Environmental Engineering Reference
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Figure 4.28 Magnetic lines of force near the Earth, which is a magnetic dipole.
This model of the Earth's magnetic field due to a magnetic dipole, as well as the
ball model of Earth, is enough for the analysis of magnetic effects for the Earth's
atmosphere.
The magnetic field of the Earth may prevent charged particles, both electrons
and protons, penetrating the Earth's atmosphere. We assume a charged particle is
moving along a magnetic line of force, as shown in Figure 4.29. Let us find at what
altitudes this takes place. In considering magnetic traps for charged particles, we
assume the Larmor frequency
of collisions
between charged particles and atmospheric nitrogen molecules. For definiteness,
we consider the motion of fast protons in the Earth's atmosphere, and take for
estimation the gas-kinetic cross section
ω
H to be large compared with the rate
ν
10 15 cm 2
σ D
3
as the cross section of
10 3 Hz for the Larmor frequency of
protons in the magnetic field of the Earth near its surface, and the criterion
proton-molecule collision. We have
ω
D
3
H
ω
H
10 10 cm 3 .
This occurs at altitudes above 140 km. Thus, the character of proton motion in
the Earth's atmosphere in accordance with Figure 4.29 takes place in the upper
atmosphere.
ν
holds true if the number density of molecules is less than N 0
D
2
Figure 4.29 Trajectory of a charged particle along a magnetic line of force which is captured by
the magnetic field of the Earth: 1 - the Earth; 2 - points of reflection of the particle.
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