CHAPTER 2
THE ELECTRIC STRUCTURE OF THE ATMOSPHERE
2.1 IONS
The existence of ions in the atmosphere is the fundamental reason for
Atmospheric electricity. An absence of ions would mean zero electric
Field in the atmosphere and most probably no thunderstorms or
Lightning. The concept of positive and negative ions as charge carriers
In the atmosphere was first put forward by J. Elster and H. Geitel
(1899) in order to explain the electric conductivity of air. Much work has
Since been done on ions and their role in atmospheric electric
Phenomena. Today we know there are mainly three classes of ions,
Namely small ions, intermediate ions and large ions. Most important
Are the small ions since their higher mobility allows them to take a more
Active part in the transfer of charge throughout the atmosphere. The
Mobility of ions can be measured in metres per second per volt per metre
Which signifies the velocity that an ion will reach when subjected to an
Electric field of one volt per metre. For small ions the mobility is of the
Order of 0.0001 with a slight edge of the negative ion over its positive
Counterpart. In fact, the negative to positive mobility ratio of small
Ions is about 1.25 (Wåhlin 1985) which is a paradox since negative ions
Are believed to be more massive than positive ions. One explanation
(Papoular, 1965) is that for part of its lifetime a negative ion is really an
16 ATMOSPHERIC ELECTROSTATICS
Electron jumping from molecule to molecule. Molecules such as NO and
NO2 are believed to dominate the negative small ion population while
Oxonium and water might make up the positive small ions in the
Atmosphere. Their true molecular structure and mass are not well
Known because it is difficult to get spectroscopic mass analysis of small
Ions in the lower atmosphere. The problem is their relative
short life
Time, about 100 seconds, which is much shorter than the transit time
Required for molecules or ions to reach the source end of a mass
Analyser.
The ionization in the lower atmosphere is mostly caused by cosmic
Rays and natural radioactivity. Ions are also produced in and near
Thunderclouds by lightning and corona processes. Cosmic rays originate
From solar flares and other galactic objects such as supernovas and
Exploding stars. One interesting thought is; do stellar events affect our
Lives here on earth? We know that cosmic rays are by far the major ion
Producers in the lower atmosphere and if thunderstorms need ions to
Feed on in order to charge, we certainly would not have thunderstorms
If there were no cosmic rays. Ancient man would not have had access to
Fires and the many thousands deaths each year from lightning strokes
Would have been avoided.
Cosmic rays originate from deep space and usually consist of very
High-velocity atoms that have been stripped of their orbiting electrons.
There are also electrons present in space that travel with near-light
Velocities, but such particles are usually absorbed at very high altitudes
In the earth’s atmosphere. However, heavy cosmic rays penetrate the
Atmosphere quite far and often reach the earth’s surface. During such
An encounter numerous secondary electrons are produced (electron
Showers) along its track from ionizing collisions with atmospheric
Molecules. The secondary electrons in turn might ionize a fair amount
Of molecules themselves before they slow down and attach themselves to
Atmospheric molecules to form negative ions. The result is that one
Cosmic particle could be responsible for the creation of as many as one
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