Environmental Engineering Reference
In-Depth Information
7
Conclusion - Plasmas in Nature and the Laboratory
Summarizing the analysis of properties of ionized gases or a weakly ionized plas-
ma, we consider briefly the problems in our environment where we encounter a
plasma. A terrestrial plasma occurs in various forms in the Earth's atmosphere
and the solar system, and these forms are different depending on the location of
this plasma. At low altitudes above the Earth's surface the plasma is maintained by
ionization of air under the action of cosmic rays and atmospheric electric fields,
whereas near the Earth's surface, some of the ionization of air arises from the de-
cay of radioactive elements in the Earth's crust. A plasma in lower layers of the
atmosphere is characterized by a low density of charged particles. The presence
of a plasma is limited by the tendency of electrons to attach to oxygen molecules
fast, with formation of negative ions. Therefore, plasmas in the lower atmosphere
acquire a negative charge in the form of negative ions. Plasma processes are re-
sponsible for the action of the electric machine of the Earth (Section 6.5) and ac-
company the circulation of water in the Earth through its atmosphere and create
electric fields in the atmosphere. This leads to various electric phenomena in the
Earth's atmosphere, such as lightning (Section 6.5.2) - electric breakdown under
the action of electric fields in the atmosphere, corona discharge in a quiet atmo-
sphere due to passage of electric currents near conductors, and Saint Elmo's fire
- a glow in the vicinity of protruding objects. Electric discharges in atmospheric
air are often associated with energetic natural phenomena of a nonelectric nature,
such as volcanic eruptions, earthquakes, sand storms, and water spouts.
The Earth's ionosphere (Section 6.4.3) is a typical plasma consisting of electrons
and ions. It results from photoionization of atomic oxygen or molecular nitrogen
under the action of the hard UV radiation from the Sun. The ionosphere reflects
radio signals from the Earth, that is, it is a mirror for electromagnetic waves (Sec-
tion 6.4.3), and allows the propagation of radio waves over large distances. The au-
rora or aurora borealis are observed in the regions of the geographical poles where
the magnetic lines of force due to the Earth's magnetic field are directed perpendic-
ular to the Earth's surface. Electrons and protons of the solar wind move along the
magnetic field lines of the Earth, and the more energetic particles proceed to the
vicinity of the Earth's magnetic poles, where they are braked as a result of collisions
with atmospheric atoms or molecules, and these protons ionize and excite atoms in
the upper atmosphere. Radiation of metastable oxygen and nitrogen atoms formed
