Radial evolution of power spectra of interplanetary Alfvénic turbulence

The radial evolution of the power spectra of the MHD turbulence within the trailing edge of high‐speed streams in the solar wind has been investigated with the magnetic field data of HELIOS 1 and 2 for heliocentric distances between 0.3 and 0.9 AU. In the analyzed frequency range 2.8 · 10 −4 ‐ 8.3 · 10 −2 Hz the computed spectra have, near the earth, values of the spectral index close to that predicted for an incompressible hydromagnetic turbulence in a stationary state. Approaching the sun, the spectral slope remains unchanged for frequencies f ≳ 10 −2 Hz, whereas at lower frequencies we find a clear evolution toward a less steep falloff with frequency. The radial gradient of the power in Alfvénic fluctuations depends on frequency and it increases upon increasing frequency. For frequencies f ≳ 10 −2 Hz, however, the radial gradient remains approximately the same. A discussion of possible theoretical implications of the observational features pointed out is given.