Summary

Solar irradiance provides the only significant source of energy input to the climate system and its variability has the potential to either mitigate or exacerbate anthropogenic change. Maintaining an unbroken record of Total Solar Irradiance (TSI) is critical in resolving ongoing debates regarding the potential role of solar variability in influencing Earth’s climate.

Space-borne instruments have acquired TSI data since 1978. Currently, the best calibrated and lowest noise source of TSI measurements is the Total Irradiance Monitor (TIM) onboard NASA’s Solar Radiation and Climate Experiment (SORCE). These TIM-era data are of higher quality than the older data in the full record. Thus, the TSI climate data record (CDR) has two components. There is the shorter, but more accurate record of the TIM era and the full (33+ year) space-based TSI measurement record. Both are important and require preservation.

SORCE is well past its design life and is encountering significant battery degradation. The Total Solar Irradiance Sensor (TSIS), a dual-instrument package that will be flown on the Joint Polar Satellite System (JPSS) Free Flyer 1 (FF-1) mission to continue the TSI record, is not scheduled to launch until late 2016 or early 2017, creating the potential for a data gap. Unfortunately, NASA’s Glory spacecraft, which carried a TIM that would have provided a gap filler, failed to reach orbit on March 4, 2011. Without a mitigation plan, the continuity of both data records (the TIM-era and full record) is threatened.

In examining options to avoid a gap in the record between SORCE and JPSS TSIS measurements, scientists at the Laboratory for Atmospheric and Space Physics (LASP), working with NASA and NOAA officials, identified an opportunity to include a TSI Calibration Transfer Experiment (TCTE) module, which includes a TIM sensor and electronics box, on an upcoming Air Force STPSat-3 launch. In early 2013, a NOAA Working Group developed a plan that describes how the agency anticipates mitigating the measurement gap through the TCTE mission.

The Committee found that the plan faithfully followed the scientific content of two studies conducted by Greg Kopp and Judith Lean for NOAA. The solution presented was a creative, rapid, and low-cost response that exploited the availability of an existing engineering instrument model, and heritage in engineering, mission architecture, and data analysis.

The CDR requirements can only be met when TCTE data overlap occurs at both ends of the gap (with both SORCE/TIM and JPSS FF-1/TSIS). The focus of the NOAA plan was on the shorter, more accurate TIM-era record, and the Committee concluded that the plan is unable to ensure the integrity of the TIM-era data record because as presented it is a 1.5 year plan to fill a 3+ year gap. The launch of the TCTE is currently scheduled for October 30, 2013, which will



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Summary Solar irradiance provides the only both data records (the TIM-era and full significant source of energy input to the record) is threatened. climate system and its variability has the In examining options to avoid a gap in potential to either mitigate or exacerbate the record between SORCE and JPSS TSIS anthropogenic change. Maintaining an measurements, scientists at the Laboratory unbroken record of Total Solar Irradiance for Atmospheric and Space Physics (LASP), (TSI) is critical in resolving ongoing debates working with NASA and NOAA officials, regarding the potential role of solar identified an opportunity to include a TSI variability in influencing Earth’s climate. Calibration Transfer Experiment (TCTE) Space-borne instruments have acquired module, which includes a TIM sensor and TSI data since 1978. Currently, the best electronics box, on an upcoming Air Force calibrated and lowest noise source of TSI STPSat-3 launch. In early 2013, a NOAA measurements is the Total Irradiance Working Group developed a plan that Monitor (TIM) onboard NASA’s SOlar describes how the agency anticipates Radiation and Climate Experiment mitigating the measurement gap through the (SORCE). These TIM-era data are of higher TCTE mission. quality than the older data in the full record. The Committee found that the plan Thus, the TSI climate data record (CDR) has faithfully followed the scientific content of two components. There is the shorter, but two studies conducted by Greg Kopp and more accurate record of the TIM era and the Judith Lean for NOAA. The solution full (33+ year) space-based TSI presented was a creative, rapid, and low-cost measurement record. Both are important response that exploited the availability of an and require preservation. existing engineering instrument model, and SORCE is well past its design life and is heritage in engineering, mission encountering significant battery architecture, and data analysis. degradation. The Total Solar Irradiance The CDR requirements can only be met Sensor (TSIS), a dual-instrument package when TCTE data overlap occurs at both that will be flown on the Joint Polar Satellite ends of the gap (with both SORCE/TIM and System (JPSS) Free Flyer 1 (FF-1) mission to JPSS FF-1/TSIS). The focus of the NOAA continue the TSI record, is not scheduled to plan was on the shorter, more accurate TIM- launch until late 2016 or early 2017, creating era record, and the Committee concluded the potential for a data gap. Unfortunately, that the plan is unable to ensure the integrity NASA’s Glory spacecraft, which carried a of the TIM-era data record because as TIM that would have provided a gap filler, presented it is a 1.5 year plan to fill a 3+ year failed to reach orbit on March 4, 2011. gap. The launch of the TCTE is currently Without a mitigation plan, the continuity of scheduled for October 30, 2013, which will 1

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2 Review of NOAA WG Report on Long-term Satellite Total Solar Irradiance Observation likely ensure overlap with SORCE. A 1.5 that the CDR requirements as posed year collection of data on orbit however represent requirements that were derived leaves a gap of more than a year between the from those relevant to understanding stated end of TCTE and beginning of the climate change. The Committee’s research TSIS on JPSS FF-1. Based on data furnished on the source of the requirements given by the spacecraft provider and LASP, the suggests they derive from empirical Committee found the likelihood of knowledge of solar variability and achieving overlap with the JPSS/TSIS is 0.56, instrumental capability and are less related or slightly better than 50 percent. If the to the energetics of the Earth system. To launch of the JPSS FF-1 is delayed, the determine the implications of these probability of overlap will decrease. requirements on the understanding of the Although the TCTE mission will not ensure Earth’s climate system, the Committee continuity of the TIM-era data record, it is considered two different pathways for more likely to ensure continuity of the full, setting these requirements based on climate lower quality data record. The Committee sensitivity, rather than on solar variability or does note that NOAA would be wise to instrumental capability. Coincidentally, the utilize all available data resources (e.g., outcome of the calculations made by the TCTE, other instruments, proxy models) to Committee agrees with the pre-defined CDR fill the gap. requirements. Hence, given that the Taken together, the NOAA Plan and the recommendations were based on these Kopp and Lean studies provided a balanced requirements, the Committee considers that discussion of strengths and weakness of the the plan, to an appreciable extent, explored proposed method to fill the TSI gap and the implications of loss of, or changes in, TSI recognized fully the limitations of TCTE. measurements on the understanding of The Committee was not initially convinced Earth’s climate system and processes.