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Magnetic ropes tie down solar eruptions
Posted: 12 April

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Using data from the Hinode spacecraft, astronomers have discovered new details of an immense structure that erupted from the Sun’s surface to produce a coronal mass ejection in December 2007.

Coronal mass ejections, or CMEs, throw billions of tons of matter into space that can lead to disruptions of telecommunications and power grids on the Earth. They therefore represent a phenomenon that scientists strive to understand. The Sun’s outbursts are related to the behaviour of magnetic field lines that thread through the solar atmosphere, varying in shape from arches to bundles of field lines known as flux ropes.

The three Hinode images reveal gases trapped in the flux rope at different temperatures, from 1.5 million degrees Celsius in the image on the left through to 2.5 million degrees Celsius in the right hand image. Image: JAXA/ISAS/NASA/STFC.

Flux ropes are thought to play a key role in the production of CMEs, and Lucie Green of University College London says, “Magnetic flux ropes have been observed in interplanetary space for many years now and they are widely invoked in theoretical descriptions of how CMEs are produced. We now need observations to confirm or reject the existence of flux ropes in the solar atmosphere before an eruption takes place to see whether our theories are correct.”

The formation of the flux rope requires that significant energy is stored in the solar atmosphere. It remains stable for as long as the local solar magnetic field is able to hold it down, but when it becomes unstable, it erupts to produce a CME.

Green used Hinode data to show that a flux rope formed in the solar atmosphere in the days that preceded the 7 December 2007 event, manifesting as an S-shaped structure in Hinode’s Extreme-Ultraviolet Imaging Telescope. Moreover, by combining the observations of the structure with data on how the magnetic field evolved, Green was able to learn more about how the flux rope became unstable in the first place. She found that over 30 percent of the magnetic field of the region had been transformed into the flux rope before it became unstable, three times what had previously been estimated.

“Flux ropes are thought to play a vital role in the evolution of the magnetic field of the Sun,” says Green, who adds that the physics of flux ropes can be applied to scenarios across the Universe. “For example, a solar physics model of flux rope ejection was recently used to explain the jets driven by the accretion discs around the supermassive black holes found in the centre of galaxy.”