Evidence for a “background” spectrum of solar wind turbulence in the inner heliosphere

Abstract

This statistical analysis of power spectra of the fluctuating Elsässer variables, δZ± = δV±δVA, covers 3408 hours of interplanetary magnetic field and plasma observations made by experiments on the Helios 1 and 2 spacecraft during the years 1975, 1976, and 1980. The averaged spectra of δZ− in the range 2×10−8 km−1 < k* < 4×10−7 km−1 (k* = k/2π, k is the wave number) are found to show little variation with heliocentric distance, solar wind velocity, and solar activity. The resulting averaged power spectrum of δZ− obeys a power law with a slope of −5/3. We suggest that it could be thought of a persistent “background” spectrum. The averaged spectrum of δZ+, however, is flatter and of higher amplitude than the invariant spectrum of δZ−, and it appears to evolve radially toward that of δZ−. The concurrent decrease of the normalized cross‐helicity with increasing heliocentric distance (e.g., Roberts et al., 1987a, b) may therefore mainly result from a decrease of δZ+ and not from an increase of δZ−. This finding places an observational constraint on theoretical models for the evolution of cross‐helicity.