Environmental Engineering Reference
In-Depth Information
by these collisions creates the observed glow of the aurora. In addition, electrons
and protons may be captured by the magnetic field of the Earth at a distance of
several Earth radii, and radiation belts are formed in this manner (see Figure 7.1).
Thus, the upper atmosphere of the Earth contains different types of plasmas whose
properties are determined by interaction of solar radiation and the solar wind with
the Earth's fields and flows.
The properties of a solar plasma depend on where in the Sun it is located. The
Sun's core contains a plasma with a temperature of about 17 million Kelvin and
a number density of the order of 10
24
cm
3
.Asonemovesfromthecenterofthe
Sun, the number density of charged particles in the plasma and the temperature
decrease. The Sun's photosphere is of principal importance both for Earth process-
es and for the solar energy balance. The photosphere is a thin layer of the Sun
whose thickness (about 1000 km) is small compared with the Sun's radius, but this
layer, with a temperature of about 6000 K, emits most of the energy radiated by
the Sun owing to electron photoattachment to hydrogen atoms (Section 3.3.7) and
determines the yellow color of the Sun.
The next layer of the solar atmosphere is the chromosphere, whose name orig-
inates from the red color observed during a total eclipse of the Sun by the Moon.
The temperature of the chromosphere decreases at first and then increases with
an increase of the distance from the Sun's center, whereas the density of the solar
atmosphere decreases with increase of distance from the Sun's center. The chromo-
sphere is transparent because it is rarefied. The next higher, more rarefied region
of the Sun's atmosphere is called the solar corona. This beautiful name is suggest-
ed by the glow in this region that is observed during a total eclipse of the Sun. The
Figure 7.1
Radiation belts near the Earth.
Curves
fluxes of protons are expressed in units of
per square centimeter per second,
correspond to regions of capture
of fast protons with energy of more 30MeV,
curves
B
correspond to capture of slow elec-
trons with energy of 0.1-5MeV. The mean
A
R
0
is the
Earth's radius,
x
is the distance from the
Earth.
