Waves and streams in the expanding solar wind
J.Geophys. Res., 101, 425, 1996
R. Grappin and M. Velli
Abstract
The expanding box model (EBM) allows the simulation of the evolution of
compressible MHD turbulence within the expanding solar wind, taking into account
the basic properties of expansion. Using the EBM we follow the evolution of
waves within a compressive stream shear and magnetic sector structure in the
range of 0.1 to 1 AU from the Sun. We analyze the physical processes which lead
in these simulations to the modulation and erosion of the wave component,
combined with WKB and non-WKB processes due to expansion. A strong erosion by
stream shear corresponds indeed to one of the observed regimes in the solar
wind; however, we are unable to reproduce the regime which holds during solar
minimum, in which the correlation between large-scale stream structure and
turbulence remains high independently from distance to the Sun. The main point
of disagreement with observations concerns the energy spectrum (it is difficult
to generate and sustain small-scale turbulence with an Alfvénic wave band
present, and even more so in an expanding medium); the main point of agreement
concerns the statistics of density fluctuations, which are independent of
distance, and matches the observed amplitudes both within slow and fast wind. At
the same time, small scales appear to be dominated in the simulations by
compressible effects, which contradicts popular ideas on solar wind turbulence.