The Sun's Sizzling Corona

Lots of theories...Not enough data...

There are many ideas to explain the extraordinary warmth of the Sun's corona. The leading theories fall into three categories: miniature solar flares, atmospheric waves, and electrical dissipation.

A typical solar flare releases 10²⁷ ergs per second, equivalent to millions of 100-megaton hydrogen bombs exploding at the same time. Although solar flares are powerful, there aren't enough of them to maintain the corona's high temperature. Instead, some solar physicists, including Ron Moore and Jason Porter of the NASA/Marshall Space Flight Centre, suspect that miniature solar flares, called microflares, are responsible. Microflares are small explosions that release less than 1% of the energy in a full-fledged flare. Although they're relatively small, microflares may be so numerous that as an ensemble they could provide the energy needed to keep the corona superheated.

Unfortunately, microflares are so small that they're near the limits of what current telescopes can see. The number of observed microflares doesn't quite add up to the energy budget of the corona but there could be many more very faint ones that can't yet be detected. The new TRACE satellite is providing better sensitivity and resolution than previous instruments, and solar physicists are looking forward to 2004, when Japan will launch Solar B with a more powerful array of telescopes to test these theories.

Another way to heat the corona involves magnetic waves called Alfvén waves. The solar atmosphere is permeated with magnetic fields that are especially intense around sunspot groups. The sun is a rumbling, boiling, dynamic place, so the magnetic field lines are constantly shaking back and forth. These oscillations send waves of magnetised plasma (ionised gas) propagating outward into the corona.

"You can think of an Alfvén wave as acting something like a bullwhip," explains Hathaway. "A magnetic Alfvén wave starts out in the dense atmosphere near the photosphere and moves upward into the corona where its amplitude grows in the tenuous gas. Eventually the wave breaks and it dumps its energy in the form of heat. It's like cracking a whip. A small shake near your hand (the photosphere) becomes a big crack near the tip (the corona)."