New study finds intense wave energy may explain sun’s hot corona

Solar physicists have grappled with a century-old inquiry regarding the temperatures in the sun’s upper atmosphere, known as the corona, which are significantly hotter than the temperatures observed at its visible surface. However, an international team of scientists has recently presented a novel explanation for this phenomenon based on new observational data gathered with the 1.6-meter Goode Solar Telescope (GST) at the Big Bear Solar Observatory (BBSO), operated by NJIT’s Center for Solar Terrestrial Research (CSTR).

Published in the journal Nature Astronomy, their study reveals the presence of intense wave energy emanating from a relatively cool, dark, and strongly magnetized plasma region on the sun. These waves have the ability to traverse the solar atmosphere and sustain temperatures of up to a million degrees Kelvin within the corona.

This discovery marks a significant milestone in comprehending various related enigmas surrounding our planet’s nearest star. Wenda Cao, BBSO director and NJIT physics professor, who co-authored the study, emphasized the importance of this breakthrough for solar physics research. The findings not only shed light on the coronal heating problem but also offer potential insights into energy transportation and dissipation within the solar atmosphere, as well as the nature of space weather. This newfound understanding has the potential to unlock a multitude of perplexing questions in the field.

A video showing high-resolution observations of transverse motion in the sunspot. Credit: NJIT-BBSO, Yuan et al., Nature Astronomy, 2023

Using the advanced imaging capabilities of the GST, Yuan Ding’s team successfully captured transverse oscillations in the sun’s darkest and coldest region known as the sunspot umbra. Sunspots form when the sun’s powerful magnetic field obstructs thermal conduction, preventing the energy from the hotter interior to reach the visible surface or photosphere, where temperatures reach approximately 5,000 degrees Celsius.

The team focused on investigating dark features within an active sunspot recorded on July 14, 2015, using the GST at BBSO. They observed oscillatory transverse motions of plasma fibrils within the sunspot umbra, an area where the magnetic field is over 6,000 times stronger than Earth’s magnetic field. Fibrils appear as cone-shaped structures with heights of 500-1,000 km and widths of about 100 km. These fibrils have a lifetime of two to three minutes and tend to reappear within the darkest parts of the sunspot umbra, where the magnetic fields are strongest.

While the presence of these dark dynamic fibrils in the sunspot umbra has been previously observed, this study marks the first time the team was able to detect their lateral oscillations, which are indicative of fast waves. These persistent and widespread transverse waves within strongly magnetized fibrils facilitate the upward transport of energy through vertically elongated magnetic conduits. They contribute to the heating of the sun’s upper atmosphere.

Using numerical simulations of these waves, the team estimates that the energy carried by them could be thousands of times stronger than energy losses in the active region plasma of the sun’s upper atmosphere. These waves dissipate energy up to four orders of magnitude stronger than the heating rate required to maintain the high temperatures in the corona.

While various waves have been observed throughout the sun, their energy levels are typically too low to effectively heat the corona. However, the fast waves detected in the sunspot umbra represent a persistent and efficient energy source that could potentially be responsible for heating the corona above sunspots.

Although this research significantly enhances our understanding of the sunspot umbra and energy transport processes, questions regarding the coronal heating problem still remain. While the energy flux emitted by sunspots may contribute to heating the loops connected to them, there are other regions without sunspots that exhibit hot coronal loops that require further explanation. The researchers anticipate that the GST at BBSO will continue to provide high-resolution observational evidence to unravel the mysteries of our star.

Source: New Jersey Institute of Technology

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