## P

Fig 23.23.

Magnetic mirror. Particles spiral around magnetic field lines. For simplicity, we look at positively charged particles, though the same argument can be carried out for negative particles. In the region where the magnetic field lines are parallel, on the left, the magnetic force on particle P is downward.This just keeps the particle orbiting. However, when the field lines become closer together, meaning that the field is increasing, the magnetic force on the particle at P' has a slightly rearward component.This is the force that slows and reverses the component of the motion parallel to the field lines.

Bpar, parallel to the z-axis, along which the particles are drifting, and BPERP, perpendicular to the z-axis. If the field were constant, BPERP would be zero.

We look at the force on a proton moving in the indicated spiral path. (The argument works just as well for negatively charged electrons. See Problem 23.14.) Note that the sense of the spiral is determined by the charge of the particle. BPAR doesn't change the z-component of the proton's speed. However, v x BPERP points to the left. This is opposite to the direction of drift of the particle. The motion along the field lines is slowed, and eventually reversed. It is as if the particle struck a mirror. In fact, this phenomenon is called a magnetic mirror. The charged particles therefore spiral back and forth, trapped in the Earth's magnetic field.

The particles come from interplanetary space, mostly from the solar wind. In fact the Earth's magnetic field shields us from most of the solar wind by trapping the particles. Because of the dipole nature of the Earth's magnetic field, these charged particles get closer to the surface near the Earth's magnetic poles. In these regions, there is more of the normal atmosphere for the charged particles to strike. The fast moving charged particles lose some energy to the air as they pass through, and the air glows. We see this glowing air as an aurora (plural aurorae). An

Fig 23.24.

When the solar wind particles penetrate the Earth's atmosphere, they cause the glowing aurorae.These are seen from above in an ultraviolet image. [NASA]

### Fig 23.24.

When the solar wind particles penetrate the Earth's atmosphere, they cause the glowing aurorae.These are seen from above in an ultraviolet image. [NASA]

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