Researchers have applied a powerful simulation tool to a fundamental question about the solar wind: how do sparse plasmas exchange energy?
A Tenuous Topic

This artist’s impression of the solar wind shows a constant torrent of particles filling the heliosphere and streaming past Earth. [NASA Goddard’s Conceptual Image Lab/Greg Shirah]
Particles bounce around equally in all directions in a substance in thermal equilibrium, but in a tenuous, magnetized plasma like the solar wind, the particles can move at very different speeds parallel and perpendicular to the magnetic field. Since temperature is a measure of the average kinetic energy of a collection of particles, this means the temperature of a plasma can be different in the direction parallel to the magnetic field lines than it is perpendicular to them!
Early plasma theory predicted that the solar wind plasma would be tens of times hotter parallel to the magnetic field than perpendicular to it. In reality, at Earth’s location, the temperature parallel to the magnetic field is, on average, just 20% hotter than the temperature perpendicular to the field lines. With few to no collisions to help the plasma come to thermal equilibrium, how is this possible?
An Unstable Solution

The Kelvin–Helmholtz instability, which plays an important role in the atmospheres of stars and planets alike, is revealed by rare, wave-like patterns in clouds. [UCAR]
Similar to these examples, instabilities can arise in a plasma in which the temperature parallel to the magnetic field is much higher than the temperature perpendicular to the magnetic field. In today’s article, López and collaborators investigated what happens to this setup when the fire hose instability comes into play. Previous work has explored the impact of the fire hose instability in plasmas where the electrons or the protons have a much higher temperature parallel than perpendicular, but relatively little work has explored what happens when both types of particles have this feature.
Approaching Equilibrium

The change in the perpendicular to parallel temperature ratio over time for the simulated electrons (top) and protons (bottom). In Case 1, the electrons start out with a temperature ratio of 1. In Cases 2 and 3, the electron temperature ratios are 0.4 and 0.3, respectively. [Adapted from López et al. 2022]
Overall, the authors’ findings confirm that the fire hose instability plays an important role in moderating the temperature of the solar wind plasma, and future work should consider the influence that electrons have on the behavior of protons in the solar wind and other sparse plasmas.
Citation
“Mixing the Solar Wind Proton and Electron Scales. Theory and 2D-PIC Simulations of Firehose Instability,” R. A. López et al 2022 ApJ 930 158. doi:10.3847/1538-4357/ac66e4