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Introduction

Solar irradiance provides the only significant source of energy input to the Earth’s climate system and its variability has the potential to either mitigate or exacerbate anthropogenic change. Because intrinsic atmospheric variability precludes solar irradiance measurement with sufficient accuracy and precision from Earth’s surface, a space-based record is essential for specifying solar forcing of climate. The current record of total solar irradiance, comprising reasonable overlapping time series of measurements from different spaceborne instruments, extends uninterrupted since 1978 and, although it is one of the longest continuous space-based climate records, it covers less than three 11-year solar activity cycles. To understand the Sun’s role in climate change, scientists need an uninterrupted irradiance record that extends over many, not just a few, solar activity cycles with sufficient precision to resolve long-term solar changes that may manifest from one activity cycle to the next. Additionally, this record is important to understanding the creation of magnetic fields on the Sun (Judge et al., 2012) and determining if the Sun is anomalous among similar stars (Shapiro et al., 2013). The magnitude of long-term solar irradiance change is highly uncertain because the observational record is, thus far, too short to reliably detect possible centennial-scale variations that may underlie the activity cycle. A reliable, uninterrupted, long-term solar irradiance record can also guide policy by constraining external climate forcing to plausible limits.

The Total Solar Irradiance Sensor (TSIS) is a dual-instrument package that was originally designed to measure solar irradiance on the National Polar-orbiting Operational Environmental Satellite System (NPOESS). It was de-manifested during the 2006 NPOESS restructuring, but was restored in 2008 to the first NPOESS satellite, because of its critical role in determining the natural forcing of the climate system and because of the high priority given by the 2007 National Research Council (NRC) Decadal Survey, Earth Science and Applications from Space. TSIS is comprised of the Total Irradiance Monitor (TIM), which measures the total solar irradiance (TSI) that is incident at the top of the atmosphere; and the Spectral Irradiance Monitor (SIM), which measures solar spectral irradiance (SSI) from 200 nm to 2400 nm (96 percent of the TSI). The TSIS TIM and SIM are successor instruments to those currently flying on NASA’s Solar Radiation and Climate Experiment (SORCE).

In 2008, the NRC Committee on a Strategy to Mitigate the Impact of Sensor Descopes and Demanifests on the NPOESS and GOES-R Spacecraft provided recommendations for continuing the total solar irradiance record in its report, Ensuring the Climate Record from the NPOESS and GOES-R Spacecraft: Elements of a Strategy to Recover Measurement



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1 Introduction Solar irradiance provides the only by constraining external climate forcing to significant source of energy input to the plausible limits. Earth’s climate system and its variability has The Total Solar Irradiance Sensor (TSIS) the potential to either mitigate or exacerbate is a dual-instrument package that was anthropogenic change. Because intrinsic originally designed to measure solar atmospheric variability precludes solar irradiance on the National Polar-orbiting irradiance measurement with sufficient Operational Environmental Satellite System accuracy and precision from Earth’s surface, (NPOESS). It was de-manifested during the a space-based record is essential for 2006 NPOESS restructuring, but was specifying solar forcing of climate. The restored in 2008 to the first NPOESS current record of total solar irradiance, satellite, because of its critical role in comprising reasonable overlapping time determining the natural forcing of the series of measurements from different space- climate system and because of the high borne instruments, extends uninterrupted priority given by the 2007 National Research since 1978 and, although it is one of the Council (NRC) Decadal Survey, Earth longest continuous space-based climate Science and Applications from Space. TSIS is records, it covers less than three 11-year comprised of the Total Irradiance Monitor solar activity cycles. To understand the Sun’s (TIM), which measures the total solar role in climate change, scientists need an irradiance (TSI) that is incident at the top of uninterrupted irradiance record that extends the atmosphere; and the Spectral Irradiance over many, not just a few, solar activity Monitor (SIM), which measures solar cycles with sufficient precision to resolve spectral irradiance (SSI) from 200 nm to long-term solar changes that may manifest 2400 nm (96 percent of the TSI). The TSIS from one activity cycle to the next. TIM and SIM are successor instruments to Additionally, this record is important to those currently flying on NASA’s SOlar understanding the creation of magnetic Radiation and Climate Experiment fields on the Sun (Judge et al., 2012) and (SORCE). determining if the Sun is anomalous among In 2008, the NRC Committee on a similar stars (Shapiro et al., 2013). The Strategy to Mitigate the Impact of Sensor magnitude of long-term solar irradiance Descopes and Demanifests on the NPOESS change is highly uncertain because the and GOES-R Spacecraft provided observational record is, thus far, too short to recommendations for continuing the total reliably detect possible centennial-scale solar irradiance record in its report, variations that may underlie the activity Ensuring the Climate Record from the cycle. A reliable, uninterrupted, long-term NPOESS and GOES-R Spacecraft: Elements solar irradiance record can also guide policy of a Strategy to Recover Measurement 3

