Geoscience Reference
In-Depth Information
or around 90km (
Figure 2.15
). This layer is
commonly termed the mesosphere, although as
yet there is no universal terminology for the upper
atmospheric layers. Pressure is very low in the
mesosphere, decreasing from about 1mb at 50km
to 0.01mb at 90km. Above 80km, temperatures
again begin rising with height and this inversion
is referred to as the 'mesopause'. Molecular
oxygen and ozone absorption bands contribute to
heating around 85km altitude. It is in this region
that
noctilucent clouds
are observed on summer
'nights', particularly over high latitudes at
80-90km altitude. Their presence appears to be
due to meteoric dust particles, which act as ice
crystal nuclei when traces of water vapor are
carried upward by high-level convection caused
by the vertical decrease of temperature in the
mesosphere. However, their formation may also
be related to the production of water vapor
through the oxidation of atmospheric methane,
since apparently they were not observed prior to
the Industrial Revolution. The layers between
the tropopause and the lower thermosphere are
commonly referred to as the
middle atmosphere
,
with the upper atmosphere designating the
regions above about 100km altitude.
diurnal and seasonal variations. They are higher
by day and are also higher during a sunspot
maximum, although the changes are only repre-
sented in varying velocities of the sparse air
molecules.
Above 100km, cosmic radiation, solar X-rays
and ultraviolet radiation increasingly affect the
atmosphere, which cause
ionization
, or electrical
charging, by separating negatively charged
electrons from neutral oxygen atoms and nitrogen
molecules, leaving the atom or molecule with a net
positive charge (an
ion
). The term
ionosphere
is
commonly applied to the layers above 80km.
The Aurora Borealis and Aurora Australis are
produced by the penetration of ionizing particles
through the atmosphere from about 300km to
80km, particularly in zones about 10-20
latitude
from the earth's magnetic poles. On occasion,
however, aurora may appear at heights up to
1000km, demonstrating the immense extension
of a rarefied atmosphere.
°
5 Exosphere and magnetosphere
The base of the exosphere is between about 500km
and 750km. Here atoms of oxygen, hydrogen and
helium (about 1 percent of which are ionized)
form the tenuous atmosphere, and the gas laws
(see B, this chapter) cease to be valid. Neutral
helium and hydrogen atoms, which have low
atomic weights, can escape into space since the
chance of molecular collisions deflecting them
downward becomes less with increasing height.
Hydrogen is replaced by the breakdown of water
vapor and methane (CH
4
) near the mesopause,
while helium is produced by the action of cosmic
radiation on nitrogen and from the slow but
steady breakdown of radioactive elements in the
earth's crust.
Ionized particles increase in frequency through
the exosphere and, beyond about 200km, in the
magnetosphere there are only electrons (negative)
and protons (positive) derived from the solar
wind - which is a plasma of electrically conducting
gas.
4 Thermosphere
Above the mesopause, atmospheric densities are
extremely low, although the tenuous atmosphere
still effects drag on space vehicles above 250km.
The lower portion of the thermosphere is com-
posed mainly of nitrogen (N
2
) and oxygen in
molecular (O
2
) and atomic (O) forms, whereas
above 200km atomic oxygen predominates over
nitrogen (N
2
and N). Temperatures rise with
height, owing to the absorption of extreme ultra-
violet radiation (0.125-0.205μm) by molecular
and atomic oxygen, probably approaching
800-1200K at 350km, but these temperatures are
essentially theoretical. For example, artificial
satellites do not acquire such temperatures owing
to the rarefied air. 'Temperatures' in the upper
thermosphere and exosphere undergo wide
