For reference, an image of the entire Sun taken by SDO/AIA in extreme ultra-violet light (false color). (Right) An image of a solar prominence at the limb of the Sun taken by Hinode/SOT in visible light (Ca II H line, false color). As shown in the image, a prominence is composed of long, thin structures called threads. A scale model of the Earth is shown on the right for reference. Credit: Image courtesy of National Institutes of Natural Sciences
Solar physicists have captured the 1st direct observational signatures of resonant absorption, thought to play an important role in solving the ‘coronal heating problem’ which has defied explanation for over 70 years. An international research team combined high resolution observations from JAXA’s Hinode and NASA’s IRIS mission, together with state-of-the-art numerical simulations and modeling from NAOJ’s ATERUI supercomputer. In the combined data, they were able to detect and identify the observational signatures of resonant absorption.
Resonant absorption is a process where 2 different types of magnetically driven waves resonate, strengthening one of them. In particular this research looked at a type of magnetic waves known as Alfvénic waves which can propagate through a prominence (a filamentary structure of cool, dense gas floating in the corona). Here, for the first time, researchers were able to directly observe resonant absorption between transverse waves (via Hinode) and torsional waves (via IRIS), leading to a turbulent flow which heats the prominence.
>>This new information can help explain how the solar corona reaches temperatures of 1,000,000 C ie “coronal heating problem.” The corona 200 times hotter than the photosphere, the layer beneath it. The key to solving the “coronal heating problem” is understanding how magnetic energy can be converted efficiently into heat in the corona. There have been two competing theories.
The 1st theory involves solar flares but is not high enough to account for all of the energy needed to heat and maintain the solar corona. The 2nd is based on magnetically driven waves. The solar atmosphere is permeated with “Alfvénic” waves which can carry significant amounts of energy along the magnetic field lines, enough energy in fact to heat and maintain the corona. For this theory to work, there needs to be a mechanism through which this energy can be converted into heat. To evaluate this both instruments targeted the same solar prominence ie a filamentary bundle of cool, dense gas (still 10,000 degrees) floating in the corona . It doesn’t sink because magnetic field lines act like a net to hold it aloft. The individual filaments composing the prominence, called threads, follow the magnetic field lines.
Hinode’s detects small motions in the 2-D plane of the image. To understand the complete 3D phenomenon, IRIS measured Doppler velocity (i.e. velocity along the line of sight, in-to/out-of the picture). IRIS spectral data also provided vital information about the temperature of the prominence. In the case of the prominence threads, the torsional motion is half-a-beat out of sync with the transverse motion driving it.
To understand this, NAOJ’s ATERUI supercomputer conducted 3D numerical simulations of an oscillating prominence thread. Of the theoretical models they tested, one involving resonant absorption provides the best match to the observed data. In this model, transverse waves resonate with torsional waves, strengthening the torsional waves. The simulations show that this resonance occurs within a specific layer of the prominence thread close to its surface. When this happens, a half-circular torsional flow around the boundary is generated and amplified. This is known as the resonant flow. Because of its location close to the boundary, the maximum speed of this flow is delayed by half-a-beat from the maximum speed of the transverse motion, just like the pattern actually observed.
This resonant flow along the surface of a thread can become turbulent, effective at converting wave energy into heat energy. The turbulence also enlarges the resonant flow predicted in the models to the size actually observed.
This model can explain the main features: 1. resonant absorption transfers energy to the torsional motions, producing a resonant flow along the surface of the prominence thread. 2. Trbulence in this strengthened resonant flow converts the energy into heat.
http://www.nao.ac.jp/en/news/science/2015/20150824-hinode.html
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