A consequence of thermodynamics is, if left alone, ordinary fluids, like water and air, will eventually reach an equilibrium where the temperature is spatially uniform throughout the fluid. Fundamental particle motions in collisionless plasmas suggest this understanding is incomplete. Using findings from NASA’s Magnetospheric Multiscale (MMS) mission, scientists solved one piece of the puzzle.

Shuster et al. demonstrated collisionless plasmas can achieve kinetic equilibrium in the presence of a temperature gradient. Utilizing electron-scale data for thin current layers detected by the four MMS spacecraft, the group found an exact solution for the Vlasov-Maxwell system of equations describing particle distributions in plasmas.

Such findings contradict ingrained intuitions about uniform temperatures of everyday systems in equilibrium. Nevertheless, MMS observations agree with their results.

“The agreement between our model and MMS measurements indicates we’re capturing key ingredients needed to understand the counter-intuitive properties of plasmas,” said author Jason Shuster.

The current layer in their Vlasov-Maxwell model includes a prominent crescent-shaped electron velocity distribution that forms at the temperature transition.

The integration of electron physics into global models of Earth’s magnetosphere remains an elusive goal because of prohibitive computational requirements. Shuster hopes their electron-scale solution will reduce this computational expense, which would ultimately improve forecasts for the plasma-driven weather of space.

“Sometimes, we need to zoom in to the smallest scales before we can understand the big picture,” Shuster said. “Just like how knowledge of a material’s atomic nature helps to understand its overall macroscopic properties, MMS gives us our first ‘microscopic’ view of the plasma all the way down to the scale of electron motion.”

This paper is part of the Plasma Physics from the Magnetospheric Multiscale Mission collection. Learn more here.

Source: “Electron-scale temperature gradients in kinetic equilibrium: MMS observations and Vlasov-Maxwell solutions,” by J. R. Shuster, N. Bessho, S. Wang, and J. Ng, Physics of Plasmas (2021). The article can be accessed at https://doi.org/10.1063/5.0069559.