The Sun as a star: observations of white-light flares

Context. Solar flares radiate energy at all wavelengths, but the spectral distribution of this energy is still poorly known. White-light continuum emission is sometimes observed, and these flares are then called white-light flares (WLFs). Aims. We investigate if all flares are WLFs and how the radiated energy is distributed spectrally. Methods. We perform a superposed epoch analysis of spectral and total irradiance measurements obtained since 1996 by the SOHO and GOES spacecraft at various wavelengths, from soft X-rays to the visible domain. Results. The long-term record of solar irradiance and the excellent duty cycle of the measurements allow us to detect a signal in visible irradiance even for moderate (C-class) flares mainly during the impulsive phase. We identify this signal as continuum emission emitted by WLFs and find that it is consistent with a blackbody emission at similar to 9000 K. We estimate the contribution of the WL continuum for several sets of flares and find it to be about 70% of the total radiated energy. We re-analyse the X17 flare that occurred on 28 October 2003 and find similar results. Conclusions. We show that most of the flares - if not all - are WLFs and that the white-light continuum is the main contributor to the total radiated energy; this continuum is consistent with a blackbody spectrum at similar to 9000 K. These observational results are important for understanding the physical mechanisms during flares and possibly suggest a contribution of flares to the variations of the total solar irradiance (TSI).