Total and Spectral Solar Irradiance Sensor (Total & Spectral Solar Irradiance Sensor (TSIS)) - 12.06.17

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Science Objectives for Everyone
Total and Spectral Solar Irradiance Sensor (TSIS) measures total solar irradiance (TSI) and solar spectral irradiance (SSI). TSI helps establish Earth’s total energy input while SSI contributes to understanding how Earth’s atmosphere responds to solar output changes. Energy input minus outgoing energy determines Earth’s climate and energy from the Sun drives atmospheric and oceanic circulations on Earth. Knowing the magnitude and variability of solar irradiance is therefore essential to understanding Earth’s climate. Solar irradiance represents one of the longest climate data records derived from space-based observations – nearing 40 years of data – and researchers anticipate maintaining continuity of that record with TSIS.
Science Results for Everyone
Information Pending

The following content was provided by Peter Pilewskie, Ph.D., and is maintained in a database by the ISS Program Science Office.
Experiment Details


Principal Investigator(s)
Peter Pilewskie, Ph.D., University of Colorado, Boulder, CO, United States

Information Pending

University of Colorado, Laboratory for Atmospheric and Space Physics (LASP), Boulder, CO, United States

Sponsoring Space Agency
National Aeronautics and Space Administration (NASA)

Sponsoring Organization
NASA Research-SMD

Research Benefits
Earth Benefits, Scientific Discovery

ISS Expedition Duration
September 2017 - February 2018

Expeditions Assigned

Previous Missions
Information Pending

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Experiment Description

Research Overview

  • Total solar irradiance (TSI) describes the entirety of solar energy summed over all wavelengths.
  • Solar spectral irradiance (SSI) is a measure of the Sun’s energy in individual wavelength bands.
  • TSI is required for establishing the total energy input to Earth, constraining the energy budget. SSI is necessary to understand how the atmosphere and climate respond to the Sun’s changes. Both TSI and SSI are required measurements of Total and Spectral Solar Irradiance Sensor (TSIS), following recommendations from the Global Observing System for Climate, the U.S. Climate Change Science Program, the National Academy of Science, and other several US and international science organizations.
  • TSI and SSI provide fundamental boundary conditions for climate, atmospheric, chemical, and radiative transfer modeling.


