In plasma where some atoms remain neutral, magnetic field lines can rearrange at different speeds. This is like two rubber bands that are initially stuck together, then suddenly snap apart and contract, releasing energy. In the new work, physicists for the first time scanned two parameters simultaneously: the fraction of ionized particles and the collision frequency between ions and neutrals.

It turned out that when ions are strongly coupled to neutrals, the reconnection rate grows as the fourth root of the ion fraction. But as soon as the coupling weakens, the system transitions to a fast regime where the rate no longer depends on the ion fraction. The current sheet becomes very thin, on the order of the ion inertial length, rather than inflating to the hybrid scale as old models predicted.

These results agree with recent kinetic simulations and laboratory experiments. Understanding the transition from slow to fast reconnection helps explain why solar flares occur so suddenly.