Origin and evolution of fluctuations in the solar wind: Helios observations and Helios‐Voyager comparisons

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Abstract
Using hour‐averaged data from the Helios and Voyager spacecraft, we have investigated the origin and evolution of low‐frequency interplanetary fluctuations from 0.3 to 20 AU. Alfvénic fluctuations in the inner solar system are found to be generally outward traveling from the Sun and at times quite pure, in general agreement with previous work. The correlation between velocity and magnetic field fluctuations can be high even on scales longer than the transit time from the Sun to the spacecraft, indicating a solar origin for the initial outward traveling waves. However, the fluctuations become substantially less Alfvénic by 1 AU, with the larger scales evolving more rapidly, and this evolution continues in the outer heliosphere. Near the Sun it is regions with small velocity gradients, rather than specifically the trailing edges of high‐speed streams, that exhibit the purest Alfvénic fluctuations. Density and magnetic field magnitude fluctuations inside 1 AU show the anticorrelation characteristic of pressure balance structures previously found in the outer heliosphere. The lower frequency positive correlation between density and field observed farther out in association with the growth of compression regions is not generally present inside 0.4 AU. Fluctuations have a somewhat higher magnetic than kinetic energy at scales of less than a day, but at lower frequencies, kinetic energy is already dominant by 0.3 AU. These results support the view that outward propagating Alfvénic fluctuations are generated near the Sun and that substantial dynamical evolution, probably involving shear‐generated nonlinear couplings, is important at all heliocentric distances examined.