A severe space weather storm began pounding Earth late Wednesday and is expected to threaten communications, satellite operations and possibly astronauts and airline passengers -- especially pregnant women -- through Sunday.

Forecasters said the event could also kick up powerful displays of the colorful northern lights as far south as California, Colorado and Virginia on Friday night, though the chances for this were very uncertain.

The event began Wednesday November 8 at 6:50 p.m. Eastern Standard Time (23:50 GMT), when a large solar flare welled up from deep within the Sun. This energy interacted with the solar atmosphere, sending a stream of charged particles called protons heading toward Earth.

"The protons measured near Earth increased 10,000 times in the matter of a few minutes," said Joseph Kunches, lead forecaster at NOAA Space Environment Center.

[inset]

Forecasters labeled the event an S4. The most severe would be an S5.

Update: On Friday morning, a NOAA forecaster told SPACE.com that the radiation storm was gradually subsiding and that activity would likely be back to normal levels sometime Sunday.

The protons buffeting Earth are a form of radiation that, with extended exposure, is thought to damage DNA and contribute to cancer. While cosmic radiation from distant sources constantly bombards Earth, the amount increases during a severe solar storm.

People on the ground are not at risk, as Earth's magnetic field and atmosphere provide a blanket of protection.

But NOAA says an airline passenger can experience as much radiation as 10 chest X-rays, though this figure is debated. Experts do agree, however, that anyone on a high-altitude jet or in space is exposed to more radiation than someone on the ground.

Wallace Friedberg, who studies the threat for the Federal Aviation Administration (FAA), said that the NOAA estimates are the best available. He said most people planning a flight during the storm should not necessarily change their plans, adding that the radiation measurements he has seen indicate that this event is not the worst possible.

"If it was my daughter, and if she was pregnant, I'd tell her she might want to wait," said Friedberg, who heads the radiobiology research team at the FAA Civil Aeromedical Institute. "If she's not pregnant, I wouldn't be that concerned.