A strong wave of charged plasma particles emanating from the Jupiter-sized sun spot, the most powerful seen in four years, has already disrupted radio communication in southern China.
The Class X flash -- the largest such category -- erupted at 0156 GMT Tuesday, according to the US space agency.
"X-class flares are the most powerful of all solar events that can trigger radio blackouts and long-lasting radiation storms," disturbing telecommunications and electric grids, NASA said Wednesday. Geomagnetic storms usually last 24 to 48 hours, but some could last for many days, said the US National Weather Service.
"Ground to air, ship to shore, shortwave broadcast and amateur radio are vulnerable to disruption during geomagnetic storms. Navigation systems like GPS can also be adversely affected."
NASA's Solar Dynamics Observatory said it saw a large coronal mass ejection (CME) associated with the flash blasting toward Earth at about 560 miles per second (900 kilometers per second).
The flare spread from Active Region 1158 in the sun's southern hemisphere, which had so far lagged behind the northern hemisphere in flash activity. It followed several smaller flares in recent days.
"The calm before the storm," says the US National Weather Service Space Weather Prediction Service.