Solar flares are the most energetic explosions in our Sun’s atmosphere. A typical solar flare releases anywhere between ~10 thousand up to ~100 million times the world’s annual energy consumption in a matter of hours. The most powerful solar flare in recorded history was the Carrington Event which took place in the year 1859. This solar flare created a large geomagnetic storm on Earth which damaged telegraph systems and produced dramatic aurorae all around the world, visible even as far as the tropical latitudes. Should such an event occur in the present day, it would be very damaging to our technological society. Even so, flares that are many times more energetic than the Carrington Event are known to occur on other Sun-like stars. These flares are known as superflares and they are typically ~10 to ~1000 times more energetic than the Carrington Event. Whether our present Sun is capable of launching a superflare is not just of astrophysical important, but also of sociological importance.
Kepler is a space telescope designed to detect Earth-like planets around other stars. It constantly monitors the brightness of over a hundred thousand stars to a high level of precision and look for periodic dips in brightness that could be indicative of a planet crossing in front of its parent star. Using data collected by Kepler from April 2009 to December 2009, a study by Maehara et al. (2012) discovered 365 superflares on 148 solar type stars. Among them, 14 superflares came from Sun-like stars with rotational periods longer than 10 days and surface temperatures of 5,600 K £ T £ 6,000 K. Note that our Sun has a rotational period of over 25 days and a surface temperature of 5778 K. Based on this, it is estimated for a Sun-like star that a superflare 100 times more energetic than the Carrington Event occur once ever 800 years and a superflare 1000 times more energetic than the Carrington Event occur once ever 5000 years. If this occurrence rate is applicable to our Sun, then sometime during the next few hundred or thousand years, a superflare could pose a serious threat to our technology dependant civilization.
The study by Maehara et al. (2012) shows that the occurrence rate of superflares on slowly rotating stars (rotational periods longer than 10 days) is much lower than on rapidly rotating stars as most of the 365 superflares observed by Kepler occurred on stars that rotated in less than 10 days. This is because the energy of stellar flares depends on stellar magnetic activity and more rapidly rotating stars have higher magnetic activity. However, the maximum energy of a superflare remains unchanged regardless of whether the star is slowly or rapidly rotating. This means that superflares from slowly rotating stars are just as powerful as those from rapidly rotating stars.
It has been proposed that the presence of a close-in hot-Jupiter around a star affects the star’s magnetic activity and superflares can only occur on stars with hot-Jupiters. This suggests that our Sun is unlikely to have a superflare since there is no hot-Jupiter orbiting our Sun. However, the study by Maehara et al. (2012) paints a different picture. Given that the probability for a transit of a hot-Jupiter in front of its parent star is about 10 percent and assuming that the superflares on all 148 stars were caused by hot-Jupiters, then Kepler should have already detected about 15 hot-Jupiters. Instead, not a single hot-Jupiter was found around the 148 solar type stars with superflares. Since Kepler has almost completed its survey of hot-Jupiters, the non-detection of hot-Jupiters shows that it is not necessary to have a hot-Jupiter for the production of superflares.
There is no record of any superflare from our Sun over the last few thousand years. The largest solar flare known to date is still the Carrington Event in the year 1859. Nevertheless, there are a number of curious events in history that might be attributed to superflares from our Sun even though there is a lack of evidence at present to form any meaningful conclusion. One such event was published in a study by Miyake et al. (2012) where they discovered a rapid increase in cosmic-rays from AD 774 to 775. This was inferred from a spike in concentration of carbon-14 found in the tree rings of Japanese cedar trees. Carbon-14 is a radioactive isotope of carbon produced when high energy radiation enters the Earth’s atmosphere. If a superflare from our Sun is responsible, the flare will need to be ~1000 times more energetic than the Carrington Event to produce the spike in carbon-14.
1. Maehara et al., “Superflares on solar-type stars”, Nature 485, 478-481 (24 May 2012)
2. Miyake et al., “A signature of cosmic-ray increase in AD 774-775 from tree rings in Japan”, Nature 486, 240-242 (14 June 2012)