Evolution of the spectrum of solar wind velocity fluctuations from 0.3 to 5 AU

Abstract

Recent work has shown that at 1 AU from the Sun, the power spectrum of the solar wind magnetic field has the −5/3 spectral slope expected for Kolmogorov turbulence but that the velocity has closer to a −3/2 spectrum. This paper traces the changes in solar wind velocity spectra from 0.3 to 5 AU using data from the Helios and Ulysses spacecraft to show that this is a transient stage in solar wind evolution. The spectrum of the velocity is found to be flatter than that of the magnetic field for the higher frequencies examined for all cases until the slopes become equal (at −5/3) well past 1 AU when the wind is relatively non‐Alfvénic. In some respects, in particular in the evolution of the frequency at which the spectrum changes from flatter at larger scales to a “turbulent” spectrum at smaller scales, the velocity field evolves more rapidly than the magnetic, and this is associated with the dominance of the magnetic energy over the kinetic at “inertial range” scales. The speed of the flow is argued to be largely unrelated to the spectral slopes, consistent with previous work, whereas high Alfvénicity appears to slow the spectral evolution, as expected from theory. This study shows that, for the solar wind, the idea of a simple “inertial range” with uniform spectral properties is not realistic, and new phenomenologies will be needed to capture the true situation. It is also noted that a flattening of the velocity spectrum often occurs at small scales.