Speed f luctuations near 60 AU on scales from 1 day to 1 year : Observations and model

[1] This paper describes the multiscale, statistical state of the speed observed near 60 AU from mid-1999 to mid-2000 by Voyager 2 (V2), and it shows that a multifluid MHD model can explain the basic features of these observations. The probability distribution functions (PDFs) of the running speed differences (dVn) on scales from 1 day to 256 days provide a relatively complete description of some important properties of the large-scale speed fluctuations. On a scale of 1 or 2 days the PDFs of the positive and negative speed differences observed by V2 are approximately exponential, which is related to jump-ramp structures but might include a contribution from intermittent turbulence. On scales greater than 26 days (the solar rotation period) the PDFs of the speed differences are approximately Gaussian, i.e., quadratic on a semilog scale. On a scale of the order of several days, on which one sees jump-ramp structures in the speed profile, the PDF of the speed differences is cubic on a semilog scale. The standard deviation of dVn increases with increasing scale. The skewness and kurtosis of dVn are relatively large at small scales and decrease to Gaussian values at scales 16 days. The PDFs of speed differences and their lower moments versus scale near 60 AU were also derived from a speed profile predicted by the deterministic, spherically symmetric, multifluid, MHD model of Chi Wang, using ACE observations at 1 AU as the inner boundary conditions. Although the projected speed profile is not the same as the observed speed profile because ACE and Voyager are not radially aligned throughout the 1-year interval, the statistical properties of the observed profiles are essentially the same as the projected speed profiles. Significant evolution of the multiscale statistical properties of the solar wind speed fluctuations occurs between 1 and 60 AU; this evolution can be explained by a deterministic model.