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The NASA Earth Observing System (EOS) is an advanced study of Earth’s long-term global changes of solid Earth, its atmosphere, and oceans and includes a coordinated collection of satellites, data systems, and modeling. The EOS program was conceived in the 1980s as part of NASA’s Earth System Enterprise (ESE). The Solar Radiation and Climate Experiment (SORCE) is one of about 20 missions planned for the EOS program, and the SORCE measurement objectives include the total solar irradiance (TSI) and solar spectral irradiance (SSI) that are two of the 24 key measurement parameters defined for the EOS program. The SORCE satellite was launched in January 2003, and its observations are improving the understanding and generating new inquiry regarding how and why solar variability occurs and how it affects Earth’s energy balance, atmosphere, and long-term climate changes.

The Solar Radiation and Climate Experiment (SORCE) satellite carries four scientific instruments that measure the solar radiation at the top of the Earth’s atmosphere. The mission is an important flight component of NASA’s Earth Observing System (EOS), which in turn is the major observational and scientific element of the U.S. Global Change Research Program. The scientific objectives of SORCE are to make daily measurements of the total solar irradiance and of spectral solar irradiance from 120 to 2000 nm with additional measurements of the energetic X-rays. Solar radiation provides the dominant energy source for the Earth system and detailed understanding of its variation is essential for atmospheric and climate studies. SORCE was launched on January 25, 2003 and has an expected lifetime through the next solar minimum in about 2007. The spacecraft and all instruments have operated flawlessly during the first 2 years, and this paper provides an overview of the mission and discusses the contributions that SORCE is making to improve understanding of the Sun’s influence on the Earth environment.

An array of empirical evidence in the space era, and in the past, suggests that climate responds to solar activity. The response mechanisms are thought to be some combination of direct surface heating, indirect processes involving UV radiation and the stratosphere, and modulation of internal climate system oscillations. A quantitative physical description is, as yet, lacking to explain the empirical evidence in terms of the known magnitude of solar radiative output changes and of climate sensitivity to these changes. Reproducing solar-induced decadal climate change requires faster and larger responses than general circulation models allow. Nor is the indirect climatic impact of solar-induced stratospheric change adequately understood, in part because of uncertainties in the vertical coupling of the stratosphere and troposphere. Accounting for solar effects on pre-industrial surface temperatures requires larger irradiance variations than present in the contemporary database, but evidence for significant secular irradiance change is ambiguous. Essential for future progress are reliable, extended observations of the solar radiative output changes that produce climate forcing. Twenty-five years after the beginning of continuous monitoring of the Sun’s total radiative output, the Solar Radiation and Climate Experiment (SORCE) commences a new generation of solar irradiance measurements with much expanded capabilities. Relative to historical solar observations SORCE monitors both total and spectral irradiance with significantly reduced uncertainty and increased repeatability, especially on long time scales. Spectral coverage expands beyond UV wavelengths to encompass the visible and near-IR regions that dominate the Sun’s radiative output. The space-based irradiance record, augmented now with the spectrum of the changes, facilitates improved characterization of magnetic sources of irradiance variability, and the detection of additional mechanisms. This understanding provides a scientific basis for estimating past and future irradiance variations, needed for detecting and predicting climate change.

Solar radiation is the primary energy source for many processes in Earth’s environment and is responsible for driving the atmospheric and oceanic circulation. The integrated strength and spectral distribution of solar radiation is modified from the space-based Solar Radiation and Climate (SORCE) measurements through scattering and absorption processes in the atmosphere and at the surface. Understanding how these processes perturb the distribution of radiative flux density is essential in determining the climate response to changes in concentration of various gases and aerosol particles from natural and anthropogenic sources, as is discerning their associated feedback mechanisms. The past decade has been witness to a tremendous effort to quantify the absorption of solar radiation by clouds and aerosol particles via airborne and space-based observations. Vastly improved measurement and modeling capabilities have enhanced our ability to quantify the radiative energy budget, yet gaps persist in our knowledge of some fundamental variables. This paper reviews some of the many advances in atmospheric solar radiative transfer as well as those areas where large uncertainties remain. The SORCE mission’s primary contribution to the energy budget studies is the specification of the solar total and spectral irradiance at the top of the atmosphere.

