A team of physicists at Northumbria University, UK, has found that magnetic waves in the corona of our Sun — its outermost layer of atmosphere — react to sound waves escaping from the inside of the star.
This image from NASA’s Solar Dynamics Observatory (SDO) shows the Sun. Image credit: NASA / SDO.
Most people are familiar with the three states of matter: solid, liquid, and gas. Less known is the fourth state of matter, plasma, when the atoms themselves break down into electrons and ions.
Plasmas are actually more common in the Universe than the better known solid, liquids and gas, because they tend to exist at temperatures and densities beyond our human experience. They exhibit behaviors similar to fluids and gases, but with added complexity of containing magnetic fields.
In 1942, Swedish physicist Hannes Alfvén combined the mathematics of fluid mechanics and electromagnetism to predict that plasmas could support wave-like variation in the magnetic field, a wave phenomenon that now bears his name, Alfvén waves.
These waves play a crucial role in transporting energy around the Sun and the Solar System. They were previously thought to originate at the Sun’s surface, where boiling hydrogen reaches temperatures of 6,000 degrees Celsius and churns the Sun’s magnetic field.
However, Northumbria University’s Dr. Richard Morton and colleagues found evidence that the magnetic waves also react — or are excited — higher in the atmosphere by sound waves leaking out from the inside of the Sun.
The researchers discovered that the sound waves leave a distinctive marker on the magnetic waves.
The presence of this marker means that the Sun’s entire corona is shaking in a collective manner in response to the sound waves. This is causing it to vibrate over a very clear range of frequencies.
This newly-discovered marker is found throughout the corona and was consistently present over the 10-year time-span examined. This suggests that it is a fundamental constant of the Sun — and could potentially be a fundamental constant of other stars.
The findings could therefore have significant implications for our current ideas about how magnetic energy is transferred and used in stellar atmospheres.
“The discovery of such a distinctive marker — potentially a new constant of the Sun — is very exciting,” Dr. Morton said.
“We have previously always thought that the magnetic waves were excited by the hydrogen at the surface, but now we have shown that they are excited by these sound waves. This could lead to a new way to examine and classify the behavior of all stars under this unique signature.”
“Now we know the signature is there, we can go looking for it on other stars.”
The Sun’s corona is over one hundred times hotter than its surface and energy stemming from the Alfvén waves is believed to be responsible for heating the corona to a temperature of around one million degrees.
They are also responsible for heating and accelerating powerful solar wind from the Sun which travels through the Solar System.
These winds travel at speeds of around a million miles per hour. They also affect the atmosphere of stars and planets, impacting on their own magnetic fields, and cause phenomena such as aurora.
“Our evidence shows that the Sun’s internal acoustic oscillations play a significant role in exciting the magnetic Alfvén waves,” Dr. Morton said.
“This can give the waves different properties and suggests that they are more susceptible to an instability, which could lead to hotter and faster solar winds.”
The research is published in the journal Nature Astronomy.
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R.J. Morton et al. A basal contribution from p-modes to the Alfvénic wave flux in the Sun’s corona. Nature Astronomy, published online January 28, 2019; doi: 10.1038/s41550-018-0668-9
