Basic properties of solar wind MHD turbulence near 0.3 AU analyzed by means of Elsässer variables

Abstract

Magnetic field and plasma data, obtained by the Helios 1 and 2 spacecraft in the solar wind near 0.3 AU during the years 1975 to 1976, have been analyzed by calculating 12 kinds of spectra related to the Elsässer variables, δZ+ = δV + δVA, and δZ− = δV − δVA, where δV and δVA are the bulk velocity and Alfvén velocity fluctuations, respectively. For small amplitude Alfvén waves the fluctuation variable δZ+ simply relates to outward propagation and δZ− to an inward sense of propagation, if the ambient magnetic field B0 is directed inward. The frequency range analysed in this paper is 6×10−6 Hz to 6×10−3 Hz. It is found that (1) the autocorrelation length for δZ− is much larger than for δZ+ in both the high‐speed and low‐speed wind. (2) The power spectra of δZ−, especially in high‐wind speed, are steeper in the low‐frequency range and flatten in the high‐frequency range. (3) In the low‐frequency range, the power spectra for the components of δZ+ tend to be isotropic with respect to the three polarization directions, while the spectra of δZ− are dominated by the radial component. In the high‐frequency domain, the spectra of both δZ+ and δZ− are dominated by the transverse component in high‐speed wind and are more isotropic in low‐speed wind. (4) The spectra related to the residual energy or the cross‐correlation in low‐speed flows have a power law with the slope near to −5/3. However, in high‐speed flows the corresponding data are widely distributed in a cloud of points with an upper envelope near to the spectrum of δZ−. The origin of all these spectra and their importance for the solar wind physics have also been discussed. Several generation mechanisms are suggested as candidates. In the flat part of e− spectrum, the fluctuations may be generated by non‐local (in wave number space) interactions with the low‐frequency part of the e+ spectrum, or just by parametric decay of the high‐frequency part of the e+ spectrum. The steep part of e− (f) may be related to small‐scale stream tubes, or be influenced by pressure waves, nonlinear cascading, and the interaction with the outgoing Alfvén waves.