Most auroras occur in far northern and southern regions. They appear chiefly as arcs, clouds, and streaks. Some move, brighten, or flicker suddenly. The most common color in an aurora is green. But displays that occur extremely high in the sky may be red or purple. Most auroras occur about 60 to 620 miles (97 to 1,000 kilometers) above the earth. Some extend lengthwise across the sky for thousands of miles or kilometers.
A bar magnet has a magnetic field like that of the sun. Field lines, which represent the field, exit the north pole and enter the south pole. Image credit: World Book diagram by Precision Graphics |
Auroras occur most frequently during the most intense phase of the 11-year sunspot cycle. During this phase, dark patches on the sun's surface, called sunspots, increase in number. Violent eruptions on the sun's surface, known as solar flares, are associated with sunspots. Electrons and protons released by solar flares add to the number of solar particles that interact with the earth's atmosphere. This increased interaction produces extremely bright auroras. It also results in sharp variations in the earth's magnetic field called magnetic storms. During these storms, auroras may shift from the polar regions toward the equator.
Contributor:
C. R. O'Dell, Ph.D., Professor of Space Physics and http://cms.nasa.gov/cm/jsp/library/libselector.jsp?
returnPIField=document.contentpage.item__926__1__3266__E__&assetType=1
&docid=60756&family=3Astronomy, Rice University.
How to cite this article:
To cite this article, World Book recommends the following format:
O'Dell, C. R. "Aurora." World Book Online Reference Center. 2005. World Book, Inc. http://www.worldbookonline.com/wb/Article?id=ar038160.
› Return to Topics | › Back to Top |