The Sun in Action
ATLAS OF THE UNIVERSE
T
he Sun is never calm. Even the photosphere is in a
constant state of turmoil. In the chromosphere we have
the prominences, some of which are violently eruptive,
and there are also spicules, narrow vertical gas-jets which
begin on the bright surface and soar to as much as 10,000
kilometres (over 6200 miles) into the chromosphere. They
are always present, and at any one time there may be as
many as a quarter of a million of them.
Even more dramatic are the flares, which usually,
though not always, occur above active spot-groups; they are
seldom seen in ordinary light, so that spectroscopic equip-
ment has to be used to study them. They are short-lived,
and generally last for no more than 20 minutes or so, though
a few have been known to persist for several hours. They
produce shock-waves in the chromosphere and the corona,
and considerable quantities of material may be blown
away from the Sun altogether; the temperatures may rocket
to many millions of degrees. Flares are essentially magnetic
phenomena, and it seems that rapid rearrangement of
magnetic fields in active regions of the corona results in
a sudden release of energy which accelerates and heats
matter in the Sun’s atmosphere. Radiations at all wave-
lengths are emitted, and are particularly strong in the X-ray
and ultra-violet regions of the electromagnetic spectrum.
The solar wind is made up of charged particles sent
out from the Sun at all times. It is made up of a plasma
(that is to say, an ionized gas, made up of a mixture of
electrons and the nuclei of atoms), and is responsible for
repelling the ion tails of comets, making them point away
from the Sun. When these charged particles reach the
Earth they are responsible for the lovely displays of aurorae
or polar lights – aurora borealis in the northern hemi-
sphere, aurora australis in the southern.
The average velocity of the solar wind as it passes the
Earth is 300 to 400 kilometres per second (190 to 250
miles per second); we are not sure how far it extends, but it
is hoped that four of the current space probes (Pioneers 10
and 11, and Voyagers 1 and 2) will keep on transmitting
until they reach the edge of the heliosphere, that is to say
the region where the solar wind ceases to be detectable.
The solar wind escapes most easily through coronal
holes, where the magnetic field lines are open instead of
looped. In 1990 a special spacecraft, Ulysses, was launched
to study the polar regions of the Sun, which have never
been well known simply because from Earth, and from all
previous probes, we have always seen the Sun more or
less broadside-on. Ulysses had to move well out of the
ecliptic plane, which it did by going first out to Jupiter
and using the powerful gravitational pull of the giant
planet to put it into the correct path.
Many solar probes have been launched, and have pro-
vided an immense amount of information, particularly in
the X-ray and ultra-violet regions of the electromagnetic
spectrum. The Solar and Heliospheric Observatory
(SOHO) was launched in 1995, and stationed at a perma-
nent vantage point 1.5 million kilometres (900,000 miles)
sunward of the Earth, keeping the Sun continuously in
view. It has obtained superb pictures of solar flares, as
well as CMEs (Coronal Mass Ejections) in which thou-
sands of millions of charged particles are hurled into space
at speeds of around 3.5 million kilometres (2.2 million
miles) per hour. CMEs are often (not always) associated
with flares.
We have come a long way since William Herschel
believed that the Sun might be inhabited, and we are learn-
ing more all the time, but we have to admit that we do
not yet have anything like a full understanding of our own
particular star. The X-Ray Sun,
28 September 1991; imaged
by the Japanese X-Ray
satellite Yohkoh. This picture
shows regions of different
X-ray emission; there is clear
evidence of coronal holes.Launch of the Japanese
X-Ray satellite Yohkoh.
The satellite was launched
by a Japanese M-3SII-6
rocket. X-rays cannot
penetrate the atmosphere,
so all research has to be
conducted from space.E152- 191 UNIVERSE UK 2003mb 7/4/03 5:42 pm Page 162