  The solar EUV irradiance is of key importance for space weather. Most of the
time, however, surrogate quantities such as EUV indices have to be used by lack
of continuous and spectrally resolved measurements of the irradiance. The
ability of such proxies to reproduce the irradiance from different solar
atmospheric layers is usually investigated by comparing patterns of temporal
correlations. We consider instead a statistical approach. The TIMED/SEE
experiment, which has been continuously operating since Feb. 2002, allows for
the first time to compare in a statistical manner the EUV spectral irradiance
to five EUV proxies: the sunspot number, the f10.7, Ca K, and Mg II indices,
and the He I equivalent width.
  Using multivariate statistical methods such as multidimensional scaling, we
represent in a single graph the measure of relatedness between these indices
and various strong spectral lines. The ability of each index to reproduce the
EUV irradiance is discussed; it is shown why so few lines can be effectively
reconstructed from them. All indices exhibit comparable performance, apart from
the sunspot number, which is the least appropriate. No single index can
satisfactorily describe both the level of variability on time scales beyond 27
days, and relative changes of irradiance on shorter time scales.
