The biggest solar eruptions to date of the current solar cycle have crossed the solar system and smacked into the Earth's magnetic field on Friday afternoon. The collision of solar particles with the Earth's atmosphere could trigger the aurora borealis, or "Northern Lights" tonight.

"My estimate is we will probably get aurorae in the northern tier of the U.S.," said Brian J. Anderson, a research physicist at the Johns Hopkins University's Applied Physics Lab near Laurel. "We might be able to see it in the Baltimore-Washington area if it [the magnetic field in the solar storm] turns due south."

That's not a guarantee, he cautioned. "If the magnetic field doesn't cooperate, this thing could be a dud... That happens half the time."

The sun is currently on the upswing of its 11-year solar activity cycle, and after a long, unusually quiet period at the solar "minimum," eruptions of solar particles and magnetic energy are becoming more common.

The website SpaceWeather.com reported that a large sunspot on the sun, numbered 1261, has hurled out three large flares in recent days, the latest on Thursday. The flares were imaged by NASA'a twin STEREO spacecraft. And as the blast of solar particles and magnetic energy, called a coronal mass ejection (CME) sped toward Earth, they were measured by the SOHO and Advanced Compositions Explorer (ACE) spacecraft.

"The first of these events, the plasma associated with it, the magnetic cloud, arrived yesterday at about 6 p.m.," Anderson said on Friday afternoon around 4 p.m. "The second one, the first hint of it arrived about two hours ago. Then the major piece of it arrived about an hour ago" as measured by instruments in geosynchronous Earth orbit and on the ground.

This kind of solar storm, rated a medium-sized "M-class" flare, can set the Earth's magnetic field ringing like a bell, accelerating ions and adding solar particles to the flow of energy around the planet. That can disturb the Earth's ionosphere and disrupt shortwave communications. It can also disrupt or disable communications and GPS satellites and electric grids. The solar blast can expand the Earth's atmosphere and bring down space junk from low orbits, and disturb the orbits of working satellites. It can also raise radiation levels aboard manned spacecraft and trigger northern lights in far northern and southern latitudes.

Analysts at the Goddard Space Flight Center said the CME has compressed the Earth's magnetic field on the sunward side of the planet to near the altitude of geosynchronous satellites, potentially exposing communications satellites to the solar wind. That could trigger outages. 

"We are seeing enhancement of the electric currents in the atmosphere as indicated by magnetic field readings in polar regions," Sullivan said. There may be more effects noted in the next day or two. "We're in the early stages of this event."

Cloudy skies and high humidity would, of course, make it impossible to observe any aurorae that do occur. But there will likely be more opportunities ahead.

"In a solar cycle there are perhaps 10 or 20 events of this size," Anderson said. "This is not a once-in-a-century type of thing. I'd say it's the first really strong one we're seeing out of this solar cycle."

Anderson is currently engaged in a research project called AMPERE, funded by the National Science Foundation. He is measuring solar-induced electric currents surrounding the Earth, using equipment on board 70 satellites flown by the Iridium satellite telephone system. In time, he said, he hopes the technology can be used to provide commercial interests, such as electric utilities, with site-specific warnings on potential impacts from solar storms.

(PHOTO: Top: Solar Dynamics Observatory, Aug. 4, 2011; Bottom: Aurora seen from Int'l Space Station, NASA/ISS, May 2010))