Observations of field‐aligned density microstructure near the Sun

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Abstract
Radio scattering observations made with multiple antennas provide a direct measure of the two‐dimensional microstructure of the solar wind. Previous multiple antenna observations have shown that the microstructure at scales of the order of 10 km becomes highly field‐aligned inside of 6 RS [e.g., Armstrong et al., 199O]. Single antenna observations, which can measure only a radial cut through the microstructure, have shown that scales larger than 1000 km have a Kolmogorov spectrum, whereas the smaller‐scale structure has a flatter spectrum and is considerably enhanced above the Kolmogorov “background” [e.g., Coles et al., 1991]. Here we present new multiple antenna “angular broadening” observations made in 1990 and 1992. These confirm that the microstructure is highly field‐aligned near the Sun, they show that it has elliptical symmetry, and they show that the axial ratio changes quite abruptly near 6 RS. We also present simultaneous measurements at 9 RS of the anisotropy on scales of 1 to 30 km and on scales of 200 to 3000 km. Significant anisotropy was seen on the smaller scales but not on the larger scales. This suggests that the process responsible for the anisotropic microstructure is distinct from the larger‐scale, more isotropic structure.