OBSERVATIONS OF FIELD-ALIGNED DENSITY MICROSTRUCTURE NEAR THE SUN

Radio scattering observations made with multiple antennas provide a di rect measure of the two-dimensional microstructure of the solar wind. Previous multiple antenna observations have shown that the microstruct ure at scales of the order of 10 km becomes highly field-aligned insid e of 6 R(S) [e.g., Armstrong et al., 1990]. Single antenna observation s, which can measure only a radial cut through the microstructure, hav e shown that scales larger than 1000 km have a Kolmogorov spectrum, wh ereas the smaller-scale structure has a flatter spectrum and is consid erably 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 microstruct ure is highly field-aligned near the Sun, they show that it has ellipt ical symmetry, and they show that the axial ratio changes quite abrupt ly near 6 R(S). We also present simultaneous measurements at 9 R(S) of the anisotropy on scales of 1 to 30 km and on scales of 200 to 3000 k m. Significant anisotropy was seen on the smaller scales but not on th e larger scales. This suggests that the process responsible for the an isotropic microstructure is distinct from the larger-scale, more isotr opic structure.