The nature and evolution of magnetohydrodynamic fluctuations in the solar wind: Voyager observations

Abstract

The magnetic field and plasma data collected by the Voyager spacecraft between 1 and 11 AU are used to study the properties of interplanetary MHD fluctuations and to attempt to answer several related questions about the Alfvénicity of solar wind fluctuations: First, to what extent are magnetic and velocity fluctuations Alfvénic? Second, does the dominant propagation direction of Alfvénic fluctuations evolve with heliocentric distance? Third, is the presence of Alfvénic fluctuations correlated with large‐scale structures, such as stream interaction regions? In addition, we investigated the contributions of compressive modes to the interplanetary fluctuations. We find that near 1 AU at most 15% of the fluctuations at the scale of a few hours or less are purely Alfvénic and these usually propagate outward from the Sun. The propagation direction becomes more inward on average with increasing heliocentric distance. Although it is commonly supposed that compression regions are not generally Alfvénic, we found that the wave propagation direction is only slightly more mixed in compression regions than in the corresponding rarefaction regions. Moreover, the evolution in propagation direction is not directly due to the growth of large‐scale compression regions. There is a tendency for magnetic fluctuations to be larger than velocity fluctuations at scales less than a day, while the reverse is true, due to the dominance of stream energy, at larger scales. While smaller‐scale density fluctuations are uniformly small (<δn/n> ≃ 0.1), they are usually negatively correlated with field magnitude variations even on time scales that preclude contributions from tangential discontinuities. In general the solar wind cannot be in a static state of superposed large‐amplitude waves. Fluctuations must be produced outside the Alfvénic critical point, perhaps generated by stream shear with subsequent nonlinear evolution.