Solar irradiance variability: a six-year comparison between SORCE observations and the SATIRE model

¹ Astrophysics Group, Blackett Laboratory, Imperial College London, SW7 2AZ UK
e-mail: william.ball08@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
⁴ School of Space Research, Kyung Hee University, Yongin, Gyeonggi 446-701, Korea

Received: 23 November 2010
Accepted: 5 April 2011

Abstract

Aims. We investigate how well modeled solar irradiances agree with measurements from the SORCE satellite, both for total solar irradiance and broken down into spectral regions on timescales of several years.

Methods. We use the SATIRE model and compare modeled total solar irradiance (TSI) with TSI measurements over the period 25 February 2003 to 1 November 2009. Spectral solar irradiance over 200−1630 nm is compared with the SIM instrument on SORCE over the period 21 April 2004 to 1 November 2009. We discuss the overall change in flux and the rotational and long-term trends during this period of decline from moderate activity to the recent solar minimum in  ~10 nm bands and for three spectral regions of significant interest: the UV integrated over 200−300 nm, the visible over 400−691 nm and the IR between 972−1630 nm.

Results. The model captures 97% of the observed TSI variation. This is on the order at which TSI detectors agree with each other during the period considered. In the spectral comparison, rotational variability is well reproduced, especially between 400 and 1200 nm. The magnitude of change in the long-term trends is many times larger in SIM at almost all wavelengths while trends in SIM oppose SATIRE in the visible between 500 and 700 nm and again between 1000 and 1200 nm. We discuss the remaining issues with both SIM data and the identified limits of the model, particularly with the way facular contributions are dealt with, the limit of flux identification in MDI magnetograms during solar minimum and the model atmospheres in the IR employed by SATIRE. However, it is unlikely that improvements in these areas will significantly enhance the agreement in the long-term trends. This disagreement implies that some mechanism other than surface magnetism is causing SSI variations, in particular between 2004 and 2006, if the SIM data are correct. Since SATIRE was able to reproduce UV irradiance between 1991 and 2002 from UARS, either the solar mechanism for SSI variation fundamentally changed around the peak of cycle 23, or there is an inconsistency between UARS and SORCE UV measurements. We favour the second explanation.