Empirical orthogonal function (EOF) analysis is applied to solar spectral irradiance (SSI) from the SIM (Spectral Irradiance Monitor) instrument on-board the SORCE (Solar Radiation and Climate Experiment) satellite. The SORCE spacecraft was launched on January 25, 2003 into a near-circular Earth orbit with inclination angle of 40 degrees to the equator and altitude near 645km. SIM measures solar spectral irradiance in the visible and near-infrared primarily from 300 nm to 2000 nm. With an additional channel from 200 nm to 300 nm, SIM observes SSI from 200 nm to 2000 nm, providing accurate daily average SSI.
EOF analysis has been widely used in meteorology and climate research. This paper demonstrates that EOF is a powerful tool for analyzing SSI. It is evident that the SSI from SIM covers almost the entire solar energy spectrum. However solar spectral irradiances at each individual wavelength are not independent from each other. It is known that SSI in different parts of the electromagnetic spectrum originates from different layers of the solar atmosphere. The degree of correlation between SSI in different spectral regions provides a measure of interactions among different physical mechanisms. Since solar spectral irradiances from same layers or same regions are well correlated, only a small number of principal components are needed to describe and understand the physical processes. This paper will describe the principal components or eigenfunctions of SSI from SIM and associated eigenvalues. The EOF eigenvalue spectra are compared with an analytical noise spectrum to determine the significant principal components. The principal components are used to examine the characteristics of SSI as a function of time.