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. The best diagnostic lines and
the relevant atomic data are discussed in detail. Radiances over the quiet Sun
are also considered, with comparisons with previous measurements. Some
departures in the shapes of the ground calibration responsivities are found at
the start of the mission. These shapes do not change significantly over time,
with the exception of the shorter wavelengths of the EIS short-wavelength (SW)
channel, which shows some degradation. The sensitivity of the SW channel at
longer wavelengths does not show significant degradation, while that of the
long-wavelength (LW) channel shows a significant degradation with time. 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.Comment: Accepted for publication in A&A (under minor revision
