Coronal sounding experiments over the solar activity cycle with HELIOS-1 and HELIOS-2: Faraday rotation observations of Alfvenic waves in the region of solar wind acceleration

The coronal Faraday rotation (FR) experiments using the linearly polarized signals of the HELIOS-1 and HELIOS-2 interplanetary probes remain as a unique investigation of the magnetic field of the solar corona and its aperiodic and quasi-periodic variations. The unexpectedly long lifetime of these spacecraft (1974-1986) enabled studies from very deep solar activity minimum (1975-1976) into the strong activity maximum (1979). Important experimental data were also obtained for the rising (1977-1978) and declining (1980-1984) branches of the solar activity cycle. Previous publications have presented results of the initial experimental data only for coronal sounding experiments performed during individual solar conjunction opportunities. This report is a more detailed analysis of the HELIOS FR measurements for the entire period 1975-1984. Radial profiles of the FR fluctuation intensity recorded during the deepest solar activity minimum in 1975-1976 are shown to differ distinctly from those during the strong solar activity maximum in 1979. In particular, the decrease of the FR fluctuation intensity with solar offset distance is substantially higher in 1979 in comparison with 1975/1976. In all cases, however, the FR data reveal the presence of quasi-periodic wave-like fluctuations in addition to aperiodic variations. The dominant period of these fluctuations, recorded during 35 percent of the total measurement time, is found to be close to five minutes. Large-scale FR variations at considerably longer periods (1.1-2.7 hours) were observed during 20 percent of the measurement time. Observations of FR fluctuations at widely-spaced ground stations have been used to estimate the velocity of coronal Alfvén waves. The velocity values range between 290 and 550 km/s at heliocentric distances between 3.5 and 4.5 solar radii and are marginally smaller (150-450 km/s) at distances between 5.5 and 6.5 solar radii.