When stars explode, their remnants drive violent shock waves that heat electrons in the interstellar medium to three orders of magnitude above the temperature expected from adiabatic compression on the shock. The mechanisms behind this heating have remained a long-standing puzzle.

In a new article published in the prestigious Physical Review Letters, Frederico Fiuza, professor at the Physics Department and principal investigator at the Group of Lasers and Plasmas of the Institute for Plasmas and Nuclear Fusion of IST, and colleagues present a new theory for the energy partition between electrons and ions in such strong shock waves.

Initially, most of the energy in these shocks is carried by the ions, which are much heavier than electrons. Because these shocks are collisionless, meaning that Coulomb collisions between particles are negligible, collective electromagnetic processes must be responsible for exchanging energy between the ions and the electrons at the shock. The authors show that the difference in inertia between electrons and ions leads to differential scattering between the two species in the turbulent magnetic field produced ahead of the shock, driving an electric field. It is this electric field that, in turn, leads to efficient electron heating in a Joule-type process, with electrons acquiring a temperature that is a significant fraction of that of the ions, as inferred from astronomical observations.

The theoretical model is validated against first-principles simulations showing very good agreement. Its generality opens up promising avenues for studying electron transport and heating in different settings dominated by magnetic turbulence, which range from stellar explosions to fusion plasmas.

This work was carried in the context of the project “Extreme Particle Acceleration in Shocks: from the laboratory to astrophysics” (XPACE), which was awarded a Consolidator Grant by the European Research Council (ERC) in 2023. The article, which includes co-authors from the Sorbonne Université in France, Princeton University in USA and the National Institute of Natural Sciences in Japan can be assessed here.