First-year continuous solar EUV irradiance from SOHO by the CELIAS/SEM during 1996 solar minimum

Abstract

The CELIAS/SEM photodiode spectrometer aboard SOHO continuously monitors the full-disk EUV solar irradiance in an 8-nm wavelength band centered at 30.4 nm (first order), and in a broad wavelength band between 0.1 and 77 nm (central order). We present the absolute solar EUV irradiances for the 1996 solar EUV minimum year at 1 AU. The uncertainty in absolute flux for each channel is approximately +/-14%. The accuracy and stability of the instrument make the data extremely useful in modeling the upper terrestrial atmosphere during this period of ``low'' solar activity. 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 lowest value of solar flux in the first order 30.4-nm band occurred on November 6, 1996, with a photon flux of 9.8×10⁹cm^-2s^-1. Using previously obtained solar spectra, we infer a photon flux of 4.7×10⁹cm^-2s^-1 within a 1-nm bandpass centered on the solar HeII 30.4-nm emission line at this time. 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 database for 1996. The lowest central-order EUV photon flux occurred on the same day (November 6, 1996) with an absolute flux of 2.2×10¹⁰cm^-2s^-1. 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 database is obtained. The long-term solar cycle variation during the 12-month smoothed data in both channels indicates that the solar EUV minimum was reached during mid 1996. Large short-term sudden increases monitored by both channels correspond to solar flares observed from the ground and from the GOES satellites. New data for two isolated flares obtained from both CELIAS/SEM channels are also presented and compared with GOES 0.1 to 0.8-nm soft X ray data.