The issue of why the Sun’s outer atmosphere, or corona, is much hotter than its surface is still being studied in astrophysics and solar science. The finding of high wave energy from a relatively chilly, dark, and strongly magnetized plasma area on the Sun capable of penetrating the solar atmosphere and maintaining temperatures of a million degrees Kelvin inside the corona has been announced by researchers. The discovery, according to researchers, is the latest clue to unlocking a slew of linked mysteries surrounding Earth’s nearest star.
A century-old conundrum for solar physicists lies over five thousand kilometers above the Sun’s surface: how are temperatures in the star’s upper atmosphere, or corona, hundreds of times hotter than temperatures at the Sun’s visible surface?
With new observational data obtained with the 1.6-meter Goode Solar Telescope (GST) at Big Bear Solar Observatory (BBSO), operated by NJIT’s Center for Solar Terrestrial Research (CSTR), an international team of scientists has a new answer to the question – commonly referred to as the Sun’s coronal heating problem.
Researchers have discovered intense wave energy from a relatively cool, dark, and strongly magnetized plasma region on the Sun, capable of traversing the solar atmosphere and maintaining temperatures of a million degrees Kelvin inside the corona, according to a study published in Nature Astronomy. The discovery, according to researchers, is the latest key to unlocking a slew of linked puzzles concerning Earth’s nearest star.
Fibrils appear as cone-shaped structures with a typical height of 500-1,000 km and a width of about 100 km. Their lifetime ranges from two to three minutes, and they tend to reappear in the same location within the darkest parts of the umbra, where magnetic fields are strongest.Vasyl Yurchyshyn
“One of the biggest mysteries in solar physics research is the coronal heating problem.” It has been around for about a century,” said Wenda Cao, BBSO director and NJIT physics professor and study co-author. “With this study, we have new answers to this problem, which may be crucial in unraveling many tangled questions in energy transportation and dissipation in the solar atmosphere, as well as the nature of space weather.”
The team lead by Yuan Ding was able to catch transverse oscillations in the Sun’s darkest and coldest zone, known as the sunspot umbra, using GST’s unique imaging capabilities. Such dark sunspot regions can form as the star’s strong magnetic field suppresses thermal conduction and hinders the energy supply from the hotter interior to the visible surface (or photosphere), where temperatures reach roughly 5,000 degrees Celsius.
To investigate, the team measured activity related to numerous dark features detected in an active sunspot recorded on July 14, 2015 by BBSO’s GST — including oscillatory transverse motions of plasma fibrils within the sunspot umbra in which the magnetic field is more than 6,000 times stronger than that of Earth’s.
“Fibrils appear as cone-shaped structures with a typical height of 500-1,000 km and a width of about 100 km,” said Vasyl Yurchyshyn, BBSO senior scientist and NJIT-CSTR research professor of heliophysics. “Their lifetime ranges from two to three minutes, and they tend to reappear in the same location within the darkest parts of the umbra, where magnetic fields are strongest.”
“These dark dynamic fibrils have been observed in the sunspot umbra for a long time, but our team was able to detect their lateral oscillations, which are manifestations of fast waves for the first time,” Cao added. “These persistent and ubiquitous transverse waves in strongly magnetized fibrils bring energy upwards through vertically elongated magnetic conduits and contribute to the heating of the upper-atmosphere of the Sun.”
The team estimates that the energy carried by these waves could be thousands of times stronger than energy losses in the Sun’s upper atmosphere’s active region plasma – dissipating energy up to four orders of magnitude stronger than the heating rate required to maintain the corona’s blazing plasma temperatures.
“Various waves have been detected everywhere on the Sun, but their energy is typically too low to be able to heat the corona,” Yurchyshyn explained. “The fast waves detected in the sunspot umbra are a persistent and efficient energy source that may be responsible for heating the corona above sunspots.”
For the time being, experts claim that the new findings not only alter our understanding of the sunspot umbra, but also represent a crucial step forward in increasing physicists’ understanding of energy transport processes and solar corona heating. However, concerns about the coronal heating issue remain.
“While these findings are a step forward toward solving the mystery, the energy flux coming out of sunspots may only be responsible for heating those loops that are rooted in sunspots,” Cao explained. “Meanwhile, other sunspot-free regions associated with hot coronal loops remain unexplained.” We anticipate that GST/BBSO will continue to provide the highest-resolution observational evidence to help us solve the riddles of our star.”