A large group of sunspots headed toward Earth during the weekend and two coronal mass ejections erupted from the Sun's surface Tuesday, they said.
The ejection could arrive on Earth late Wednesday and cause moderate geomagnetic storms and auroras in the higher altitudes, the National Oceanographic and Atmospheric Administration's Space Weather Prediction Center reported.
Because the coronal mass ejections were not aimed directly at Earth they are not likely to affect communications satellites or other equipment, researchers said.
The Sunspot Region 1476 is a "monster sunspot" because of its size, 11 times wider than the Earth, NASA's Space Dynamics Observatory said.
The spots are so large they have been visible without the aid of telescopes, although special filters are required for viewing the sun directly to protect the eye from damage, astronomers warn.