The storm, the most disruptive to hit Earth since 1989, was unleashed by the fourth-most powerful solar flare ever seen, NASA said.
The gigantic cloud of highly charged particles hurled from the sun posed a threat to electric utilities, high frequency radio communications, satellite navigation systems and television broadcasts. Continued turbulence on the sun remains a concern for the next week, space forecasters say.
The biggest immediate effect was the blackout of high-frequency voice-radio communications for planes flying far northern routes.
But airliners in an emergency could still communicate through VHF contact with another aircraft or military monitoring station, said Louis Garneau, a spokesman for the company that handles Canada's civil aviation navigation service.
British controllers were keeping trans-Atlantic jets on more southerly routes than usual to avoid the problem.
The particle storm, measuring 13 times larger than Earth, was rated a G5, the highest intensity on scientists' scale of space weather. Space observers have measured G5 storms five times in the past 15 years, but few of them have hit Earth so directly.
It whipped through the solar system at about 5 million mph, taking just 19 hours to travel the 93 million miles from the sun to envelop the planet. Federal scientists said it collided with Earth's magnetic field at 1:13 a.m. EST on Wednesday, about 12 hours earlier than predicted.
Last week, a weaker solar flare erupted on the sun's surface, but scientists said the particle cloud from that event largely spared the planet.
Such storms pose no direct threat to people on the ground because the Earth's thick atmosphere deflects and absorbs incoming charged particles. But the storm may produce colorful auroras in the northern night sky visible as far south as El Paso, Texas, beginning late Wednesday.
The last time a G5 storm hit Earth was in 1989, which damaged the power grid and caused electrical blackouts in the Canadian province of Quebec.
"It is extremely rare to get this level of geomagnetic storming," said Larry Combs, forecaster for the Space Weather Center at the National Oceanic and Atmospheric Administration in Boulder. "This is one of the strongest storms that we have received during this cycle."
There were few immediate reports of damage related to the geomagnetic storm. However, Combs said, "We know that our power grids are definitely feeling the effects of this."
That's because disruptions in the magnetic field caused by the incoming particles can induce power surges and other current fluctuations in electrical systems.
In Princeton, N.J., officials at the North American Electric Reliability Council, which oversees the power grid, said this storm had not caused any failures so far. Geomagnetic storms have caused power disturbances in the United States and Canada at least 11 times since 1940.
Among the precautions taken by various utilities: making less electricity at generating stations, removing vulnerable transmission lines from service and adding voltage control equipment.
The increased solar activity is also affecting the international space station. The Expedition 8 crew, Commander Mike Foale and Russian cosmonaut Alexander Kaleri, briefly retreated to the aft end of the station's service module, which is shielded from higher levels of radiation.
The pair will spend about 20 minutes there, twice on each orbit of the Earth for about three orbits, until the station phases out of the high radiation areas.
The sun generates flares and particle storms in 11-year cycles as sunspots develop. The current solar cycle peaked nearly three years ago, so such a powerful event occurring on the cycle's downside is especially surprising.
In Tokyo, Japan's space agency said the Kodama communications satellite malfunctioned because of the flare, so it was temporarily shut down. There was no major communication disruption, the agency said.
Space scientists in the U.S. and Europe, as well as commercial satellite operators, shut down some delicate instruments and turned them away from the storm's blast. Solar panels are particularly vulnerable.
Researchers said Earth was protected from the storm's full impact because the magnetic field of the storm cloud was pointed north in the same direction as Earth's magnetic field.
But if the cloud's magnetic field shifted southward — something that still could happen, scientists say — its opposing force would, in effect, open a "hole" in the planet's magnetic shield. That could result in later disruptions to electrical systems, they said.
In southern California, wildfires already have knocked out many microwave communication antennas on the ground, making satellite communications important to emergency efforts. Researchers said safety personnel might encounter communications interference.