Radiative energy from the Sun establishes the basic structure of the Earth’s surface and atmosphere and defines its external environment. Solar radiation is the Earth’s primary source of energy, exceeding by four orders of magnitude the next largest source, which is radioactive decay from the Earth’s interior. Solar radiation powers the complex and tightly coupled circulation dynamics, chemistry, and interactions among the atmosphere, oceans, ice, and land that maintain the terrestrial environment as humanity’s habitat. Natural variability on a wide range of temporal and spatial scales is ubiquitous in the Earth system, and this constant change combines with anthropogenic influences to define the net system state, in past, present, and future climates. A reliable, continuous record of solar irradiance is essential for understanding the Earth’s climate, how it changes and for attributing those changes to their underlying causes.
Although the difference between the Sun’s radiative forcing on the present climate compared to a pre-industrial baseline is small, seemingly small variations in solar irradiance represent large impacts on the total energy input to the Earth. Total Solar Irradiance (TSI) measurements provide the only quantitative record that reliably substitute the physical models used for estimating historical solar irradiances, which are essential for a definitive understanding of the historical record of climate change. Establishing the baseline provides the foundation for evaluating all other forcings of climate change, particularly those caused by human activities. All such forcings act to change the climate by perturbing the planetary radiation balance.
A reliable, continuous record of solar irradiance is essential for climate change understanding and attribution. NASA identified solar irradiance as one of 23 crucial measurements by the NASA EOS program and it is a designated Climate Data Record measured by TSIS. The Global Observing System for Climate GCOS, (a joint undertaking of the World Meteorological Organization, the Intergovernmental Oceanographic Commission of the United Nations Educational Scientific and Cultural Organization, the United Nations Environment Programme, and the International Council for Science) has designated solar irradiance (including both total and spectral) as one of 27 Global Essential Climate Variables and recommends its continuity in the Implementation Plan for the Global Observing System for Climate.
A continuous 39-year record of TSI exists from space-based observations. Evident in this combined record is an 11-year cycle with peak-to-peak amplitude of approximately 0.1% and larger variations that are associated with the short-term transits of active regions over the disk of the Sun. Variability in TSI occurs over a broad range of time scales, from day-to-day variations, to the 11-year solar cycle and longer. Because the Sun’s energy input to the Earth is so large, even the small relative fluctuations that occur during the 11-year solar activity cycle can cause detectable climate responses. Variability of similar magnitude likely occurs on longer time scales, and may have been the chief contributor to warming in the first half of the twentieth century. However, the amplitude of long-term change must be deduced indirectly from proxy records tied to the existing TSI data record, which is too short to fully identify the long-term physical mechanisms of solar variability. The Fourth Assessment Report of the Intergovernmental Panel on Climate Change estimates the direct radiative forcing due to changes in the solar output since 1750 to be 0.12 W m–2 (from its baseline value of 1361 W m-2) with a factor of two uncertainty and a low level of scientific understanding. Prior to 1750, the Maunder Minimum, which corresponded with the Little Ice Age in Europe, may have caused even greater changes in solar forcing.
The measurements made by individual radiometers providing the 39-year observational data record exhibit a spread of nearly 1% that is of instrumental rather than solar origin, far exceeding the 11-year or rotational solar variability. The individual TSI datasets from 1978 to the present time include observations made by ERB on Nimbus-7; ACRIM-I on SMM, ACRIM-II in UARS, and ACRIM III on ACRIMSAT; ERBS on the ERBE satellite; SOVA on EURICA; VIRGO on SOHO; TIM on SORCE; and PREMOS on PICARD. While instrument offsets are large, each instrument has high precision and is able to detect small changes in the TSI caused by variability in solar activity. Increases of 0.1% in TSI during times of high solar activity over the 11-year solar cycle are unambiguous. These data were all recorded with ambient temperature sensors, each of which has its own stated instrumental uncertainty, typically on the order of 0.1% (1000 ppm), with the exception of the Total Irradiance Monitor on SORCE, which has a 350 ppm uncertainty. Most of these instruments have internal degradation tracking methods, giving them the best stability of any on-orbit solar sensor so that long-term (secular) changes in solar variability can be monitored given measurement continuity.
Continuous measurements of solar ultraviolet radiation began in 1978 with the Nimbus-7 Solar Backscatter Ultraviolet (SBUV). These measurements were followed by those from the Solar Mesosphere Explorer (SME), NOAA-9 SBUV/2, NOAA-11 SBUV/2, the Upper Atmospheric Research Satellite (UARS) Solar Stellar Intercomparison Experiment (SOLSTICE) and the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM), and the present-day SORCE SOLSTICE and SORCE Spectral Irradiance Monitor (SIM), providing a continuous record of the solar ultraviolet and its variability, albeit with different spectral coverage, resolution, and instrumental accuracies and stabilities. Although the ultraviolet region of the spectrum provides only a small fraction of the TSI, ultraviolet irradiance changes over the solar cycle can be several percent, and thus represent an important source of modulation of the energy deposition and composition in the middle and upper atmosphere. This changes both the radiative balance of the atmosphere and affects the shape of the spectrum of radiation reaching the lower atmosphere. This area of inquiry is known as “top-down coupling”. 
The measurement of the full solar irradiance spectrum from space is a much shorter record than TSI, commencing with the SIM on SORCE in 2003. These observations include the visible through near-infrared portions of the spectrum, which form the primary contributions to “bottom-up” mechanisms of climate response. There are a number of open issues in the SSI observation record to be explored with Total and Spectral Solar Irradiance Sensor (TSIS), including higher than expected ultraviolet variability in SORCE observations that may have been compensated by opposing trends in other spectral bands.
TSIS maintains continuity of the TSI and SSI data records with unprecedented accuracy and instrument stability. For the first time, TSI during consecutive solar-cycle minima is measured with climate quality accuracy. This new record of SSI improves with the second-generation SIM and along with it, our understanding of the mechanisms of climate response to solar variability. Mounted on the EXPRESS Logistics Carrier 3 site 5, TSIS is to be launched in the SpaceX Dragon trunk externally on an Express Pallet Adapter using a Flight Releasable Attachment Mechanism.

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Space Applications
Improved understanding of solar variability at all wavelengths may enhance space weather predictions, including solar winds and geomagnetic storms. These predictions, which are developed from solar radiation measurements, could help protect humans and satellites in space as well as electric power transmissions and radio communications on the ground.

Earth Applications
Increased understanding of the energy that the Earth receives from the Sun is far-reaching and multidisciplinary, including a number of practical applications such as renewable energy and water resources.

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Operational Requirements and Protocols
TSIS acquires solar irradiance during the sunlit portion of all orbits. During eclipse, it acquires dark measurements required for correcting thermal offsets.

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Decadal Survey Recommendations

Information Pending

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Results/More Information

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Related Websites
Quick Facts: Total and Spectral Solar Irradiance Sensor (TSIS)

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