The Sun's tenuous outer atmosphere is called the corona. The corona
is not normally visible since the Sun's disk is so bright that the relatively
faint light from the corona is overwhelmed. However, during a total solar
eclipse, the corona is visible. Until quite recently, a total eclipse was the
only time the corona could be observed. However, in 1930, Bernard Lyot of
France, invented the coronagraph, a telescope specially designed to
create an artificial solar eclipse by blocking the light from the Sun's disk.
Until 1930, scientists journeyed far and wide to observe the solar corona during
The corona seen during a total eclipse shows beautiful structures shaped
by the Sun's magnetic field. The bright, pointed structures visible
in this image taken by scientists from the High Altitude Observatory in
Boulder, Colorado, are known as helmet streamers. These streamers
are affixed to the magnetic field of the Sun as it connects the
magnetic fields on the surface of the Sun to the interplanetary magnetic field
in our solar system.
The corona also emits energy of many different wavelengths, from long wavelength radio waves, to short wavelength X-rays.
By designing special telescopes we are able to look at the corona in many different ways. Below is a picture of X-rays coming
from our Sun taken by an X-ray telescope on the Yohkoh satellite when it was orbiting Earth.
The X-rays we see all come from the corona. But not all the corona emits the same amount of
The most visible structures are the loops and
arches around the bright areas called active regions. Also visible are
much smaller regions called "bright points." (The name gives
away their appearance.) Movies made from X-ray pictures show that the
corona is a very stormy place, constantly changing and erupting. In X-ray
images we may also see areas called coronal
holes, regions of low brightness at the north and south poles.
Spacecraft orbiting the Earth can pass through the Moon's shadow, just as the
Earth does. Below is an image obtained by the Yohkoh spacecraft of a partial
eclipse seen in X-rays.
Because the spacecraft is moving rapidly in its orbit about the Earth, the
path of the Moon across the Sun, as seen from space, can be very complex
and a spacecraft may even see the same eclipse several times.