The Facts About Bounce Motion:

Not only do particles spiral around magnetic field lines but they also move along the field lines toward the Earth. The crowded magnetic field lines near the poles cause particles to "reflect" and move back the same direction the from which they came. They bounce back and forth between reflection points in each hemisphere. Only a small portion of these particle penetrate into the lower atmosphere.

Click on the image for a quicktime movie (325 kb) showing the bounce motion in 3D