From
http://www.sec.noaa.gov/advisories/bulletins.html
SPACE WEATHER ADVISORY BULLETIN #03- 5
2003 November 04 at 05:25 p.m. MST (2003 November 05 0025 UTC)
**** EXTREME SOLAR FLARE ****
Powerful Active Region 486 has produced one of the most intense solar flares ever measured. The event began at 12:29 P.M. MST (1929 UTC) on November 4th and rose quickly to exceed X17 on the GOES-12 X-ray sensor. By 12:44 P.M. MST (1944 UTC), the GOES sensor was saturated and remained that way until 12:56 P.M. (1956 UTC), suggesting this event extended well in to the very highest flare category, the X20 plus range. This massive flare produced a category R5 (extreme) radio blackout. All short-wave communications through the sunlit hemisphere of the Earth experienced complete blackout conditions.
An associated solar radiation storm is underway. The current storm is at category S1 (minor) levels, and rising. The radiation storm is expected to reach category S2 (moderate) levels, and there is a small chance for a category S3 (strong) radiation storm.
High latitude aviation interests are advised to closely monitor radiation levels over the next 24 hours. High latitude short-wave communications will likely experience considerable degradation for the next 24 to 48 hours. All agencies with space interests should also carefully monitor radiation levels.
A powerful, and extremely fast (over 5 million miles per hour) mass ejection occurred from this flare site, but is mostly directed away from Earth. As a result, only category G1 (minor) to G2 (moderate) geomagnetic storm levels are likely on November 6th.
This solar flare was the largest in a series of very large flares that have occurred in Active Region 486 over the past two weeks. At its peak, this sunspot cluster was approximately 15 times larger than Earth. This active region will rotate to the backside of the sun over the next 24 hours. Another large flare is still possible from this region, but large flare activity will become much less likely after the next 24 to 48 hours.