The solar wind as a turbulence laboratory

The solar wind is the continuous flow of plasma leaving the hot solar corona and permeating the whole heliosphere. During its expansion, the solar wind develops a turbulent character that evolves towards a state resembling hydrodynamic turbulence, described by Kolmogorov's theory.

However, the magnetic field has non-trivial effects on the turbulence dynamics. Because of the strong magnetic field that the solar wind carries, low-frequency fluctuations are better described within the physical-mathematical framework of MHD. Interesting analogies also emerge between fluid and MHD turbulence.

Within the EU-funded project SOLWINDCAS (Cascade rates of magnetohydrodynamic turbulence in the solar wind), scientists explored an alternative method to describe the turbulent energy cascade: Yaglom's law for MHD turbulence.

SOLWINDCAS scientists have established Yaglom's law for MHD turbulent energy cascade in a uniformly expanding solar wind. Their theoretical analysis was based on a two-scale expansion model of MHD turbulence and resulted in the addition of two new terms to Yaglom's law.

The first term appearing in Yaglom's law is related to the decay of MHD turbulent energy due to non-linear interactions. The second term is caused by the interaction between large-scale magnetic fields and small-scale inward and outward propagating Alfven waves.

Using magnetic field and plasma measurements from WIND and Helios 2 spacecraft, the scientists showed that at low frequencies these terms become comparable to Yaglom's third-order mixed moment. Therefore, they should be considered in estimating the energy cascade rate in the solar wind.

Next, the focus of SOLWINDCAS was set on the negative residual energy in the turbulent solar wind. Specifically, in situ measurements of fluctuating solar wind flow show that the energy of magnetic field fluctuations exceeds that of kinetic energy. Numerical simulations reveal the same behaviour.

The results of theoretical analysis describe for the first time how negative residual energy arises from strong MHD turbulence. Even if residual energy is absent initially, negative residual energy will always be generated by non-linearly interacting Alfven waves.

SOLWINDCAS provided a solid explanation for observed properties of the solar wind and numerical simulations of MHD turbulence. The solar wind offered a natural laboratory to test theories and improve the current understanding of MHD turbulence that also emerges in practical applications, like plasma confinement devices.