Northern lights occur roughly every eleven years. Record-keeping of the sun's activity began in 1749. Since then, there have been 22 full cycles. 
Particles ejected from the sun travel 93 million miles toward Earth before they are drawn toward the magnetic north and south poles. As the particles move through the Earth's magnetic shield, they mix with the oxygen, nitrogen, and other elements that result in the display of lights.

Cycles and particles

Auroras also occur on planets such as Jupiter, Saturn, Uranus, and Neptune. These gas giants have thick atmospheres and strong magnetic fields. These auroras are a little different from Earth's as they are formed under different conditions.
Venus has an aurora generated by its magnetotail. Mars experiences local auroras due to magnetic fields in the crust. There are also northern hemisphere auroras caused by particles hitting the Maritan atmosphere.

Auroras on other worlds

The colours of a aurora boralis are pink, green, yellow, blue, violet, and occasionally orange and white. 
<ul>
<li>When the particles mix with oxygen, yellow and green are produced.</li>
<li>When the particles interact with nitrogen, red, violet, and blue colours are produced.</li>
<li>Atomic nitrogen causes blue displays.</li>
<li>Molecular nitrogen results in purple.</li>
<li>Green light typically appears in areas up to 150 miles high, red above 150 miles, blue at up to 60 miles, and purple and violet above 60 miles.</li>
</ul>

The display of colors

The magnetic fields of the sun distort and twist as the Earth rotates on its axis. When these fields become knotted together, they create sunspots. Usually, these sunspots occur in pairs.
As the temperature on the surface of the sun rises and falls, the sun boils and bubbles. Particles escape from the sun from the sunspot regions on the surface, throwing particles of plasma, known as solar wind, into space. These winds take about 40 hours to reach Earth, causing the magical displays.

The northern lights, or aurora borealis

The best places to see the northern lights are Alaska and northern Canada, Norway, Sweden, and Finland. During periods of particularly active solar flares, the lights can be seen in Scotland and northern England.
Winter is usually the best time to see the northern lights, due to lower levels of light pollution. September, October, March, and April are some of the best months to view the aurora borealis. Agencies such as NASA and the National Oceanic and Atmospheric Administration issue aurora alerts when the displays are expected to be very impressive.

The best places to see the northern lights

The northern lights and southern lights are a dazzling spectacle for skywatchers lucky enough to see them, but fascinating science is behind what causes these impressive light shows. 

The display of colors

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