FIRST SOLAR EUV IRRADIANCES OBTAINED FROM SOHO BY THE CELIAS/SEM D. L. JUDGE, D. R. McMULLINand H. S. OGAWA Department of Physics and Astronomy and Space Sciences Center, University of Southern California, Los Angeles, CA 90089-1341, U. S. A. D. HOVESTADT, B. KLECKER and M. HILCHENBACH Max-Planck Institut f¨ ur extraterrestrishe Physik, D-85740 Garching, Germany E. M ¨ OBIUS University of New Hampshire, Durham, New Hampshire 03824-3525, U.S.A. L. R. CANFIELD, R. E. VEST, R. WATTS andC. TARRIO Physics Laboratory, National Institute of Standards and Technology, Gaithersburg, MA 20899, U. S. A. M. K ¨ UHNE Physikalisch-Technische Bundesanstalt, D-10587 Berlin, Germany P. WURZ Physikalisches Institut, University of Bern, 3012 Bern, Switzerland (Received 3 December 1996; accepted 8 January 1997) Abstract. The first results obtained with the Solar EUV Monitor (SEM), part of the Charge, Element, and Isotope Analysis System (CELIAS) instrument, aboard the SOlar and Heliospheric Observatory (SOHO) satellite are presented. The instrument monitors the full-disk absolute value of the solar He II irradiance at 30.4 nm, and the full-disk absolute solar irradiance integrated between 0.1 nm and 77 nm. The SEM was first turned on December 15, 1995 and obtained ‘first light’ on December 16, 1995. At this time the SOHO spacecraft was close to the L-1 Lagrange point, 1 5 10 6 km from the Earth towards the Sun. The data obtained by the SEM during the first four and a half months of operation will be presented. Although the period of observation is near solar minimum, the SEM data reveal strong short-term solar irradiance variations in the broad-band, central image channel, which includes solar X-ray emissions. 1. Introduction The Solar EUV Monitor (SEM) is a highly stable transmission grating solar extreme ultraviolet (EUV) spectrometer that is integrated into the Charge, Element, and Isotope Analysis System (CELIAS) instrument which measures the mass, ionic charge, and energy of the low- and high-speed solar wind, suprathermal ions, and low energy solar flare particles (Hovestadt et al., 1995). The SEM solar irradiance data will not only supplement the CELIAS helium pick-up ion data by provid- ing the ionization rate of inflowing interstellar neutral helium atoms, but will also provide a reliable solar flux data base for inter-calibration of the Extreme ultraviolet Paper presented at the SOLERS22 International Workshop, held at the National Solar Observat- ory, Sacramento Peak, Sunspot, New Mexico, U.S.A., June 17–21, 1996. Solar Physics 177: 161–173, 1998. c 1998 Kluwer Academic Publishers. Printed in Belgium.