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
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 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.