How a simple magnetic field configuration could trigger solar eruptions
Solar eruptions, including flares and coronal mass ejections, are powered by magnetic fields in the Sun’s corona. The challenge is pinpointing a specific mechanism that triggers them. As a precondition for eruption, many models invoke complex magnetic field structures, such as twisted ropes of flux, but astronomers have rarely observed such topologies in the pre-eruption corona. Now Chaowei Jiang at the Harbin Institute of Technology’s campus in Shenzhen, China, and colleagues have demonstrated through three-dimensional simulations that magnetic field lines that are arranged in a simple configuration near sunspots are sufficient to initiate a solar eruption.
Comparison of an observed flare eruption by NASA’s Solar Dynamics Observatory (left, courtesy of NASA) with a simulation of an eruptive structure (right, adapted from C. Jiang et al., Nat. Astron., 2021, doi:10.1038/s41550-021-01414-z ).
The work by Jiang and his colleagues builds on a 1980s theory proposing that a single loop of magnetic flux pinned at both ends to sunspots on the solar surface could disrupt the stability of the corona and lead to an eruption. Using China’s National Supercomputing Center in Tianjin, the researchers designed a magnetohydrodynamic simulation driven by slow rotational flow of the plasma at the Sun’s surface. They found that horseshoe-like loops of flux ballooned outward while remaining connected to the solar surface. The horseshoe’s pinned sides sheared against one another, forming a sheet of electrical current. As the current sheet became thinner, the field lines pushed toward each other and then abruptly rearranged themselves in a way that converted magnetic energy into kinetic energy, a process known as reconnection. That reconnection drove the eruption. A twisted rope of magnetic flux appeared only after the eruption had initiated—dispelling the notion that it was a necessary precursor. On closer analysis, the researchers found that strong curvature in the newly reconnected field lines created a slingshot effect that resulted in extremely strong magnetic tension forces directed away from the Sun. Those forces provided an efficient means of driving particles and radiation out into space.
Magnetic field lines approach each other just before reconnection (left). The slingshot effect occurs just after reconnection (right). The newly reconnected field line is curved strongly, and thus has extremely strong magnetic tension forces, as denoted by the arrows. The force represented by the red arrow, which is pointing away from the Sun’s surface, can drive an eruption.
Chaowei Jiang
Key to the simulation’s success was maintaining the pinning of an untwisted loop at the solar surface long enough for a current sheet to form, the researchers say. The morphology of the simulations is consistent with the structures captured by NASA’s Solar Dynamics Observatory. The finding could lead to a universal model of solar eruptions and provide better understanding of how the phenomenon influences space weather. (C. Jiang et al., Nat. Astron., 2021, doi:10.1038/s41550-021-01414-z .)