Comparison of the Sacramento Peak Fe XIV Index with a Model Index Computed from Differential Emission Measure Maps

We compare the Sacramento Peak Fe XIV 5303 Å green line index with a model index time series for the period of operations of the EUV Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory (SOHO), covering the years 1996-2002, from cycle minimum past the peak of the current activity cycle 23. We compute a differential emission measure (DEM) map for each day using images from the four channels of EIT at 171, 195, 284, and 304 Å. From the daily DEM map we then calculate a daily synthetic Fe XIV 5303 Å intensity image. The Sacramento Peak index is an average intensity, measured using a circular aperture 1.1 arcmin in diameter, sampling the off-limb corona in 3° steps around disk center. It is taken at several different heights beyond the daily white light limb. We modeled the daily index values, for the aperture center at 1.15 and 1.25 R_solar from disk center, as the weighted average intensity within an annulus covering 1.11-1.19 and 1.21-1.29 R_solar superposed on the daily synthetic intensity image. We compare the observed index with our model results and find a high correlation of the short-term values but a long-term systematic difference in the absolute values. We examine the accuracy of the respective calibrations and argue that the model results, based on the calibration of the EIT images used to produce the daily DEM maps, are more plausible in absolute value.