Modelling the Sun's explosive corona

The sun's atmosphere is a violent, chaotic place, threaded with ropes of twisted magnetic field that build up stress and cause vast explosions more powerful than any man-made bomb. Direct measurements of the magnetic field are impossible, so scientists resort to complex computer models to explore it. Researchers at the University of St Andrews have developed their best computer model yet.

Atoms as we know them cannot survive in the Sun’s atmosphere, called the corona, where temperatures can exceed 2 million degrees Celsius. Instead they are stripped of their electrons to become plasma. The positively and negatively-charged particles of the plasma spiral along the magnetic field and move with it, like children on a helter-skelter. But large-scale movements of the plasma can also move the magnetic field, equivalent to the children moving the helter-skelter whilst sliding down it.

Emerging from the Sun’s surface, called the photosphere, are many ropes of magnetic field that turn into and away from the Sun. Large clumps of plasma move about on the surface, dragging and stretching the magnetic field as plasma travels from the equator towards the poles.

The new model can track the emerging magnetic ropes in 3D over many months. The scientists aim to track the build-up of potentially explosive stresses in the magnetic field that could eventually release huge amounts of energy as coronal mass ejections. They have tested their predictions by comparing them with the real thing – telescope observations of ‘filaments’, clouds of dense cool gas suspended in the hot corona by the powerful magnetic field. These move with the magnetic structures, showing the scientists how well their model compares to reality.