ON 2-COMPONENT MODELS OF SOLAR-WIND FLUCTUATIONS

Matthaeus et al. first suggested a two-component scenario for solar wind fluctuations which contains both parallel propagating Alfven 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 Alfven waves and convective structures, in an attempt to account for observed radial evolutions of energy density, normalized cross helicity sigma(c), and Alfven ratio r(A) of solar wind fluctuations. According to a recent analysis on two-dimensional Alfvenic 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 Alfvenic disturbances in the solar wind, the wave and convective components coexist in a naturally intermixed manner. Our analysis specifically describes behaviors of Alfvenic disturbances of all scales, including the intermediate ones. In a fully developed spiral magnetic field at large radii, the criterion for separating radially propagating Alfven waves and radially convective Alfvenic structures depends on whether \f + mfs\ is greater or less than the characteristic frequency f(c) equivalent to (FMU3/F-B(2))(1/2), where f is the perturbation frequency, f(S) is the solar rotation frequency, m Is an integer for the azimuthal wavenumber, F-M is the solar wind mass flux, F-B is the radial magnetic flux, and U is the asymptotic wind speed. For typical solar wind parameters, the timescale corresponding to f(c) is greater than or similar to 15 hours.