.