Complex sentence: As particles move toward the poles, the magnetic field line density increases and their "longitudinal" velocity is slowed and can be reversed, reflecting the particle and causing them to bounce back and forth between the Earth's poles.
Split into simpler sentences: As particles move toward the poles, the magnetic field lines density increases and their "longitudinal" velocity is slowed and can be reversed, reflecting the particle. This causes the particles to bounce back and forth between the Earth's poles.