Using data from the NASA/ESA Solar and Heliospheric Observatory (SOHO), solar physicists have found evidence of a type of seismic wave (gravity waves, or g-waves) in the Sun. These low-frequency waves reveal that the deepest part of the solar body, its hydrogen-burning core, rotates about 4 times faster than the solar surface.
Flaring, active regions of the Sun are highlighted in this image combining observations from several telescopes. High-energy X-rays from NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) are shown in blue; low-energy X-rays from Japan’s Hinode spacecraft are green; and extreme ultraviolet light from NASA’s Solar Dynamics Observatory (SDO) is yellow and red. All three telescopes captured their solar images around the same time on April 29, 2015. Image credit: NASA / JPL-Caltech / GSFC / JAXA.
Just like seismologists use the way earthquakes travel through Earth’s interior to study our planet’s structure, solar physicists use helioseismology to study the Sun’s interior structure by tracking the way waves move throughout the star.
On Earth, it is usually one event that is responsible for generating the seismic waves at a given time, but the Sun is continuously ‘ringing’ owing to the convective motions inside the giant gaseous body.
Higher frequency waves, also called pressure waves (p-waves, or p-modes), are easily detected as surface oscillations owing to sound waves rumbling through the upper layers of the Sun. They pass very quickly through deeper layers and are therefore not sensitive to the Sun’s core rotation.
Conversely, lower frequency g-waves (or g-modes) that represent oscillations of the deep solar interior have no clear signature at the surface, and thus present a challenge to detect directly.
In contrast to p-waves, for which pressure is the restoring force, buoyancy (gravity) acts as the restoring force of the gravity waves.
“The solar oscillations studied so far are all sound waves, but there should also be gravity waves in the Sun, with up-and-down, as well as horizontal motions, like waves in the sea,” explained lead author Dr. Eric Fossat, an astronomer at the Côte d’Azur Observatory.
“We’ve been searching for these elusive g-waves in our Sun for over 40 years, and although earlier attempts have hinted at detections, none were definitive.”
“Finally, we have discovered how to unambiguously extract their signature.”
Dr. Fossat and co-authors used 16.5 years of data collected by SOHO’s Global Oscillations at Low Frequencies (GOLF) instrument.
By applying various analytical and statistical techniques, they were able to pick out a regular imprint of the g-modes on the more easily detected p-modes.
In particular, they looked at a p-mode parameter that measures how long it takes for an acoustic wave to travel through the Sun and back to the surface again — a journey known to take four hours and seven minutes.
They detected a series of modulations in this p-mode parameter: the signature of g-waves shaking the structure of the Sun’s core.
The imprint of these g-waves suggests that the solar core is rotating once every week, nearly four times faster than the Sun’s surface and intermediate layers, which have rotation periods anywhere from 25 days at the equator to 35 days at the poles.
This cutaway diagram shows key regions of the Sun, starting with the outer chromosphere and then the photosphere, in which cool dark features known as sunspots can be seen. Inside the Sun, there is a turbulent outer convection zone and a more stable inner radiative zone. Image credit: NASA / ESA / SOHO.
“The most likely explanation is that this core rotation is left over from the period when the Sun formed, some 4.6 billion years ago,” said co-author Roger Ulrich, professor emeritus of astronomy at the University of California at Los Angeles.
“It’s a surprise, and exciting to think we might have uncovered a relic of what the Sun was like when it first formed.”
“The rotation of the solar core may give a clue to how the Sun formed. After the Sun formed, the solar wind likely slowed the rotation of the outer part of the Sun.”
“The rotation might also impact sunspots, which also rotate. Sunspots can be enormous; a single sunspot can even be larger than the Earth.”
“G-modes have been detected in other stars, and now thanks to SOHO we have finally found convincing proof of them in our own star,” Dr. Fossat said.
“It is really special to see into the core of our own Sun to get a first indirect measurement of its rotation speed. But, even though this decades-long search is over, a new window of solar physics now begins.”
“Although the result raises many new questions, making an unambiguous detection of gravity waves in the solar core was the key aim of GOLF,” said Dr. Bernhard Fleck, ESA’s SOHO project scientist.
“It is certainly the biggest result of SOHO in the last decade, and one of SOHO’s all-time top discoveries.”
The core of the Sun differs from its surface in another way as well.
The core has a temperature of approximately 29 million degrees Fahrenheit, which is 15.7 million Kelvin. The solar surface is ‘only’ about 30,000 degrees Fahrenheit, or 5,800 Kelvin.
The findings were published this week in the journal Astronomy & Astrophysics.
_____
E. Fossat et al. 2017. Asymptotic g modes: Evidence for a rapid rotation of the solar core. A&A 604, A40; doi: 10.1051/0004-6361/201730460
