Letter to the Editor

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

¹ Department of Physics, University of Warwick, Coventry CV4 7AL, UK e-mail: {greenh,sandrac,valery}@astro.warwick.ac.uk; g.rowlands@warwick.ac.uk
² UKAEA Culham Division, Culham Science Centre, Abingdon, Oxfordshire, OX14 3DB, UK e-mail: richard.dendy@ukaea.org.uk

Corresponding author: J. Greenhough, greenh@astro.warwick.ac.uk

Received: 7 May 2003
Accepted: 22 August 2003

Abstract

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 power-law-tailed distributions. We show that (i) during the high activity period 2000 Jan.–June the tail exponent ; (ii) during the low activity period 1996 Jan.–June ; and (iii) in general a_T decreases with increasing activity.