Mechanics of coronal mass ejections revealed


The genesis of enormous explosions on the sun has long been a mystery, but no more. Lauren Fuge reports.


A coronal mass ejection captured by NASA's Solar Dynamics Observatory in September, 2017.

NASA/SDO

An international team of astronomers has untangled new insight into the birth of coronal mass ejections, the most massive and destructive explosions from the sun.

In a paper published in the journal Science Advances, a team led by Tingyu Gou from the University of Science and Technology of China was able to clearly observe the onset and evolution of a major solar eruption for the first time.

From a distance the sun seems benevolent and life-giving, but on closer inspection it is seething with powerful fury. Its outer layer – the corona – is a hot and wildly energetic place that constantly sends out streams of charged particles in great gusts of solar wind.

It also emits localised flashes known as flares, as well as enormous explosions of billions of tons of magnetised plasma called coronal mass ejections (CMEs).

These eruptions could potentially have a big effect on Earth. CMEs can damage satellite electronics, kill astronauts on space walks, and cause magnetic storms that can disrupt electricity grids.

Studying CMEs is key to improving the capability to forecast them, and yet, for decades, their origin and evolution have remained elusive.

“The underlying physics is a disruption of the coronal magnetic field,” explains Bernhard Kliem, co-author on the paper, from the University of Potsdam in Germany.

Such a disruption allows an expanding bubble of plasma – a CME – to build up, driving it and the magnetic field upwards. The “bubble” can tear off and erupt, often accompanied by solar flares.

The magnetic field lines then fall back and combine with neighbouring lines to form a less-stressed field, creating the beautiful loops seen in many UV and X-ray images of the sun.

“This breaking and re-closing process is called magnetic reconnection, and it is of great interest in plasma physics, astrophysics, and space physics,” says Kliem.

But the reason why the coronal magnetic field is disturbed at all is a matter of continuing debate.

“To many, an instability of the magnetic field is the primary reason,” says Kliem. “This requires the magnetic field to form a twisted flux tube, known as magnetic flux rope, where the energy to be released in the eruption can be stored.”

The theory holds that turbulence causes the magnetic flux ropes to become tangled and unstable, and if they suddenly rearrange themselves in the process of magnetic reconnection, they can release the trapped energy and trigger a CME.

Others in the field think that it’s the other way around – magnetic reconnection is the trigger that forms the flux rope in the first place.

It’s a tricky question to tease out because flux ropes and reconnection are so intertwined. Recent studies even suggest that there’s another layer of complexity: smaller magnetic loops called mini flux ropes, or plasmoids, which continuously form in a fractal-like fashion and may have a cascading influence on bigger events like a CME.

To get a better handle on this complex process, the team observed the evolution of a CME that erupted on May 13, 2013. By combining multi-wavelength data from NASA’s Solar Dynamics Observatory (SDO) with modern analysis techniques, they were able to determine the correct sequence of events: that a magnetic reconnection in the solar corona formed the flux rope, which then became unstable and erupted.

Specifically, they found that the CME bubble continuously evolved from mini flux ropes, bridging the gap between micro- and macro-scale dynamics and thus illuminating a complete evolutionary path of CMEs.

The next step, Kliem says, is to understand another important phenomenon in the eruption process: a thin, elongated structure known as a “current sheet”, in which the mini flux ropes were formed.

“We need to study when and where the coronal magnetic field forms such current sheets that can build up a flux rope, which then, in turn, can erupt to drive a solar eruption,” he concludes.

  1. http://advances.sciencemag.org/content/5/3/eaau7004
  2. https://doi.org/10.1038/nature14556
  3. https://sdo.gsfc.nasa.gov/mission/
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