Authors: Pap, J. M., D. Judge, D. McMullin and L. Floyd
Reference: SOHO 11 Workshop, Davos, Switzerland (2002)
Solar EUV irradiance has been measured by the CELIAS/SEM on the SOHO experiment since January 1996, providing data from solar minimum to maximum conditions. These measurements cover two spectral ranges: the broader 0.1 – 50 nm (central order channel, often called as XUV) and in the 26 – 34 nm (first order channel, often called as EUV). To study the relation betwen the XUV and EUV irradiances, a ratio (Ratio = XUV/EUV) has been derived. This XUV/EUV ratio is compared to the 0.1 – 5 nmspectral range (so-called _ value) as calculated from the SEM measurements, representing the soft X-ray part of the SEM irradiance measurements. It is shown that the XUV/EUGV ratio and _ are closely related and both indices rise slowly over the solar cycle. The most interesting events in the derived ratio and _ are the sharp spikes which are associated with the increased sunspot areas derived from the SOHO/MDI images and the rapid increases in the GOES soft X-ray data. Results on the SEM irradiance variations and their interpretation are presented in this paper.