Auroras

Problem

Explain in a few lines why auroras are observed at the poles and not at the equator, citing the Lorentz force on solar-wind particles and the geometry of the Earth’s field.

Solution

The charged particles of the solar wind feel the Lorentz force $\vec F=q\vec v\times\vec B$ and, unable to freely cross the Earth’s field lines, spiral around them.

The Earth’s field lines converge towards the poles, where they enter the atmosphere almost vertically: there the particles are channelled and penetrate down to the upper layers of the atmosphere, exciting the gases that emit light (the auroras).

At the equator, on the other hand, the lines are horizontal and the magnetic force deflects the particles, preventing them from descending: this is why auroras are not observed there.

$\ev{\text{particles channelled along the lines converging at the poles}}$

Links

Topics: Magnetismo Concepts: Forza di Lorentz · Campo magnetico