Two swift streams of charged particles collide, and magnetic field lines begin to tear and reconnect. This process powers solar flares and auroras. It was usually assumed that all particles behave alike, like bullets from the same gun. But each particle has a tiny top: its intrinsic rotation, or spin. In a new study, scientists showed that under extreme conditions, this top can radically change the picture.
They introduced a simple parameter, denoted by the Greek letter Ξ (xi). It compares how much spin shifts a particle sideways across the flow with the radius of its circular motion in the magnetic field. If the parameter is small, spin can be neglected, and particles simply drift together. That is how the solar wind and most jets, narrow plasma streams erupting from galactic centers, behave. But when Ξ approaches unity or exceeds it, magnetic sorting begins. Particles with opposite spin directions are pushed apart, like fans of two rival clubs at a stadium. This creates charge separation and an additional electric current that boosts reconnection.
Computer simulations of plasma made of electrons and positrons (their antiparticles) confirmed the idea. At a fixed flow speed, increasing Ξ accelerated the reconnection rate, adding a new “spin-kinetic” energy injection channel. This is like a conveyor belt that not only moves faster but also pushes parts in the right direction by itself. In the classical picture, reconnection depended only on pressure and field geometry, as if the conveyor speed were determined solely by belt tension. Now we have a lever that astrophysicists can use to explain the most powerful explosions in the Universe.
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