Geoscience Reference
In-Depth Information
ions with electrons
. Figure 1.10 shows that the influence of nega-
tive ions on recombination becomes significant below 75 km. Not shown, and
only recently appreciated, are charged smoke, dust, and ice particles that create
interesting dusty and icy plasma physics behavior in the D region (see Chapter 7).
(α
eff
=
α
d
)
1.4 The Earth's Magnetic Field and Magnetosphere
To first order the earth's magnetic field is that of a dipole whose axis is tilted
with respect to the spin axis of the earth by about 11
◦
. This offset, which is com-
mon to several planetary magnetic fields, is presently such that the dipole axis
in the northern hemisphere is tilted toward the North American continent. The
magnetic field
B
points down toward the surface of the earth in the Northern
hemisphere and away from it in the Southern hemisphere. The dipole position
wanders with time, and paleomagnetic studies show that it flips over with irreg-
ular time differences. The field is created by currents in the molten, electrically
conducting core of the earth, currents that are in turn driven by thermal con-
vection in the core. This convection is certainly quite complex, but the magnetic
field contributions that are of higher order than the dipole term fall off faster
with distance, leaving the dipole term dominant at the surface.
Some useful equations for a dipole field are gathered in Appendix B. The field
at the earth's surface varies from about 0
10
−
4
tesla (0.25 gauss) near the
.
25
×
10
−
4
tesla near the poles. Sketches of magnetic
field lines are useful when considering the ionosphere and magnetosphere, since
plasma particles move very freely along field lines. A dipole field is sketched in
Fig. 1.11. The equation for the magnetic field lines can be written in a modified
spherical coordinate system as
magnetic equator to about 0
.
6
×
L
cos
2
r
=
θ
N
S
Figure 1.11
Sketch of dipole magnetic field lines extending into a vacuum.
Search WWH ::
Custom Search