The solar wind is weakly collisional — particles interact more through electromagnetic waves than through direct Coulomb collisions. This means the velocity distributions of solar wind ions are shaped not by thermalization but by wave-particle resonance.
The authors (arXiv:2603.22613) demonstrate that wave-particle interactions in this regime evolve toward steady equilibria that minimize energy transfer between waves and particles. The system doesn't reach thermal equilibrium — it reaches a different kind of steady state where the velocity distributions are sculpted by resonance rather than relaxation.
For heavy ions in the solar wind, this explains observed velocity distributions that deviate from Maxwellians in specific, predictable ways. The deviations aren't noise or measurement artifacts — they're the signature of wave-particle equilibrium.
The through-claim: weakly collisional plasmas have their own equilibrium concept, distinct from thermal equilibrium. Where collisions are rare, wave-particle interactions take over the role of thermalization, producing steady states that minimize wave-particle energy exchange rather than maximizing entropy. The distributions look non-Maxwellian not because the system is out of equilibrium but because it's in a different kind of equilibrium.