Authors: H. S. Ogawa, D. L. Judge, D. R. McMullin, P. Gangopadhyay, A. B. Galvin
Reference: Journal Geophysical Research, 103,1-6,1998
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 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 band pass centered on the solar He II 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 data base for 1996. The lowest central-order
EUV photon flux occurred on 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
variability are ignored,a variation between +45% and -30% from the smoothed
least squares fit to the central-order data base 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.