SOHO and Hinode offer new insight into solareruptions23 January 2015, by Karen C. Fox

Scientists are trying to understand the precise details ofwhat creates giant explosions in the sun's atmosphere,such as this solar eruption from Oct. 14, 2012, as seenby NASA's Solar Dynamic Observatory.Credit: NASA/SDO/Amari

The sun is home to the largest explosions in thesolar system. For example, it regularly produceshuge eruptions known as coronal mass ejections when billions of tons of solar material erupt off thesun, spewing into space and racing toward thevery edges of the solar system. Scientists knowthat these ejections, called CMEs, are caused bymagnetic energy building up on the sun, whichsuddenly releases. But the details of what causesthe build up and triggers the release are notprecisely understood.

A journal paper in Nature magazine on Oct. 23,2014, used data from NASA missions to presentan example of how something called a magneticflux rope builds up over time until it is so unstablethat even the slightest perturbation will send it

flying. Understanding what triggers CMEs is crucialnot only for better understanding of our sun, butalso to lay the groundwork for predicting when suchgiant explosions might happen.

"We looked at a well-studied CME from 2006," saidTahar Amari first author on the Nature paper atEcole Polytechnique in France. "We knew thatthere had been a great deal of data available forthis CME and much analysis already done, but noone had created a comprehensive picture of whathappened."

Amari and his colleagues used a traditionalmeteorology technique to examine the event:Gather observations from the days before the CMEto track how the event grew over time. They usedobservations from the European Space Agency andNASA's Solar and Heliospheric Observatory, orSOHO, and the Japanese Aerospace ExplorationAgency and NASA's Hinode, as well as from theParis-Meudon Observatory.

The team wanted to see if they could distinguishbetween two broad theories about how themagnetic energy develops. The first modeldescribes a situation in which a series of loops ofmagnetic fields on the sun known as an arcade is the start of every active region CME. This arcadehas a weak point at the top, a place where theenergy from below can burst through once it's greatenough. During the eruption a flux rope forms,which can be seen inside the CME as it surgesaway from the sun.

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Scientists created this model to examine the magneticfield before a giant solar eruption a coronal massejection which occurred on Dec. 13, 2006. The orangelines show magnetic field lines. The grey line representswhat's called a flux rope, which built up the day beforethe event. Credit: Amari/Ecole Poytechnique

The second model assumes that the flux rope isthere before the CME erupts. In this theory, noweak point is required. Instead, the flux rope gainsmore and more energy, and becomes increasinglyunstable until a disturbance on the sun causes it torelease the energy out into space.

Amari and his team used magnetic data from thesurface obtained by Hinode, but they also neededmagnetic data for the sun's atmosphere, thecorona, which is strongly affected by its magneticfield.

"The corona is so hot that most of the techniques tomeasure the magnetic field don't work," said Amari."So we developed an efficient and accurate modelto compute the magnetic field there, based on thedata we had from the surface, and the equationsgoverning the physics of the low corona aboveactive regions."

With these two data sets in hand, the teamexamined what happened in the four days beforethe 2006 CME erupted. They could see themagnetic energy building; it was clear somethingwas emerging. Only, however, on the last day did aflux rope develop and only then did it have enoughenergy built up to power a CME eruption. At thispoint, some small disruption was enough of anudge to make the flux rope erupt.

A model of the eruption of a giant magnetic rope that ledto a coronal mass ejection on the sun in December 2006.The model showed that magnetic fields built up forseveral days before the eruption. Credit: Amari/EcolePoytechnique

"In this case no weak point up in the atmospherewas needed to allow the energy to be released,"said Amari. "There is, instead, a kind of criticalvalue of energy, a value we can compute based onseeing an active magnetic region on the sun.Beneath that value the magnetic field will stayquiet. Above that, it is likely to erupt. There is also acritical height for rising flux rope, beyond which themagnetic loops above can no longer keep itconfined."

The team explored the initial conditions from thisevent and put the information into anotherdynamical model the team had developed. Thesimulation mirrored what was actually seen, with aneruption occurring only when the critical energy andheight were reached on the last day.

Amari points out that just because this CMEcontained a flux rope prior to eruption, it doesn'tmean that other CMEs can't erupt based on otherphysical catalysts. But it clearly describes one

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mechanism that is at work on the sun.

By measuring and calculating the magnetic fieldson the sun, coupled with determining how tomeasure the critical tipping point where a CME canerupt, the paper offers new ways to determine thepossibility of eruption from any given active area onthe sun.

More information: For more information about theEuropean Space Agency and NASA's SOHOMission, visit:www.nasa.gov/soho/. For more information aboutthe Japanese Aerospace Exploration Agency andNASA's Hinode, visit: www.nasa.gov/hinode

Provided by NASA

APA citation: SOHO and Hinode offer new insight into solar eruptions (2015, January 23) retrieved 16 April2015 from http://phys.org/news/2015-01-soho-hinode-insight-solar-eruptions.html

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