Why do the aurorae appear in rings around the poles of planets and not at the poles??
I suspect it is because right at the magnetic pole where the magnetic field points exactly straight down, the charged particles don’t actually traverse much atmosphere (they travel a path going just straight down through the atmosphere). A little ways off the pole, the particles spiralling along the field lines are traveling at an angle through the atmosphere, and have a chance to traverse a longer pathlength. Hence they will excite more molecules in the air and create more aurora light.
