Lights in the Northern Sky
On clear winter nights in northern Canada, curtains of green light sometimes ripple across the sky. These lights are called the aurora borealis, or northern lights. For a long time, people wondered what caused them.
Scientists now know that the story begins at the Sun. The Sun constantly sends out a stream of tiny charged particles called the solar wind. When these particles reach Earth, our planet's magnetic field steers many of them toward the North and South poles.
High above the ground, the particles crash into gases in the atmosphere. The collisions give the gases extra energy, which they release as light. Oxygen usually glows green, while nitrogen can add touches of blue and purple.
Because the magnetic field guides the particles toward the poles, the lights are seen most often in places far to the north or far to the south. A similar display near the South Pole is called the aurora australis.