Resonant Waves Might Heat Corona

Astronomers have detected waves working together in the solar atmosphere, potentially heating the gas through turbulence.

A superheated halo called the corona surrounds the Sun. It’s the diffuse, outermost part of our star’s atmosphere. And somehow, even though it’s the farthest out, it’s more than a hundred times hotter than the layer below it.

solar filament from the side

This image from the Hinode solar observatory shows a solar filament, a giant ribbon of relatively cool plasma, threading through the Sun's corona. This high-resolution image shows the prominence from the side, revealing the individual threads that make it up. Combined with spectra from the IRIS satellite and computer simulations, this image of plasma threads spied on October 19, 2013, suggests that turbulence created by resonant waves heats the corona.
Credit: JAXA / NASA / Hinode

One explanation for this temperature surge is Alfvénic waves, motions in the solar plasma governed by the magnetic field. The corona is essentially a sea of these waves, and astronomers think they carry enough energy to heat the corona. But physics-wise, it’s unclear how the waves transfer their energy into the ionized gas around them.

New observations, coupled with computer simulations, might solve the problem. To try to understand what’s going on, Takenori Okamoto (JAXA–Institute of Space and Astronautical Science), Patrick Antolin (National Astronomical Observatory of Japan), and their team looked at how threads moved in a plasma filament in the corona. These so-called prominences are cooler and denser than the surrounding corona, making them easier to observe. Using images from the Hinode spacecraft and spectra from the Interface Region Imaging Spectrograph (IRIS) satellite, the researchers watched the prominence’s strands wiggle in 3D.

What they found is that it’s how the waves add together that matters. Seen from the side, the threads vibrate with transverse waves, akin to those rippling along a plucked guitar string. But the threads are also twisting toward and away from us, back and forth like your hand twists one way, then the other, on your wrist. These are torsional waves. When these two types of vibrations travel at the same speed, they resonate and keep the same beat, Antolin explains.

Guided by simulations, the team suggests the waves’ resonance causes the guitar-string waves to transfer their energy to the twist waves, pumping up their motion. This in turn exacerbates the shear between the threads’ boundaries and the thinner, hotter gas around them, creating eddies. The eddies degenerate into larger-scale turbulence. And the turbulence, due to the friction and electrical currents it creates — remember, we’re dealing with moving charged particles here, so electrical currents are going to happen — steals the wave energy from the threads and unceremoniously dumps it into the coronal plasma, heating it.

Shears and turbulence both exist in the corona, but it’s the resonance that speeds up the heating process and makes it effective, Antolin says. Both the simulations and observations suggest that the whole thing happens in a few hundred seconds.

Daniel Savin and Michael Hahn (both Columbia University), who work on solar Alfvénic waves, think the results are exciting. But whether or not resonance solves the coronal heating problem remains unclear. The new work convincingly demonstrates that this resonant interaction heats prominences, they agree, but the same mechanisms don’t necessarily operate in other structures in the corona.

But they are expected to operate throughout the corona, Antolin points out. The team focused on prominences because current instruments can detect at very high resolution the wavelengths this “cooler” plasma emits. The prominence was about 10,000 kelvin, and the resonance-spurred turbulence heats the threads’ boundaries to roughly ten times that, but the corona itself is yet another ten times hotter. Technologically speaking, it’s a lot harder to achieve Hinode and IRIS’s high resolution at such temperatures.

The team reports the results in two papers in the August 10th Astrophysical Journal. You can also read more about what the team found in NASA’s press release.

 

References:

J. Okamoto et al. “Resonant Absorption of Transverse Oscillations and Associated Heating in a Solar Prominence. I: Observational Aspects.” Astrophysical Journal. August 10, 2015.

Antolin et al. “Resonant Absorption of Transverse Oscillations and Associated Heating in a Solar Prominence. II: Numerical Aspects.” Astrophysical Journal. August 10, 2015.


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