Helios 2 observations of solar wind turbulence decay in the inner heliosphere

TL;DR

This study analyzes Helios 2 data and MHD simulations to understand how turbulence energy transfer in the solar wind decreases with distance from the sun, revealing a decay consistent with observed temperature profiles.

Contribution

It provides the first combined observational and numerical analysis of turbulence decay in the inner heliosphere, linking energy transfer rates to radial distance and temperature profiles.

Findings

Turbulence energy transfer rate decays as a power law with distance.

Decay law matches the observed radial temperature profile.

Numerical simulations support the interpretation of turbulence decay.

Abstract

The linear scaling of the mixed third-order moment of the magnetohydrodynamic fluctuations is used to estimate the energy transfer rate of the turbulent cascade in the expanding solar wind. In 1976 the Helios 2 spacecraft measured three samples of fast solar wind originating from the same coronal hole, at different distance from the sun. Along with the adjacent slow solar wind streams, these represent a unique database for studying the radial evolution of turbulence in samples of undisturbed solar wind. A set of direct numerical simulations of the MHD equations performed with the Lattice-Boltzmann code FLAME is also used for interpretation. We show that the turbulence energy transfer rate decays approximately as a power law of the distance, and that both the amplitude and decay law correspond to the observed radial temperature profile in the fast wind case. Results from magnetohydrodynamic numerical simulations of decaying magnetohydrodynamic turbulence show a similar trend for the total dissipation, suggesting an interpretation of the observed dynamics in terms of decaying turbulence, and that multi-spacecraft studies of the solar wind radial evolution may help clarifying the nature of the evolution of the turbulent fluctuations in the ecliptic solar wind.