Spectral and spatial evolution of compressible turbulence in the inner solar wind

Abstract

Magnetic field and plasma data collected by the Helios spacecraft between 0.3 and 1 AU near the activity minimum of solar cycle 21 have been analyzed to establish spectral characteristics of compressive fluctuations in the inner solar wind and their relation to the morphology of the plasma flow and magnetic field. The compressive turbulence level is found to be closely related to the stream structure or latitudinal location with respect to the heliospheric current sheet. Compressive turbulence in low‐speed flows is more fully developed and intense. The spectra are radially invariant and come close to a −5/3 spectral law. In contrast, fast stream turbulence becomes increasingly compressive, in terms of radially growing amplitudes of δn/n and δB/B, with increasing heliocentric distance. The spectra reveal a flatter high‐frequency part, which gradually seems to get straightened out at large solar distances. Single case studies, as well as averaged spectra and their spectral features, are presented and discussed in the context of the theoretical turbulence literature.