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index, I
ln(DST), where DST is the global midlatitude average magnetic
deflection in units of nT.
It is clear that photon sources such as galactic x-rays and scattered Lyman
=
K p +
α
(from the geocorona) cannot produce much ionization above 105 km. Par-
ticle ionization sources are thus surprisingly important at midlatitudes, and it
seems that the primary component involves positively charged particles rather
than electrons, which produce most of the auroral ionization. Evidence in sup-
port of this hypothesis is given in Fig. 6.11c for a rocket flight over Wallops
Island, Virginia ( L
=
.
6). In comparing the two curves, electrons contribute
only about 10% of the count rate in the energy range surveyed. A schematic lati-
tude profile of precipitating particles based on satellite observations is reproduced
in Fig. 6.12. Positive particles dominate in three bands: equatorial, midlatitude,
and the subauroral zone. Electrons dominate in the auroral zone proper and in a
“low” latitude region between 20 and 30 . The schematic picture is valid when
averaged over longitude. However, in the Northern Hemisphere, the midlati-
tude zone disappears almost entirely between 45 W and 75 E. Conversely, the
low-latitude electron zone disappears between 135 W and 150 W. These latter
effects are due to the South Atlantic anomaly, a region of unusual magnetic field
strength that modifies the mirror heights of energetic particles quite drastically.
The equatorial zone is due to the charge exchange of ring current ions with the
geocorona. The energetic neutrals cross the field lines and are focused in the
equatorial zone (Pröllss, 1973).
In summary, particle precipitation plays an important role in creating structure
in the mid- and low-latitude sector by providing an ionizing source for the F-layer
2
1
2
3
4
5
Electron auroral zone
Proton auroral zone
Midlatitude zone
Low-latitude zone
Electron auroras
Hydrogen arcs
10 1
Equatorial zone
1
2
1
Electrons
Ion
10 1
Plasmapause
10 2
5
10 3
3
4
10 4
80
60
40
20
0
Geomagnetic latitude
Figure 6.12 Particle precipitation as a function of latitude. [Adapted from Voss and
Smith (1980b). Reproduced with permission of Pergamon Press.]
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