Hydro- and Electrodynamics of the Midlatitude Ionosphere - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 224

Height and critical frequency at 45°

180

160

140

120
100
80
60
40

h m F2
f 0 F2

20
0

20

2

40

2

60

2

80

2

100

2

120

2

140

2

17 18 19 20 21 22 23 24 25 26 27 28

April 1979 (days)

Figure 5.1 A plot of the height of the F peak and the value of the peak plasma frequency
at middle latitudes relative to the long-term hourly average for April as a function of time
for eleven days in April 1979. A chain of stations near the Japanese longitude was used.
[After Forbes et al. (1988). Reproduced with permission of the American Geophysical
Union.]

h m F2 relative to average hourly values are presented in Fig. 5.1 for an 11-day
interval. The first four days were very quiet geomagnetically, but the next six
days were very active. Also plotted is the maximum value of the plasma fre-
quency in the F region of the ionosphere relative to its hourly average.

The electron density n is related to the plasma frequency f p by n

( f 0 F2 )

2
(see Appendix B). In the plot, both parameters vary drastically after April 21
in a manner called an ionospheric storm. The general trend is for a decrease in
plasma density, an effect referred to as a negative ionospheric storm. A nega-
tive storm is thought to be due to a composition change and, more specifically,
to a decrease in the ratio [O]/[N 2 ], which affects both the production and the
recombination rates of the plasma. But superimposed on the density decrease
are variations of f 0 F2, which correlate with the height variations—that is, a high
ionosphere corresponds to high electron density. This correlation is due in part
to reduced recombination at high altitudes but is complicated by an exchange of
plasma with the plasmasphere.
The plasma content also affects the electrical conductivity of the ionosphere
(and thus the electric field) as well as ion drag (and thus the neutral wind).
Throwing in the conjugate hemisphere, which in general will have a different
neutral wind and conductivity (but presumably the same electric field), one has
a very complex coupled electrodynamic/dynamic system (without even mention-
ing external influences such as high-latitude electric fields and aurorally driven
winds!). We consider some of these complexities later in this chapter.

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