The eruption of a powerful flare on the surface of the sun yesterday has triggered what scientists are calling the biggest solar radiation event since the fall of 1989.
The flare was followed by a coronal mass ejection - a blast of billions of tons of electrically charged atomic particles and magnetic energy - hurled in the Earth's direction at 3 million miles an hour.
The blast was expected to trigger "strong" to "severe" geomagnetic disturbances this weekend, affecting power grids, pipelines, navigation systems, shortwave radio communications and satellite operations.
And skywatchers were advised to look for displays of the northern lights around midnight at latitudes spanning much of the United States.
Space weather forecasters were describing the solar storm as the space equivalent of a Category 4 hurricane on the five-point Saffir-Simpson scale.
"I think it's probably the second-largest [storm] in the last 20 years," said Bill Murtaugh, a space weather forecaster at the federal Space Environment Center in Boulder, Colo. "We're certainly looking at the potential for a major geomagnetic storm with this event."
More major solar storms are expected this year as the sun reaches "solar maximum," the peak in its 11-year cycle of activity.
Yesterday's event began at 6:24 a.m. when a Class-X5 solar flare - the most powerful category - burst from a region of sunspots now crossing the center of the sun's disk.
Solar flares are triggered by the tearing and reconnection of magnetic field lines on the sun's surface. They are the solar system's most powerful explosive events, equivalent to 40 billion Hiroshima-type atomic bombs.
Over a period of minutes or hours, a solar flare releases an intense burst of atomic particles - mostly protons - and electromagnetic energy ranging from gamma and X-rays through the visible light spectrum to longer radio waves.
The radiation from the flare sweeps across the solar system at the speed of light, arriving just minutes after leaving the sun.
Particles and magnetic disturbances from the subsequent coronal mass ejection is much slower, taking about 30 hours to cross the 93 million miles from the sun to the Earth.
People on the ground are protected from these events by the Earth's magnetic field. But the hazards rise with higher altitudes, with proximity to the magnetic poles, and in space.
The shower of radiation from yesterday's solar flare was strong enough to give the passengers and crews of airlines flying over the Earth's poles a dose of radiation equivalent to one chest X-ray, according to the Space Environment Center.
It prompted NASA to calculate the risk the storm would have posed to astronauts had the shuttle been flying, or had the International Space Station been occupied. NASA found the proton shower would have quadrupled the astronauts' radiation exposure.
"We would have been telling them during the most recent sequence of orbits to stay in the more heavily shielded areas" of the space station, said Michael J. Golightly, chief of space science at NASA's Johnson Space Flight Center in Houston.
Vyacheslav Shurshakov, deputy chief of space radiation safety for the Russian Institute of Biomedical Problems, said cosmonauts on the Mir space station would have received the equivalent of the annual radiation limit for radiation workers on the ground.
"We fortunately have no crew on board the Mir station at the moment," he said.