Reconciling Parker Solar Probe observations and magnetohydrodynamic theory

TL;DR

This paper analyzes Parker Solar Probe data to understand how solar wind turbulence evolves with distance from the Sun, reconciling observations with magnetohydrodynamic theory and turbulence models.

Contribution

It provides the first reconciliation of observational spectral transitions with MHD theory, highlighting the impact of magnetic fluctuations and plasma expansion on turbulence.

Findings

Spectral transition occurs around 0.4-0.5 au from the Sun.

Magnetic and velocity fluctuations are influenced by plasma thermal expansion.

Results support evolving coupling between magnetic and velocity fields in turbulence.

Abstract

The Parker Solar Probe mission provides a unique opportunity to characterize several features of the solar wind at different heliocentric distances. Recent findings have shown a transition in the inertial range spectral and scaling properties around 0.4-0.5 au when moving away from the Sun. Here we provide, for the first time, how to reconcile these observational results on the radial evolution of the magnetic and velocity field fluctuations with two scenarios drawn from the magnetohydrodynamic theory. The observed breakdown is the result of the radial evolution of magnetic field fluctuations and plasma thermal expansion affecting the distribution between magnetic and velocity fluctuations. The two scenarios point towards an evolving nature of the coupling between fields that can be also reconciled with Kraichnan and Kolmogorov pictures of turbulence. Our findings have important implications for turbulence studies and modeling approaches.