The Unsaintly Halo of the Sun
Common sense and general experience teach that the farther something is from a heat source, the cooler it is. When it comes to the sun, though, common sense fails. The solar corona, a halo of incandescent gases that stream out into space for distances several times the sun's diameter, is hotter than the sun's surface.
Nobody is quite sure why the corona seems to violate the basic rules of thermodynamics. It's a question of more than abstract interest, too. Figuring out how the corona works is of special interest to Alaskans, because upheavals in the corona are responsible for some of our favorite phenomena, such as the aurora. They also produce some of our troubles, such as failures in radio and satellite communications, some power outages, and even corrosion in the Trans-Alaska Pipeline. The solar wind, which triggers these blessings and banes, is an extension of the solar corona.
The issue of Science for February 11 of this year reported on some recent debates about the hot corona. According to that article, at least one scientist---solar physicist Jack Scudder, of the University of Iowa---thinks the corona's unusual heat comes from the action of what one might call a gravity filter. Scudder theorizes that particles in the sea of ionized gas at the base of the corona have different energy levels, and naturally some will be of higher energy---hotter---than others. The tremendous gravity of the sun would make it impossible for any but the highest-energy particles to surge out to form the corona, and hence it is hotter than the sun's surface layer, which still houses many cooler, lower-energy particles.
Scudder's critics assert that he's assuming the existence of far too many high-energy particles to explain the corona. Most researchers instead believe that mechanical energy from the churning of the sun's outer layers, which seethe and roil nonstop, gets pumped up into the corona and there becomes heat energy. They disagree only about the exact mechanism whereby the energy gets changed from one form to another.
Many of the theories center on the bundles of magnetic field lines that arch far above the sun's surface, forming a kind of energy skeleton for the corona. Three postulated mechanisms lead the pack of theories. Turbulence or waves in the field lines might produce heat; "nanoflares," sparklike releases of energy from the bundles, might generate heat by spitting high-energy particles into the corona; or electric currents flowing along the magnetic field lines might turn them into heating elements, like the electric filaments in a toaster.
The argument over these theoretical mechanisms might have been settled only in theory if it weren't for two fortunate circumstances. It turns out that the magnetic bundles are outlined by x-ray emitting gas and they are now being studied by two new spaceborne instruments. The Soft X-Ray Telescope (known as SXT) has been observing the sun from the Japanese satellite known as Yohkoh since 1991; the Normal Incidence X-Ray Telescope (NIXT) flies on sounding rockets, takes high-resolution snapshots, and returns to earth again.
The two instruments have not fully resolved the debate, but they have produced a new favorite. The SXT data indicate that there aren't enough nanoflares to heat the corona. The evidence from NIXT, on the other hand, seems to show strong electric currents forced along the bundles by magnetic activity.
The argument won't be settled fully until the next generation of instruments is up and working, but so far it looks as if the solar corona gets its heat from essentially the same mechanism that browns a muffin in a toaster: electrical resistance.
