Geoscience Reference
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60
40
20
Slope 1 night/year
0
1970
1980
Year
1990
Figure 7.17
Observations of NLC over nearly three decades. [After Gadsden (1990).
Reproduced with permission of Pergamon Press.]
agree most of the time. Thus, PMSE are of particular interest in view of their
frequent coincident occurrence with NLC and the possible association of recent
increased detection of NLC with global warming trends (Gadsden, 1990). If we
could monitor PMSE for long periods of time and observe increases/decreases
in these events (and their relative strength) over time, we could use such infor-
mation as a possible indicator of atmospheric change. In fact, these clouds have
been called the miner's canary of Global Change (Thomas, 1996a, b). Unfortu-
nately, the injection of water vapor by the space shuttle in the last twenty years
may distort this development.
7.8 Upward-Propagating Lightning
The earth is a good electrical conductor compared to the atmosphere and effec-
tively forms a “ground” potential. Thunderstorms charge the earth negatively,
which then discharges through the weakly conducting atmosphere. Thus, the
system can be modeled as a leaky spherical capacitor. About 250,000V exist
between the earth and the lower ionosphere. The average current density over
the earth is about one pA/m
2
and the total is about 1500A.
Ninety-nine percent of the time, lightning striking the ground brings negative
charge, which is how the earth becomes charged. Every such flash consists of
several strikes following the same channel. Each such current pulse creates an
electromagnetic wave similar to that of a dipole antenna. The fields have electro-
static, inductive, and radiation characteristics. Although the electrostatic field is
largest in the near field and right over the strike, all three field types are important
at the base of the ionosphere.
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