Studying the Corona
The corona is that region of the solar atmosphere lying above the chromosphere. As seen in photographs taken during a total eclipse, it is the large halo of white, glowing gas extending out a few solar radii (millions of kilometers) beyond the dark limb of the Moon. At times when an eclipse is not in progress, specially designed refracting telescopes, called coronagraphs, that block out light from the photosphere are used to observe the corona.
Compared with the many hours of almost continuous surveillance accumulated in satellite studies over the last 20 years, eclipse studies have yielded not more than a few hours of observations (an eclipse seldom lasts longer than a few minutes). Even coronagraphic studies, which have greatly increased observing time, cannot match the time coverage and resolution of satellite observations from above the atmosphere. In February of 1980, the launching of the Solar Maximum Mission spacecraft, one of the most sophisticated and complex satellites ever built, provided the opportunity to keep watch on the Sun during its period of maximum surface activity. However, in November of that year, Solar Max lost its attitude-control system, so that the craft could no longer point its eight instruments at interesting portions of the solar surface. Then in April of 1984, the crew of the Space Shuttle Challenger were able to wrestle Solar Max into the cargo bay of Challenger for repair (chapter opening). This first successful repair of a satellite in orbit opens a new era for space observatories, in that in the future they will be repaired or serviced in orbit or even returned to the ground for modification. Properly attended, Solar Max may well last into the 1990s or almost a complete solar cycle of 11 years.
The Coronal Spectrum
Approximately 30 emission lines have been identified in the visible part of the coronal spectrum, and many hundreds of emission lines are known in the ultraviolet and X-ray spectrum. They originate in highly excited ions of familiar elements, such as iron, from which several to as many as 15 electrons have been stripped in the corona's extremely hot, tenuous gases. (It takes temperatures from many hundreds of thousands up to several million degrees to sustain such a degree of ionization.)
From millimeter to meter wavelengths there is a wide spectral window in the Earth's atmosphere that lets in radio radiation. The Sun, when quiet and undisturbed, normally emits thermal (blackbody) radiation, which is characteristic of a million-degree corona. When the Sun is disturbed, as when solar flares are occurring, nonthermal radio emission is added to the thermal component, and it can be quite intense.
Coronal Structure
There are several lines of evidence, besides the coronal spectrum, that confirms a rise in temperature through the chromosphere into the corona. Since heat flows from high to low temperature regions, then clearly energy must be pumped by some mechanism from the low-temperature photosphere to the high-temperature corona. For a number of years astronomers thought that the corona's high temperature resulted from energy carried into the corona by mechanical waves starting in the turbulent hydrogen convection zone below the photosphere.
As evidence grew that the magnetic fields of the photosphere and chromosphere were highly localized and very intense, it seemed hard to ignore the possibility that these magnetic fields extending up into the corona were part of the coronal heating process. When X-ray pictures showed that the corona was divided into active regions and hole regions primarily because of the structures of their magnetic fields, it became readily apparent that most, if not all, of the heating involves magnetic fields. The heating is produced by the direct dissipation of the energy stored in magnetic fields into thermal energy in the coronal gas. The lower parts of the chromosphere, however, are still thought to be heated by mechanical waves.
Eclipse pictures of the corona provide evidence for the importance of magnetic fields in structuring the corona. In white-light photographs one can see that the corona is irregular and structured. Beautiful, long streamers extend outward in the Sun's equatorial regions. Near sunspot maximum the corona is nearly circular, with streamers radiating out in all directions. Near sunspot minimum, the corona extends farther out in the equatorial region and terminates rather abruptly, with short, thin plumes curving out of polar areas.
Because coronal gases are almost transparent, we often are looking through several structures at once in eclipse pictures, which blurs the details. This is why direct photographs in X-ray and extreme ultraviolet wavelengths, where we look down on top of coronal structures, are so valuable to the study of the corona. To photograph the corona directly, we must observe in the 10 to 900 A (X-ray to extreme ultraviolet) wavelength region, where radiation from the much hotter corona overwhelms the short-wavelength radiation of the photosphere. But X-ray or ultraviolet pictures must be taken from space because the Earth's atmosphere absorbs these short wavelengths. In X-ray and ultraviolet pictures the corona appears highly inhomogeneous and generally asymmetrical, and it varies over time on both short and long time scales.
There are three different types of structural regions that collectively characterize the entire solar corona: Coronal holes and coronal active regions are two of them, and they are reasonably well defined in terms of observational characteristics; the third is the coronal quiet regions, which are not well defined. Coronal holes are regions of slightly lower temperatures and significantly lower densities, with magnetic fields of about 10 gauss whose lines open out into interplanetary space. Rays and polar plumes extend out of them away from the Sun. Coronal holes are also thought to be the source of most of the subatomic particles in the solar wind. Coronal active regions, however, are extremely different from coronal holes. They consist of loop structures, they are somewhat hotter and much denser regions of the corona, and their magnetic fields of about 100 gauss having field lines that loop back into the Sun instead of extending outward as in the case of coronal holes. Between these two extremes are the ill-defined coronal quiet regions, which appear to be something in between. There are also many small, bright points visible in X-ray pictures.