NASA - National Aeronautics and Space Administration

Scientists may have at last found a way to explore the heart of the sun with the detection of a special type of wave generated deep in the solar interior. The heart, or core, of the sun is the location of the sun's nuclear furnace, where fusion reactions power the sunlight that supports almost all life on Earth.

The Solar and Heliospheric Observatory (SOHO) spacecraft may have glimpsed these waves in the form of ripples on the sun’s surface. Analysis of the ripples will reveal details about the hidden core of our central star. Such information contains clues about how the Sun formed, 4.6 billion years ago.

Image right: This is an artist's concept of the SOHO spacecraft exploring the center of the sun. In reality, the spacecraft does this indirectly, by analyzing ripples on the solar surface that come from the deep interior. Print-resolution copy Credit: ESA and NASA.

The subtle waves reveal themselves as a minuscule ripple in the overall movement of the solar surface. "Solar astronomers have been searching for this kind of signal since the 1970s," notes NASA's Joe Gurman, US Project Scientist for SOHO.

The waves, called g modes, are driven by gravity and provide information about the deep interior of the Sun. They are thought to occur when churning gas below the solar surface plunges even deeper into our star and collides with denser material, sending ripples propagating through the Sun’s interior and up to the surface. It is the equivalent of dropping a stone in a pond.

Unfortunately for observers, these waves are badly degraded by their passage to the solar surface. By the time g modes reach the exterior, they are little more than ripples a few meters (yards) high. To make matters more difficult, the g modes take between two and seven hours to rise and fall just once. So astronomers are faced with having to detect a swell on the surface that rises a meter or two over several hours.

Now, however, astronomers using the Global Oscillation at Low Frequency (GOLF) instrument on SOHO think they may have caught glimpses of this behavior. Instead of looking for an individual ripple, they looked for the signature of the cumulative effect of a large number of these ripples. By analogy, imagine that the Sun was an enormous piano playing all the notes simultaneously. Instead of looking for a particular note (middle C for instance) it would be easier to search for all the ‘C’s, from all the octaves together. In the piano their frequencies are related to each other and in the Sun one class of g modes are separated by about 24 minutes. “So that’s what we looked for, the cumulative effect of several g modes,” says Rafael A. García, of DSM/DAPNIA/Service d’Astrophysique, France.

They combined ten years of data from GOLF and then searched for any hint of the signal at 24 minutes. They found it. “We must be cautious but if this detection is confirmed, it will open a brand new way to study the Sun’s core,” says García.

Until now, the rotation rate of the solar core was uncertain. If the GOLF detection is confirmed, it will show that the solar core is definitely rotating faster than the surface.

The rotation speed of the solar core is an important constraint for investigating how the entire Solar System formed, because it represents the hub of rotation for the interstellar cloud that eventually formed the Sun and all the planets, asteroids, etc., around it. The next step for the team is to refine the data to increase their confidence in the detection. To do this, they plan to incorporate data from other instruments, both on SOHO and at ground-based observatories.

“By combining data from other SOHO helioseismology instruments and ground-based helioseismology networks, we hope to improve this detection and open up a new branch of solar science,” says García.

SOHO is a cooperative project between the European Space Agency and NASA.