What causes
Aurora?
This IMAGE satellite web page clarifies a common misconception about what causes the Aurora Borealis and Aurora Australis.
Most textbooks state that the Northern Lights are caused by particles from the Sun that flow into the polar regions of Earth. For decades, scientists have known that this is not the actual cause of these spectacular night-time light shows over Arctic and Antarctic regions.
Let's take a closer look at the correct scientific explanation!
[Photo by Dick Hutchinson]
The story begins with a solar storm that hurls gas into space. The Sun is a stormy star. Every minute, the surface changes in spectacular ways. Modern satellite observatories can photograph the surface of the sun and see many kinds of explosions and ejections of matter and energy going on all the time. Most of these have no effect on Earth because they are too puny, and Earth is too far away. But every week or so, the Sun launches from its surface a spectacular storm of energy, in a loop of gas and twisted magnetism that soon swells in size to rival the Sun itself!
[Picture: NASA/SOHO Satellite]
These puffs of gas are called Coronal Mass Ejections. They travel millions of miles per hour and can travel to Earth in only a few days. Although they contain billions of tons of gas, Earth only intercepts a few hundred tons of this gas! Scientists have been watching these CMEs for several decades since their discovery in the early 1970's - thanks to satellite technology. When the Sun is most active at the height of the Sunspot Cycle, hundreds of these CMEs are launched into space every month. Only a few of them are directed exactly at Earth so we are mostly protected from them. The ones that are pointed right at Earth, however, can cause all sorts of problems for satellites, and for electricity utilities on the ground! But they also explain another natural phenomenon we have known about for thousands of years - Aurora! [Photo: NASA/SOHO]
When this CME gas arrives at Earth, it disturbs Earth's magnetic field. This field is shaped like a comet, with an invisible magnetic tail that stretches over a million miles behind Earth, in a direction opposite the Sun. The CME stretches and pulls this tail until it snaps. This releases stored magnetic energy, and causes trapped atoms of gas to pick up some of this energy. Some of these particles flow out the 'back door' of Earth and flow out into interplanetary space. The rest of them flow inwards towards Earth at speeds of over 1,000 miles per second. The Space Shuttle's top speed is only 10 miles/second!
The particles flow towards Earth, riding along its invisible magnetic lines of force into the polar regions. As they collide with atoms of nitrogen and oxygen in the upper atmosphere, they cause the atoms to give off the colored lights that we see in the Aurora. Both the poles of Earth acquire a luminous halo of auroral light until the CME storm passes and the magnetic field of Earth calms down.
[Photo: NASA/IMAGE satellite]
So you see, Aurora aren't caused by particles from the Sun that directly enter the poles of Earth! This is what textbooks and science journalists often say because they don't have enough room to really describe the process in detail. Scientists still don't know exactly where the auroral current particles come from, originally. Some may come from the Sun and get trapped in Earth's magnetic field. Others may come from Earth's upper atmosphere. Scientists are studying this whole process in detail using satellites like Geotail, Cluster and IMAGE. In the next few decades we may finally understand the entire process from cradle to grave!
For more information visit:
Aurora Primer Beginner (Grade 3-6)
Aurora Primer Intermediate (Grade 7-9)
Questions about Aurora FAQs (Various Levels)