Researchers studying sun-like stars as well as historical evidence from tree rings and glacial ice suggest that extreme solar events, including superflares, occur more frequently than previously thought. The findings, derived from data from thousands of stars observed by Kepler, indicate that violent solar storms are a normal part of solar activity, underscoring the need for effective space weather forecasting and protecting Earth's technological infrastructure.
The Sun is an unstable star, as evidenced by this year's unusually strong solar storms, which produced stunning auroras even at low latitudes. But will our star become more extreme? Clues about the most violent solar outbursts are preserved in ancient tree rings and glacial ice dating back thousands of years. However, these natural records provide only indirect evidence, making it difficult to determine how often such events occurred. Direct measurement of solar radiation has only been possible since the beginning of the Space Age.
To better understand the sun's long-term behavior, scientists have turned to the stars. Modern space telescopes monitor thousands of stars, tracking changes in their brightness over time. Superflares, which release more than a quadrillion joules of energy in a short period of time, appear as spikes in these observations.
"We cannot observe the Sun for thousands of years," explains MPS director and co-author Professor Sami Solanki, explaining the basic idea behind the investigation. "However, we can monitor the behavior of thousands of stars very similar to our Sun over short periods of time. This helps us estimate how often superflares occur," he added.
In the current study, a team including researchers from the University of Graz (Austria), the University of Oulu (Finland), the National Astronomical Observatory of Japan, the University of Colorado Boulder (USA), as well as the Paris-Saclay Commission for Atomic and Alternative Energies and the Université de la Cité in Paris analyzed data on 56,450 sun-like stars observed by NASA's Kepler space telescope between 2009 and 2013. "The Kepler data give us evidence of stellar activity over 220,000 years," said Professor Alexander Shapiro of the University of Graz.
The key to this study is careful selection of stars to consider. After all, the chosen star should be a particularly close "relative" of the Sun. Therefore, scientists only accept stars with surface temperatures and brightness similar to those of the Sun. The researchers also ruled out many sources of error, such as cosmic radiation, passing asteroids or comets, and non-Sun-like stars that might accidentally explode near Sun-like stars in the Kepler images. To do this, the team carefully analyzed each image of a potential superflare (just a few pixels in size) and counted only those events that could be reliably attributed to one of the chosen stars.
In this way, the researchers found 2,889 superflares on 2,527 of the 56,450 observed stars. This means that on average, a Sun-like star produces a superflare about once every hundred years.
"High-performance dynamo calculations of these Sun-like stars readily account for the magnetic origin of the intense energy release during such superflares," said co-author Dr. Allan Sacha Brun from the Paris-Saclay Commission for Atomic and Alternative Energies and Université Paris-Cite.
surprisingly often
"We were very surprised that Sun-like stars experience superflares so frequently," said first author Dr. Valeriy Vasilyev of MPS. Previous investigations by other research groups have found that superflares occur on average between a thousand and even 10,000 years. However, earlier studies were unable to determine the exact source of the observed flares and had to limit their studies to stars that did not have too close neighbors in telescope images. The current study is the most precise and sensitive to date.
Studies looking for evidence of violent solar storms affecting Earth have also shown that the average time between extreme solar events is longer. When the flow of energetic particles from the sun is particularly high, they produce detectable radioactive atoms such as the radioactive carbon isotope 14C. These atoms are then deposited in natural archives such as tree rings and glacial ice. Therefore, even thousands of years later, a sudden influx of high-energy solar particles can be inferred by measuring the amount of 14C using modern technology.
In this way, the researchers were able to identify five extreme solar particle events and three candidate events over the past 12,000 years of the Holocene, with an average occurrence rate of one every 1,500 years. The most violent one is believed to have occurred in AD 775. However, it is likely that many more such violent particle events and superflares have occurred in the past. "It is still unclear whether giant flares are always accompanied by coronal mass ejections, and what is the relationship between superflares and extreme solar particle events. This requires further study," noted co-author Professor Ilya Usoskin from the University of Oulu, Finland. Therefore, looking at terrestrial evidence for past extreme solar events may underestimate the frequency of superflares.
Predicting dangerous space weather
The new study doesn't reveal when the sun will erupt again. However, the findings urge caution. "The new data are a clear reminder that even the most extreme solar events are part of the sun's natural activity," said co-author Dr. Natalie Krivova, MPS. The Carrington Event of 1859 was one of the most violent solar storms of the past 200 years, during which telegraph networks collapsed across much of northern Europe and North America. The associated flares are estimated to release only one percent as much energy as superflares. Today, in addition to infrastructure on the Earth's surface, satellites are also at risk.
Therefore, the most important preparation for severe solar storms is reliable and timely forecasts. For example, satellites can be switched off as a precaution. Starting in 2031, ESA's space probe Vigil will help with the forecasting effort. From its vantage point in space, it will view the Sun from the side and detect processes that could lead to dangerous space weather brewing on our planet much earlier than Earth-based probes do. MPS is currently developing polarimetric and magnetic imagers for this mission.
Compiled from /scitechdaily