It's been 40 years since space physicists realized that cyclic throbbings they'd seen on the Sun's visible surface were caused by periodic pressure waves — literally sounds — reverberating through the solar interior. The discovery created a new scientific discipline: helioseismology.
You won't hear this ringing in your ears if you listen closely on a sunny day. For one thing, the sound waves are trapped inside the Sun and can't propagate out into space. And the "loudest" frequencies cluster around 0.003 hertz, or one vibration every 5 minutes. That's more than 12 octaves below the lowest notes we humans can hear.
The Sun isn't so much singing as it is humming constantly, the kind of ostinato you'd get from a bell hung out in the desert that constantly being pelted with sand grains.
But a new study, to appear in the May 1st edition of Astrophysical Journal Letters, reports that ol' Sol is coaxed into throaty solos whenever a solar flare erupts. Christoffer Karoff and Hans Kjeldsen, both at the University of Aarhus in Denmark, chanced upon this connection as they analyzed data from the workhorse Solar and Heliospheric Observatory, a spacecraft launched jointly by the European Space Agency and NASA in 1995.
“The signal we saw was like someone occasionally walking up to the bell and striking it, which told us that there was something missing from our understanding of how the Sun works,” Karoff notes in an ESA press release.
As a second ESA press release points out, researchers have suspected for decades that flares beget sunquakes. The new SOHO findings confirm that when solar-flare counts go up, so does the strength of the 5-minute oscillations. It's a situation akin to the weeks of ringing that geophysicists observe deep in our planet's interior after a strong earthquake.
The Sun's rippling surface is evident in a captivating SOHO video just released by ESA. The 56-second-long clip shows the Sun's activity over several days in 2003 and a titanic flare (a few seconds from the end) that erupted on November 4th.
But the correlation between the flares and sunquakes is ironclad in data returned from a SOHO instrument called VIRGO, short for Variability of solar IRradiance and Gravity Oscillations. You can see this for yourself in a special report on NASA's SOHO webpage.
Karoff and Kjeldsen don't yet understand why flares trigger the enhanced oscillations. They're working on that now. And they're also looking for similar tunes from other stars — both with sensitive ground-based instruments and with the ESA's COROT spacecraft.