On two‐component models of solar wind fluctuations

Abstract

Matthaeus et al. first suggested a two‐component scenario for solar wind fluctuations which contains both parallel propagating Alfvén waves together with a population of quasi‐two‐dimensional turbulence. On the basis of these earlier ideas and further evidence, Tu and Marsch developed a two‐component model, in which small‐scale incompressible fluctuations are decomposed into Alfvén waves and convective structures, in an attempt to account for observed radial evolutions of energy density, normalized cross helicity σc, and Alfvén ratio rA of solar wind fluctuations. According to a recent analysis on two‐dimensional Alfvénic disturbances in the equatorial solar wind with a spiral magnetic field, we discuss the theoretical basis of this important two‐component concept and further indicate necessary ingredients that a more self‐consistent two‐component turbulence model should contain. From the perspective of Alfvénic disturbances in the solar wind, the wave and convective components coexist in a naturally intermixed manner. Our analysis specifically describes behaviors of Alfvénic disturbances of all scales, including the intermediate ones. In a fully developed spiral magnetic field at large radii, the criterion for separating radially propagating Alfvén waves and radially convective Alfvénic structures depends on whether |ƒ + mƒS| is greater or less than the characteristic frequency ƒc ≡ (FMU³/FB²)1/2, where ƒ is the perturbation frequency, ƒS is the solar rotation frequency, m is an integer for the azimuthal wavenumber, FM is the solar wind mass flux, FB is the radial magnetic flux, and U is the asymptotic wind speed. For typical solar wind parameters, the timescale corresponding to ƒc is ≳ 15 hours.