Page 167 - Handbook of Gold Exploration and Evaluation

Gold deposition in the weathering environment 145

Temperature drops uniformly in the troposphere at around 6.4 ëC per 1000 m of
ascent. It then changes abruptly at the tropopause (transition into the strato-
spheric zone) at about 14 km. After rising in the stratosphere, another abrupt
change takes place in the stratopause at about 50 km before again falling sharply
in the mesosphere. At about 80 km elevation the temperature levels out again in
the mesopause, again rising rapidly in the thermosphere before entering into an
electrified region, the ionosphere, then through the exosphere, into space. The
heights of the various boundaries vary with latitude and season. In the
troposphere, where most of what happens of direct importance to mankind
occurs, the height of the tropopause is 5 to 10 km higher in equatorial regions
than in polar regions where it is fairly stable at an elevation of about 10 km.
The intensity of solar radiation when the Earth is at its mean distance from
the Sun just outside of the Earth's atmosphere is termed the `solar constant'. It is
measured as the strength of the energy received on a unit area of horizontal
surface perpendicular to the Sun's rays in unit time. The mean value of the solar
constant is 1.94 langleys/min, the lower values occurring in the shortest part of
the spectrum. Changes have also been recorded up to 2.04 langleys/minute
(Whittow, 1984) due, probably, to deviation from regular motion of the Earth's
orbital elements as a result of gravitational attraction from other bodies in the
solar system (e.g. Jupiter and Saturn).

The magnetosphere
The Earth also has a magnetic atmosphere from which lines of force of the
Earth's magnetic field pass outward into surrounding space thus creating the
Earth's external magnetic field. The entire region within the limit of the
magnetic field is called the `magnetosphere'; its outer boundary is termed the
`magnetopause'. The plane of the magnetosphere lies in the plane of the
magnetic equator. The Earth occupies a position that is determined by the action
of the solar wind, a more or less continual flow of ionising radiation from the
sun, which acts to press the magnetopause close to the Earth on the side nearest
to the Sun. The result is that lines of force are concentrated in this region while
the force lines on the opposite side are greatly attenuated (Fig. 3.6). The
magnetosphere shields the inner atmosphere and surface of the Earth from the
fast-moving stream of electrons and protons of the solar wind that would
otherwise destroy all life on Earth. These particles are trapped and retained
within the force lines of the magnetic field upon encountering the
magnetosphere and concentrated into a belt of intense ionising radiation
surrounding the Earth termed the `Van Allen Radiation Belt'. Trapped particles
are continually discharged from the tail of the magnetosphere.
Solar flares of ionised hydrogen gas produced by sunspots on the Sun's
surface reach the Van Allen Belt about 13±26 hours after they are first sighted.
Electrons trapped by the Earth's magnetic field are then directed downward in