Large‐scale speed fluctuations at 1 AU on scales from 1 hour to ≈1 year: 1999 and 1995

This paper describes the solar wind speed fluctuations, V(τ i ), observed on a wide range of scales (from 1 hour to ≈1 year) at 1 AU, both during 1999 approaching solar maximum and during 1995 during the descending phase of solar activity. The fluctuations extend from the inertial turbulence range, throughout the scales dominated by interaction regions and streams, and through the largest scales where the fluctuations reflect the variability of the characteristics of the streams. The general properties of the fluctuations of the speed differences, dVn, on all of these scales, τ n , are described by the three functions: skewness S(τ n ), kurtosis K(τ n ), and standard deviation SD(τ n ). These three functions have the same qualitative form for both the 1999 data and the 1995 data, in the range of scales of τ n = 10.7–341 days (the “Gaussian range”), S(τ n ) ≈ 0, and K(τ n ) ≈ 3, consistent with Gaussian distribution functions for both 1999 and 1995. The skewness is positive for τ n < 10.7 days and is maximum at a characteristic scale, which is 4 hours for the 1999 data and ≈8 hours for the 1995 data. A more detailed description of the fluctuations is provided by the probability distribution functions (PDFs) for dVn. In the range τ n > 10.7 days, the PDFs are Gaussian for the 1999 data and approximately Gaussian for the 1995 period. For scales defined by streams, the PDFs are like Gaussians but fatter on the dVn > 0 side. The PDFs are cubic on a semilog scale for the 1999 data and are approximately cubic for the 1995 data. At scales characteristic of the sizes of interaction regions, the PDFs have a large tail for dVn > 0 that is produced by the large positive dVn at leading edges of streams. At the smallest scales, the PDFs have a form that is characteristic of intermittent turbulence. The power spectra for the 1999 and 1995 data are power laws in frequency throughout the interaction region range and the stream range, the slopes being β = −2.02 ± 0.01 and −2.12 ± 0.04, respectively, consistent with the dominance of shocks and jumps over turbulence. The fluctuations of V(τ n ) have a multifractal scaling structure, described by a function s(q), the exponent of the structure function versus moment number q in the inertial range, the interaction region range, and extending to the stream range. In the interaction region and stream ranges, s(3) > 1.