Tonight's forecast calls for fair skies and continued cool, with a chance of geo-magnetic storms.
Government scientists say the eruptions of three huge flares on the surface of the sun since last Saturday have sent blasts of high-energy particles toward Earth.
These solar wind "gusts" have caused auroras, or Northern Lights displays as far south as Philadelphia, and threaten to briefly disrupt electrical service, navigation gear and shortwave radio signals.
Energy from the most recent flare, on Wednesday night, should reach Earth by tonight, scientists say.
So far, little more than auroras have been reported. But managers at the Pennsylvania, New Jersey and Maryland Interconnection, which controls the distribution of electricity from New York to Washington, have been placed on alert, ready take action to prevent major blackouts if the solar storms begin to knock out key electrical facilities.
X-ray sensors on weather satellites 22,300 miles above Earth went off the scale for 20 minutes on Saturday when the first blast of solar energy swept past, said Joe Hirman, operations manager at the Space Environment Services Center, in Colorado.
"This event was followed on Tuesday by another just about the same size," he said,"
Energy from the most recent flare,on " and late [Wednesday] night there was a third event of equivalent magnitude."
The blasts appear to be coming from a huge sunspot now pointed toward the Earth. As the sun rotates, the spot will turn its aim away from Earth a week from now, but will reappear two weeks after that.
Because the sun is now at the peak of its 11-year cycle of activity, these big solar flares, and geo-magnetic storms, were expected, said Mario H. Acuna, project scientist with the International Solar Terrestrial Physics Program at the NASA Goddard Space Flight Center in Greenbelt.
The flares send vast amounts of energy into space in the form of hydrogen and helium atoms, stripped of their electrons and accelerated to high speeds, Acuna said.
The fastest reach Earth within eight minutes at nearly the speed of light. Captured by Earth's magnetic field, they streak into the outer layers of the atmosphere near the poles, exciting the gas molecules there and causing them to glow. That creates the shimmering Aurora Borealis, or Northern Lights in northern latitudes, and Aurora Australis in far southern latitudes.
This week, Hirman said, "there have been lots of aurora sightings, all the way across the U.S., where it isn't cloudy, from Philadelphia to Wyoming."
The slower-moving blasts of energy, or solar wind, arrive two or three days after the flares erupt. They push against Earth's magnetic field, blowing it back like a comet's tail, or smoke from a match, Acuna said.
This distortion of the magnetic field can disrupt vulnerable navigation systems, satellite operations, and shortwave radio communications.
But, most importantly, it induces a flow of electricity through Earth's surface, Acuna said. And when the electricity can't flow easily through the rock, it looks for a better conductor.
Among the best are long distance power lines. When the solar storms' high-voltage direct currents flow into the lines, they can overheat big transformers and trip circuit breakers. "The result is a power outage," Acuna said.
After the Space Environment Service Center in Colorado issued its first warning last Saturday, power controllers at the PJM
Interconnection's command center in Valley Forge, Pa., watched their line monitors closely.
"We saw some activity, more than normal, but it did not trigger any remedial actions on our part to put the system in a safer state," said Charles Woodward, the PJM operations manager.
Had transmission lines or transformers overheated or failed, power managers would have acted to isolate the trouble, or adjust the output of generators across the grid to reduce the load on power lines.