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# Magnetic field spectral evolution in the inner heliosphere

...+22
Sep 2022

The radial evolution of the magnetic field fluctuations spectral index andits dependence on plasma parameters is investigated using a merged Parker SolarProbe ($PSP$) and Solar Orbiter ($SO$) dataset covering heliocentric distancesbetween $0.06 ~ \lesssim R ~\lesssim 1$ au. The spectrum is studied as afunction of scale, normalized to the radially dependent ion inertial scale$d_{i}$. In the vicinity of the Sun, the magnetic spectrum inertial range islimited with a power law exponent $\alpha_{B}$ consistent with theIroshnikov-Kraichman phenomenology of Alfv\'enic turbulence, $\alpha_{B} =-3/2$, independent of plasma parameters. The inertial range of turbulence growswith distance from the Sun, progressively extending to larger spatial scales,while at the same time steepening towards a Kolomogorov scaling, with a meanvalue of $\alpha_{B} =-5/3$. Highly alfv\'enic intervals seem to retain theirnear-Sun scaling and only show a minor steepening with distance. In contrast,intervals, where turbulence is characterized by large magnetic energy excessand no dominance of outwardly propagating Alfv\'enic fluctuations, appear tohave spectra that steepen significantly with distance from the Sun, resultingin slightly anomalously steep inertial range slopes at $1~au$. Thoughgenerically slower solar wind streams exhibit steeper spectra, the correlationcan be attributed to the underlying positive correlation between solar windspeed and alfv\'enicity, i.e. to the relatively rare occurrence of highlyAlfv\'enic slow wind.

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