Sounding Rocket Absolute Full Disk Extreme Ultraviolet (EUV) Solar Flux Measurements
and the
Implications for the SOHO CELIAS/SEM EUV Data Base

ABSTRACT

    The SOHO Solar EUV Monitor (SEM) has been in operation since launch in December 1995. This instrument is a highly stable transmission grating solar extreme ultraviolet (EUV) spectrometer that has since launch made nearly continuous full disk solar irradiance measurements within an 8 nm bandpass centered at 30.4 nm, as well as broadband solar soft X-ray and EUV flux measuremens in the spectral region between 0.1 and 50 nm.

    Using a nominally identical SOHO SEM clone three sounding rocket SOHO calibration flights have been completed since June 1996 in order to assure that appropriately calibrated absolute data are being published on the Web and in the scientific literature. These sounding rocket calibration data and the absolute solar flux measured by the SOHO SEM from December 1995 up to the present are presented.






BACKGROUND

    An increased knowledge of the absolute solar flux and its variability in the EUV and X-ray regions over a long time scale is a sine qua non for a better understanding of the solar-terrestrial connection. Significant advances in our knowledge of the long term variation of solar radiation in the EUV and X-ray regions have in fact been achieved during the last few decades through satellite and sounding rocket observations (Schmidtke et al., 1992; Hinteregger et al., 1981; Heroux and Higgins, 1977; Kreplin and Horan, 1992; Timothy and Timothy, 1970). Nevertheless, the time variation and the absolute value of the solar radiation between 0.1 and 50 nm over the entire solar cycle is very poorly known.


SOHO CELIAS/SEM IN FLIGHT CALIBRATION

    Sounding rocket calibration underflight measurements of the 26 - 34 nm solar flux suggest that there has been a modest amount of degradation in the CELIAS/SEM instrument. A detailed model analysis has shown that carbon deposition on the optical surfaces could be responsible for the observed degradation. It is assumed that the carbon deposited within the instrument is due primarily to outgassing of hydrocarbons from the SOHO satellite. We have ruled out solar wind particles as a cause for degradation due to the very low deposit formation rate associated with untrapped solar wind particles.





THE SOHO CELIAS/SEM DATA BASE

    In recent years the CELIAS/SEM photodiode spectrometer (figures 1 and 2) on board SOHO has provided nearly continuous observations of the absolute solar flux in three channels comprising two distinct wavelength regions, 0.1 - 50 nm (central channel) and 26 - 34 nm (30.4 nm first order channels)(Judge et al., 1998; Ogawa et al., 1998).









SOLAR REFERENCE SPECTRUM

    The reference spectrum (Woods, Ogawa, Tobiska, and Farnik, 1998) is shown in figure 3. This composite solar EUV and XUV spectrum was constructed during the 1996 SOLERS22 Workshop. The data have been organized into 1 nm bins from 0.1 - 50 nm, centered every 0.5 nm. The spectrum is a composite of three spectra measured during low to moderate solar activity. The data were obtained from Kreplin and Horan, 1992 for the 0.1 - 2 nm irradiance, from Freeman and Jones, 1970 for the 2 - 5 nm irradiance and Van Tassel et al., 1981 for the 5 - 50 nm irradiance, for which the solar 10.7 cm radio fluxes (F10.7) were 70, 127, and 158 respectively.





CONCLUDING REMARKS

    The absolute solar EUV and soft X-ray flux in the spectral range from 0.1 to 50 nm has been determined in two bands: 0.1 - 50 nm and 26 - 34 nm to an accuracy of ~ 10% and a precision of ~ 1%. These (L1) observations provide the first such data from outside the Earth's magnetosphere and the first continuous data set (excluding a brief hiatus when SOHO controllers misdirected the spacecraft) from solar minimum to near solar maximum. It is accordingly now possible to produce critical tests of proxy models which provide estimates of the solar flux in the absence of direct measurements. These data, also provide a direct measure of the full disk solar flux which drives physical phenomena as diverse as the photochemistry of planetary atmospheres and ionization of the inflowing interstellar gas. The present absolute solar flux data thus make possible a quantitative analysis of the myriad solar driven phenomena observed in our heliosphere.


REFERENCES

 Heroux, L., and J.E. Higgins, Summary of Full-disk Solar fluxes Between 250 and 1940 Å, J. Geophys. Res., 82, 3307, 1977.
 Hinteregger, H.E., EUV Flux Variation During end of Solar Cycle 21, Observed from AE-C Satellite, Geophys. Res. Lett., 4, 231, 1977.
 Kreplin, R.W., The Solar Cycle Variation of Soft X-Ray Emission, Annales de Geophysique, 26, 567, 1970.
 Kreplin, R.W., K.P. Dere, D.M. Horan, and J.F. Meekins, The Solar Spectrum Below 10 Å in The Solar Output and Its Variation, edt by O.R. White, Colorado Associated University Press, Boulder, 287, 1977.
 Schmidtke, G., EUV Indices for Solar-Terrestrial Relations, Geophys. Res. Lett., 3, 573, 1976.
 Schmidtke, G., T.N. Woods, J. Worden, G.J. Rottman, H. Doll, C. Wita, and S.C. Solomon, Solar EUV Irriadiance from the San Marco ASSI: A Reference Spectrum, Geophys. Res. Lett., 19, 2175, 1992
 Timothy, A.F., and J.G. Timothy, Long Term Intensity Variations in the Solar Helium II Ly-alpha Line, J. Geophys. Res., 75, 6950, 1970.
 Woods, T., H. Ogawa, K. Tobiska, and F. Farnik, Solers 22 WG-4 and WG-5 Report for The 1996 Solers 22 Workshop, Solar Physics, 511, 1998.