COSMIC RAYS AND EARTH
A cosmic ray approaching Earth first encounters
Earth's magnetic field. The magnetic field repels some particles
altogether. Those that get through are deflected by the magnetic field.
Computers are used to track cosmic ray paths through Earth's magnetic
field, and to determine how the starting direction ("asymptotic direction")
is related to the impact point.
When the primary cosmic ray strikes an atom
in Earth's atmosphere, the collision may produce one or more new energetic
particles called "secondary" cosmic rays. These secondary particles strike
other atmospheric atoms producing still more secondary cosmic rays.
The whole process is called an
atmospheric cascade. If the primary cosmic ray has enough energy -- greater
than 500 million electron volts -- the nuclear byproducts of the cascade
can reach Earth's surface.