  One of the important open questions in solar irradiance studies is whether
long-term variability (i.e. on timescales of years and beyond) can be
reconstructed by means of models that describe short-term variability (i.e.
days) using solar proxies as inputs. Preminger and Walton (2005, GRL, 32,
14109) showed that the relationship between spectral solar irradiance and
proxies of magnetic-flux emergence, such as the daily sunspot area, can be
described in the framework of linear system theory by means of the impulse
response. We significantly refine that empirical model by removing spurious
solar-rotational effects and by including an additional term that captures
long-term variations. Our results show that long-term variability cannot be
reconstructed from the short-term response of the spectral irradiance, which
cautions the extension of solar proxy models to these timescales. In addition,
we find that the solar response is nonlinear in such a way that cannot be
corrected simply by applying a rescaling to sunspot area.