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4 Review of NOAA WG Report on Long-term Satellite Total Solar Irradiance Observation Capabilities Lost in Program Restructuring. To ensure effective transfer calibration Ranking TSI measurements in the second between SORCE and TCTE, a daily highest of four tiers, the committee observation with both instruments is recommended that the “agencies should necessary for 50 days, which corresponds to consider use of an appropriate combination two solar rotation periods; a similar of small, low-cost satellites and flights of calibration period would be required to opportunity to fly TSIS (or at least TIM) as transfer calibration from TCTE to the TSIS needed to ensure overlap and continuity of that will be on the JPSS Free Flyer 1 (FF-1) the measurement of total solar irradiance.” mission. In the interim, weekly A subsequent NPOESS restructuring measurements may be used to maintain a assigned TSIS flight responsibility to the measurement record. This option represents Joint Polar Satellite System (JPSS). a feasible, current solution to maintaining a On March 4, 2011, NASA’s Glory well-calibrated TSI record from space, spacecraft, which carried a TIM, failed to although it relies on a less than ideal reach orbit after liftoff aboard a Taurus XL platform and will substantially decrease the rocket. The best calibrated and lowest noise number of solar observations that are made. source of solar irradiance measurements In addition, neither the TCTE module nor used to support climate research and the spacecraft bus on STPSat-3 are applications therefore remains the anticipated to have a long lifespan, with the instruments onboard SORCE (Kopp and likelihood of insufficient overlap with Lean, 2011), which is well past its design life SORCE and/or FF-1, thus leaving the and is encountering significant battery community with the prospect of a gap in TSI degradation. Without a mitigation plan, a measurements in the event that both SORCE gap in the record of TSI will occur, causing a and TCTE end before the start of JPSS TSIS serious obstruction to the creation of a data collection. consistent data record. In examining options to avoid a gap in STUDY CONTEXT AND CHARGE the record between SORCE and JPSS TSIS TO THE COMMITTEE measurements, scientists at the Laboratory for Atmospheric and Space Physics (LASP), Maintaining an unbroken record of TSI working with NASA and NOAA officials, is critical in resolving ongoing debates recently identified an opportunity to include regarding the potential role of solar a TSI Calibration Transfer Experiment variability in influencing Earth’s climate. (TCTE) module, which includes a TIM Available evidence indicates that solar sensor and electronics box, on an upcoming irradiance variations are responsible for only Air Force STPSat-3 launch. However, once a small part of overall climate forcing over on orbit, the TCTE module would be the the past 150 years but the magnitude is only solar-looking instrument among the controversial and the record is not long payloads (the others are Earth-viewing) and enough to discern longer time scale would require re-orientation of the processes that may play an important role in spacecraft. As a result, TSI measurements Earth’s climate. A gap in the record would would not be made continuously. also make the contribution of solar

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Introduction 5 BOX 1.1 Committee on Evaluating NOAA’s Plan to Mitigate the Loss of Total Solar Irradiance Measurements from Space Statement of Task Measurement of total solar irradiance is an important long-term climate record. An ad hoc committee appointed by the National Research Council will evaluate NOAA’s plan for mitigating the loss of total solar irradiance measurements from space, given the likelihood of losing this capacity from instruments currently on the SORCE satellite in coming years and the short- term/experimental nature of the currently identified method of filling the data gap (interim observations from TCTE). The committee will evaluate NOAA’s plan for mitigating the gap in total solar irradiance data (including consideration of two commissioned papers upon which NOAA is basing its plan). The committee’s evaluation will include consideration of:  Whether the plan appropriately reflects the scientific content of the commissioned papers,  Whether the potential alternate method in the plan maintains the integrity of the data record,  Whether the plan adequately summarizes the strengths and weaknesses of the proposed approach, and  Whether the background documents and plan together fully explore the implications of loss of or changes in measurement on the understanding of Earth’s climate system and processes. variability to any observed changes to the Based on two background papers, a climate system difficult to characterize. One NOAA Working Group developed a plan of the most important roles of the TSI that describes how the agency anticipates record has been as a null argument, mitigating the measurement gap. These two providing evidence that it is not the Sun background papers and other relevant driving observed global warming. Without a information from validated references are reliable mechanism in place to the basis for this NRC Committee’s measure/model TSI, it will be difficult for independent evaluation of the NOAA plan scientists to accurately assess the natural (Box 1.1). components of the Earth’s primary climate forcing agents. Finally, successive TSI instruments are calibrated in part against STUDY APPROACH AND overlapping TSI measurements provided METHODOLOGY from reliable heritage instruments. Thus, if the TCTE mission proves to be unsuccessful, To carry out its charge, the committee or short-lived, the resulting gap could held one in-person meeting during which introduce additional uncertainties in the they heard input from NOAA staff and the calibration of future TSI instruments. authors of the background papers upon

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6 Review of NOAA WG Report on Long-term Satellite Total Solar Irradiance Observation which NOAA based its plan. The committee review process and preparation of this reviewed the literature and other relevant report, interaction among the committee documents, which are listed in the members was maintained via email and References section. During the rest of the teleconference.