AN EXTENDED STRUCTURE-FUNCTION MODEL AND ITS APPLICATION TO THE ANALYSIS OF SOLAR-WIND INTERMITTENCY PROPERTIES

An extended structure-function model is developed by including the new effect in the p-model of Meneveau and Sreenivasan which shows that th e averaged energy cascade rate changes with scale, a situation which h as been found to prevail in nonfullydeveloped turbulence in the inner solar wind. This model is useful for the small-scale fluctuations in t he inner heliosphere, where the turbulence is not fully developed and cannot be explained quantitatively by any of the previous intermittenc y turbulence models. With two model parameters, the intrinsic index of the energy spectrum alpha, and the fragmentation fraction P-1, the mo del can fit, for the first time all the observed scaling exponents of the structure functions, which are calculated for time lags ranging fr om 81 s to 0.7 h from the Hellos solar wind data. From the cases we st udied we cannot establish for P-1 either a clear radial evolution tren d, or a solar-windspeed or stream-structure dependence or a systematic anisotropy for both the flow velocity and magnetic field component fl uctuations. Generally, P-1 has values between 0.7 and 0.8. However, in some cases in low-speed wind P-1 has somewhat higher values for the m agnetic components, especially for the radial component. In high-speed wind, the inferred intrinsic spectral indices (alpha) of the velocity and magnetic field components are about equal, while the experimental spectral indices derived from the observed power spectra differ. The magnetic index is somewhat larger than the index of the velocity spect rum. For magnetic fluctuations in both high- and low-speed winds, the intrinsic exponent alpha has values which are near 1.5, while the obse rved spectral exponent has much higher values. In the solar wind with considerable density fluctuations near the interplanetary current shee t near 1 AU, it is found that P-1 has a comparatively high value of 0. 89 for V-x. The impact of these results on the understanding of the na ture of solar wind fluctuations is discussed, and the limitations in u sing structure I functions to study intermittency are also described.