New Research Indicates the Sun may be More Prone to Flares Than we Thought

This past year saw some significant solar activity. This was especially true during the month of May, which saw more than 350 solar storms, solar flares, and geomagnetic storms. This included the strongest solar storm in 20 years that produced aurorae at far lower latitudes than usual and the strongest solar flare observed since December 2019. Given the threat they pose to radio communications, power grids, navigation systems, and spacecraft and astronauts, numerous agencies actively monitor the Sun’s behavior to learn more about its long-term behavior.

However, astronomers have not yet determined whether the Sun can produce “superflares” or how often they might occur. While tree rings and samples of millennia-old glacial ice are effective at records of the most powerful superflares, they are not effective ways to determine their frequency, and direct measurements of solar activity have only been available since the Space Age. In a recent study, an international team of researchers adopted a new approach. By analyzing Kepler data on tens of thousands of Sun-like stars, they estimate that stars like ours produce superflares about once a century.

The study was conducted by reseMax-Planck-Institut for Solar System Research (MPS), the Sodankylä Geophysical Observatory (SGO) and the Space Physics and Astronomy Research unit at the University of Oulu, the National Astronomical Observatory of Japan (NAOJ), the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado Boulder (UCF), the National Solar Observatory (NSO), the Commissariat of Atomic and Alternative Energies of Paris-Saclay and the University of Paris-Cité, and multiple universities. The paper that addresses their research recently appeared in the journal Science.

Superflares are notable for the intense amount of radiation they emit, about 1032 erg, or 6.2444 electron volts (eV). For comparison, consider the Carrington Event of 1859, one of the most violent solar storms of the past 200 years. While this solar flare caused widespread disruption, leading to the collapse of telegraph networks in northern Europe and North America, it released only a hundredth of the energy of a superflare. While tree rings and glacial samples have recorded powerful events in the past, the ability to observe thousands of stars at a time is teaching astronomers a lot about how often the most powerful flares occur.

This is certainly true of the Kepler Space Telescope, which monitored about 100,000 main-sequence stars continuously for years for signs of periodic dips indicating the presence of exoplanets. These same observations recorded countless solar flares, which appeared in the observational data as short, pronounced peaks in brightness. As Prof. Dr. Sami Solanki, a Director at the MPS and a co-author of the paper, explained in a MPS press release:

“We cannot observe the Sun over thousands of years. Instead, however, we can monitor the behavior of thousands of stars very similar to the Sun over short periods of time. This helps us to estimate how frequently superflares occur.”

For their study, the team analyzed data obtained by Kepler from 56,450 Sun-like stars between 2009 and 2013. This consisted of carefully analyzing the images for signs of potential superflares, which were only a few pixels in size. The team was also careful in their selection of stars, taking into account only those whose surface temperature and brightness were similar to the Sun’s. The researchers also ruled out potential sources of error, including cosmic radiation, transient phenomena (asteroids or comets), and other types of stars flaring up near a Sun-like star.

In total, the Kepler data provided the team with evidence of 220,000 years of stellar activity. From this, they were able to identify 2,889 superflares from 2,527 of the observed stars, producing an average of one superflare per star per century. While previous surveys have found average intervals of a thousand or even ten thousand years, these studies could not determine the exact source of the observed flares. They also had to limit themselves to stars without any close neighbors, making this latest study the most precise and sensitive to date.

Nevertheless, previous studies that considered indirect evidence and observations made in the past few decades have yielded longer intervals between superflares. Whenever the Sun has released a high level of energetic particles that reached Earth’s atmosphere in the past, the interaction produced a detectable amount of radioactive carbon-14 (C14). This isotope will remain in tree and glacial samples over thousands of years of slow decay, allowing astronomers to identify powerful solar events and how long ago they occurred.

This method has allowed researchers to identify five extreme solar particle events and three candidates within the past twelve thousand years – suggesting an average rate of one superflare per 1,500 years. However, the team acknowledges that it is possible that more violent solar particle events and superflares occurred in the past. “It is unclear whether gigantic flares are always accompanied by coronal mass ejections and what is the relationship between superflares and extreme solar particle events,” said co-author Prof. Dr. Ilya Usoskin from the University of Oulu. “This requires further investigation.”

While the new study does not reveal when the Sun will experience its next superflare, the results urge caution. “The new data are a stark reminder that even the most extreme solar events are part of the Sun’s natural repertoire,” said co-author Dr. Natalie Krivova from the MPS. In the meantime, the best way to stay prepared is to monitor the Sun regularly to ensure reliable forecasting and advanced warning. By 2031, these efforts will be bolstered by the ESA’s Vigil probe, which the MPS is assisting through the development of its Polarimetric and Magnetic Imager (PHI) instrument.

Further Reading: MPS, Science