The third most powerful solar X-ray flare on record, a remarkable X17.2 category explosion, erupted from sunspot 10486 on Tuesday, 28 October 2003. This is the second largest X-ray flare ever seen by the ESA/NASA SOHO spacecraft, after the major X20 solar flare of April 2001. Regular observations of X-ray flares began in the 1970s.
More information at:
http://www.esa.int/export/esaCP/SEMY3X7O0MD_Protecting_0.html