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Solar flares: l’X researchers make the front page of Nature

A team of researchers led by Tahar Amari, research director at the Center for Theoretical Physics (CNRS / École Polytechnique) featured in Nature on February 8, 2018. Their work, combining astronomical observation and theoretical models, has enabled them to estimate the maximum potential energy released during a solar eruption.

Major flare on October 24, 2014 observed by the AIA Instrument of the Solar Dynamics Observatory mission at NASA. An image of the Earth has been included to demonstrate the enormous scale of the flare. © Tahar Amari et al. / Center for Theoretical Physics (CNRS / École Polytechnique)

The atmosphere of the Sun is constantly disrupted by solar flares, the most intense of which have the potential to affect the technological installations of our planet (satellite networks for communication, geolocation, electricity distribution, air traffic management, etc.). In order to better predict these eruptions, a team of researchers has developed a model that can be used to estimate the maximal energy potentially released during a solar eruption. This research, led by Tahar Amari and Aurélien Canou from the Center for Theoretical Physics (CNRS / École Polytechnique)—in collaboration with Jean-Jacques Aly from the Laboratory of Astrophysics: Interpretation – Modeling (CNRS / CEA / Paris Diderot), François Delyon from the Laboratory of Condensed Matter Theoretical Physics (CNRS / Sorbonne University) and Frédéric Alauzet from INRIA—made the front page of Nature on February 8, 2018.

A research team under the direction of Tahar Amari also made the front page of the prestigious journal back in 2014, after proving the existence of a specific structure in magnetic field lines, which form a twisted rope in the hours leading up to a solar eruption. With his 2014 research having been focused on "eruptive" solar flares that are associated with coronal mass ejections; this time, Amari has looked into the mechanism of "confined" solar flares without coronal mass ejections, which can be just as powerful, but which release energy solely in the form of potent rays.

By chance, the object of study for this latest research was a solar flare that took place on October 24, 2014; the same day the previous study had been published in Nature. This confined solar flare occurred in the largest active region of the Sun since 1990, in a sunspot as large as the planet Jupiter, whereas spots usually have the same size as the Earth.

"Making the invisible visible"

"To access the magnetic field, we studied the final stages of ‘pregnancy’ that gave birth to the eruption," explains Tahar Amari. The researcher creates an image of the phenomenon so that the team can better understand the method, a practice that could be compared to ultrasound imagery. This allows them to deduce the properties of the magnetic field within the Sun’s atmosphere. "Conventional" observation methods, which consist of analyzing the Sun’s light spectrum to deduce its magnetic field, do not work for the solar atmosphere as the spectrum is damaged by very high temperatures.

In order to observe the eruption, this so-called "magnetic baby", Tahar Amari gathers data, which he then presents as a type of magnetic ultrasound. He explains: "By measuring the magnetic field on the Sun’s surface, we are able to deduce the properties of magnetic fields within its atmosphere. But the analogy stops here because, contrary to an ultrasound, which observes the inside of the body, our method lets us see the magnetic baby outside of the Sun, using measurements at its surface level."

Satellite measurements are combined with solutions of mathematical equations in a theoretical model developed by the researchers that bears a similarity to models for weather prediction. With this methodology, the researchers have managed to determine the shape of magnetic fields within the solar corona. Whereas conventional models measure the behavior of the Earth’s atmosphere, in their model, this is replaced by that of the Sun’s atmosphere, which obeys the rather specific physical laws of magnetohydrodynamics (the study of the magnetic properties of electrically conducting fluids). Through a series of echographic studies (comprising measurements and solutions using the team’s theoretical model), and continuing on their work from 2014, the research team was able to bring to light the existence of a magnetic rope that also forms during confined solar flares.

Nature of solar flares determined by magnetic power balance

The researchers made a further discovery: the rope is formed within a "cocoon", a cage of magnetic fields, which supports the rope’s development by keeping it in close proximity to the sun (see simulation below). Once the rope is strong enough, an equilibrium is established between it and the cage. In the case of the solar eruption observed on October 24, 2014, the researchers could ascertain that the cage had become strong enough to keep the rope inside and prevent it from ejecting matter. Their dynamic theoretical model goes even further, as the researchers were able to estimate the maximum potential energy to be released by the solar flare using data measured 10 minutes before the eruption.

This method has helped them understand the power equilibrium between the cage and the rope. Therefore, just as in the case of the flare they studied, a reinforced cage prevents any coronal mass ejections from occurring. But this model is also valuable for eruptive solar flares. By weakening the cage in their model, the scientists were able to demonstrate that if it is not powerful enough in comparison with the rope, the eruption will break the cage and eject a magnetic bubble of plasma.

The work of Tahar Amari and his research partners has improved our comprehension of the mechanisms that control solar eruptions and allowed for the possibility of making early predictions of their intensities if they make measurements ahead of a solar flare. This could prove essential for turning off satellites in time and preventing their electrical apparatus from damage; damage that would cut off access to GPS systems or communication networks. Such predictions will only be possible if there is enough data, which necessitates therefore more mission launches in which France is involved, in order to monitor the Sun from a variety of angles.

Magnetic ultrasound image produced using data from the magnetic field of the surface of the Sun (NASA SDO satellite) and an efficient multiscale model, a few minutes before the start of a solar flare. The result reveals the presence of a reinforced, multilayer magnetic cage (indicated here in yellow, pink and white) inside which the magnetic rope forms during the final hours before the eruption (indicated in blue). ©Tahar Amari et al. / Center for Theoretical Physics (CNRS / École Polytechnique)