On the radial evolution of MHD turbulence in the inner heliosphere

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Abstract
The radial evolution of MHD turbulence in the inner heliosphere, as observed by the Helios probes between 0.3 and 1 AU, is reanalyzed and newly evaluated by means of Elsässer variables. It is found that the spectra of the energy of the outward (e+) and inward (e−) oriented fluctuations, of the cross helicity and cross correlation and of related dimensionless ratios, like the ones named after Alfvén or Elsässer, all undergo considerable radial evolution with an overall trend for the spectra to steepen. These evolutionary tendencies are established both by individual typical case studies for a recurrent high‐speed stream and neighboring low‐speed flows as well as by a statistical analysis of many sample spectra pertaining to various heliocentric distances. In particular, the e+ spectrum considerably steepens in the low‐frequency part. Whereas the fluctuations in the Alfvénic domain are more transverse in the perihelion, they radially evolve toward an equipartition of the energy in the three spatial dimensions at larger heliocentric distances, the e− spectrum does not change a lot at low frequencies but strongly steepens beyond about 2×10−4Hz. The spectral results on e− suggest that isotropic turbulence with an index of −5/3 is probably the ultimate state toward which solar wind MHD fluctuations seem to evolve. The radial evolution of the cross correlation eR is also analyzed.