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Scientists using NASA's twin STEREO spacecraft have developed a theoretical model to describe some of the Sun's most powerful explosions called coronal mass ejections or CMEs.

CMEs are massive eruptions of trillions of tonnes of million-degree ionized hydrogen plasma from the Sun. They induce strong electric currents in the Earth's magnetosphere and ionosphere, which can trigger spectacular auroral displays in the higher latitudes.

But they can also destroy satellites, endanger the lives of astronauts in space, and cause both power blackouts and communications problems on Earth.

These magnetic plasma clouds can be enormous, spanning millions of kilometres and can be rapidly accelerated by magnetic forces to speeds of thousands of kilometres per second.

Scientists first observed solar flares (energy erruptions on the Sun's surface which can produce CMEs) in 1859 and have been working to understand them and coronal mass ejections ever since.

One theory, by Dr. James Chen from the United States Naval Research Laboratory describes CMEs as giant magnetic flux ropes, like "a rope of twisted magnetic field lines shaped like a hoop or partial donut expanding outwards from the Sun."

This theory is now supported by data from the NASA STEREO mission.
STEREO Spacecraft

The 2 STEREO (Solar TErrestrial RElations Observatory) spacecraft were launched in 2006 to map the flow of energy and matter from the Sun to the Earth.

They orbit the Sun along the same track as the Earth, one just ahead of the planet in its orbit, the other trailing just behind.

From these positions they monitor CMEs and build a 3 dimensional map of their structure as they expand out from the Sun.

STEREO allows us to to construct a more complete picture of these enormous these enormous and complicated plasma structures accelerating away from the Sun.

This made it difficult to test theoretical models to understand the underlying mechanisms causing these eruptions.
STEREO and the Chen theory

Chen and Valbona Kunkel, a doctoral student at George Mason University, applied Chen's model to the new STEREO data and found that the theoretical solutions agree with the actual measured trajectories of the ejected particles.

Chen describes the underlying magnetic structure as a flux rope, and the basic driving force as a toroidal Lorentz force looking like a hoop emerging from the Sun.

This is the first model to replicate directly observed quantities near the Sun and the Earth, as well as the actual trajectories of CMEs.

Professor Iver Cairns from the University of Sydney says the model will help scientists predict of how CMEs are likely to move towards the Earth.

He says the model does a good job of explaining the three dimensional shape and speed of a CME heading for Earth.

But he still has some questions.

He asks what parameters Chen's team were able to fit and input into their model, to predict their observations.

"Specifically what the initial magnetic configuration on the Sun was, and how that structure was broken?"

"Coronal mass ejections are the primary cause of space weather and could cause trillions of dollars in damage", Cairns says, "so the more warning time we get the more time we have to prepare for the next such event."

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