Coronal radio sounding with Ulysses: dual-frequency phase scintillation spectra in coronal holes and streamers.

Abstract

Dual-frequency Doppler measurements of the Ulysses downlink carrier signals were recorded during the spacecraft's 1995 solar conjunction. In this paper we present power spectra derived from phase fluctuations of the radio signals passing through the corona for different solar latitudes. It is possible to (a) distinguish clearly between spectra for which the ray paths were entirely embedded in a coronal hole or traversed a coronal streamer for the same solar offset distance of 25 solar radii and (b) characterize the evolution of the slope of the phase fluctuation spectra p-1 as a function of heliolatitude, where p is the spectral index of the three-dimensional wavenumber spectrum of the electron density fluctuations. There is an obvious increase from p=3.2 at high southern latitudes (coronal hole) toward the Kolmogorov value of p=11/3 at those heliolatitudes coinciding with the extension of the heliospheric current sheet. The spectra show a local flattening with break frequencies at 0.06Hz and 0.2Hz for the hole and the streamer spectra, respectively. The shift from 0.06Hz to 0.2Hz is not gradual, but rather abrupt when the ray path becomes embedded in the streamer. The derived electron density fluctuations and fractional electron density fluctuations at 25 solar radii are larger in the coronal streamer than in the coronal hole by factors of ~20 and ~5, respectively.