Superflares once per century

(Press-News.org) There is no question that the Sun is a temperamental star, as alone this year’s unusually strong solar storms prove. Some of them led to remarkable auroras even at low latitudes. But can our star become even more furious? Evidence of the most violent solar “tantrums” can be found in prehistoric tree trunks and in samples of millennia-old glacial ice. However, from these indirect sources, the frequency of superflares cannot be determined. And direct measurements of the amount of radiation reaching the Earth from the Sun have only been available since the beginning of the space age.

Another way to learn about our Sun’s long-term behavior is to turn to the stars, as is the approach of the new study. Modern space telescopes observe thousands and thousands of stars and record their brightness fluctuations in visible light. Superflares, which release amounts of energy of more than one octillion joules within a short period of time, show themselves in the observational data as short, pronounced peaks in brightness. “We cannot observe the Sun over thousands of years,” Prof. Dr. Sami Solanki, Director at the MPS and coauthor, explained the basic idea behind the investigation. “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,” he adds.

Looking for close relatives of the Sun

In the current study, the 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) and the Commissariat of Atomic and Alternative Energies of Paris-Saclay and the University of Paris-Cité, analyzed the data from 56450 sun-like stars as seen by NASA’s space telescope Kepler between 2009 and 2013. “In their entirety, the Kepler data provide us with evidence of 220000 years of stellar activity,” said Prof. Dr. Alexander Shapiro from the University of Graz.

Crucial for the study was the careful selection of the stars to be taken into account. After all, the chosen stars should be particularly close “relatives” of the Sun. The scientists therefore only admitted stars whose surface temperature and brightness were similar to the Sun’s. The researchers also ruled out numerous sources of error, such as cosmic radiation, passing asteroids or comets, as well as non-sun-like stars that in Kepler images may by chance flare up in the vicinity of a sun-like star. To do this, the team carefully analyzed the images of each potential superflare - only a few pixels in size - and only counted those events that could reliably be assigned to one of the selected stars.

In this way, the researchers identified 2889 superflares on 2527 of the 56450 observed stars. This means that on average, one sun-like star produces a superflare approximately once per century.

“High performance dynamo computations of these solar-type stars easily explain the magnetic origins of the intense release of energy during such superflares”, said coauthor Dr. Allan Sacha Brun of the Commissariat of Atomic and Alternative Energies of Paris-Saclay and the University of Paris-Cité.

Surprisingly frequent

“We were very surprised that sun-like stars are prone to such frequent superflares”, said first author Dr. Valeriy Vasilyev from the MPS. Earlier surveys by other research groups had found average intervals of a thousand or even ten thousand years. However, earlier studies were unable to determine the exact source of the observed flare and therefore had to limit themselves to stars that did not have any too close neighbors in the telescope images. The current study is the most precise and sensitive to date.

Longer average time intervals between extreme solar events have also been suggested by studies looking for evidence of violent solar storms impacting Earth. When a particularly high flux of energetic particles from the Sun reaches the Earth's atmosphere, they produce a detectable amount of radioactive atoms such as the radioactive carbon isotope 14C. These atoms are then deposited in natural archives such as tree rings and glacial ice. Even thousands of years later, the sudden influx of high-energy solar particles can thus be deduced by measuring the amount of 14C using modern technologies. 

In this way, researchers were able to identify five extreme solar particle events and three candidates within the past twelve thousand years of the Holocene, leading to an average occurrence rate of once per 1500 years. The most violent is believed to have occurred in the year 775 AD. However, it is quite possible that more such violent particle events and also more superflares occurred on the Sun 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. This requires further investigation”, co-author Prof. Dr. Ilya Usoskin from the University of Oulu in Finland pointed out. Looking at the terrestrial evidence of past extreme solar events could therefore underestimate the frequency of superflares.

Forecasting dangerous space weather

The new study does not reveal when the Sun will throw its next fit. However, 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 coauthor Dr. Natalie Krivova from the MPS. During the Carrington event of 1859, one of the most violent solar storms of the past 200 years, the telegraph network collapsed in large parts of northern Europe and North America. According to estimates, the associated flare released only a hundredth of the energy of a superflare. Today, in addition to the infrastructure on the Earth's surface, especially satellites would be at risk.

The most important preparation for strong solar storms is therefore reliable and timely forecasting. As a precaution, satellites, for example, could be switched off. From 2031, ESA’s space probe Vigil will help in the endeavor of forecasting. From its observation position in space, it will look at the Sun from the side and notice sooner than Earth-bound probes when processes that might drive dangerous space weather are brewing up on our star. The MPS is currently developing the Polarimetric and Magnetic Imager for this mission.

 

 

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