NASA says the sun emitted a significant solar flare Wednesday, its fourth X-class flare this week as it nears the maximum in its 11-year activity cycle. Solar flares are powerful bursts of radiation. Earth\'s atmosphere protects humans from harmful radiation from a flare, but flares can be intense enough to disturb the atmosphere in the layer where GPS and communications signals travel, causing disruption. The latest flare has been classified as an X2.3 class flare, NASA said. X-class denotes the most intense flares, while the number provides more information about its strength -- with an X2 is twice as intense as an X1, an X3 is three times as intense, and so on. The flare and its radiation are being tracked by the National Oceanographic and Atmospheric Administration\'s Space Weather Prediction Center in Boulder, Colo. The center is the U.S. government\'s official source for space weather forecasts, alerts, watches and warnings. In the sun\'s solar cycle -- first discovered in 1843 -- it is normal for there to be many flares a day during peak activity, NASA said.