SHORT TERM, DIRECT INDICES OF SOLAR VARIABILITY J. L. LEAN E. O. Hulburt Center for Space Research Naval Research Laboratory, Washington, DC 20375, U.S.A. Received: 20 October 1999; Accepted in final form: 25 February 2000 Abstract. Indices of solar activity relevant for understanding and modelling solar irradiance variabil- ity are identified, and their temporal characteristics compared. Reproducing observed solar irradiance variability requires a minimum of two different types of indices — an index for irradiance depletion by sunspots and an index for global irradiance enhancement by faculae and network. When com- bined with appropriate wavelength-dependent parameterizations of sunspot and facular contrasts and center-to-limb functions, these indices permit the construction of empirical models of daily, monthly and annual solar total and spectral irradiances. The models are compared with observations at selected wavelengths and for the total irradiance. While the models replicate much of the rotational and 11-year cycle variance in contemporary irradiance databases, differences exist because of either the presence of variability mechanisms additional to solar magnetism, or of unresolved instrumen- tal effects in the databases. The reconstruction of solar irradiance in the past requires speculation about the extent of intercycle fluctuations in the global facular index, or in other, as yet unspecified, variability mechanisms. 1. Introduction A balance between incoming shortwave solar radiation (which peaks in the visible spectrum) and outgoing longwave terrestrial radiation (which peaks in the vicinity of 10 µm) establishes the equilibrium surface temperature of the Earth. The Sun’s irradiance at the top of the Earth’s atmosphere, shown in Figure 1, is therefore a critical determinant of Earth’s climate. Both the solar spectral irradiance variability, also shown in Figure 1, and the processes which facilitate climate response to solar radiative forcing, are strongly wavelength dependent. Comparison in Figure 1 of the spectrum of radiation incident at the top of the Earth’s atmosphere with that in- cident at the surface (0 km altitude) indicates considerable atmospheric absorption in the ultraviolet (UV) and near infrared (IR) spectral regions. Climate model simu- lations of Earth’s surface temperature response to solar variability in past centuries require knowledge of historical irradiance. Irradiance variability models based on solar activity indices estimate these inputs. Past climate change simulations have used reconstructions of total solar irradiance, alone; future global change studies are planned using reconstructions of the spectral irradiance changes that comprise total irradiance variability, at wavelengths from the UV to the far infrared IR. Space-based observations of total solar irradiance exist for only about twenty years. A composite record compiled recently from multiple, independent measure- Space Science Reviews 94: 39–51, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands.