First Solar EUV Irradiances Obtained from SOHO by the CELIAS/SEM

Authors: Judge, D.L., D.R. McMullin, H.S. Ogawa, D. Hovestadt, B. Klecker, M. Hilchenbach, E. Moebius, L.R. Canfield, R.E. Vest, R. Watts, C. Tarrio, M. Khne, and P. Wurz
Reference: Solar Physics 177,161-173. 1998

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 x 106 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 absolute solar EUV irradiance data. The uncertainty in absolute flux for each channel is approximately " 14% with a maximum noise level of < 0.5 %. The data show evidence of persistent solar EUV/soft X-ray active regions throughout this solar minimum period which give rise to both 27-day and short- term (minutes to hours) solar EUV irradiance variations. The irradiance variation of the first-order channel was between + 15% and -10% as measured from a smoothed quadratic least squares fit to the entire first-order channel data base. The minimum flux value, 1.1 x 10^10 cm-2 s-1, of the quadratic fit occurs on November, 6, 1996 (DOY 311). When sharp increases of short-term flux variability are ignored, a variation between +45% and -30% from the smoothed least squares fit to the central-order data base is obtained. The minimum flux value, 3.1 x 10^10 cm-2 s-1, of the quadratic fit occurs on July 2, 1996 (DOY 184). The long-term solar cycle variation during the 15 month smoothed data in both channels indicates that the solar EUV minimum was reached during the latter half of 1996. Large short-term sudden increases monitored by both channels correspond to solar flares observed from the ground and from the GOES satellites. Two of the largest isolated flares will be presented. An increase of nearly a factor of 7 for a M-class flare, and an increase greater than a factor of 13 for a X-class flare was observed in the central-order channel between the pre-flash and flash peak fluxes. Modest increases of 11% and 23%, respectively, were observed in the narrow 30.4 nm band for the same flares.

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Last Update: April 2, 1999, James Weygand