The Solar Corona
The extended outer atmosphere of the Sun is called the
corona, which comes from the Greek word for "crown." It has
a temperature of millions K, but it is
tens of billions times less dense than the atmosphere of the Earth at sea level.
The Corona in an Eclipse
The
glow of the corona results from both continuum and line emission (see the right panel). It is
a million times less bright than the photosphere, so it can be
seen only when the disk of the Sun is blocked off in a
total solar eclipse,
or by
using a special instrument called a
coronagraph (or, depending on design, a
coronameter) that blocks the
disk of the Sun artificially
so that
regions near the
Sun can be imaged.
The top right image in this frame shows the corona as observed from Mauna Kea during the 1991
total solar eclipse.
In this image we see clearly the intricate structure of the corona during a period when the
Sun was relatively active.
