Geomagnetic Storm Engulfs Earth...07/14/00
By Lee Siegel

A major geomagnetic storm enveloped Earth early Thursday after solar flares earlier in the week helped hurl material off the sun’s outer atmosphere. The activity means tonight could be a good opportunity for viewing auroras in the far northern

Storm conditions settled down to minor late in the day, although more major activity is possible next week.

"It’s waning now," space weather forecaster Bill Murtagh said of the geomagnetic storm that hit Earth’s magnetic field about 5:54 a.m. Eastern Daylight Time (09:54 GMT) Thursday. "It’s starting to weaken."

A major, or category-G3 magnetic storm is midway on NOAA’s scale of a minor, or G1 storm to an extreme, or category G5 storm.

Forecasters at the National Oceanic and Atmospheric Administration’s (NOAA) Space Environment Center in Boulder, Colorado did not have reports of specific effects of the storm.

But NOAA says a G3 storm can trigger on Earth voltage fluctuations on power systems; cause false alarms on power-system protection devices; make the auroras visible as low as the mid latitudes and disrupt satellite navigation and high-frequency radio communication. For satellites in space it has the potential to create orientation problems, increase atmospheric drag that speeds orbital decay and cause electrical charges. There is no known health risk to people on Earth's surface.

NASA reported the aurora borealis, or northern lights, put on vivid displays across Alaska, Canada and Greenland. But Murtagh said it was unlikely the latest geomagnetic storm would push the phenomenon far enough south to be visible from the continental United States.

He said forecasters believe the geomagnetic storm was caused by coronal mass ejections – large amounts of electrically charged gas or plasma exploding off the sun’s surface. They were triggered by a moderate solar flare at 5:42 p.m. EDT (21:42 GMT) Monday July 10 and/or by a major, or X-class, solar flare at 9:10 a.m. EDT (13:10 GMT) Tuesday July 11.

NASA said the U.S.-European Solar and Heliospheric Observatory (SOHO) spacecraft spotted the second mass ejection as "a glowing halo of material" in the sun’s atmosphere.

It generally takes 48 to 98 hours for the big bubbles of electrified gas to reach Earth’s magnetic field. Forecasters believe the shock wave from one mass ejection overtook the other en route to Earth so they combined to cause Thursday’s geomagnetic storm, Murtagh said.

He said that if forecasters were wrong about the two shock waves combining, geomagnetic activity would increase early Friday before subsiding again.

When the shock wave that triggered the storm was detected by NASA’s Advanced Composition Explorer (ACE) spacecraft on Thursday, solar-wind speeds quickly jumped from about 1.16 million m.p.h. (520 kilometers per second) to about 1.57 million m.p.h. (700 kilometers per second).

The geomagnetic storm was not caused by another X-class or major flare that erupted on the sun at 6:37 a.m. EDT (10:37 GMT) Wednesday July 12, Murtagh said. That event did not produce a coronal mass ejection big enough to trigger a geomagnetic storm.

However, Wednesday’s big flare did produce X-rays and other electromagnetic radiation that moved toward Earth at the speed of light and caused category R3, or strong radio blackout conditions on Earth’s daylit side. Such blackouts can cause mariners and aviators to lose high-frequency radio contact for about an hour or cause hour-long degradation of low-frequency signals used for navigation.

Many of the flares are erupting from sunspot group 9077, which now is near the center of the disk of the sun as viewed from Earth. If more big solar flares occur in that region and trigger coronal mass ejections, they will be in prime position to produce more geomagnetic storms on Earth two to four days later, Murtagh said.

The forecasters predicted a 90-percent chance of medium solar flares and a 50-percent chance of major solar flares through Saturday.