Solar explosions held by magnetic “ropes”

A coronal mass ejection on the Sun.

Coronal mass ejections (CMEs) are huge explosions on the surface of the Sun that should billions of tonnes of matter into space.

Over the last century, astronomers have become very aware of how just dynamic the Sun really is. One of the most dramatic manifestations of this is a coronal mass ejection (CME) where billions of tons of matter are thrown into space.

If a CME reaches the Earth it creates inclement ‘space weather’ that can disrupt communications, power grids and the delicate systems on orbiting satellites. This potential damage means there is a keen interest in understanding exactly what triggers a CME outburst.

Now a team of researchers from University College London (UCL) have used data from the Japanese Hinode spacecraft, revealing new details of the formation of an immense magnetic structure that erupted to produce a CME on the December 7, 2007.

An eruption on the Sun known as a coronal mass ejection.

NASA's STEREO space probe saw an eruption on the Sun known as a coronal mass ejection.

Lead researcher Dr Lucie Green will present the team’s results on Monday, April 12, at the Royal Astronomical Society (RAS) National Astronomy Meeting in Glasgow.

The Sun’s behaviour is shaped by the presence of magnetic fields that thread through the solar atmosphere. The magnetic fields may take on different shapes from uniform arches to coherent bundles of field lines known as ‘flux ropes’.

Understanding the exact structure of magnetic fields is a crucial part of the effort to determine how the fields evolve and the role they play in solar eruptions.

In particular, flux ropes are thought to play a vital role in the CME process, having been frequently detected in interplanetary space as CMEs reach the vicinity of the Earth.

The formation of the flux rope requires that significant energy is stored in the solar atmosphere. The rope is expected to remain stable whilst the solar magnetic field in the vicinity holds it down.

But at some point the structure becomes unstable and it erupts to produce a CME. Using data from the Hinode spacecraft Dr Green has shown that a flux rope formed in the solar atmosphere over the 2.5 days that preceded the December 2007 event. Evidence for the flux rope takes the form of S shaped structures which are clearly seen by one of the Hinode instruments, the Extreme-Ultraviolet Imaging Telescope.

The key point to understanding and predicting the formation of CMEs is to know when the flux rope becomes unstable. Combining the observations of the S shaped structure with information on how the magnetic field in the region evolves has enabled Dr Green to work out when this happened.

The work shows that over 30% of the magnetic field of the region had been transformed into the flux rope before it became unstable, three times what has been suggested in theory.

Adapted from information issued by RAS. Images courtesy the STEREO Science Centre / NASA / SOHO.

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