In September 1859, a coronal mass ejection struck Earth and caused the most intense geomagnetic storm ever recorded. Aurorae appeared around the world, and telegraph systems failed in North America and Europe. Some operators received electric shocks; sparking lines also caused fires. Known as the Carrington event after observer Richard Carrington, the storm has inspired grim scenarios of what a similar disruption could do to our society today. In 2013, Lloyd’s and Atmospheric and Environmental Research said a Carrington-level event is inevitable and could cause up to $2.6 trillion in damage. Researchers in Japan say they have devised a method to accurately predict a key driver of such events: solar flares.
Finding the Trigger
Solar flares form when magnetic fields on and around the Sun reconnect. Although most solar flares are harmless, they can release enormous amounts of energy that accumulates around sunspots, sending plasma and high-energy particles into space. Even flares that are not Carrington-level events can cause serious disruptions to electrical, communication, and transport networks. A reliable method of predicting dangerous solar flares could help governments mitigate their impact.
Previous attempts to predict solar flare activity have been based on observations of sunspots, for instance, estimating flare size on the basis of sunspot and magnetic field properties. In a paper published this summer in Science, the researchers from Nagoya University and the National Astronomical Observatory of Japan describe what they call “a physics-based method that can predict imminent large solar flares.”
The method is based on the insight that relatively small reconfigurations in the Sun’s magnetic field lines can cause significant instabilities, not unlike how small cracks in the snow covering a mountain can trigger avalanches.
The method uses only data related to the solar magnetic field. Two-dimensional surface magnetic field data are fed into a supercomputer to calculate the three-dimensional field above the surface. The calculations are based on magnetohydrodynamics (MHD), the study of electrically conductive fluids and plasma under the influence of magnetic and electrical fields. The scientists found that a new type of MHD instability called a double-arc instability—a sort of M-shaped formation of magnetic field lines—can act as an initial driver of solar flares. The study also describes how the scientists used a new parameter called magnetic twist flux density, a reading taken near the magnetic polarity inversion line on the solar surface.
“If the magnetic twist flux density is strong near the polarity inversion line, a small-scale reconnection can trigger a flare,” said Kanya Kusano, lead author of the study and director of the Institute for Space–Earth Environmental Research at Nagoya University. “Magnetic twist flux density is a source of instability which can drive a flare.”
Need for a Breakthrough
This finding sheds light on the conditions that are needed to generate solar flares, but the team also wanted to see whether they could predict such explosions. They tested their theory against 10 years of solar flare observations from NASA’s Solar Dynamics Observatory, which is aimed at understanding the Sun’s magnetic field. Researchers focused on nine flares that were larger than X2 in seven active solar regions. X is the most intense class of solar flare; the majority of such explosions have strength ratings between 1 and 9. The method was able to accurately predict the emergence of seven flares, as well as their precise locations in six regions. This result contrasts with a hit rate of less than 50% for the current method for predicting X-class flares, according to the researchers.
“Instability and magnetic reconnection were separately investigated as the initial driver of solar flares,” said Kusano. “We combined them and developed the theory of triggered instability. As a result, we developed a new scheme (the ‘kappa scheme’), which can calculate how small trigger reconnections can cause a large flare and how a large flare is imminent. This is a new type of challenge in which the observation of magnetic fields on the solar surface and high-performance computation with supercomputers are combined to predict large flares.”
Being able to recognize the precursors to catastrophic large-scale solar flares “would be a major breakthrough in our understanding,” said David B. Jess, a scientist in the Solar Physics Group at Queen’s University Belfast who was not involved with the Japanese study.
“Kusano and colleagues provide evidence that increasingly twisted magnetic fields provide a critical length over which instabilities can occur, hence for the first time giving a numerical threshold that can identify the imminent risk of large flares erupting in the Sun’s atmosphere,” added Jess, who previously worked with colleagues to explain why our star’s magnetic waves strengthen as they emerge from its surface.
“Initial tests with a sample of existing data look promising. However, in order to alleviate false positives, and perhaps more importantly false negatives, more statistical testing of the method is required. This becomes even more important as we embark on the newest solar cycle, where activity levels are likely to increase over the coming years as we head towards solar maximum.”
Applying the Theory
Putting the kappa scheme into use will be a challenge. Even though the scientists used a supercomputer, predictions still took several hours to generate, and the process is longer than the current method to predict flares.
To reduce the computation time, Kusano is working with the Space Weather Forecast Center at Japan’s National Institute of Information and Communications Technology. Although he and his colleagues have been using findings from the Solar Dynamics Observatory, which can constantly monitor the entire solar magnetic field, Kusano wants to use the more sensitive instruments aboard the Japan Aerospace Exploration Agency spacecraft Hinode, another magnetic field observatory. Kusano hopes to have the kappa scheme implemented in 2–3 years.
“One of the next challenges is to extend our scheme to predict coronal mass ejections,” said Kusano. “That is more difficult than the prediction of onset of flares because the formation of CMEs depends on the nonlinear dynamics of MHD instability.”
—Tim Hornyak (@robotopia), Science Writer