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Technion researchers offer a solution to a long-standing mystery in the sun

The chemical composition of the Sun's corona may have originated from past and present flares

An illustration of the Chandra space telescope launched in 1999. Its greatness is its ability to measure X-ray spectra with a higher resolution than any other instrument. Photo credit: NASA
An illustration of the Chandra space telescope launched in 1999. Its greatness is its ability to measure X-ray spectra with a higher resolution than any other instrument. Photo credit: NASA

Technion physics faculty researchers offer a solution to a long-standing mystery - how was the sun's corona formed and how did it reach temperatures of millions of degrees, a thousand times that of the sun itself? In an article that will be published in the scientific journal Astronomy & Astrophysics, they say that it is possible that the origin of the sun's corona is in past and present flares that enriched it.

The sun's crown (corona) is a hot and thin shell of the star around which we circle. The temperature of the Sun's corona reaches several million degrees, about a thousand times more than the yellow face of the Sun. The chemical composition of the corona is also different from that of the sun's surface. How was the sun's corona formed, how did it reach such high temperatures, and what is the process responsible for a different chemical composition in the corona than in the sun? These are puzzles that have occupied scientists for decades.

Ra'anan Nordon from the Physics Faculty at the Technion and his PhD supervisor, Dr. Ehud Bachar, analyzed a series of observations made with the help of the American Space Agency's Chandra space telescope on six stars neighboring the Sun and in which there is active coronal activity, several thousand times more energetic than that of the Sun.

"The enormous temperatures of the star's corona mean that they cannot be seen with ordinary telescopes in visible light," they explain. "Indeed, Chandra is a telescope that is sensitive to much more energetic radiation, X-ray radiation. "Flares sometimes occur in the coronas of stars, in which the flux of X-ray radiation from the star and the temperature of the gas increase several times."

Ionized gas was ejected from the Sun following an eruption, as captured by the cameras of the SOHO satellite designed for UV observations of the Sun. The length of the trail is about thirty earth radii.
Ionized gas was ejected from the Sun following an eruption, as captured by the cameras of the SOHO satellite designed for UV observations of the Sun. The length of the trail is about thirty earth radii.

Using the sensitive spectrometers on Chandra, the Technion researchers were able to measure the chemical composition of the radiation sources, namely the concentrations of different chemical elements, such as oxygen, neon, silicon, and iron. To their surprise, it turned out that the chemical composition of the star's corona changes during the flare in a different way than expected. Moreover, the chemical change in the stars was found to be identical in pattern to the difference in chemical composition between our Sun and its corona. It has been known for a long time that the sun's corona is richer in atoms from which it is relatively easy to remove an electron (ion) than the sun itself. Even in flares observed by Technion researchers on other stars, it was found that the concentration of these elements increases significantly during flares.

"The full meaning of the result is still not entirely clear, but it is possible that the chemical pattern discovered in the stars hints at the solution to the long-standing puzzle of the chemical composition of the Sun's corona," say the researchers. "If, as was discovered in this study, flares bring to the corona in preference elements that are easy to ionize, it is possible that the origin of the sun's corona is from past and present flares that enriched it with these elements."

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