Why Is The Sun's Outer Layer 200 Times Hotter Than Its Surface?

A solar prominence erupting into space. NASA/SDO/Steele Hill.

The "coronal heating problem" stumped scientists for more than 70 years. Instinctively, the corona, the outermost layer of the Sun, should be the coolest since it is farthest away from the intense nuclear reactions producing heat in the core. However, this couldn't be further from the truth. Instead of being the coolest layer, the corona is around 200 times hotter than the layer beneath it, the photosphere. 

There have been clues in the past: it was suggested that tiny "nanoflares," which could produce high-speed electrons, may have caused these intense temperatures. Light emission lines have suggested this to be the case. However, they haven't yet been observed directly and the evidence is "inconclusive" according to Dr Patrick Antolin, co-leader of the research with Dr Joten Okamoto, who spoke to IFLScience. Now, an international team of scientists from Japan, the U.S. and Europe have taken data from the Sun and found another piece of this solar puzzle. And it's all tangled up with the Sun's magnetic field.

You can read their results in the Astrophysical Journal.

They found that the corona reaped the benefits of a process known as resonant absorption. If two different waves, driven by magnetic fields, have some sort of synchronized pattern then one of them gets stronger – much like if two gymnasts on a trampoline timed their bounces together, meaning one can jump higher.

The team spotted resonant absorption between two wave types: transverse waves (up and down motion) and torsional waves (twisting motion). Two satellites were required to detect them, with the transverse waves observed by the Hinode satellite and the torsional waves detected by the IRIS satellite. 

In order to create a map of how the Sun turned magnetic energy into heat, both satellites observed the a solar prominence. A solar prominence is a bright, tendril-like feature that extends out from the surface of the Sun. The strands that form it snake along the Sun's magnetic field lines.

Both satellites observed the same solar prominence to figure out its motion, with the Hinode observing transverse waves and the IRIS observing torsional waves. Amazingly, their data sets showed synchronization. They also indicated that the temperature of the prominence increased from 10,000°C (18,000°F) all the way to 100,000°C (180,000°F). 

Curiously, the waves aren't perfectly synchronized. The torsional flow is slightly behind the transverse waves. This is unlike what we experience on Earth. If you run a spoon through a cup of coffee then circular waves are produced around the spoon. The transverse waves and the torsional waves on the spoon are perfectly in sync. However in the Sun's prominences, the torsional waves peak after the transverse waves. "The flow becomes turbulent. It is able to very efficiently convert the magnetic energy of the wave into heat," Antolin told IFLScience.

As seen in the figure below, the combination of transverse and torsoidal waves creates vortices at the edges of the prominence. These swirling vortices form eddy currents and lots of friction which transfers kinetic energy into heat energy, causing the incredible temperature rise that perplexed scientists for years. 

Evolution of a solar prominence. The addition of transverse and torsional motion creates turbulence, then heat.  JAXA/NAOJ.

The resonant flow turns out to be a two-step process. First, the resonant absorption gives the torsional motion an extra boost of energy. This resonates along the prominence thread. Second, this resonant thread creates destructive turbulence that produces heat, causing the epic increase in temperature along the thread.

Antolin summarized that "this work is unique because we can detect, for the very first time, coronal heating mechanisms in action, directly."

Image in text: a solar prominence compared to the Earth. NASA/JAXA/NAOJ.

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