Very long baseline interferometer measurements of plasma turbulence in the solar wind

Abstract

Turbulence in the solar wind plasma was studied using angular broadening measurements of 10 extragalactic compact radio sources (quasars) with a very long baseline interferometer (VLBI) at 4.99 GHz. Unlike other angular broadening studies, the measured broadening size was corrected for intrinsic source structures which were obtained from a separate VLBI observation. The solar elongations of the sources ranged from 18 RS to 243 RS, and five sources with elongations ≤ 61 RS showed varying degrees of broadening. The measured angular sizes are considerably less than predicted by the well‐known empirical relationship of Erickson [1964], as well as two other models for strength of scattering as a function of solar elongation. However, the data are in good agreement with a model for the spatial power spectrum of the turbulence proposed by Coles and Harmon [1989]. This model consists of a Kolmogorov spectrum at large scales, but with an enhancement of power near the wavenumber corresponding to the ion inertial length. Two of our sources, 1148–001 and 1253–053 (3C279), show substantial differences in the amount of scattering, even though they are at similar solar elongations (29 versus 35 RS). Data to which we have access indicate that the state of the corona along the lines of sight to these sources may have been quite different. Angular broadening measurements with VLBI interferometers currently under development (primarily the very long baseline array) will allow a global view of plasma turbulence out of the ecliptic plane and thus be complementary to the point in situ measurements with Ulysses.