1211.6771 (G. Del Zanna)
G. Del Zanna
A preliminary assessment of the in-flight radiometric calibration of the Hinode EUV Imaging Spectrometer (EIS) is presented. This is done with the line ratio technique applied to a wide range of observations of the quiet Sun, active regions and flares from 2006 until 2012. Radiances over the quiet Sun are also considered. The responsivity of the EIS short-wavelength (SW) channel does not show significant degradation, with the exception of its shorter wavelengths. The responsivity of the EIS long-wavelength (LW) channel instead shows an overall degradation with time, with values at the start of the mission already lower by 30% than those measured on the ground. Some departures in the shapes of the ground calibration responsivities are also found. The net effect is that by the beginning of 2010 the responsivity of the LW channel was already a factor of two or more lower than the values measured on the ground. A first-order correction is proposed. With this correction, the main ratios of lines in the two channels become constant to within a relative 20%, and the He II 256 A radiances over the quiet Sun also become constant over time. This correction removes long-standing discrepancies for a number of lines and ions, in particular those involving the strongest Fe X, Fe XIII, Fe XIV, Fe XVII, and Fe XXIV lines, where discrepancies of factors of more than two were found. These results have important implications for various EIS science analyses, in particular for measurements of temperatures, emission measures and elemental abundances.
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http://arxiv.org/abs/1211.6771
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