One of the largest sunspot clusters in years developed over the past three days and produced a coronal mass ejection, similar to a solar flare, at 3 a.m. EDT Wednesday, forecasters at the National Oceanic and Atmospheric Administration said.
The disturbance was expected to produce a geomagnetic storm rated G3. A G5 storm is the strongest.
The storm could make the aurora visible as far south as Oregon and Illinois.
A coronal mass ejection is an explosion of gas and charged particles into space from the corona, the outermost layer of the sun's atmosphere.
A second sunspot cluster not yet visible from Earth could produce more geomagnetic storms in the next two weeks, NOAA said.