An Analysis of the EUV Time Series from the First Order Channel of the SOHO CELIAS SEM Experiment

摘要

A 2.5 year time series produced by the first order channel (FOC) of the Solar Heliospheric Observatory (SOHO) CELIAS Solar Extreme Ultraviolet Monitor (SEM) (Hovestadt et al., 1995) is analyzed. The FOC optical path comprises two 150 nm Al filters, a transmission diffraction grating, and diode detector. It measures the solar EUV flux in an 10 nm range centered on the strong He II 30.4 nm transition region emission line. Using least-squares regression techniques, a six parameter model is found to account for about 92% of the time series variance. Instrumental effects include responsivity degradation and occasional contamination from energetic protons during solar proton flares. The solar irradiance portion of the FOC time series variation is explained by a linear combination of the Mg II core-to-wing ratio (Heath and Schlesinger, 1986) and the F_(10.7) radio flux (Tapping, 1987) proxies. The determined parameters from the fits predict the responsivity for future times as well as allow projection of the solar part of the SEM FOC signal for times where the Mg II and F_(10.7) proxies are available. The responsivity determination roughly agrees with measurements of a second SEM on two rockets flights in 1996 and 1997 (Judge et al., 1999). Using this solution, the decline SEM FOC responsivity is expected to be far more gradual in the next few years, falling by less than 5% in the three years 1999-2001.