Linked solar eruptions explained

Ron Cowen

Solar eruptions can trigger other explosions thousands of kilometres away on the other side of the Sun.
Solar Dynamics ObservatoryAugust 2010 began with a spectacular fireworks show on the Sun: gas ignited in the extreme ultraviolet and billion-tonne clouds of solar mass exploded in a chain of eruptions that went off like popcorn across an entire hemisphere of the Earth's nearest star.

The mayhem brought with it a revelation, thanks to images recorded by NASA's Solar Dynamic Observatory (SDO): some of these far-flung explosions were set off in sequence, with one eruption somehow triggering another, even over distances of several thousand kilometres1.

Now, Tibor Török of Predictive Science in San Diego, California, and his colleagues describe a model2 of how such an unlikely chain of events could happen. The model drives home a lesson for anyone interested in how solar eruptions affect Earth, says Karel Schrijver of the Lockheed Martin Advanced Technology Center in Palo Alto, California. To understand when and how a solar eruption may occur, activity over the entire Sun must be taken into account.

The model, posted on the arXiv preprint server this month and set to appear in an upcoming Astrophysical Journal Letters, simulates several magnetically turbulent regions in the Sun's corona, or outer atmosphere, that are ready to erupt but are confined by an overlying magnetic field, much as a pot of water boiling under a lid. If one of the regions manages to push aside its 'lid', the ensuing explosion can dislodge the lid on another magnetically volatile region — triggering a sequence of events in which the nearest region ready to blow may not be the next to erupt. Indeed, that's just what SDO and NASA's twin STEREO craft found during the August 2010 event.

"This is one of the new developments that is changing the way we think about the Sun and space weather," says Schrijver. "Because we have now both observations and models that say you can no longer ignore the coupling" from distant regions in predicting solar fireworks.

Even if one parcel of highly magnetized gas from the Sun's corona isn't headed towards Earth, the explosion could trigger another, more threatening coronal mass ejection elsewhere. Aimed at Earth, such an event could damage electrical power grids or harm Earth-orbiting craft, Schrijver notes.

Theorists first proposed some 75 years ago that widely separated solar eruptions might be linked, but solar physicists assumed that such events were rare. Only since the February 2010 launch of the SDO, which provides pictures of the corona at several different temperatures about every 10 seconds (rather than previous solar probes' every 1,000), has observation of an event like the one in August 2010 been possible. It was following his group's observation of that event1 that Schrijver asked Török and his colleagues to model it.

The team has "successfully simulated a model that several folks have speculated about", says Craig DeForest of the Southwest Research Institute in Boulder, Colorado, who was not part of the study. "Knowing that one event can in principle destabilize another energized knot of magnetic flux at some distance from the first gives more insight into how and why some systems erupt and others don't".

References

1.Schrijver, C. J. & Title, A. M. J. Geophys. Res. 116, A04108 (2011)
2.Török, T. et al. preprint at http://arxiv.org/abs/1108.2069 (2011).nature.com