When a model sun starts to look like the real one

Everything we know about the hidden heart of the Sun needs to be deduced from the external clues that we can actually observe. Complex mathematical modelling is an essential aid. Researchers simulate the deeper physical processes that might be responsible for the more obvious manifestations of solar activity: the dramatic flares, coronal mass ejections (CMEs) and storms. When the models begin to generate outputs that closely match the observed truth, they tell us more about what really does happen inside the Sun.

Scientists at the Mathematical Institute, University of St Andrews, are investigating the suspected links between magnetic fields and the creation of solar prominences, flares and plasma ejections. Recent observations have revealed many properties of these events, but the knowledge of the physics behind their formation and evolution is still limited. What are the underlying processes that tie everything together?

For the first time, the University of St Andrews team have modelled two large-scale emerging magnetic fields as they burst from the interior of the sun: the first flux system rises through the highly stratified solar atmosphere and expands above the visible surface, forming a magnetized enviroment into which the second flux system expands. The results of the interaction of the two magnetic flux systems show that many dynamical phenomena are produced by this new model, in a manner that is consistent with real-life observations. This means that the ‘model’ is telling us something real.

Firstly, a ‘current sheet’ is formed at the interface between the two interacting flux systems when they come into contact. Current sheets are fascinating structures, because intense heating due to magnetic field reconnection occurs in their neighborhood. Also, many models for flares and CMEs are based on the theory that the energy which drives them may come from magnetic energy stored in the current sheets.

Then, plasmoid structures are formed inside the current sheet due to instabilities. Eventually, the plasmoids are ejected to the outer atmosphere of the Sun, carrying with them cool and dense material from the lower layers of the solar atmosphere. The ejection of small-scale plasmoids may account for solar flares, while larger ones may be associated with the major eruption of a CME.

Later on in the process, high temperature plasma is emitted sideways from the top and bottom ends of the current sheet. The hot plasma is ejected along the reconnected fieldlines. According to the model, this creates structures very similar to the observed X-Ray jets we actually detect emerging from our sun.