In addition to flashes of light and waves of magnetically charged particles, it happens that a solar flare releases a plasma bubble. This event, also known as coronal mass ejection, is usually not directly directed towards the Earth, or at a low intensity. But these phenomena are taken very seriously because they present a danger of "magnetic storm": disturbance of radio signals and communications (GPS, UHF...), current peaks on electrical installations, danger for astronauts etc.
Thanks to satellites such as SOHO, DSCOVR or SDO, it is now possible to pick up these plasma bubbles and even to observe the eruptions, but also to predict them thanks to the work of a CNRS team, the school Polytechnique Paris-Saclay, CEA and INRIA. It was even possible to reveal, thanks to supercomputers, the mechanisms behind the evolution of magnetic fields' properties during solar flares. "We are trying to find out how this energy accumulates to understand what is responsible for these eruptions", says Amari.
A matter of magnetic field
Backing up a little bit. In 2006, the Japanese satellite Hinode (also called Solar B) observes a particular region of the Sun during an eruption occurring from 12 to 13 December. For the first time, the team of researchers led by Tahar Amari observes a magnetic disturbance connecting the photosphere (the inner layer sometimes called the "surface" of the Sun) to its crown (the highest layer of the solar atmosphere, which dilutes in space). These lines of forces being twisted, scientists speak of magnetic rope.
In 2014, in a first article that made the headline of Nature, they show through a digital simulation that the appearance of a rope is in fact a prerequisite for a solar flare. Without this "magnetic lasso" whose energy increases as it passes the outer layers of the Sun, a coronal mass ejection could not happen!
But to study this phenomenon, one must understand how the magnetic field of the solar corona works. It is a complicated area: by a mechanism still poorly understood, the temperatures are phenomenal (between 1 and 3 million degrees Celsius!) and this induces particularly powerful radiation (X-rays, ultraviolet etc.) ... What about the magnetic field? "Unfortunately, we cannot directly observe it, so our goal was to reveal it from the magnetic field data of the photosphere”, explains Amari.
In 2014, the American SDO probe (Solar Dynamics Observatory) observes a particularly active region of the sun with the HMI instrument (Helioseismic and Magnetic Imager). This area, as wide as Jupiter, is conducive to solar flares...
On 24 October, a major event produces a "flash" that would disrupt some terrestrial communications. This time, Tahar Amari and his colleagues have better data, valid for the entire Sun. "Using equations from magnetohydrodynamics applied to the case of the Sun, we obtain something similar to a snapshot of the magnetic field. By repeating this operation, it enables us to visualise the magnetic field at each point of the crown all along the development that precedes the solar flare". A bit like an ultrasound on the stomach of a pregnant woman.
Towards a solar meteorology
Understanding how fields evolve over time has allowed researchers to set up a method that can predict the evolution of interactions between the magnetic rope, its environment and the energy that could be released. Like a model of "magnetic weather" in the solar corona!
Numerical simulation using magnetic field data at the Sun's surface (NASA SDO satellite) and a powerful multi-scale model a few minutes before the onset of the eruption. The result reveals the presence of a reinforced multilayered magnetic cage in which the magnetic rope develops during the last hours before the eruption.
© Tahar Amari et al./Centre de physique théorique, CNRS-Ecole Polytechnique, FRANCE
During an eruptive phenomenon, this helps to understand why sometimes there is no plasma bubble ejection: the magnetic fields of the solar corona form a very strong magnetic "cage" in several layers. "This cage is both the ground for the development of a rope phenomenon, an ideal envelope (conditions of pressure and magnetic tension) and the environment that will contain it", says Mr. Amari. However, it happens that the rope breaks its cage and destroys this balance (it is the solar flare, with coronal mass ejection), or that its torsion disturbs the envelope to the point of releasing an important energy "flash" as observed in October 2014.
By successfully observing and understanding these cage-rope interactions, the team not only opens the door for better predicting solar flares, but also for estimating the maximum energy they could release.
So, what is the next step? "Having data from the future ESA Solar Orbiter probe with the participation of CNES, with its orbit out of the plane of the ecliptic, will provide us with a different point of view and allow us to better characterise the magnetic fields. Especially with STIX, its X-band spectrometer. "A crucial study to understand the mechanisms of our sun and its crown but also to better characterise the stars of neighbouring systems, which we examine in search for exoplanets...
- Amari, T., et al. (2018), Magnetic cage and rope as the key for solar eruptions, Nature, 554, 211–215
- Press release from the CNRS : http://www2.cnrs.fr/presse/communique/5427.htm
- Tahar Amari, researcher at the Centre de physique théorique, CNRS-Ecole Polytechnique : tahar.amari at polytechnique.edu
- Kader Amsif, head of the programme Sun, Heliosphere, Magnetosphere au CNES : kader.amsif at cnes.fr