The aim of the Working Group is to produce an absolutely
calibrated measure of the solar EUV irradiance, and to provide a
long-term record of the solar EUV irradiance and its
variability. This is accomplished by validating the EUV
irradiance products from various instruments, understanding
their calibration and degradation. The solar Extreme Ultraviolet
(EUV) radiation is totally absorbed in the Earth's atmosphere
and drives the photo-chemistry of the Earth's (and other
planets) upper atmosphere. Even though the EUV is only a small
fraction of the total solar irradiance, it is highly variable on
many time scales from minutes (solar flares), hours (flares and
active region evolution), days (solar rotation modulation) and
years (the 22 year magnetic cycle). The amount of variability
depends on the wavelength (and hence temperature) we are looking
at. The Total Solar Irradiance variation over an 11 year cycle
is ~0.1%. In the mid-UV (200 – 300 nm) the variability is ~1.5%
, at Lyman-α (121.6 nm) it is almost a f