The solar corona is a violent and highly structured region of plasma where magnetic fields rule. In independent articles in Astrophysical Journal, Yuhong Fan1 and Tibor Török and Bernhard Kliem2 present numerical simulations that show how coronal magnetic field lines can become twisted and lead to solar eruptions known as coronal mass ejections (CMEs).

CMEs are large ‘explosions’ of material (108–1011 tonnes) that are expelled from the Sun at supersonic speeds of up to 2,000 km s-1. Once free of the Sun’s gravitational pull, the ejected material travels outwards through interplanetary space, often causing shockwaves that can trigger activity within planetary magnetospheres (the region of space around a planet that contains its magnetic field). The rate at which CMEs occur is closely linked to the 11-year cycle of the solar magnetic field and visible sunspot patterns.

A feature commonly observed in images of the corona is that of closed magnetic ‘loops’, connected at both ends to the solar surface. It has long been suspected, owing to their similarity in overall structure, that these coronal loops may be associated with CMEs.

In their current work, Fan1 and Török and Kliem2 use magnetohydrodynamics to model the evolution of a pre-existing coronal loop. Both studies find that an effect known as the ‘kink instability’ is able to accelerate the loop away from the Sun, such that it can become free of the solar magnetic field. This instability is directly analogous to the ‘knots’ that appear in an elastic band when it is twisted past a certain limit.

Figure 1: Filament eruption, real and simulated.
figure 1

Images from the TRACE satellite showing the evolution of a filament eruption on the solar surface (left) are well matched by new numerical simulations of the magnetic field lines of a coronal loop2 (right). Reproduced by permission of the AAS.

As well as closely reproducing the structural and dynamical evolution of CMEs observed by imaging instruments aboard spacecraft (Fig. 1), these simulations also replicate the 'sigmoid' current layer that is thought to be the origin of distinctive S-shaped X-ray emissions that radiate from the solar surface.