The Sun's coldest regions may be responsible for its million-degree corona

An answer to the Sun's coronal heating problem

An international team of scientists came up with the new, paradoxical answer to the Sun's coronal heating problem using data obtained by the 1.6-meter Goode Solar Telescope (GST) at Big Bear Solar Observatory (BBSO).

The researchers, who detailed their findings in a new paper published in the journal Nature Astronomy, pinpointed intense wave energy coming from the cool region of the Sun known as the sunspot umbra.

These waves, which originate in a relatively cool, dark region of the Sun, are capable of traversing the solar atmosphere and maintaining temperatures of a million degrees Kelvin. The new findings could help to shed new light on the mechanisms surrounding solar phenomena such as coronal mass ejections (CMEs).

"The coronal heating problem is one of the biggest mysteries in solar physics research," "It has existed for nearly a century," Wenda Cao, BBSO director, New Jersey Institute of Technology (NJIT) physics professor, and co-author of the study, explained in a press statement. "With this study we have fresh answers to this problem, which may be key to untangling many confusing questions in energy transportation and dissipation in the solar atmosphere, as well as the nature of space weather."

Measuring energetic waves from the Sun

The team specifically measured activity linked to dark features in an active sunspot that was recorded on July 14, 2015, by GST. In their measurements, they detected oscillatory transverse motions of plasma fibrils within the sunspot umbra.

"Fibrils appear as cone-shaped structures with a typical height of 500-1,000 km and a width of about 100 km," Vasyl Yurchyshyn, NJIT-CSTR research professor of heliophysics and BBSO senior scientist explained in the statement. "Their lifetime ranges from two to three minutes and they tend to reappear at the same location within the darkest parts of the umbra, where magnetic fields are strongest."

"These dark dynamic fibrils had been observed in the sunspot umbra 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 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 created computer models of the energetic waves, allowing them to estimate that the energy they carried could be thousands of times stronger than the energy losses in active region plasma of the Sun’s upper atmosphere. In other words, these waves are likely responsible for the immense heat in the Sun's upper atmosphere.

"Various waves have been detected everywhere on the Sun, but typically their energy is 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."

Study abstract:

The solar corona is two to three orders of magnitude hotter than the underlying photosphere, and the energy loss of coronal plasma is extremely strong, requiring a heating flux of over 1,000 W m−2 to maintain its high temperature. Using the 1.6 m Goode Solar Telescope, we report a detection of ubiquitous and persistent transverse waves in umbral fibrils in the chromosphere of a strongly magnetized sunspot. The energy flux carried by these waves was estimated to be 7.52 × 106 W m−2, three to four orders of magnitude stronger than the energy loss rate of plasma in active regions. Two-fluid magnetohydrodynamic simulations reproduced the high-resolution observations and showed that these waves dissipate significant energy, which is vital for coronal heating. Such transverse oscillations and the associated strong energy flux may exist in a variety of magnetized regions on the Sun, and could be the observational target of next-generation solar telescopes.