The Solar Radiation and Climate Experiment, SORCE, is a satellite carrying four scientific instruments that measure the total solar irradiance and the spectral irradiance from the ultraviolet to the infrared. The instruments were all developed by the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado, Boulder. The spacecraft carrying and accommodating the instruments was developed by Orbital Sciences Corporation in Dulles, Virginia. It is three-axis stabilized with a control system to point the instruments at the Sun, as well as the stars for calibration. SORCE was successfully launched from the Kennedy Space Center in Florida on 25 January 2003 aboard a Pegasus XL rocket. The anticipated lifetime is 5 years, with a goal of 6 years. SORCE is operated from the Mission Operations Center at LASP where all data are collected, processed, and distributed. This paper describes the SORCE spacecraft, integration and test, mission operations, and ground data system.

The Total Irradiance Monitor (TIM) instrument is designed to measure total solar irradiance with an absolute accuracy of 100 parts per million. Four electrical substitution radiometers behind precision apertures measure input radiant power while providing redundancy. Duty cycling the use of the radiometers tracks degradation of the nickel-phosphorous absorptive black radiometer interiors caused by solar exposure. Phase sensitive detection at the shutter frequency reduces noise and simplifies the estimate of the radiometer’s equivalence ratio. An as-designed uncertainty budget estimates the instrument’s accuracy goal. The TIM measurement equation defines the conversion from measured signal to solar irradiance.

The calibrations of the SORCE Total Irradiance Monitor (TIM) are detailed and compared against the designed uncertainty budget. Several primary calibrations were accomplished in the laboratory before launch, including the aperture area, applied radiometer power, and radiometer absorption efficiency. Other parameters are calibrated or tracked on orbit, including the electronic servo system gain, the radiometer sensitivity to background thermal emission, and the degradation of radiometer efficiency. The as-designed uncertainty budget is refined with knowledge from the on-orbit performance.

The solar observations from the Total Irradiance Monitor (TIM) are discussed since the SOlar Radiation and Climate Experiment (SORCE) launch in January 2003. The TIM measurements clearly show the background disk-integrated solar oscillations of generally less than 50 parts per million (ppm) amplitude over the ∼2 ppm instrument noise level. The total solar irradiance (TSI) from the TIM is about 1361 W/m, or 4–5 W/m lower than that measured by other current TSI instruments. This difference is not considered an instrument or calibration error. Comparisons with other instruments show excellent agreement of solar variability on a relative scale. The TIM observed the Sun during the extreme activity period extending from late October to early November 2003. During this period, the instrument recorded both the largest short-term decrease in the 25-year TSI record and also the first definitive detection of a solar flare in TSI, from which an integrated energy of roughly (6 ± 3) × 10 ergs from the 28 October 2003 X17 flare is estimated. The TIM has also recorded two planets transiting the Sun, although only the Venus transit on 8 June 2004 was definitive.

The Spectral Irradiance Monitor (SIM) is a dual Fèry prism spectrometer that employs 5 detectors per spectrometer channel to cover the wavelength range from 200 to 2700 nm. This instrument is used to monitor solar spectral variability throughout this wavelength region. Two identical, mirror-image, channels are used for redundancy and in-flight measurement of prism degradation. The primary detector for this instrument is an electrical substitution radiometer (ESR) designed to measure power levels ∼1000 times smaller than other radiometers used to measure TSI. The four complementary focal plane photodiodes are used in a fast-scan mode to acquire the solar spectrum, and the ESR calibrates their radiant sensitivity. Wavelength control is achieved by using a closed loop servo system that employs a linear charge coupled device (CCD) in the focal plane. This achieves 0.67 arcsec control of the prism rotation angle; this is equivalent to a wavelength positioning error of / = 150 parts per million (ppm). This paper will describe the scientific measurement requirements used for instrument design and implementation, instrument performance, and the in-flight instrument operation modes.

The Spectral Irradiance Monitor (SIM) is a satellite-borne spectrometer aboard the Solar Radiation and Climate Experiment (SORCE) that measures solar irradiance between 200 and 2700 nm. This instrument employs a Fèry prism as a dispersing element, an electrical substitution radiometer (ESR) as the primary detector, and four additional photodiode detectors for spectral scanning. Assembling unit level calibrations of critical components and expressing the sensitivity in terms of interrelated measurement equations supplies the instrument’s radiant response. The calibration and analysis of the spectrometer’s dispersive and transmissive properties, light aperture metrology, and detector characteristics provide the basis for these measurement equations. The values of critical calibration parameters, such as prism and detector response degradation, are re-measured throughout the mission to correct the ground-based calibration.