Hydro- and Electrodynamics of the Midlatitude Ionosphere - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 231

angle, neutral waves, and plasma instabilities, are what make the midlatitude
ionosphere interesting. In the tropical zone, one can generalize these results by
projecting the forces parallel to the magnetic field.

5.1.6 The Appleton Anomaly: An Equatorial Electric Field Effect
Before discussing midlatitude dynamics and electrodynamics per se, there is an
interesting and important tropical ionospheric effect arising from equatorial elec-
trodynamics. As discussed in Chapter 3, the zonal electric field at the magnetic
equator is eastward during the day, which creates a steady upward E

B 2
plasma drift. Just after sunset this eastward electric field is enhanced and the
F-region plasma can drift to very high attitudes where recombination is slow,
while the low-altitude plasma decays quickly once the sun sets. The result is
called the fountain effect, since the dense equatorial plasma rises until the pres-
sure forces are high enough in (5.1) that it starts to slide down the magnetic field
lines, assisted by gravity, toward the tropical ionosphere. This “sliding” results
in a region of enhanced plasma density referred to as the equatorial or Appleton
Anomaly. The various forces acting on the plasma are illustrated schematically
in Fig. 5.4a.
If the zonal electric field is taken as a given quantity based, for example, on
the experimental data shown in Chapter 3, then the actions of production and
recombination can be combined with the vertical E

× /

B

× B motion of the ionosphere

FOF2

E 3 B

B

= p , g

p , g

=

FOF2

E

Equator

20° N

20° S

(a) (b)

Figure 5.4 (a) Schematic diagram of how plasma uplift via electric fields transports
plasma from equatorial to tropical zones. (b) Contour plots of FOF2 in megahertz (the
peak F-region electron plasma frequency, which is proportional to the square of the
electron density) for zero zonal electric field (upper plot) and for a typical diurnal variation
of the zonal (lower right) electric field. (Figure courtesy of D. Anderson.)