[Should this page be in this module or in the "Auroras" module? Perhaps put this discussion and applet here, and in the Auroras module have a Shockwave movie illustrating the trapping of charges at mirropoint where the field lines come close together?]
A fundamental property of magnetic fields is that they exert forces on moving electrical charges. Here is a Java applet illustrating the motion of charged particles in magnetic fields. Thus, a magnetic field can trap charged particles such as electrons and protons as they are forced to execute a spiraling motion back and forth along the field lines.
[Replace with a new applet that shows the motion of charges in fields, but also provides controls to allow user to vary field strength and direction?]