The Solar Radius Might Be Slightly Smaller Than We Thought

A pioneering method suggests that the size of our Sun and the solar radius may be due revision.

Our host star is full of surprises. Studying our Sun is the most essential facet of modern astronomy: not only does Sol provide us with the only example of a star we can study up close, but the energy it provides fuels life on Earth, and the space weather it produces impacts our modern technological civilization.

Now, a new study, titled The Acoustic Size of the Sun suggests that a key parameter in modern astronomy and heliophysics—the diameter of the Sun—may need a slight tweak.

The study out of the University of Tokyo and the Institute of Astronomy at Cambridge was done looking at data from the joint NASA/ESA Solar Heliospheric Observatory (SOHO’s) Michelson Doppler Imager (MDI) imager. The method probes the solar interior via acoustics and a cutting edge field of solar physics known as helioseismology.

Interior of the Sun
A cutaway diagram of the Sun. NASA/ESA/SOHO

‘Hearing’ the Solar Interior

How can you ‘hear’ acoustic waves on the Sun? In 1962, astronomers discovered that patches on the surface of the Sun oscillate, or bubble up and down, like water boiling on a stove top. These create waves that ripple in periodic 5-minute oscillations across the roiling surface of the Sun.

A view of the Sun, courtesy of SOHO’s MDI instrument. Credit: NASA

What’s more, astronomers can use what we see happening on the surface of the Sun to model the solar interior, much like terrestrial astronomers use seismic waves traveling through the Earth to model its core. Thanks to helioseismology, we can even ‘see’ what’s going on on the solar farside, and alert observers of massive sunspots before they rotate into view.

Solar Ffarside
Seeing the solar farside modeling using helioseismology. Credit: NSF/GONG

“The most comprehensive measurements are carried out by measuring the Doppler velocity across the image of the Sun in time, and then projecting the data at each instant onto spherical harmonics (theoretical spatial structures of ‘resonant modes’),” researcher on the study Douglas Gough (University of Cambridge) told Universe Today. “To obtain accurate frequencies, one must observe for long periods of time.”

The study looked at p-mode waves as they traversed the solar interior. Previous studies relied on less accurate f-mode waves, which are surface waves considerably shorter than the solar radius.

“There have been other observational projects using ground-based instruments distributed about the world (to obtain near-continuous observations), principally a U.S.-organized mission called GONG (the Global Oscillation Network Group), in which I was deeply involved, and BISON (the Birmingham Solar Oscillation Network),” says Gough. “There have been other space-borne and ground-based projects, which have produced more limited fruit.”

The study defines the solar radius (half the diameter) as 695,780 kilometers… only slightly smaller than the generally accepted radius of 696,000 kilometers obtained by direct optical measurement. This is only smaller by a few hundredths of a percent, or 100-200 kilometers.

Is the solar radius a mean value, that varies along with the solar cycle? “That’s a very important question, the answer to which will give us clues to the dynamics responsible for the solar cycle,” says Gough. “We know already from oscillation confined near the surface that there is an expansion and contraction of the outer layers of the Sun. The acoustic waves that Masao Takata and I analyzed penetrate deeply, and we intend to address the question in the near future.”

An artist’s conception of SOHO in space. Credit: ESA/SOHO

The solar radius is a deceptively simple but crucial factor in astronomy. The Sun is a glowing ball of hydrogen and helium plasma without a distinct surface boundary. The photosphere—the glowing visible layer we see shining down on us on a sunny day—is what we generally refer to as the surface of the Sun.

But what does this recent study say, in terms of the interior of our sun?

“It implies a slight modification to our view particularly of the structure of the inner core where the nuclear reactions are taking place,” says Gough, adding that this is “a modification that is quite small yet which affects the production of different kinds of neutrinos that with (perhaps imminent) more accurate neutrino measurements would test particle-physics theory.”

The Solar Radius: A Brief History

From a terrestrial viewpoint, the apparent diameter of the Sun seems to vary from 31.6’ to 32.7’, as we approach the Sun at Earth perihelion in January, and pass aphelion in July. The Greeks made the first stab at measuring the solar diameter, with Aristarchus of Samos arriving at 1/720th of a circle or half a degree. This tidy number is basically right, as the Sun appears about 30’ across, about the size of the Full Moon. Later measurements in the 18th and 19th centuries refined the solar radius down to 959.63” as seen from one astronomical unit (AU) distant, and today, the accepted International Astronomical Union (IAU) standard is 959.22”.

Perihelion versus Aphelion.
The Sun at perihelion versus aphelion. The apparent diameter of one is overlaid on the other (red circles). Credit: Dave Dickinson.

Traditional methods to measure the Sun include timing transits across the meridian, eclipses, transits of Mercury and Venus, and more. Today, missions such as SOHO and NASA’s Solar Dynamics Observatory allow astronomers to get above the murk of Earth’s atmosphere, and study the Sun in unprecedented detail. The French CNES space agency actually fielded the dedicated Picard mission (named after the French astronomer, not the Star Trek captain) in 2010-2014 in part to resolve the solar diameter mystery.

Enigmatic Sol

You can make a rough estimate of the Sun’s diameter yourself by timing sunrises and sunsets. You can also track the Sun in a filtered telescope and note how long it takes the Earth’s rotation to take it out of view.

Certainly, the Sun isn’t a true sphere, as it’s flatted or oblate at the poles. Being a ball of plasma, the Sun also rotates unevenly at 25.4 days at the equator, versus 34.4 days near the poles. We’re also headed towards the peak of Solar Cycle 25 in the 2024-2025 time frame.

Though it may seem active, the Sun is actually a uniquely stable and placid star versus what’s out there. Perhaps, this tranquility in what’s known as the Solar Constant is a key reason that life evolved here in the first place. The emerging field of astroseismology seeks to apply the methods of helioseismology to probe the surfaces and interiors of distant stars.

What About Eclipses?

What does this new measurement mean for eclipse predictions? Well, a few hundredths of a percent is negligible when it comes to something over 865,000 miles across. Certainly, the elliptical ballet of the Earth at the Moon play a much larger role.

“My reaction is that this paper does not impact eclipse predictions,” Michael Zeiler (Great American Eclipse) told Universe Today, citing that “The abstract differentiates between a photospheric radius and an acoustic radius.”

It’s fascinating to think, that modern heliophysics now has a way to probe the interior of the Sun. What other mysteries does Sol have in store?