Radio synthesis imaging of anisotropic angular broadening in the solar wind

Abstract

We present Very Large Array observations at wavelengths of 2, 3.5, 6, and 20 cm, of angular broadening of radio sources due to the solar wind in the region 2-16 solar radii. Angular broadening is anisotropic with axial ratios in the range 2-16. Larger axial ratios are observed preferentially at smaller solar distances. Assuming that anisotropy is due to scattering blobs elongated along magnetic field lines, the distribution of position angles of the elliptically broadened images indicates that the field lines are non-radial even at the largest heliocentric distances observed here. At 5R_⊙, the major axis scattering angle is ~ 0.7" at λ= 6 cm and it varies with heliocentric distance as R^-1.6. The level of turbulence, characterized by the wave structure function at a scale of 10 km along the major axis, normalized to λ= 20 cm, has a value 20 ± 7 at 5R_⊙ and varies with heliocentric distance as R^-3. Comparison with earlier resu lts suggest that the level of turbulence is higher during solar maximum. Assuming a power-law spectrum of electron density fluctuations, the fitted spectral exponents have values in the range 2.8-3.4 for scale sizes between 2-35 km. The data suggests temporal fluctuations (of up to 10%) in the spectral exponent on a time scale of a few tens of minutes. The observed structure functions at different solar distances do not show any evidence for an inner scale; the upper limits are l k m at 2R_⊙ and 4 km at 13R_⊙. These upper limits are in conflict with earlier determinations and may suggest a reduced inner scale during solar maximum.