Effect of Spherical Polarization on the Magnetic Spectrum of the Solar Wind

Abstract

Magnetic fluctuations in the solar wind are often observed to maintain constant magnitude of the magnetic field in a manner consistent with spherically-polarized large-amplitude Alfvén waves. We investigate the effect of spherical polarization on the magnetic spectral index through a statistical survey of magnetic fluctuations observed by Parker Solar Probe between 20$R_{\odot }$ and 200$R_{\odot }$. We find that deviations from spherical polarization, i.e., changes in $|\mathbf{B}|$ (compressive fluctuations) and one-dimensional discontinuities, have a dramatic effect on the scaling behavior of the turbulent fluctuations. We show that shallow $k^{-3/2}$ spectra are only observed for constant magnetic field strength, three-dimensional structures, which we identify as large amplitude Alfvén waves. The presence of compressive fluctuations coincides with a steepening of the spectrum up to $k^{-5/3}$. Steeper power law scalings approaching $k^{-2}$ are observed when the fluctuations are dominated by discontinuities. Near-sun fluctuations are found to be the most spherically polarized, suggesting that this spherical state is fundamental to the generation of the solar wind. With increasing distance from the Sun, fluctuations are found to become less three dimensional and more compressive, which may indicate the breakdown of the Alfvénic equilibrium state.

10 pages, 5 figures. Submitted to The Astrophysical Journal

References in corpus (16)

• Array Programming with NumPy
• The Astropy Project: Sustaining and Growing a Community-oriented Open-source Project and the Latest Major Release (v5.0) of the Core Package
• Anisotropic scaling of magnetohydrodynamic turbulence
• The Evolution and Role of Solar Wind Turbulence in the Inner Heliosphere
• Switchbacks in the near-Sun magnetic field: long memory and impact on the turbulence cascade
• Parker Solar Probe: Four Years of Discoveries at Solar Cycle Minimum
• Role of cross helicity in magnetohydrodynamic turbulence
• Ion kinetic energy conservation and magnetic field strength constancy in multi-fluid solar wind Alfvénic turbulence
• Cross Helicity Reversals In Magnetic Switchbacks
• Switchbacks: statistical properties and deviations from alfvénicity
• Evolution of switchbacks in the inner Heliosphere
• Turbulence characteristics of switchbacks and non-switchbacks intervals observed by \emph{Parker Solar Probe}
• Kolmogorov and Irosnikov-Kraichnan scaling in the anisotropic turbulent solar wind
• Multiscale Solar Wind Turbulence Properties inside and near Switchbacks measured by Parker Solar Probe
• Magnetic field spectral evolution in the inner heliosphere
• Scaling anisotropy with stationary background field in the near-Sun solar wind turbulence

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