An observational limit to the amplitude of Alfvén waves in the solar wind and comparison with an acceleration model

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Abstract
Measurements of angular scattering of cosmic radio sources observed near the solar limb indicate that the density microstructure can be highly anisotropic (Armstrong et al., 1990). Here we point out that Alfvén waves, which are known to be present in the solar wind, will tend to isotropize the angular scattering measurements. Thus such observations set an upper bound on the amplitude of Alfvén waves in the scattering region. Recently measurements of velocity versus distance were made near the Sun (using interplanetary scintillations). In about half of the observed streams the velocity profiles were found to be consistent with an MHD model of the solar wind in which Alfvén waves are used to increase the flow velocities (Coles et al., 1991). In this paper we demonstrate that the Alfvén wave amplitudes necessary to match the measured velocities are consistent with the bounds derived here from angular scattering observations.