Hs. Ogawa et al., FIRST-YEAR CONTINUOUS SOLAR EUV IRRADIANCE FROM SOHO BY THE CELIAS SEM DURING 1996 SOLAR MINIMUM/, J GEO R-S P, 103(A1), 1998, pp. 1-6
The CELIAS/SEM photodiode spectrometer aboard SOHO continuously monito
rs the full-disk EUV solar irradiance in an 8-nm wavelength band cente
red 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 irradi
ances for the 1996 solar EUV minimum year at 1 AU. The uncertainty in
absolute flux for each channel is approximately +/-14%. The accuracy a
nd stability of the instrument make the data extremely useful in model
ing the upper terrestrial atmosphere during this period of ''low'' sol
ar activity. The data show evidence of persistent solar EUV/soft X ray
active regions throughout this solar minimum period which give rise t
o bath 27-day and short-term (minutes to hours) solar EUV irradiance v
ariations. The lowest value of solar flux in the first order 30.4-nm b
and occurred on November 6, 1996, with a photon flux of 9.8 x 10(9) cm
(-2) s(-1). Using previously obtained solar spectra, we infer a photon
flux of 4.7 x 10(9) cm(-2) s(-1) within a 1-nm bandpass centered on t
he solar He II 30.4-nm emission line at this time. The irradiance vari
ation 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 o
ccurred oil the same day (November 6, 1996) with an absolute flux of 2
.2 x 10(10) cm(-2) s(-1). When sharp increases of short-term flux vari
ability are ignored, a variation between +45% and -30% from the smooth
ed least squares fit to the central-order database is obtained. The lo
ng-term solar cycle variation during the 12-month smoothed data in bot
h channels indicates that the solar EUV minimum was reached during mid
1996. Large short-term sudden increases monitored by both channels co
rrespond to solar flares observed from the ground and from the GOES sa
tellites. New data for two isolated flares obtained from both CELIAS/S
EM channels are also presented and compared with GOES 0.1 to 0.8-nm so
il X ray data.