Solar EUV and UV spectral irradiances and solar indices

Several experiments have measured solar EUV/UV flux in the last 10-15 years including SUSIM UARS, SOHO CELIAS SEM, and SOHO EIT and have generated multi-year spectral irradiance time series. Empirical models of these important sources of radiant energy are often based on solar activity proxies, most often, the solar 10.7 cm radio flux (F10.7). The short- and long-term correspondence of four solar activity index time series International Sunspot Number, the He 1083 Equivalent Width, F10.7, and the Mg II core-to-wing ratio are analyzed. All of these show well-correlated long-term behavior with F10.7 and Mg II showing the greatest long-term agreement among all of the index pairs. However, during the recent maximum period of solar cycle 23, both the ISN and He 1083 have diverged significantly from the others. Recent UV and EUV measurements are compared with Mg II and F10.7 to assess their value as solar activity proxies. In every case, Mg II was found to correlate more strongly than F10.7 with the UV and EUV time series which correspond to a range of solar atmospheric temperatures of 4000K-2 MK. This correspondence indicates that the mechanisms underlying irradiances changes from upper photospheric chromospheric, transition region, and lower coronal solar atmospheric layers are closely linked.