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 path
s 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.
Protons (ionized hydrogen) in the solar wind frequently exhibit distinct temperatures ($T_{perp p}$ and $T_{parallel p}$) perpendicular and parallel to the plasmas background magnetic-field. Numerous prior studies of the interplanetary solar-wind hav
e shown that, as plasma beta ($beta_{parallel p}$) increases, a narrower range of temperature-anisotropy ($R_pequiv T_{perp p},/,T_{parallel p}$) values is observed. Conventionally, this effect has been ascribed to the actions of kinetic microinstabilities. This study is the first to use data from the Magnetospheric Multiscale Mission (MMS) to explore such $beta_{parallel p}$-dependent limits on $R_p$ in Earths magnetosheath. The distribution of these data across the $(beta_{parallel p},R_p)$-plane reveals limits on both $R_p>1$ and $R_p<1$. Linear Vlasov theory is used to compute contours of constant growth-rate for the ion-cyclotron, mirror, parallel-firehose, and oblique-firehose instabilities. These instability thresholds closely align with the contours of the data distribution, which suggests a strong association of instabilities with extremes of ion temperature anisotropy in the magnetosheath. The potential for instabilities to regulate temperature anisotropy is discussed.
