The most spectacular feature of the ionosphere is the nighttimeaurora. Spectacular light
displays with streamers, arcs, or foglike glows are visible on many nights in the polar regions.
The lights can be white, green, blue, red or purple. The display is called theaurora borealis
or northern lights in the Northern Hemisphere (Figure15.8). It is called theaurora australis
or southern lights in the Southern Hemisphere.
Figure 15.8: The Northern Lights above Bear Lake, Alaska. ( 7 )The aurora is caused by massive storms on the Sun that release great quantities of protons
and electrons. These electrically charged particles fly through space and spiral in along lines
of Earth’s magnetic field. Earth’s magnetic field guides the charged particles toward the
poles, which explains why the auroras are seen primarily in the polar regions. When the
protons and electrons enter the ionosphere, they energize oxygen and nitrogen gas molecules
and cause them to light up. Each gas emits a particular color of light. Depending on where
they are in the atmosphere, oxygen shines green or red and nitrogen shines red or blue. The
frequency and intensity of the aurora increases when the Sun has more magnetic storms.
The outermost layer of the atmosphere is theexosphere. There is no real outer limit to
the exosphere. If you continued traveling farther out from the Earth, the gas molecules
would finally become so scarce that you would be in outer space. There is so little gravity
holding gas molecules in the exosphere that they sometimes escape into outer space. Beyond
the atmosphere is thesolar wind. The solar wind is made of high-speed particles, mostly
protons and electrons, traveling rapidly outward from the Sun.
Lesson Summary
- Different temperature gradients create different layers within the atmosphere. The
lowest layer is the troposphere, where most of the atmospheric gases and all of the
planet’s weather are located.