A model of solar wind fluctuations with two components: Alfvén waves and convective structures

Abstract

A two‐component incompressible fluctuation model is presented to explain the radial evolution of the solar wind fluctuations. The basic idea is to consider the small‐scale fluctuations in the solar wind as being composed of Alfvén waves and convective structures. The major Alfvén waves are believed to be created near the coronal base and to propagate outward along the magnetic field lines. The convective structures are defined as the small‐scale variations perpendicular to the local magnetic field direction. They are either quasi‐static or turbulent and slowly evolving in the plasma frame of reference. The small‐scale perpendicular variations are connected, in the parallel direction, with large‐scale magnetic field variations, which are convected by the solar wind as quasi‐static structures during the wind expansion time. The decomposition of the original fluctuations can be done by using special space and time averages, which are defined by space averaging along the directions parallel and perpendicular to the local magnetic field vector and by time averaging in the plasma frame of reference. The equations of motion of the fluctuations and of the correlation functions for both Alfvén waves and convective structures have been derived from the one‐fluid MHD equations. A combination of the correlation functions of these two components is then used for a comparison with observational results. The influence of the angle between the sampling direction and the magnetic field vector on the final results has also been considered. As a first step to apply these equations, a simple model has been suggested that is based on the assumption that the fluctuations are only composed of outward propagating Alfvén waves and static magnetic structures. For comparison with the observations, new statistical results from data obtained by Helios 1 during days 1–95, 1975, and Helios 2 during days 19–109, 1976, are presented. The numerical solutions are shown to describe well the basic evolution trend of the fluctuation energy, the normalized cross helicity, and the Alfvén ratio. It is also shown that the basic physical process of the evolution of the convective structures is the convection of the fluctuating velocity vortex lines and the magnetic field lines by the expanding solar wind.