The Solar Wind as a Turbulence Laboratory

TLDR

This review examines large‑amplitude, low‑frequency plasma field fluctuations in the expanding solar wind, a topic of fundamental importance to plasma physics and astrophysics. It synthesizes Ulysses high‑latitude observations and complex‑systems‑based numerical simulations, including MHD turbulence theory, to elucidate the mechanisms of turbulence generation and spectral energy transfer. These studies provide a comprehensive phenomenological picture of solar‑wind turbulence, revealing how turbulent fluctuations behave and how energy cascades across scales.

Abstract

In this review we will focus on a topic of fundamental importance for both plasma physics and astrophysics, namely the occurrence of large-amplitude low-frequency fluctuations of the fields that describe the plasma state. This subject will be treated within the context of the expanding solar wind and the most meaningful advances in this research field will be reported emphasizing the results obtained in the past decade or so. As a matter of fact, Ulysses' high latitude observations and new numerical approaches to the problem, based on the dynamics of complex systems, brought new important insights which helped to better understand how turbulent fluctuations behave in the solar wind. In particular, numerical simulations within the realm of magnetohydrodynamic (MHD) turbulence theory unraveled what kind of physical mechanisms are at the basis of turbulence generation and energy transfer across the spectral domain of the fluctuations. In other words, the advances reached in these past years in the investigation of solar wind turbulence now offer a rather complete picture of the phenomenological aspect of the problem to be tentatively presented in a rather organic way.