Geoscience Reference
In-Depth Information
roral displays. At the top of the atmosphere, the hydrogen ions of the solar wind itself pro-
duce a pink haze. Lower down, oxygen ions produce a ruby-red glow, while nitrogen ions
in the stratosphere cause violet blue and red auroras. Occasionally, magnetic field lines in
the solar wind are forced close to those of the Earth, causing them to reconnect, often with
spectacular releases of energy which extend the auroral displays.
The fragile veil
There's no clearly defined height that marks the top of the atmosphere; 260 kilometres
above the ground, in low Earth orbit where the space shuttle flies, you're above almost
all the air and the pressure is a billion times less than it is on the ground. But there are
still about a billion atoms in a cubic centimetre up there, and they are hot and electrically
charged and hence can have a corrosive effect on space vehicles. At times of maximum
solar activity, the atmosphere expands slightly, exerting more frictional drag on low space-
craft, which have to be boosted up to stay in orbit. The upper atmosphere, above 80 kilo-
metres, is sometimes known as the thermosphere because it is so hot, even though it is so
rarefied that you would not burn your skin on it.
This region of the atmosphere also absorbs dangerous X-rays and some of the ultraviolet
radiation from the Sun. As a result, many atoms become 'ionized', that is they lose an
electron. For this reason, the thermosphere is also called the ionosphere. Because the iono-
sphere is electrically conducting, it will reflect certain frequencies of radio waves, making
it possible for short-wave radio transmissions to be heard around the world, well over the
horizon from the transmitter.
Even a mere 20 kilometres up, below the thermosphere, the mesosphere, and most of the
stratosphere, we are still above 90% of the air in the atmosphere. It is at around this height
that we encounter the tenuous ozone layer, molecules containing three oxygen atoms.
Ozone forms when ordinary oxygen molecules of two atoms are split by solar radiation and
some recombine in threes. Ozone is a highly effective sunscreen for the planet. If all the
ozone in the Earth's atmosphere were concentrated at ground level, it would form a layer
only about three millimetres thick. But it still filters out virtually all of the most danger-
ous short-wave UV C radiation from the Sun and most of the medium-wavelength UV B
rays as well. Thus it protects life from sunburn and skin cancer. The ozone layer has been
severely depleted by chemicals such as CFCs (chlorofluorocarbons) released by human
activity, leading to a generalized thinning of the layer and more specific holes over polar
regions in the still, cold air of spring. International agreements have slowed the release of
CFCs and the ozone layer should recover, but the chemicals are long-lived and it will be
some time yet before it does.
