Simulations suggest magnetic reconnection slows down in bigger systems

Magnetic reconnection, in which magnetic field lines in a plasma break and rejoin in a new arrangement and release stored magnetic energy, has long been thought to proceed at a fixed fast pace in hot, thin plasmas, regardless of how big the system is. A simulation study published in Physical Review Letters challenges that picture. Its authors report that when larger systems are built as faithful scaled-up copies of smaller ones, the supposedly universal rate disappears and reconnection slows as the system grows.

A plasma is a gas of electrically charged particles. In the collisionless regime those particles are so sparse and energetic that they rarely bump into one another. Without collisions to act as a kind of friction, what happens at the reconnection site is governed by the small-scale motions of individual ions and electrons. The widely accepted view, as the abstract describes it, is that collisionless reconnection runs at a rate of about 0.1 once properly normalised. Roughly speaking, that number compares the speed at which magnetic field is carried into the reconnection region with the Alfvén speed, the characteristic speed at which magnetic disturbances travel through a plasma.

According to the study, that consensus rests mainly on kinetic simulations, which track particle behaviour directly, of a classic starting configuration called a Harris current sheet: a layer of electric current separating magnetic fields that point in opposite directions. In those simulations the sheet was given a 'kinetic-scale' thickness, comparable to the tiny lengths on which ion and electron motions matter. Yet other set-ups told a different story. In forced reconnection, where the field is pushed together from outside, and in island coalescence, where neighbouring loops of magnetic field merge, the rate depends significantly on the size of the system.

To reconcile the two, the authors carried out a systematic scaling study using two kinds of computer model. Particle-in-cell simulations follow huge numbers of representative charged particles as they move through electric and magnetic fields that the particles themselves help to generate. Hall magnetohydrodynamic simulations instead treat the plasma as a fluid, while retaining an effect that arises when ions and electrons move differently. The crucial choice was how to enlarge the system. Rather than keeping the current sheet thin while making the surrounding region bigger, the team increased the sheet's initial thickness in proportion to the system size, so that the overall magnetic configuration stayed the same at every scale.

Under that condition, the authors report, the universal fast rate no longer appears. Instead, the reconnection rate falls as the system size increases. They argue that this places the Harris sheet alongside forced reconnection and island coalescence, and that dependence on large-scale dimensions is not a quirk of particular geometries but a fundamental property of collisionless reconnection.

The abstract does not say how steeply the rate declines with size, or whether the trend continues indefinitely, and the findings come from simulations rather than measurements. But if the authors are right, the familiar figure of 0.1 reflects a particular choice of starting conditions rather than a universal constant, and work that assumes a size-independent fast rate may need to be revisited.