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Mirror reflection
points
Particle
trajectory
Electron
drift
Guiding
field line
Instantaneous center
of particle rotation
Fig. 1.10
Motion of charged particles trapped in the geomagnetic field. The particles spiral around
the geomagnetic field line concurrently with eastward and westward drift. Taken from the site
The particle velocity V
k
parallel to the magnetic field is not influenced by the field
because the Lorentz force is zero in that direction. The center of the particle rotation
moves at this velocity along the guiding field line as shown in Fig.
1.10
.
In addition to the rapid spiraling around field lines the trapped particles also
undergo a slow eastward and westward drift around the Earth's magnetic axis.
Positive ions drift to the West while the electrons drift to the East. The charged
particles move in the Earth's magnetic field in such a way that its guiding center
belongs to the same magnetic shell/L-shell the whole time. As the gyrating particles
move closer to regions of stronger magnetic field, where field lines converge,
it produces the particle reflection from this region, which has been termed the
magnetic mirroring. At this point the particle velocity drops to zero and then reverses
that results in the particle to move back towards the conjugate point in another
hemisphere (Fig.
1.10
). A proton with energy
100 MeV makes one vibration
along the field line from Northern hemisphere to Southern one for the time
0:3 s.
The proton captured by the Earth's magnetic trap can make about 10
10
vibrations
during its lifetime, which can reach
3
10
9
s (about 100 years).
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