When the National Oceanic and Atmospheric Administration (NOAA) sponsored the Total solar irradiance Calibration Transfer Experiment (TCTE) project launched November 2013, the solar science community was relieved a critical 36-year continuity of solar irradiance measurements was saved. This was accomplished by swiftly preparing a spare solar irradiance instrument for spaceflight.
"Initial results from TCTE are excellent," said William Denig, NOAA's project scientist for the first Total and Spectral Solar Irradiance Sensor (TSIS-1) at Boulder. TSIS-1 is currently planned for launch in 2017 to the International Space Station.
NOAA is supporting TSIS-1 which will measure spectral distribution of the solar irradiance as well as the total solar irradiance measurements that are now made by the Total Irradiance Monitor (TIM) instrument on TCTE. Following TSIS-1, NASA intends to resume support for the continuity of both total and spectral solar irradiance measurements with future TSIS missions.
The TIM for the TCTE project was launched in time to compare its solar irradiance measurements to those from the aging NASA Solar Radiation and Climate Experiment (SORCE), a satellite deployed in 2003 which is now well past its expected lifetime.
“TCTE is showing almost unbelievably good agreement with the SORCE instrument,” said Greg Kopp, a senior research scientist at the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado, Boulder and the instrument scientist for TCTE. “Especially considering that the two instruments were calibrated independently nearly a decade apart.”
Solar irradiance, or solar output from the sun, is measured by the TIM instrument on both SORCE and TCTE. Natural conditions on Earth such as the surface temperature or air temperature depend on energy that comes from the sun in the form of electromagnetic radiation.
Scientists have noted changes in the sun's energy by observing from Earth's surface for more than a hundred years, but were only able to begin to determine their magnitude and impact on Earth's climate with more accurate measurements from space, starting in 1978 with measurements of the total solar irradiance made by NASA's Nimbus 7 satellite. By continuing the total solar irradiance measurements from space, TCTE is designed to prevent a break in these measurements to determine how solar changes are influencing Earth's climate.
How TCTE Readied for Launch Quickly
The TIM instrument at the heart of the TCTE project is one of three nearly identical instruments built as part of NASA's investment in the SORCE mission for NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
For TCTE, the spare TIM instrument was modified for NOAA under a NASA contract. TCTE was envisioned to fill the possible measurement gap from SORCE until the next generation of solar sensors can be launched on TSIS-1. Along with the TIM, TSIS-1 will have a next-generation Spectral Irradiance Monitor to measure the spectral distribution of the solar irradiance.
At the time of the TCTE launch on November 19, 2013, the aging SORCE satellite was in a power-saving mode to preserve its extremely limited battery life. Scientists hoped both instruments would work simultaneously in space for at least 10 days to allow scientists on the ground to cross-calibrate the readings from the two nearly identical solar irradiance instruments. In late December 2013, the very important overlap took place when the SORCE computer was able to operate through a period of short eclipses to provide calibration opportunities.
Not only did the scientists get their wish for 10 days of simultaneous observations, but SORCE is now functioning again due to the programming efforts at LASP. In February, the project operations team at LASP reprogrammed the SORCE satellite to quickly power up its computer for all four SORCE instruments during each orbit when there is sunlight on SORCE's solar panels.
"This resurrected mode is meeting many of the mission's original objectives, albeit ones that are scaled back from the primary mission, and will continue into the foreseeable future," said Kopp. "SORCE was designed for a five-year life and is still functioning 11 years later. Because it could have a mission-ending failure without any advance notice, it is very comforting that we now have a second instrument in place and have been able to cross-calibrate the two."
In addition to SORCE lasting well beyond its expected lifetime, the TCTE team is benefitting from another advantage. The TIM instrument for the TCTE project was one of several instruments launched aboard the U.S. Air Force Space Test Program Satellite-3. While NOAA only required a single 100-minute observation per week, the Air Force provided an extended calibration period of two observations daily. The TIM on TCTE is currently conducting observations once weekly as originally planned, saving some mission cost while the SORCE instruments continue to function.
Why Scientists Need Solar Measurements
"The more frequent TCTE measurements allow tracking of solar variability with time, which improves the solar climate data record as well as comparisons with the TIM on SORCE," Kopp said. The science community is eager to continue the 36 years of uninterrupted total solar irradiance measurements initiated in 1978 with NASA's Nimbus 7 satellite.
These measurements are important because scientists and the public want to know how and why Earth’s climate is changing, said Judith Lean, senior scientist for sun-Earth system research at the Naval Research Laboratory in Washington, D.C. She said most researchers studying climate change over the last 150 years attribute at least 70 percent of the warming to greenhouse gases. Continued space observations of the sun will help researchers better determine the role of the sun versus greenhouse gases as a determining factor in climate change.
To align modern day space-based solar irradiance observations with models used to evaluate solar changes going back 150 years, well before the era of space-based solar observations, Lean has a NOAA grant in partnership with scientists at LASP to incorporate this information into the NOAA climate data record to make the information accessible to the public.
Lean pointed out the TIM on TCTE only measures total irradiance while climate models also need spectral irradiance. "We are getting spectral irradiance readings from SORCE, but the new TSIS is going to be so much better," she said.
For more information and data from TCTE, please visit, http://lasp.colorado.edu/home/tcte/.