Review of NOAA Working Group Report on Maintaining the Continuation of Long-term Satellite Total Solar Irradiance Observation (2013)

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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

Capabilities Lost in Program Restructuring. Ranking TSI measurements in the second highest of four tiers, the committee recommended that the “agencies should consider use of an appropriate combination of small, low-cost satellites and flights of opportunity to fly TSIS (or at least TIM) as needed to ensure overlap and continuity of the measurement of total solar irradiance.” A subsequent NPOESS restructuring assigned TSIS flight responsibility to the Joint Polar Satellite System (JPSS).

On March 4, 2011, NASA’s Glory spacecraft, which carried a TIM, failed to reach orbit after liftoff aboard a Taurus XL rocket. The best calibrated and lowest noise source of solar irradiance measurements used to support climate research and applications therefore remains the instruments onboard SORCE (Kopp and Lean, 2011), which is well past its design life and is encountering significant battery degradation. Without a mitigation plan, a gap in the record of TSI will occur, causing a serious obstruction to the creation of a consistent data record.

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, recently 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. However, once on orbit, the TCTE module would be the only solar-looking instrument among the payloads (the others are Earth-viewing) and would require re-orientation of the spacecraft. As a result, TSI measurements would not be made continuously.

To ensure effective transfer calibration between SORCE and TCTE, a daily observation with both instruments is necessary for 50 days, which corresponds to two solar rotation periods; a similar calibration period would be required to transfer calibration from TCTE to the TSIS that will be on the JPSS Free Flyer 1 (FF-1) mission. In the interim, weekly measurements may be used to maintain a measurement record. This option represents a feasible, current solution to maintaining a well-calibrated TSI record from space, although it relies on a less than ideal platform and will substantially decrease the number of solar observations that are made. In addition, neither the TCTE module nor the spacecraft bus on STPSat-3 are anticipated to have a long lifespan, with the likelihood of insufficient overlap with SORCE and/or FF-1, thus leaving the community with the prospect of a gap in TSI measurements in the event that both SORCE and TCTE end before the start of JPSS TSIS data collection.

STUDY CONTEXT AND CHARGE TO THE COMMITTEE

Maintaining an unbroken record of TSI is critical in resolving ongoing debates regarding the potential role of solar variability in influencing Earth’s climate. Available evidence indicates that solar irradiance variations are responsible for only a small part of overall climate forcing over the past 150 years but the magnitude is controversial and the record is not long enough to discern longer time scale processes that may play an important role in Earth’s climate. A gap in the record would also make the contribution of solar

variability to any observed changes to the climate system difficult to characterize. One of the most important roles of the TSI record has been as a null argument, providing evidence that it is not the Sun driving observed global warming. Without a reliable mechanism in place to measure/model TSI, it will be difficult for scientists to accurately assess the natural components of the Earth’s primary climate forcing agents. Finally, successive TSI instruments are calibrated in part against overlapping TSI measurements provided from reliable heritage instruments. Thus, if the TCTE mission proves to be unsuccessful, or short-lived, the resulting gap could introduce additional uncertainties in the calibration of future TSI instruments.

Based on two background papers, a NOAA Working Group developed a plan that describes how the agency anticipates mitigating the measurement gap. These two background papers and other relevant information from validated references are the basis for this NRC Committee’s independent evaluation of the NOAA plan (Box 1.1).

STUDY APPROACH AND METHODOLOGY

To carry out its charge, the committee held one in-person meeting during which they heard input from NOAA staff and the authors of the background papers upon

which NOAA based its plan. The committee reviewed the literature and other relevant documents, which are listed in the References section. During the rest of the review process and preparation of this report, interaction among the committee members was maintained via email and teleconference.