Statistical characterisation of full-disk EUV/XUV solar irradiance and correlation with solar activity

We investigate the distribution of fluctuations in solar irradiance when integrated over the full disk, obtained using extreme ultraviolet/soft X-ray observations from the SOHO CELIAS/SEM instrument. This time series sums over both the contributions of single distinguishable flares, and of many other processes. By detrending we select events with timescales of less than a few hours such as waves, slow flows, and CMEs. The statistics generated by this range of phenomena can be characterised by powerlaw-tailed distributions. We show that (i) during the high activity period 2000 Jan.-June the tail exponent aT= 1.5 +- 0.1; (ii) during the low activity period 1996 Jan.-June aT= 3.0 +- 0.2; and (iii) in general aT decreases with increasing activity.