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Volume 554 Issue 7691, 8 February 2018

The cover shows a model of a solar eruption in progress. In this week's issue, Tahar Amari and his colleagues suggest that one phenomenon could control the nature and behaviour of all such eruptions. There are two types of eruption: eruptive, which result in coronal mass ejections, and confined, which do not. The exact origin of confined eruptions has been hotly debated between two alternatives: topological complexity in the magnetic structure above the Sun's surface, or an unstable twisted magnetic flux rope. Amari and his team show that the latter process is more likely. To study this, the researchers focused on an eruption that took place in October 2014, predicting its evolution using a two-stage model. Their work reveals a strong multilayer magnetic cage (orange) in which a twisted flux rope (blue) develops. The magnetic energy of the rope increases over the course of several hours before the eruption, but is still not enough to break all of the layers of the cage. However, the twist in the rope is enough to trigger an instability that results in partial destruction of the cage. The resistance of the cage to the assault from the rope determines the type and amount of energy released in the eruption. If the rope is stronger than the cage and can break free, the result is eruptive; if the cage is stronger than the rope, the eruption is confined. Understanding the conditions that lead to solar eruptions may help to predict the events that might affect satellites, communications and ground-based power generation. Cover image: Tahar Amari/Centre de Physique Théorique, CNRS-Ecole Polytechnique


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