Abstract
Abstract
The nature of the Elsässer variable
z
− plays a significant role in the understanding of solar wind turbulence. The radial spectral evolution of
z
− observed by the Helios spacecraft suggests that
z
− has a persistent background spectrum in the low-frequency range from 0.3 to 1 au. However, the radial evolution in the near-Sun solar wind remains unknown. Here we use Parker Solar Probe measurements to analyze the averaged spectra of
z
− in the slow solar wind from 0.1 to 0.5 au and compare the results with the WIND observations at 1 au. We find that the averaged spectra of
z
− do not show the low-frequency break at the low-frequency breakpoint on the power spectra of
z
+, magnetic field, and velocity. The energy in the frequency range of 10−4–10−2 Hz demonstrates a radial decay from 0.1 to 0.5 au and keeps nearly constant to 1 au. The spectral index of
z
− gradually decreases from −1.27 at 0.1–0.2 au to −1.59 at 0.4–0.5 au and then slowly declines to −1.70 at 1 au. The radial behaviors of the ∥, ⊥1, and ⊥2 components of
z
− in the field-aligned coordinates are similar to that of the trace of
z
−. Our findings suggest a faster radial spectral evolution of
z
− in the near-Sun solar wind and provide strong observational constraints for the solar wind turbulence model.
Funder
MOST ∣ National Natural Science Foundation of China
Publisher
American Astronomical Society
Cited by
1 articles.
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