Evolution of the multiscale statistical properties of corotating streams from 1 to 95 AU

Abstract

This paper discusses the multiscale structure of the large‐scale speed fluctuations between 1 and 95 AU during the declining phase of the solar cycle, when corotating streams are dominant structures close to the Sun. A deterministic, multifluid, one‐dimensional MHD model with the WIND data at 1 AU during 1995 as input was used to compute the time series of daily averages of speed, V(ti), with a length on the order of 1 year at R = 5, 10, … 95 AU, from which we calculated various statistical functions describing the solar wind speed. The probability distribution functions (PDFs) of the running speed differences dVn(ti) were calculated at scales τ from 1 to 64 days. The theoretical PDFs at 50 AU have the same qualitative forms as those observed by Voyager 2 (V2) between 46 and 49 AU. The theoretical PDF at a scale of 1 day at 15 AU agrees with the corresponding PDF observed by V2 between 13 and 16 AU. With increasing distance from the Sun, the width of the predicted PDF with τ = 1 decreases and the tail becomes more prominent. The standard deviation of dVn(ti) at various R computed from the model, SD(τ, R), decreases nearly exponentially with increasing R, and it is consistent with the observations of SD(τ, R) made by V2 near 15 and 50 AU. The skewness at a scale of 1 day increases almost linearly with R beyond ≈35 AU, but it shows structure between 5 and 35 AU; it is consistent with the observations of V2 near 15 and 50 AU. The theoretical power spectral density of the speed fluctuations, PSD(f, R), agrees with the observations of V2 near 15 and 50 AU. For frequencies f > 8.5 × 10−7 Hz, PSD(f, R) ∝ f−s(R). The observed and predicted s are ≤−2 beyond 40 AU, consistent with the jump‐ramp structure of V(t) in the distant heliosphere. We predict that the PDF for τ = 1 which will be observed by V2 at 70 AU in 2003 will have a core whose width is only ≈5 km/s and a tail extending up to 60 km/s that represents the major jumps in the speed profile. The predicted PDF at 95 AU is very similar to that at 70 AU. The model predicts that most speed jumps at 15 and 70 AU will have the signature of forward shocks and reverse shocks. The amplitudes of the large and moderate speed jumps diminish as they move from 45 to 95 AU. Some speed jumps move through this region without interacting with other speed jumps. In a few cases, neighboring speed jumps merge between 45 and 95 AU. The structure of the solar wind changes slowly as it moves from 45 to 95 AU.