Spectral irradiance variations: comparison between observations and the SATIRE model on solar rotation time scales

¹ Astrophysics Group, Blackett Laboratory, Imperial College London, SW7 2AZ, UK e-mail: y.unruh@imperial.ac.uk
² Max-Planck-Institut für Sonnensystemforschung, 37191 Katlenburg-Lindau, Germany
³ Laboratory for Atmospheric and Space Physics, 1234 Innovation Drive, Boulder, Colorado 80303-7814, USA

Received: 4 August 2007
Accepted: 27 February 2008

Abstract

Aims. We test the reliability of the observed and calculated spectral irradiance variations between 200 and 1600 nm over a time span of three solar rotations in 2004.

Methods. We compare our model calculations to spectral irradiance observations taken with SORCE/SIM, SoHO/VIRGO, and UARS/SUSIM. The calculations assume LTE and are based on the SATIRE (Spectral And Total Irradiance REconstruction) model. We analyse the variability as a function of wavelength and present time series in a number of selected wavelength regions covering the UV to the NIR. We also show the facular and spot contributions to the total calculated variability.

Results. In most wavelength regions, the variability agrees well between all sets of observations and the model calculations. The model does particularly well between 400 and 1300 nm, but fails below 220 nm, as well as for some of the strong NUV lines. Our calculations clearly show the shift from faculae-dominated variability in the NUV to spot-dominated variability above approximately 400 nm. We also discuss some of the remaining problems, such as the low sensitivity of SUSIM and SORCE for wavelengths between approximately 310 and 350 nm, where currently the model calculations still provide the best estimates of solar variability.