# The multi-scale nature of the solar wind

## TL;DR

This paper reviews the multi-scale behavior of the solar wind, emphasizing the importance of scale couplings in its dynamics and thermodynamics, based on spacecraft measurements and plasma theory.

## Contribution

It provides a comprehensive overview of the methods and findings related to the multi-scale processes in the solar wind, highlighting the role of various physical effects.

## Key findings

- Couplings across scales are crucial for solar wind dynamics.
- Expansion effects and non-equilibrium distributions influence plasma evolution.
- Waves, turbulence, and microinstabilities are key to understanding multi-scale behavior.

## Abstract

The solar wind is a magnetized plasma and as such exhibits collective plasma behavior associated with its characteristic spatial and temporal scales. The characteristic length scales include the size of the heliosphere, the collisional mean free paths of all species, their inertial lengths, their gyration radii, and their Debye lengths. The characteristic timescales include the expansion time, the collision times, and the periods associated with gyration, waves, and oscillations. We review the past and present research into the multi-scale nature of the solar wind based on in-situ spacecraft measurements and plasma theory. We emphasize that couplings of processes across scales are important for the global dynamics and thermodynamics of the solar wind. We describe methods to measure in-situ properties of particles and fields. We then discuss the role of expansion effects, non-equilibrium distribution functions, collisions, waves, turbulence, and kinetic microinstabilities for the multi-scale plasma evolution.