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Sea ice is frozen seawater floating on the surface of the ocean. Some sea ice is semi-permanent, persisting from year to year, and some is seasonal, melting and refreezing from season to season. The sea ice cover reaches its minimum extent at the end of each summer and the remaining ice is called the perennial ice cover. The AMSR-E instrument on the Aqua satellite acquires high resolution measurements of the 89 GHz brightness temperature near the poles. Because this is a passive microwave sensor which is not so sensitive to atmospheric effects, this sensor is able to observe the entire polar region every day, even through clouds and snowfall. The false color of the sea ice, derived from the AMSR-E 6.25 km 89 GHz brightness temperature, highlights the fissures or divergence areas in the sea ice cover by warm brightness temperatures (in blue) while cold brightness temperatures, shown in brighter white, represent consolidated sea ice. The sea ice edge identifies areas containing at least 15% ice concentration in the three-day moving average of the AMSR-E 12.5 km sea ice concentration data. The animations below show the continuos motion of the Arctic sea ice during 2008 up to the point at which the Arctic sea ice reached its minimum extent. The 2008 minimum extent of 4.52 sq km (1.74 sq miles) is the second lowest extent recorded since 1979.
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