Diverse recipes shine in the sun's corona.

Diverse recipes shine in the sun's corona

"Cosmic abundances" are what many scientists call the list of ingredients in the recipe for the universe. The sun, as part of the universe, presumably contains the same list of elements as the universe as a whole. Yet cosmic abundances do not always match the solar mixture.

In fact, reports Keith T. Stron of Lockheed Palo Alto (Calif.) Research Laboratory, the abundances measured with the Lockheed-built X-ray polychromator (XRP) aboard the Solar Maximum Mission satellite differ in different parts of the sun, sometimes varying even from minute to minute. He described the findings this week at the Johns Hopkins University Applied Physics Laboratory near Laurel, Md., during a meeting of the American Astronomical Society's Solar Physics Division.

"Abundances have always been the 'known' in one's equation," says Joan T. Schmelz of Applied Research Corp. in Landover, Md. For many research questions, "you just go look them up, plug them in and solve for your 'unknown.'"

But as Schmelz notes, a number of "known" abundances exist for the same solar elements, derived by different researchers from different observations. These abundances often stand quite at odds with one another.

Astronomers have published numerous such lists of abundances for the sun's corona, again based on different observations -- and few if any match in detail. Some researchers have just taken a standard set by averaging many other sets, says Strong. Typically, he notes, the view has been: "I'll just take this value, and realize that there's an uncertainty associated with it. You have to make an assumption somewhere; otherwise you make no progress."

This has advantages but creates problems as well. For example, complications arise when scientists try to measure solar temperatures by comparing the relative abundances of different elements, all of which have been ionized by the sun's heat. As the temperature rises, Strong says, some elements are ionized more quickly than others. Iron, for example, shows almost the same abundance in two different measurements, while oxygen and neon are more than twice as plentiful in one measurement as in the other.

In numerous spectra measured by the XRP, the relative brightnesses of light emitted by ions of iron-17 and iron-18 are almost the same, while the ratios between iron and neon measured in the same spectra differ widely. In fact, notes Strong, different spectra showing the same amount of iron show the relative abundances of some elements to differ by as much as a factor of 20.

If a scientist uses the relative abundance of iron and something else to calculate the temperature of a certain part of the sun, the "something else" can make a big difference in the answer, Strong says.

The Solar Max satellite has been in a unique position to make this point because the XRP made all the observations. Though spectra similar to those of XRP have been collected from many observatories, numerous differences can affect the results, such as variations among instruments. Strong and his colleagues have rechecked their theoretical analyses, the resulting temperature measurements and other details. Other factors may have been overlooked, but at present, says Strong, the message seems to be that the relative abundances of the elements simply differ from one spectrum to the next. No single list of cosmic abundances, it seems, can describe the whole sun at once.

A major goal for future study is to explain the wide abundance variations at different locales and times. In 1972 and 1973, measurements made from the Skylab space station hinted that such differences might exist, but according to Strong, these clues went unnoticed.

The sun is a place of constant uncertainty. Two weeks ago, researchers estimated the sun's effect on Earth's atmosphere might make Solar Max uncontrollable by early August. Now they estimate early September.