Statistical Studies on the Excess Peak Flux in Soft X-rays and EUV Bands from Solar Flares

Based on the solar X-ray data in the band of 0.1??C?0.8?nm observed by Geostationary Operational Environmental Satellites (GOES), the XUV and EUV data in the bands of 26??C?34?nm and 0.1??C?50?nm observed by the Solar EUV Monitor (SEM) onboard the Solar and Heliospheric Observatory (SOHO), a statistical analysis on the excess peak flux (the pre-flare flux is subtracted) in two SEM bands during M- and X-class flares from 1998 to 2007 is given. The average ratio of the excess peak flux to the pre-flare flux for the M-class flares is 5.5?%±3.7?% and that for the X-class flares is 16?%±11?%. The excess peak fluxes in two SEM bands are positively correlated with the X-ray flare class; with the increase in the X-ray flare class, the excess peak flux in two SEM bands increases. However, a large dispersion in the excess peak flux in the SEM bands and their ratio is found for the same X-ray flare class. The relationship between the excess peak fluxes of the two SEM bands also shows large dispersion. It is considered that the diversity we found in the flare spectral irradiance is caused by many variable factors related to the structure and evolution of solar flares.