Why do cosmic rays deliver a higher radiation dose at the poles than at the equator?

1. A charged particle feels no magnetic force along a field line, so it can travel freely along one. 2. Charged particles approaching field lines therefore get trapped in spiral orbits about the lines rather than crossing them. 3. Some cosmic rays follow the field lines of the Earth and enter the atmosphere near the magnetic poles. 4. Particles arriving at middle latitudes must cross field lines instead, and many are turned away. 5. More particles reaching the poles means a higher background dose there than at the equator. 6. The field near the poles is stronger, not weaker, so a weak-field explanation is wrong. 7. Air thickness is roughly the same at both places and is not what makes the difference. _Source: OpenStax College Physics (CC BY 4.0), Ch 22 "Magnetism", section 22.5 Force on a Moving Charge in a Magnetic Field: Examples and Applications_