Podcaster: Richard Drumm
Title: Space Scoop: The Mystery of the Sun’s Too-Hot Halo
Organization: 365 Days Of Astronomy
Link : astrosphere.org ; http://unawe.org/kids/unawe1538/
Description: Space scoop, news for children.
Bio: Richard Drumm is President of the Charlottesville Astronomical Society and President of 3D – Drumm Digital Design, a video production company with clients such as Kodak, Xerox and GlaxoSmithKline Pharmaceuticals. He was an observer with the UVa Parallax Program at McCormick Observatory in 1981 & 1982. He has found that his greatest passion in life is public outreach astronomy and he pursues it at every opportunity.
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Transcript:
This is 365 Days of Astronomy. Today we bring you a new episode in our Space Scoop series. This show is produced in collaboration with Universe Awareness, a program that strives to inspire every child with our wonderful cosmos.
Today’s story is
The Mystery of the Sun’s Too-Hot Halo
We know so much about the Universe that it’s hard to believe there are any big mysteries left to solve…but there are! One of the biggest mysteries in astronomy is about one of our closest neighbors: our very own Sun!
The Sun’s surface is covered in interesting features. And just like the Earth, the Sun has an atmosphere. It’s called the Corona.
The image in today’s episode artwork is a historical photograph of the corona from a total solar eclipse that happened on May 28th, 1900.
Here I’ll quote from the Wikipedia entry on the expedition and Thomas Smillie, the Smithsonian’s first staff photographer:
“In 1900 the Smithsonian Astrophysical Observatory, then based in Washington, D.C., loaded several railroad cars with scientific equipment and headed to Wadesboro, North Carolina.
Scientists had determined that this small town would be the best location in North America for viewing an expected total solar eclipse, and the Smithsonian Solar Eclipse Expedition hoped to capture photographic proof of the solar corona during the event for further study.
The team included Smithsonian photographer Thomas Smillie, who headed up the mission’s photographic component.
Smillie rigged cameras to seven telescopes and successfully made eight glass-plate negatives, ranging in size from eleven by fourteen inches to thirty by thirty inches.
At the time, Smillie’s work was considered an amazing photographic and scientific achievement.”
Today our Sun’s corona poses one of the biggest mysteries in astronomy. To understand this mystery, imagine a flame coming out of an ice cube. In a way, a similar effect occurs on the Sun!
Nuclear fusion in the center of the Sun heats its core to 15 million degrees centigrade. By the time the heat arrives at the surface of the Sun, it has cooled down to 6,000 degrees C. But the temperature of the corona soars back up to over 1 million degrees.
This unexpected extreme rise in temperature has puzzled scientists for over 70 years. However, astronomers think they just got one step closer to an answer.
Astronomers know that the Sun has a magnetic field, much like the Earth and the magnets stuck to my fridge. And we’ve long suspected that it plays an important role in this mystery. But the million dollar question is: how? How can a magnetic field create heat?
One possible answer to this riddle is: magnetically driven Alfvén waves in the ions of the solar plasma. They’re named for Sweedish physicist Hannes Alfvén. Astronomers have recently seen these waves in the Sun’s magnetic field.
To look for this conversion mechanism, the research team combined data from two state-of-the-art space missions: Japan’s Hinode satellite and NASA’s IRIS imaging and spectroscopic satellite, launched in 2013.
This is key: both instruments targeted the same solar prominence at the same time — a coordinated multi-instrument study.
A prominence is so called because it stands out prominently from the edge of the Sun. It is a filamentary bundle of cool, dense gas floating in the corona. Here, ‘cool’ is a relative term; a prominence is typically about 10,000 degrees! Ouch!
Although denser than the rest of the corona, a prominence doesn’t sink back down to the Sun’s surface because magnetic field lines act like a net to hold it up. The individual filaments composing the prominence, called threads, follow the magnetic field lines.
Hinode’s very high spatial and temporal resolutions allowed researchers to detect small motions in the 2-dimensional plane of the image, that’s up/down and left/right.
To understand the complete 3-dimensional phenomenon, researchers used the IRIS satellite to measure the Doppler velocity, which is the velocity along the line of sight, toward and away from us.
The IRIS spectral data also provided vital information about the temperature of the prominence.
These different instruments allow the satellites to detect different varieties of Alfvén waves: Hinode detects transverse waves while IRIS detects torsional waves.
Comparing the two concurrent data sets shows that these two types of waves are indeed synchronized, and that at the same time there is a temperature increase in the prominence from 10,000 degrees to more than 100,000 degrees.
This is the first time that such a close relationship has been established between Alfvén waves and prominence heating.
But the waves aren’t synchronized in the way scientists expected. Think of moving a spoon back-and-forth in a cup of coffee: the half-circular torsional flows around the edges of the spoon appear instantly as the spoon moves.
But in the case of the prominence threads, the torsional motion is half-a-beat out of sync with the transverse motion that’s driving it.
There is a delay between the maximum speed of the transverse motion and the maximum speed of the torsional motion, like the delay between the motion of the hips of a dancer in a long skirt and the motion down at the skirt’s hem.
To understand this unexpected pattern the team used NAOJ’s ATERUI supercomputer to conduct 3D simulations of an oscillating prominence thread. Of the theoretical models they tested, one involving resonant absorption provided the best match to the observed data.
In this model, transverse waves resonate with torsional waves, strengthening the torsional waves; similar to how a kid on a swing can add energy to the swing, causing it to swing higher and faster, by kicking his feet out in time with the motion.
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 resonant flow is delayed by half-a-beat from the maximum speed of the transverse motion, just like the pattern that was actually observed.
The simulations further reveal that this resonant flow along the surface of a thread can become turbulent. The appearance of turbulence is of great importance since it is effective at converting Alfvén wave energy into heat energy.
Bingo! There you have it! Coronal heating.
Another important effect of this turbulence is that it enlarges the resonant flow predicted by the models to the size that was observed.
This model explains the main features of the observations as the result of a two-step process:
- First resonant absorption transfers energy to the torsional motions, producing a resonant flow along the surface of the prominence’s thread.
- Then turbulence in this strengthened resonant flow converts the energy into heat.
This work shows how the power of multiple satellites, such as Hinode and IRIS, can be combined to investigate long-standing astrophysical problems and will serve as an example for other researchers looking for similar heating in other solar observations.
Hey, Here’s a Cool Fact:To our eyes, the corona is about a million times dimmer than the Sun. We can only see it during a solar eclipse, when it appears around the Sun like a silvery halo. There’s a great solar eclipse coming to the United States in August 2017. Stay uh, “tuned” here for more information as that time comes closer!
End of podcast:
365 Days of Astronomy
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