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Extreme ultraviolet emission of solar coronal plasma at millions of degrees. Credit: Atmospheric Imaging Assembly (AIA) aboard NASA’s Solar Dynamics Observatory (SDO) spacecraft

Nearly 5,000 kilometers above the sun’s surface lies an age-old question for the solar physicist: Are the temperatures in the star’s upper atmosphere (corona) hundreds of times hotter than the temperatures on the sun’s visible surface?

An international team of scientists has a new answer to the question commonly referred to as the sun’s coronal heating problem with new observational data obtained with the 1.6-meter Goode Solar Telescope (GST) at the Big Bear Solar Observatory (BBSO), managed by NJIT’s Center for Solar Terrestrial Research (CSTR).

In a study published in Nature astronomyResearchers have unveiled the discovery of intense wave energy from a relatively cool, dark, and strongly magnetized region of plasma on the sun, capable of traversing the solar atmosphere and maintaining temperatures of one million degrees Kelvin within the solar atmosphere. crown.

Researchers say the discovery is the latest key to unlocking a number of related mysteries relating to the closest star to Earth.

“The coronal heating problem is one of the biggest mysteries in solar physics research. It has been around for almost a century,” said Wenda Cao, director of the BBSO and NJIT professor of physics, who co-authored the study. ‘With this study we have new answers to this problem, which could be the key to untangling many confusing questions about energy transport and dissipation in the solar atmosphere, as well as the nature of space weather.’

A video showing high-resolution observations of transverse motion in the sunspot. Credit: NJIT-BBSO, Yuan et al., Nature astronomy2023

Using GST’s unique imaging capabilities, the team led by Yuan Ding was able to initially capture transverse oscillations in the sun’s darkest and coldest region, called the sunspot umbra.

Such dark sunspot regions can form when the star’s strong magnetic field suppresses thermal conduction and hinders the delivery of energy from the hotter interior to the visible surface (or photosphere), where temperatures reach around 5,000 degrees Celsius.

To investigate, the team measured activity related to several dark features detected in an active sunspot recorded on 14 July 2015 by BBSO’s GST, including oscillatory transverse motions of plasma fibrils within the sunspot umbra in whose magnetic field is more than 6,000 times stronger than the Earth’s.

‘The fibrils appear as cone-shaped structures with a typical height of 500-1,000 km and a width of about 100 km,’ explained Vasyl Yurchyshyn, NJIT-CSTR research professor of heliophysics and BBSO senior scientist. “Their life spans from two to three minutes and they tend to reappear in the same spot within the darkest parts of the shadow, where the magnetic fields are strongest.”

“These dark dynamic fibrils have been observed in sunspot shadows for a long time, but for the first time, our team was able to detect their lateral oscillations that are manifestations of fast waves,” Cao said. “These persistent and ubiquitous transverse waves in strongly magnetized fibrils carry energy upward through vertically elongated magnetic conduits and contribute to the heating of the sun’s upper atmosphere.”

Through numerical simulation of these waves, the team estimates that the energy carried could be up to thousands of times stronger than the energy losses in the plasma of the active region of the sun’s upper atmosphere, dissipating energy up to four orders of magnitude more. strong in the warm-up speed needed to maintain the pace. fiery plasma temperatures in the corona.

“Various waves have been detected all over the sun, but generally their energy is too low to heat up the corona,” Yurchyshyn said. “The fast waves detected in the sunspot umbra are a persistent and efficient energy source that may be responsible for heating the corona above the sunspots.”

For now, the researchers say the new findings not only revolutionize our view of Umbrian sunspots, but offer another important step forward in physicists’ understanding of the processes of energy transport and heating of the solar corona.

However, questions about the coronal heating problem persist.

“While these findings are a step closer to solving the mystery, the flow of energy out of sunspots may be responsible for heating only those circuits that are rooted in sunspots,” Cao said. “Meanwhile, there are other sunspot-free regions associated with hot coronal loops still waiting to be elucidated. We expect GST/BBSO will continue to provide the highest resolution observational evidence to further unravel our star’s mysteries.”

More information:
Ding Yuan et al, Transverse Oscillations and an Energy Source in a Strongly Magnetized Sunspot, Nature astronomy (2023). DOI: 10.1038/s41550-023-01973-3

About the magazine:
Nature astronomy

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