Characterizing and predicting the magnetic environment leading to solar eruptions

• Published in: Nature (London) 2014-10, Vol.514 (7523), p.465-469
• Main Authors: Amari, Tahar, Canou, Aurélien, Aly, Jean-Jacques
• Format: Article
• Language: English
• Subjects: 639/33/525; Astrophysics; Boundary conditions; Equilibrium; Fluctuations; Humanities and Social Sciences; letter; Magnetic fields; Magnetism; Meteorology; multidisciplinary; Science; Sciences of the Universe; Solar and Stellar Astrophysics; Sun
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature13815

Modelling the solar magnetic field using observations of the photospheric field in the four-day period preceding a coronal mass ejection shows that the formation and later ejection of a twisted rope of magnetic flux provides the physical mechanism responsible for the ejection. Magnetic ropes power solar eruptions Coronal mass ejections are large-scale eruptions in the solar atmosphere that consist of solar plasma confined in a magnetic field. They have the potential to produce solar storms here on Earth that can damage artificial satellites and disrupt ground-based power generation. Using observations of the photospheric magnetic field made during the four days leading up to the coronal mass ejection of 13 December 2006, together with numerical modelling, Tahar Amari et al . show that the physical mechanism responsible for such ejections is best explained as the appearance and the later ejection of a 'twisted rope' of magnetic flux. The physical mechanism responsible for coronal mass ejections has been uncertain for many years, in large part because of the difficulty of knowing the three-dimensional magnetic field in the low corona 1 . Two possible models have emerged. In the first, a twisted flux rope moves out of equilibrium or becomes unstable, and the subsequent reconnection then powers the ejection 2 , 3 , 4 , 5 . In the second, a new flux rope forms as a result of the reconnection of the magnetic lines of an arcade (a group of arches of field lines) during the eruption itself 6 . Observational support for both mechanisms has been claimed 7 , 8 , 9 . Here we report modelling which demonstrates that twisted flux ropes lead to the ejection, in support of the first model. After seeing a coronal mass ejection, we use the observed photospheric magnetic field in that region from four days earlier as a boundary condition to determine the magnetic field configuration. The field evolves slowly before the eruption, such that it can be treated effectively as a static solution. We find that on the fourth day a flux rope forms and grows (increasing its free energy). This solution then becomes the initial condition as we let the model evolve dynamically under conditions driven by photospheric changes (such as flux cancellation). When the magnetic energy stored in the configuration is too high, no equilibrium is possible and the flux rope is ‘squeezed’ upwards. The subsequent reconnection drives a mass ejection.