Magnetic field spectral evolution in the inner heliosphere

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, ${\alpha }_B=-3\mathrm{/}2$, independent of plasma parameters. The inertial range grows with distance, progressively extending to larger spatial scales, while steepening towards a ${\alpha }_B=-5\mathrm{/}3$ 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.