The Sun appears capable of releasing flares far more violent than any directly observed since the beginning of the space age.
To verify this, researchers studied the 300 most powerful solar flares recorded by NASA's Solar Dynamics Observatory space observatory between 2010 and 2016. They compared the energy released with the size of the active region at the origin of each event. These areas of the solar surface concentrate powerful magnetic fields and often contain dark sunspots.
A relationship emerges: large active regions can fuel more energetic flares. The researchers then used this relationship to look further back into the past. Sunspots have been observed regularly for about four centuries, making it possible to identify regions much larger than those recently monitored by satellites.
One case clearly stands out. In April 1947, a giant sunspot covered about 0.6% of the Sun's visible disk. Its diameter reached nearly 40 times that of Earth. Yet it did not produce any known superflare.
The new analysis nevertheless indicates that a region of this size could, under rare circumstances, have released around 1027 joules. This is the realm of “superflares.” Such energy would be far greater than that of the most powerful solar flares directly measured in modern times.
Energy can build up in an active region and then be released suddenly when the configuration of the field changes. A large sunspot therefore does not automatically cause a giant explosion: it mainly indicates that a very large amount of energy may be available.
This distinction matters for Earth. Strong solar activity can disrupt satellites, communications and certain power grids. The Carrington Event of 1859 notably caused disturbances in telegraph networks and made auroras visible at very low latitudes. However, there is no basis for claiming that it was a superflare comparable to the maximum calculated here.
One major unknown remains: no solar superflare has ever been observed directly. Scientists now know that the Sun can form sunspots large enough to supply the energy. It remains to be determined how often the conditions needed to release all this energy can actually come together.