45 Curtains of Light-Mid

Curtains of Light FOCUS Question What are auroras, and how do they form?

NATURAL LIGHT SHOW Near Earth's North and South Poles, people sometimes see a beautiful, natural light show in the sky at night. The light looks like wavy colors hanging on the horizon. This light is not coming from the Moon. What is it, and where does it come from? These lucky sky watchers are seeing auroras (uh-ROAR-uhs) —natural events that occur in Earth's atmosphere. In the Arctic region, these magical light shows are known as the aurora borealis (bore-ee-AL-us). They are nicknamed the northern lights. When seen near the Antarctic, they are called the aurora australis (aw-STRAL-us), or southern lights. Both kinds of auroras are beautiful wavy bands of light They are created by magnetic and electrical reactions high in Earth's atmosphere. Word Wise The famous astronomer Galileo Galilei connected two ancient words to describe the aurora borealis. Aurora was the Roman goddess of the dawn Boreas was the Greek name for the north wind.

EARTH'S ATMOSPHERE To understand how auroras form, it helps to know that Earth's atmosphere is made up of five main layers. The layer closest to Earth is the troposphere. Gas molecules are tightly packed together in this layer. Above the troposphere is the stratosphere. The ozone layer is part of the stratosphere. This layer protects Earth by blocking ultraviolet (UV) rays from the Sun. Next comes the mesosphere. This layer is where we see falling stars—meteoroids that burn up as they cross the night sky. Even higher above Earth is the thermosphere. Air in this layer is thin with few gas molecules. This is where auroras form. Beyond the thermosphere is the exosphere. This final layer gradually becomes outer space.

STORMY SUN How do auroras develop in Earth's thermosphere? Energy from the Sun is one of the key ingredients. The Sun is an enormous ball of burning gases. As it burns, the Sun gives off solar wind—a stream of charged particles that constantly flows into outer space. Solar wind flows away from the Sun in all directions, including toward Earth. During occasional solar storms, the Sun sends out even stronger energy flows into the solar wind. These bursts of energy are called solar flares—giant clouds of gas made up of charged particles To reach Earth, the particles. have to travel 150 million kilometers (93 million mi.). This trip takes from one to five days. Do You Know? Auroras can also occur during the day. We just can't see them because the Sun is too bright.

EARTH'S MAGNETIC SHIELD Most solar flares miss Earth entirely, but some smash right into Earth's magnetic field. Because Earth has an iron core, it acts like a giant bar magnet with lines of force that move from the South Pole to the North Pole. These lines of force create a magnetic field that surrounds Earth. The magnetic field acts like a shield that protects Earth from most of the solar wind. But during strong solar storms, some of the charged particles from the Sun squeeze in between Earth's magnetic lines of force. The particles follow the lines of force, toward the poles where they penetrate Earth's atmosphere.

A RAINBOW OF REACTIONS Near the North and South Poles, charged particles from the Sun collide with gas molecules in the thermosphere. The particles bang into each other with such force that a chemical reaction takes place. Each collision produces a tiny burst of light. All those little lights put together form a huge curtain of light—an aurora! The color of the light in an aurora depends upon the kind of gas molecule involved in the reaction. When the solar particles collide with oxygen, green and yellow auroras are created. When the particles collide with nitrogen, we see red, blue, and purple auroras. Some auroras look like motionless rings or bands of light. Others are wavy and dance in beautiful curtains of light.