Image of the sun from the SOHO spacecraft of the intense solar activity taken May 15, 2005, at 7:50 a.m. EDT.May 15, 2005 — Forecasters at the NOAA Space Environment Center in Boulder, Colo., observed a geomagnetic storm on Sunday, May 15, which they classified as an extreme event, measuring G-5—the highest level—on the NOAA Space Weather Scales.
"This event registered a 9 on the K-Index, which measures the maximum deviation of the Earth's magnetic field in a given three-hour period," said Gayle Nelson, lead operations specialist at NOAA Space Environment Center. "The scale ranges from 0 to 9, with 9 being the highest. This was a significant event."
Possible impacts from such a geomagnetic storm include widespread power system voltage control problems; some grid systems may experience complete collapse or blackouts. Transformers may experience damage. Spacecraft operations may experience extensive surface charging; problems with orientation; uplink/downlink and tracking satellites. Satellite navigation may be degraded for days, and low-frequency radio navigation can be out for hours. Reports received by the NOAA Space Environment Center indicate that such impacts have been observed in the United States.
NOAA forecasters said the probability of another major event of this type is unlikely, however, other minor level (G-1) geomagnetic storms are possible within the next 24 hours.
This event was forecast by NOAA as the result of a solar flare that occurred on Friday, May 13.
http://www.noaanews.noaa.gov/stories2005/s2437.htm