The Earth’s Atmosphere and Magnetosphere
ATLAS OF THE UNIVERSE
A
s seen from space the Earth is truly magnificent, as
we have been told by all the astronauts – particularly
those who have observed it from the Moon, although it is
quite impossible to see features such as the Great Wall of
China, as has often been claimed! The outlines of the seas
and continents show up clearly, and there are also clouds
in the atmosphere, some of which cover wide areas.
The science of meteorology has benefited greatly
from space research methods, because we can now study
whole weather systems instead of having to rely upon
reports from scattered stations. The atmosphere is made
up chiefly of nitrogen (78 per cent) and oxygen (21 per
cent), which does not leave much room for anything else;
there is some argon, a little carbon dioxide, and traces of
gases such as krypton and xenon, together with a variable
amount of water vapour.
The atmosphere is divided into layers. The lowest of
these, the troposphere, extends upwards for about 8 kilo-
metres (5 miles) out to more than 17 kilometres (over 10
miles) – it is deepest over the equator. It is here that
we find clouds and weather. The temperature falls with
increasing height, and at the top of the layer has dropped
to 44 degrees C; the density is, of course, very low.Above the troposphere comes the stratosphere, which
extends up to about 50 kilometres (30 miles). Sur-
prisingly, the temperature does not continue to fall;
indeed it actually rises, reaching 15 degrees C at the top
of the layer. This is because of the presence of ozone, the
molecule of which is made up of three oxygen atoms
instead of the usual two; ozone is warmed by short-wave
radiations from the Sun. However, the rise in temperature
does not mean increased heat. Scientifically, temperature
is defined by the rate at which the atoms and molecules
fly around; the greater the speeds, the higher the tempera-
ture. In the stratosphere, there are so few molecules that
the ‘heat’ is negligible. It is the ‘ozone layer’ which
shields us from harmful radiations coming from space.
Whether it is being damaged by our own activities is a
matter for debate, but the situation needs to be watched.
Above the stratosphere comes the ionosphere, which
extends from about 50 to 600 kilometres (30 to 370
miles); it is here that some radio waves are reflected back
to the ground, making long-range communication possi-
ble. In the ionosphere we find the lovely noctilucent
clouds, which are quite unlike ordinary clouds, and may
possibly be due to water droplets condensing as ice on
to meteoritic particles; their average height is around
80 kilometres (50 miles). The ionosphere is often divided
into the mesosphere, up to 80 kilometres (50 miles), and
the thermosphere, up to 200 kilometres (125 miles).
Beyond comes the exosphere, which has no definite
boundary, but simply thins out until the density is no
more than that of the interplanetary medium. There is also
the Earth’s geocorona, a halo of hydrogen gas which
extends out to about 95,000 kilometres (60,000 miles).
Aurorae, or polar lights – aurora borealis in the north-
ern hemisphere, aurora australis in the southern – are
also found in the ionosphere; the usual limits are from
100 to 700 kilometres (60 to 440 miles), though these
limits may sometimes be exceeded. Aurorae are seen in
various forms: glows, rays, bands, draperies, curtains and
‘flaming patches’. They change very rapidly, and can be▼The Earth’s
magnetosphereis the
region of space in which
Earth’s magnetic field is
dominant. On the sunward
side of the Earth, the solar
wind compresses the
magnetosphere to within
eight to ten Earth radii (RE).
On the opposite side,
interaction with the solar
wind draws the field lines
out into a magnetotail,
extending well beyond
the orbit of the Moon.
The boundary of the
magnetosphere, across
which the solar wind
cannot easily flow, is the
magnetopause; a bow shock
is produced in the solar wind
preceding the magnetopause
by three to four Earth radii. Aurora: 18 April 2001, as
seen from Québec, Canada,
by Dominic Cantin. During
the 2000–2001 period, when
the Sun was near the
maximum of its cycle of
activity, there were several
exceptionally brilliant
aurorae. The Earth’s atmosphere
consists of the troposphere,
extending from ground
level to a height of between
8 and 17 km (5–10 miles);
the stratosphere extends up
to around 50 km (30 miles);
the mesosphere, between
50 and around 80 km
(50 miles); the thermosphere
from around 80 up to 200 km
(125 miles); beyond this
height lies the exosphere.B Atl of Univ Phil'03stp* 1/4/03 12:03 pm Page 40