SOUNDS OF THE SUN / RADIO ADVISORY 

Doctors listen to our heartbeats to determine health, seismologists track the waves from earthquakes to learn about the Earth’s interior, and solar scientists are listening to the Sun to answer some of its great mysteries. Known as helioseismologists, these scientists are using a special instrument aboard NASA’s Solar and Heliospheric Observatory (SOHO) spacecraft to learn more about the temperature, chemical makeup, pressure, density, and motions of material within the Sun.

WHAT HAVE WE LEARNED?

* Rivers of plasma transport material beneath the solar surface, much like Earthly trade winds 

While Earth’s weather is caused by uneven heating of the surface by sunlight, the solar "weather" patterns are driven by heat rising from the solar core and by the twisting and contorting of magnetic fields that interact with the electrically charged plasma. These magnetic fields play an important role in the creation of solar flares and coronal mass ejections (CMEs), which in turn can affect communications satellites, power, and navigation systems.

* Different points on the Sun rotate at different speeds 

Unlike the solid Earth, different points on the Sun have different revolution rates that result in the solar equator completing a rotation cycle in 27 days and the poles in 35 days. Helioseismology revealed the phenomenon to extend below the surface, with the Sun’s gaseous (plasma) composition to thank for the different rates.

* There are temperature variations within the Sun’s layers 

When sounds traveled through regions quickly, the layers were shown to have a hotter temperature than expected, and a cooler temperature when sound traveled slowly.

* Magnetic loops can explain why the Sun’s atmosphere is over 100x hotter than its surface 

The difference in temperature from the corona (outer atmosphere) and the solar surface is attributed to magnetic fields on the surface that create loops and subsequently "short circuit". The corona is over 50 million degrees, while the visible surface of the Sun is 11,000 degrees.

* Advanced warning of solar storms before they can be spotted from Earth  

MDI has also enabled a sort of X-ray vision that has allowed scientists to see through the Sun to its far side. This ability has proved important in watching for emerging solar storms up to a week before they are visible from and a risk to Earth.

New mysteries being pursued include more questions about convection, what powers various layers of the Sun, and what accelerates the solar wind. Do activities within the Sun influence activity on the Surface, and if so, how? Helioseismology also aims to further explain and understand the Sun’s 11-year sunspot cycle.

LEARNING TO LISTEN

So how do scientists listen to the Sun when sound can’t travel through the vacuum of space, particularly from 93 million miles away? The Michelson Doppler Imager (MDI) instrument aboard the SOHO satellite, stationed one million miles above Earth’s atmosphere, measures the movements of the surface of the Sun under the influence of the sound waves. When these waves strike the surface, they move toward and away from the Earth, causing that part of the surface to appear microscopically more reddish or bluish than normal. These motions allow scientists to reconstruct the sounds from the Sun. Because the frequencies they construct are much too low for the human ear, helioseismologists speed them up some 42,000 times and compress 40 days’ worth of vibrations into a few seconds.

Unlike seismic waves from an earthquake that may include one or a few sources of agitation (like an individual earthquake), no one source generates the "seismic" solar waves. Rather, they are continuously occurring, leaving scientists with millions of different frequencies to separate, much like tiny grains of sand constantly striking a bell.

USING THE SOUNDS

Like a bell, the Sun resonates in many different patterns, each one associated with a single note. The precise pitch of each resonance depends on the physical conditions in a particular portion of the Sun. Scientists examine the vibration of the Sun's entire surface at once, then sort out the individual patterns and piece together a picture of the entire solar interior. In this particular case, the lower tones (as in CD track 3) are used to study the deep interior; the higher frequency (like tracks 1 & 2) tells more about the layers closer to the surface. (For storytelling purposes, track 2 is probably the best representation. Track 3, while truer to the sound of the Sun, may be too jumbled to understand and track 1 too pure.)

Helioseismology is currently considered the best method for verifying theories of stellar structure and evolution of the Sun and the only way we have to get a good look into the heart of a star.