The corona is the uppermost layer of the solar atmosphere, and like the chromosphere is only visible to the unaided eye during a total eclipse of the Sun. For this reason, an instrument called a coronograph was invented. The coronograph uses a metal disk to block the disk of the Sun in the light train of the telescope. This in effect produces an artificial eclipse allowing study of the corona to take place at any time the Sun is visible.
The corona resembles a pearly white halo of intricate design surrounding the body of the Sun. It is many times larger than the two layers beneath it, being roughly 2 million kilometers in thickness. Its shape is closely associated with the eleven-year sunspot cycle. During periods of sunspot minimums, the corona is nearly circular around the solar disk, but during sunspot maxima it becomes elongated around the equator and thin at the poles with large streamers radiating from it. Within this gaseous envelope, great fiery prominences and flares can be seen.
Around 1940, scientists began to realize that the corona was
very hot. This was based upon spectral analyses which showed
that highly ionized elements where all that where present. Such
ionization can occur only if the effective temperature is over
1,000,000oC. Although the why and how of these temperatures in
the corona are not clearly understood, astronomers believe that
the heating is directly related to solar activity in the
photosphere and below. Prominences and flares moving into the
corona inject a tremendous amount of energy into the thin gases
of the upper solar atmosphere. The release of magnetic energy
coupled with the fact that the corona is nearly a vacuum allows
for energy storage and slow loss into the surrounding space.
Questions:
1. When can the corona of the Sun be studied by scientists?
2. What is a coronograph?
3. Describe the corona during sunspot maximum and sunspot minimum.
4. What is a vacuum? Why would we consider the corona a vacuum?
5. In size, how does the corona compare with the chromosphere
and photosphere?
