Analysis of the Long‐Term Stability of TSI Instruments Using Allan Deviation and the Generation of a New TSI Composite

Abstract The dominant external forcing to Earth's climate system is solar radiation, referred to as the solar spectral irradiance (SSI) and the total solar irradiance (TSI), which is the integral of SSI over all wavelengths. Accurate measurements of TSI and SSI are made from space because solar radiation is absorbed within Earth's atmosphere and that absorption would otherwise need to be corrected for, with added uncertainty, in ground or airborne solar irradiance observations. Composite solar irradiance records, developed from multiple instruments, can represent solar forcing over timescales longer than the lifetime of any individual instrument. We develop a new TSI composite record from pairs of overlapping individual measurement records with the highest relative stability over the longest timescales as determined from Allan Deviation analysis. Allan Deviation is a time‐domain measurement standard analysis technique that was originally established by the metrology community to quantify the stability of atomic clocks. Version 0 of the Time Variance TSI (TV TSI) Composite begins in early 2003 and includes all Total Irradiance Monitor (TIM) instrument records beginning with the NASA Solar Radiation and Climate Experiment (SORCE) mission and continuing through today with the Total and Spectral Solar Irradiance Sensor (TSIS‐1) mission. In future work, we plan to use Allan Deviation analysis to guide the incorporation of additional TSI measurement records and their time‐dependent expression of uncertainties in order to develop a new version of the TV TSI composite that spans an even longer period of time for climate studies.