#17.     The Sun's Corona

Eclipsed Sun
The Sun's corona
in eclipse

During a total eclipse of the Sun, when for a few minutes the Moon completely covers the Sun's face, a glow appears around the darkened Sun--the solar corona, the Sun's outermost atmosphere.

Structures visible in the corona at such times suggest that they are shaped by magnetic fields, and therefore, that the corona consists of plasma. For instance, short "plumes" rising from the polar regions of the Sun look very much like field lines coming out of the end of a bar magnet, and they therefore suggest that the Sun, in addition to the intense fields of sunspots, also has a global magnetic field like the Earth's.

Arch-like structure rising from the Sun
Arches above the
Sun's surface

Structures observed in the corona above sunspots often have horseshoe-shaped outlines, again suggesting that they follow magnetic field lines. From the tops of such "arches" long streamers may extend, to distances of the Sun's diameter or even more, looking like pulled taffy, as if some process was pulling material away from the tops of the arches into space, (which is of course what the solar wind actually does). Astronomers have named them "helmet streamers" because of their resemblance to spiked helmets worn by knights (and also to those used by some German soldiers up to 1918).

The High Temperature of the Corona

The most remarkable aspect of the corona is its high temperature, deduced by the Swedish astronomer Bengt Edlen in 1942 after a study of the corona's light. Much of that is sunlight scattered by coronal dust, but some light is also produced by the corona itself, in narrowly defined colors ("spectral lines") characteristic of its emitting atoms. In the 19th century, some of the spectral lines of sunlight did not match the lines of any substance on Earth, and it was proposed that they came from a new unknown chemical element, named helium (from the Greek helios--Sun). Later, in 1895, Norman Ramsey actually discovered helium on Earth.

Unknown spectral lines emitted by the corona were similarly credited to a new element "coronium" until Edlen showed that they came from the familiar atoms of iron, nickel and calcium, after they had lost an appreciable number of electrons (e.g. 13 or 14 for iron). Such high levels of ionization require the atoms to be buffeted around by extremely high temperatures, around 1,000,000 C (1,800,000 F).

The source of the corona's heat remains a puzzle. It is almost certain that its energy comes from the Sun's internal furnace, which also supplies the rest of the Sun's heat. However, as a rule, temperatures are expected to drop the further one gets from the furnace, whereas the million-degree corona lies outside the surface layer where sunlight originates, whose temperature is less that 6000 C.

The Corona in X-ray Light

All hot objects emit electromagnetic waves--for example, visible light is emitted by the hot filament inside a lightbulb. The hotter the object, the shorter the wavelength, which is why the corona emits "soft x rays," whose waves are much shorter than those of visible or ultra-violet light.

The corona has been observed in these wavelengths by, among others, the space station Skylab in 1973-4 and more recently by the Japanese spacecraft Yohkoh, which provided the picture shown below. The corona in such pictures appears quite uneven. It is brightest near sunspots, whose arched field lines apparently hamper the outflow of solar wind which carries away energy and helps cool the corona. It is darker in "coronal holes" in between, where field lines apparently extend out to distant space, making it easier for the solar wind to escape.

X-ray Sun from Yohkoh Click here for a full size version
of this image.

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Last updated March 13, 1999