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Solar sheaths make or break plasma eruptions
Posted: 18 April 2011

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Our Sun is a violent object, regularly erupting material into space and occasionally into the path of the Earth, but understanding why these dramatic events take place remains a hot topic in solar physics. Dr Vasilis Archontis of the University of St Andrews thinks that clouds of plasma bound by the Sun’s magnetic field could hold the answer.

Archontis and colleagues modelled the development and propagation of plasmoids breaking through magnetic field lines. Image: Vasilis Archontis.

Active regions on the solar surface are a result of magnetic fields rising from the solar interior and gradually expanding into Sun’s outer atmosphere, the corona, in a process known as magnetic flux emergence. Archontis and colleagues modelled this process using three dimensional computer simulations and found that as magnetic field lines draw closer to each other due to the motion of plasma in the Sun’s lower atmosphere, they “reconnect” and build a new magnetic flux system, called a plasmoid. But whether these expanding 70-80,000 kilometre wide pods of dense, cool plasma erupt into space or not depends on the level of interaction between the new emerging field and the old, pre-existing magnetic field in the corona.

When the new field expands into the corona it forms a magnetic sheath with loops of field lines anchored to the solar surface, trapping the plasmoids.

“It’s like the plasmoid is in jail, and it can’t escape,” says Archontis, who explains that the plasmoids will remain trapped and even fade away if the protective sheath is not removed by a process such as reconnection – where other field lines in the vicinity snap open the established field lines to let the plasma escape. “At the same time, reconnection occurs underneath the plasmoid, which helps push it out “like a piston”, accelerating it up out through the corona at speeds of around 500 kilometres per second.”

Archontis says that the runaway plasmoid could easily be detected by orbiting solar spacecraft such as STEREO, Hinode or SDO, by the fact that it originates in the cool lower atmosphere – it would look like a dark bubble rising through the solar atmosphere. He also speculates that the ejection of plasmoids could even be precursors to larger events such as CMEs, but he needs larger computing capabilities in order to test this idea, and to see how the plasmoids develop over larger scales.