Solar EUV flux variation and flare brightenings

Abstract

On 2010 February 8, the Extreme ultraviolet (EUV) flux variation in 195Å and flare brightening has been examined in different sizes of active regions by using SOHO/EIT, MDI and Hα observational data. These three active regions represent a large active region with a sunspot group, a moderate active region without a sunspot and a small region with weak plage in Hα band respectively. Our study shows that the main full disk EUV flux comes from active regions, especially from large active regions. The sudden increases of EUV flux are corresponding to the EUV flare brightenings. For the large active region, the local EUV 195Å flux peaks are well correlated to that of the GOES X-ray flux. The EUV 195Å flux peaking time of M-class flares delay GOES X-ray flux a few minutes. For the moderate active region, the local EUV 195Å flux is not well correlated to GOES X-ray flux. The EUV 195Å flare brightenings in the moderate active region appeared in the duration of sudden increase of its own local EUV flux. For the small active region, the local EUV 195 Å flux varied almost independently of the GOES X-ray flux. Our study suggests that for an active region its local EUV 195Å flux is more closely correlated to the EUV flare brightening than the full disk GOES X-ray flux.