Space weather forecasters revised their predictions for storminess Wednesday after a major flare erupted on the Sun overnight, threatening damage to communication systems and power grids while offering up the wonder of the Northern Lights.

"We're looking for very strong, severe geomagnetic storming" to begin probably around mid-day Thursday, Joe Kunches, lead forecaster at the NOAA Space Environment Center, told SPACE.com.

The storm is expected to generate aurora or Northern Lights as far south as the northern United States Thursday night.

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Astronauts aboard the International Space Station are not expected to be put at additional risk, Kunches said.

Radio communications, satellites and power grids could face potential interruptions or damage, however.

Solar flares send radiation to Earth within minutes. Some are also accompanied by coronal mass ejections (CME), clouds of charged particles that arrive in a day or two.

This flare — categorized as a major X-3 solar flare — unleashed a strong CME that's aimed squarely at Earth.

"It's got all the right stuff," Kunches said.

However, one crucial component to the storm is unknown: its magnetic orientation.

If it lines up a certain way with Earth's magnetic field, then the storm essentially pours into our upper atmosphere.

If the alignment is otherwise, the storm can pass by the planet with fewer consequences.

Kunches and his team are advising satellite operators and power grid managers to keep an eye on their systems.

In the past, CMEs have knocked out satellites and tripped terrestrial power grids.

Engineers have learned to limit switching at electricity transfer stations, and satellite operators sometimes reduce operations or make back-up plans in case a craft is damaged.

Another aspect of a CME involves protons that get pushed along by the shock wave.

Sometimes these protons break through Earth's protective magnetic field and flood the outer reaches of the atmosphere — where the space station orbits — with radiation.

The science of it all is a gray area, Kunches said. But the best guess now is that there will only be a slight increase in proton activity. That's good news for the astronauts.

"When the shock goes by, we don't expect significant radiation issues," he said.

The astronauts were ordered to a protective area of the space station as a precaution last night.

Now that sunspot number 930 has flared so significantly — after several days of being quiet — the forecast calls for a "reasonble chance" of more major flares in coming days, Kunches said.

The sunspot, numbered 930, has been rotating across the face of the Sun for several days.

Such storms are fairly common when the Sun is at its most active, but they are rare during the current low point in the 11-year cycle of solar activity.

Sunspots are dark regions of the Sun where intense magnetic activity caps the upwelling of material from below.

Sometimes a cap blows, and a visible flare results. The flares are loaded with X-rays and other radiation, all of which reaches Earth moments after the eruption and can be accompanied by a shower of protons.

These storms can arrive in moments with little warning and can be deadly.

Last week, the same sunspot generated what astronomers described as a rarely imaged solar tsunami. The activity began with an X-9 flare Dec. 5.

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