Can we predict solar flares?

Flares from the Sun are the strongest explosions in our Solar System. They can cause severe space weather disturbances, posing a hazard to astronauts and technological systems in space and on the ground. Solar flares have an immediate impact in the form of enhanced radiation and energetic particles in as little as 8 min after the start of the event. Reliable prediction methods for flares are needed to provide longer warning times. However, pinning down the flare onset conditions is necessary for reliable predictions and is still a struggle ([ 1 ][1]). On page 587 of this issue, Kusano et al. ([ 2 ][2]) introduce a method to predict and successfully test for large imminent flares. Since their discovery more than 160 years ago by Carrington and Hodgson ([ 3 ][3], [ 4 ][4]), flares have been associated with sunspots on parts of the Sun with strong magnetic fields called active regions. The vast amount of flare energy is stored in complex (nonpotential) active region magnetic fields. The energy is impulsively released by magnetic reconnection, a fundamental plasma physics process that changes the topology of the magnetic field and converts magnetic energy into kinetic energy, thermal energy, and the acceleration of high-energy particles ([ 5 ][5]). Solar flares have been extensively studied for many decades. Now, regular observations are made at various wavelengths from a large number of ground- and space-based observatories. Regular measurements are made of the magnetic field and its vector components in the photosphere, which is considered the “surface” of the Sun. Despite extensive observations, the specific onset conditions and what triggers a flare are not understood. The lack of magnetic field measurements in the corona, where the field reconfiguration causing the sudden energy release in flares takes place, is a major limitation. Analytical and numerical methods are used instead to reconstruct the coronal magnetic field ([ 6 ][6]), model the instability and evolution of the field using magnetohydrodynamics ([ 5 ][5], [ 7 ][7], [ 8 ][8]), and indirectly infer magnetic reconnection signatures from the spatial and thermal distribution of the flaring plasma ([ 9 ][9]). Magnetohydrodynamic models are an important means for understanding the physics of solar eruptions and their onset conditions, but they cannot be used to predict the time of a flare. Current flare prediction schemes are mostly empirical and based on parameterizations of the surface magnetic field, such as the total magnetic flux of active regions, strong field gradients, and so on ([ 10 ][10], [ 11 ][11]). However, the forecast accuracy of empirical flare prediction schemes is still rather low, as measured by metrics like the skill score. Kusano et al. chose a different, physics-based approach for predicting imminent large flares through a critical condition of a magnetohydrodynamic instability that is triggered by magnetic reconnection. The method of Kusano et al. is based on “double-arc instability” ([ 12 ][12]) that allows a stability assessment of the sigmoidal field that is formed by reconnection between two sheared magnetic flux systems. The instability starts with magnetic reconnection on small spatial scales, called “trigger-reconnection,” between the sheared magnetic loops to create a double-arc loop, which contains the magnetic free energy that can be released during a flare. When the instability grows, the double-arc loops move upward, causing further magnetic reconnection that provides a positive feedback to the instability (see the figure). The double-arc instability is initiated when the ratio of the poloidal flux in the double arc to the flux of the stabilizing overlying magnetic field exceeds a limit. Based on this criterion, Kusano et al. determine the critical length-scale for the trigger-reconnection to destabilize the double arc and estimate the energy that can be released. The predictive model was tested on 200 active regions during solar cycle 24 that contained the largest sunspots but which did not produce large flares, in comparison with all seven active regions that produced large flares above class X2. X-class flares are powerful enough to trigger long-lasting radiation storms. Magnetic field vector measurements of the solar photosphere by the Helioseismic and Magnetic Imager ([ 13 ][13]) onboard NASA's Solar Dynamics Observatory were used for data. The authors found that the location and the time evolution of the identified critical regions provide a precursor to large flares, with lead times of 1 to 24 hours. The two large flares for which this scheme did not work were from a very specific large active region (AR12192) present on the Sun in October 2014. AR12192 was the source of numerous large flares, including six X-class flares, but none of them associated with a large cloud of plasma and embedded magnetic field ejected from the Sun known as a coronal mass ejection ([ 14 ][14]). This is a strong exception to the general statistics, because >90% of all large flares are accompanied by coronal mass ejections ([ 15 ][15]). However, the failure of the Kusano et al. prediction scheme for these events is also relevant because it shows that flare prediction methods have specific difficulties for large flares that are not accompanied by coronal mass ejections. This may be related to flare reconnection occurring relatively high in the corona or to strong overlying fields preventing an eruption ([ 2 ][2], [ 14 ][14]). ![Figure][16] Forecasting solar flares A physics-based model helps better estimate where and when large solar flares (shown above) will erupt. CREDIT: N. CARY/ SCIENCE Predicting solar flares is a very challenging task. The physics is complex and covers a large range of spatial scales, and key observables like the coronal magnetic field are lacking. Finally, the potential that flares are inherently stochastic processes cannot be ruled out. Nonetheless, tackling this issue has occurred by working along different paths. The diverse set of approaches will gain enormously from the upcoming observations of the 4-m Daniel K. Inouye Solar Telescope (DKIST), which had first light in December 2019. DKIST will provide improved resolution of the solar magnetic field fine structure and its dynamics and provide measurements of the coronal magnetic field. These key data are vital for a better understanding and probing of the onset and physics of solar flares. 1. [↵][17]1. L. Green, 2. T. Török, 3. B. VrŠnak, 4. W. Manchester IV, 5. A. Veronig , Space Sci. Rev. 214, 46 (2018). [OpenUrl][18] 2. [↵][19]1. K. Kusano et al ., Science 369, 587 (2020). [OpenUrl][20][CrossRef][21] 3. [↵][22]1. R. A. Carrington , Mon. Not. 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