Research on the causes and effects of climate change is primarily focused on the many different elements of the Earth’s interconnected systems: atmosphere, hydrosphere, biosphere, and even the lithosphere. For that reason, most of NASA’s earth observation satellites make their measurements by looking downward at the Earth from their vantage points in orbit. The NASA Goddard Earth Sciences Data and Information Services Center (GES DISC) is the archive of data from many of these missions.
However, the GES DISC also holds data from a mission that looks away from the Earth, toward the vital source of energy to the Earth system – the Sun. This mission is the Solar Radiation and Climate Experiment (SORCE), which was launched to collect a continuous record of the Sun’s Total Solar Irradiance (TSI) and Spectral Solar Irradiance (SSI). The SORCE spacecraft carries four observational instruments: Total Irradiance Monitor (TIM), Solar Stellar Irradiance Comparison Experiment (SOLSTICE), Solar Irradiance Monitor (SIM), and soft X-ray Ultraviolet Photometer System (XPS).
Like most NASA satellite missions, the SORCE mission commenced with launch, a nail-biting, heart-stopping display of sound and light and motion. Following launch and entry into orbit, for every mission, the check-out phase begins. Solar panels unfold to provide power for the instruments and satellite systems. Each subsystem of the satellite is powered up and tested. Hatches covering sensitive, technologically advanced detection systems are opened, allowing observations to begin. The satellite sends its first data down to the ground station, where they are examined by engineers and scientists. Calibration adjustments are radioed back to the satellite, which uses them to refine the observations made by its instruments. After all of those operations and many others are performed, the satellite can get down to the business that it was launched for – making observations and collecting data.
For SORCE, launch day was January 25, 2003. The check-out phase of the mission extended to March 5, 2003. On March 6, 2003, the normal operations phase of the mission began.
SORCE mission personnel cited two top achievements of the mission as the following:
- SORCE established a new level of total solar irradiance (TSI) at the top of Earth's atmosphere of 1360.8 W/m2 , which is 0.34% (4.6 W/m2 ) lower than previously measured (Kopp and Lean, 2011), during the recent solar minimum.
- SORCE acquired the first continuous measurements of solar spectral irradiance (SSI) in the 115 to 2400 nanometer spectral range.
The SORCE mission is managed by the University of Colorado’s Laboratory for Atmospheric and Space Physics (LASP). From the beginning of the mission, LASP has partnered with the GES DISC for the archive and distribution of all public SORCE Level 3 data products. Initial data processing of downlinked SORCE data takes place at LASP, and the processed data are then sent to the GES DISC. The GES DISC features a Web site dedicated to SORCE mission data: http://disc.sci.gsfc.nasa.gov/SORCE
A graphic image puts the data record from SORCE in context with other solar TSI observations:
Just as January 25, 2013 marked the 10th anniversary of the start of the SORCE mission, March 6, 2013 marks the 10th anniversary of the start of normal mission operations — and thus the initiation of the full SORCE instrument data stream. The GES DISC provides five Level 3 SSI data products and two Level 3 TSI data products, listed below:
- SORCE Solar Spectral Irradiance Daily Means (310-2400 nm range at 1-27 nm intervals)
- SORCE SOLSTICE Far-Ultraviolet (FUV) Solar Spectral Irradiance Daily Means (115-180 nm range at 1 nm intervals)
- SORCE SOLSTICE Mid-Ultraviolet (MUV) Solar Spectral Irradiance Daily Means (180-310 nm range at 1 nm intervals)
- SORCE XPS Solar Spectral Irradiance 6-Hour Means & Daily Means (0.1 - 27nm, and 121nm)
- SORCE Total Solar Irradiance 6-Hour Means & Daily Means
In addition to the data products, the GES DISC provides software tools for the analysis of the SORCE data.
One of the primary results of the SORCE mission is the daily record of Total Solar Irradiance (TSI), above the atmosphere of the Earth. TSI is a critical, fundamental variable for the Earth’s climate system, because even small variations in the amount of energy received by the Earth globe from the Sun can affect Earth’s climate and weather systems. TSI is slightly lower during solar minimum periods and higher during solar maximum periods, with higher variability evident during solar maximum periods. Solar flares and sunspots, which occur more frequently during solar maxima, cause measurable variability in TSI.
The figure accompanying this article demonstrates the variability of TSI with the solar cycle. SORCE was launched during a solar maximum period; as the mission progressed, the Sun moved into a solar minimum period that was considerably longer than predicted. The next cycle did finally get started, and solar activity is now nearing its apex for this current cycle (though current observations suggest that the maximum may have a double peak). The TSI daily record demonstrates this clearly, with higher values and increased variability at both ends of the 10-year record, and a period of quiescence in the middle. The TSI record is shown with a backdrop of two NASA Solar Dynamics Observatory (SDO) images of the Sun acquired in October 2010 and October 2012, showing an increase in the surface activity of the Sun over those two years. The red arrows on the TSI plot show the October 2010 and October 2012 locations in the decade-long data record. An artist’s depiction of the SORCE satellite in orbit is shown at the top.
SORCE has made many significant discoveries in its 10-year mission, several of which are highlighted in an article in the NASA January-February 2013 issue of The Earth Observer (PDF)
. As a partner of this successful mission, the GES DISC is pleased to provide the public with SORCE data, which give scientists a unique understanding of how the Sun varies both slowly and rapidly, affecting Earth’s weather and climate systems and ultimately all the life on Earth that depends on the flow of energy from the solar orb.
Kopp, G. and Lean, J.L. (2011) A New, Lower Value of Total Solar Irradiance: Evidence and Climate Significance. Geophysical Research Letters Frontier article, Vol. 38, L01706, doi:10.1029/2010GL045777.
Article by James Acker. Editing by Bill Teng, additional information provided by Suraiya Ahmad. Web formatting by James Acker.
The SORCE mission is managed by the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado-Boulder. Tom Woods is the SORCE Principal Investigator. Robert Cahalan is the SORCE Project Scientist at Goddard Space Flight Center. James Johnson and Irina Gerasimov of the GES DISC manage the SORCE data archive.
The GES DISC is a NASA earth science data center, part of the NASA Earth Science Data and Information System (ESDIS) Project.