Magnetic field spectral evolution in the inner heliosphere

Abstract

Parker Solar Probe and Solar Orbiter data are used to investigate the radial evolution of magnetic turbulence between $0.06\lesssim R\lesssim 1$ au. The spectrum is studied as a function of scale, normalized to the ion inertial scale $d_i$. In the vicinity of the Sun, the inertial range is limited to a narrow range of scales and exhibits a power-law exponent of, ${\alpha }_B=-3/2$, independent of plasma parameters. The inertial range grows with distance, progressively extending to larger spatial scales, while steepening towards a ${\alpha }_B=-5/3$ scaling. It is observed that spectra for intervals with large magnetic energy excesses and low Alfvénic content steepen significantly with distance, in contrast to highly Alfvénic intervals that retain their near-Sun scaling. The occurrence of steeper spectra in slower wind streams may be attributed to the observed positive correlation between solar wind speed and Alfvénicity.

arXiv commentAccepted to APJ letters with minor revisions

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