{"id":774676,"date":"2023-11-29T10:20:49","date_gmt":"2023-11-29T15:20:49","guid":{"rendered":"http:\/\/spaceweekly.com\/?p=774676"},"modified":"2023-11-29T10:20:49","modified_gmt":"2023-11-29T15:20:49","slug":"the-solar-radius-might-be-slightly-smaller-than-we-thought","status":"publish","type":"post","link":"https:\/\/spaceweekly.com\/?p=774676","title":{"rendered":"The Solar Radius Might Be Slightly Smaller Than We Thought"},"content":{"rendered":"


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Two astronomers use a pioneering method to suggest that the size of our Sun and the solar radius may be due revision.<\/em><\/p>\n

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.<\/p>\n

Now, a new study, titled The Acoustic Size of the Sun <\/em>suggests that a key parameter in modern astronomy and heliophysics\u2014the diameter of the Sun\u2014may need a slight tweak.<\/p>\n

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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\u2019s) Michelson Doppler Imager (MDI) imager. The method probes the solar interior via acoustics and a cutting edge field of solar physics known as helioseismology.<\/p>\n

A cutaway diagram of the Sun. NASA\/ESA\/SOHO<\/figcaption><\/figure>\n

\u2018Hearing\u2019 the Solar Interior<\/h2>\n

How can you \u2018hear\u2019 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. <\/p>\n

\"SOHO
A view of the Sun, courtesy of SOHO\u2019s MDI instrument. Credit: NASA<\/figcaption><\/figure>\n

What\u2019s 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 \u2018see\u2019 what\u2019s going on on the solar farside, and alert observers of massive sunspots before they rotate into view.<\/p>\n

\"Solar
Solar farside modeling using helioseismology. Credit: NSF\/GONG<\/figcaption><\/figure>\n

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.<\/p>\n

The study defines the solar radius (half the diameter) as 695,780 kilometers\u2026 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.<\/p>\n

\"SOHO\"
An artist\u2019s conception of SOHO in space. Credit: ESA\/SOHO<\/figcaption><\/figure>\n

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\u2014the glowing visible layer we see shining down on us on a sunny day\u2014is what we generally refer to as the surface of the Sun.<\/p>\n

The Solar Radius: A Brief History <\/h2>\n

From a terrestrial viewpoint, the apparent diameter of the Sun seems to vary from 31.6\u2019 to 32.7\u2019, 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\u2019 across, about the size of the Full Moon. Later measurements in the 18th<\/sup> and 19th<\/sup> centuries refined the solar radius down to 959.63\u201d as seen from one astronomical unit (AU) distant, and today, the accepted International Astronomical Union (IAU) standard is 959.22\u201d.<\/p>\n

\"Perihelion
The Sun at perihelion versus aphelion. The apparent diameter of one is overlaid on the other (red circles). Credit: Dave Dickinson.<\/figcaption><\/figure>\n

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\u2019s Solar Dynamics Observatory allow astronomers to get above the murk of Earth\u2019s 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<\/em> captain) in 2010-2014 in part to resolve the solar diameter mystery.<\/p>\n