Caged fury: study sheds light on solar flares

Magnetic ropes and cages determine how the sun erupts. Lauren Fuge reports.

Magnetic loops forming a solar flux rope extending from the surface of the sun, captured by NASA's Solar Dynamics Observatory (SDO) in 2012.
Magnetic loops forming a solar flux rope extending from the surface of the sun, captured by NASA's Solar Dynamics Observatory (SDO) in 2012.
NASA/Goddard Space Flight Center/SDO

French astrophysicists have found that solar eruptions may be controlled by one unique phenomenon, in a discovery that could help predict these powerful and potentially destructive events.

Solar flares are spectacular bursts of radiation from the sun. They are sometimes, but not always, followed by eruptions called coronal mass ejections (CME): massive explosions of magnetised plasma that can extend well into the solar system.

Scientists have long puzzled over why CMEs do not occur after all flares. There are two leading theories.

The first links the triggering of a CME to the “magnetic topology” of the flaring structure -- the shape the resulting magnetic field, which determines how and when magnetic energy is converted into kinetic energy and particle acceleration.

The second relies on the formation of a series of twisted magnetic loops on the sun, collectively known as a flux rope. Flux ropes have been observed by the NASA Solar Dynamics Laboratory (SDL), confirming theoretical predictions, and researchers have established that the structures can exist in stable equilibrium for weeks, even months, before suddenly undergoing a violent phase change and erupting at high speed.

Limits to observational accuracy, however, have made it impossible to establish which explanation is correct.

Now, in a paper published in the journal Nature, a team of French physicists have shown that the second explanation is more likely. Using data from the SDL, a team led by Tahar Amari of the Université Paris-Saclay, observed the growth of a large flare in October 2014 in an active region on the sun that has produces many such flares, but no major CMEs.

The team predicted the evolution of the flare by developing state-of-the-art numerical models of the solar coronal magnetic field. They found that the flare was surrounded by a strong, multi-layered magnetic “cage” with a flux rope developing inside it.

Effectively, a battle ensues between the rope and the cage. If the rope, twisting and flexing, becomes powerful enough to trigger an instability in the structure enclosing it, the cage is destroyed and a CME results.

These findings are not surprising because they were previously seen in numerical simulations, but they have now been observed on the sun itself. The model is also able to predict the maximum possible amount of energy released during a solar eruption.

According to Michael Wheatland, solar physicist at the University of Sydney, Australia, who was not involved in the study, “the results are relatively convincing, and the level of detail is really ahead of what other people are achieving”.

Lauren Fuge is an Adelaide-based author and science communicator.
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