Faraday rotation fluctuation spectra observed during solar occultation of the Helios spacecraft Available to Purchase

Faraday rotation (FR) measurements using linearly polarized radio signals from the two Helios spacecraft were carried out during the period from 1975 to 1984. This paper presents the results of a spectral analysis of the Helios S-band FR fluctuations observed at heliocentric distances from Fig. 3 to $16 R⊙$ during the superior conjunctions 1979–1984. The mean intensity of the FR fluctuations usually does not exceed the noise level for solar offsets greater than $≃15 R⊙.$ The rms FR fluctuation amplitude increases rapidly as the radio ray path approaches the Sun. Good representations of the radial variation were obtained with a two-term empirical formula. The equivalent two-dimensional FR fluctuation spectra are well modeled by a single power-law over the frequency range from 1 to 50 mHz. There is a tendency for an increase of the spectral index from 1.2 at solar offset distances $R>10 R⊙$ to 1.6 at $R<6 R⊙,$ corresponding to a range for the three-dimensional spectral index $p=2.2–2.6$⁠. FR fluctuations thus display a somewhat lower spectral index compared with phase and amplitude fluctuations. Finally, a quasiperiodic component, believed to be associated with Alfvén waves, was discovered in selected fluctuation spectra observed simultaneously at two ground stations. Characteristic periods and bulk velocities of this component were $240±30 sec$ and $300±60 km/s,$ respectively.