The Sun's Sizzling Corona

Solar sound waves might work in much the same way. The Sun is churning with seismic activity that produces a roar of gaseous compression waves in the solar atmosphere. Sound waves that propagate from the dense lower atmosphere into the rarefied corona break, like an ocean wave on the beach, and convert wave energy into thermal energy.

Hathaway and colleagues have studied acoustic vibrations in the sun's atmosphere with periods slower than about 1 minute. They find that the total energy in such waves cannot deliver enough heat to the corona to keep it warm. That doesn't mean that sound waves can't do the job. The instrument that Hathaway et al used for their study was not sensitive to vibration periods faster than about 60 seconds. If there is a great deal of energy in shorter period sound waves and if they can propagate all the way out to the corona - two very big ifs - then acoustic waves could be a important heating source.

"In principle, in situ measurements by spacecraft are great for characterising all kinds of waves, but none of the satellites in orbit now are close enough to reveal what's really happening in the corona," continued Hathaway. "The Solar Probe Mission could change all that. Solar Probe will have a special heat shield that lets it fly right through the corona within a few solar radii of the sun. With direct measurements we might finally understand what's going on in there."

Meanwhile, here on Earth, impatient scientists aren't waiting for the next wave of high-tech spacecraft to solve the problem. Hundreds of astronomers and solar physicists positioned themselves along the path of totality of the August 11, 1999, solar eclipse to gain a fleeting view of the corona with state-of-the-art observing hardware. Prof. Jay Pasachoff of William's College led an expedition of over 30 scientists and students to Romania where the eclipse lasted longest, 2 minutes and 29 seconds. Using high-time-resolution digital cameras, Pasachoff's team observed the Sun through filters sensitive to the Fe XIV green line in hopes of detecting the signatures of high-frequency Alfvén waves in the corona.