The Total and Spectral Solar Irradiance Sensor: Response to the National Academy of Science Decadal Strategy for Solar and Space Physics

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1 The Total and Spectral Solar Irradiance Sensor: Response to the National Academy of Science Decadal Strategy for Solar and Space Physics Peter Pilewskie, Greg Kopp, and Erik Richard Laboratory for Atmospheric and Space Physics, University of Colorado at Boulder 1. Introduction Robert Cahalan NASA Goddard Space Flight Center William F. Denig National Geophysical Data Center, NOAA/NESDIS Radiative energy from the Sun establishes the basic climate of the Earth s surface and atmosphere and defines the terrestrial environment that is humanity s habitat. Solar radiation powers the complex and tightly coupled atmospheric dynamics and chemistry, driving interactions among the atmosphere, oceans, ice, and land. External solar variability on a wide range of temporal and spatial scales ubiquitously affects the Earth system, and combines with forcings internal to the climate system, including anthropogenic changes in greenhouse gases and aerosols, natural modes such as ENSO, and volcanic forcing, to define past, present, and future climates. Understanding these effects requires: 1) Measurement continuity via a 2014 launch of the JPSS/TSIS; 2) climate model refinements using recently available total and spectral solar irradiances; and 3) interagency support. Although relative variations in the Sun s radiative inputs since the Industrial Revolution are small, the climate sensitivity uncertainties from solar forcing are comparable to those associated with greenhouse gases and aerosols [IPCC Fourth Assessment, 2007]. Seemingly small variations in solar irradiance represent large impacts on, or uncertainties in, the total energy input to the Earth. Total solar irradiance (TSI) measurements provide the only quantitative record constraining proxy and physical models used for estimating past solar irradiances that are essential for a definitive understanding of the historical record of climate change. Establishing the input solar energy baseline provides the foundation for evaluating all other climate forcings, including those caused by human activities. The current uninterrupted 32-year TSI climate data record is the result of several overlapping instruments flown on different missions. This record clearly exhibits variability over the 11 year solar cycle and on shorter time scales. Measurement continuity has allowed successive instruments to be linked to the existing data record despite offsets between instruments, but does make this important climate data record susceptible to loss in the event of a gap in measurements. Future improvements in instrument accuracy will reduce this risk, but a timely 2014 launch of the JPSS/TSIS, intended to provide continuity after the Glory/TIM, is critical. A newer record of solar spectral irradiance (SSI) commenced with the launch of the NASA SORCE mission in The measurement of SSI is vital for understanding how solar variability impacts climate and for validating climate model sensitivity to spectrally varying

2 solar forcing. These data also provide a basis for improving synthetic solar spectra, which can be used to model paleoclimate and validate hypotheses on past and future climate change. The continuous high-precision TSI record acquired from research missions over the last three decades constrains the total energy available to drive the climate system. The newer SSI measurements provide the details to elucidate the underlying mechanisms and terrestrial interactions responsible for Sun-induced climate changes. Both measurements are essential for constraining the energy input to the climate system and for interpreting the response of climate to external forcing. Prior decadal surveys have been influential in supporting these measurements with recommendations to move them from research to operational realms to guarantee their long-term continuity, but the ongoing transition to operations relies on further interdisciplinary and interagency support. Many solar irradiance missions are supported by NASA Earth Sciences programs with intent to transition to NOAA for operations. Meanwhile, the physical understanding of why the solar irradiance varies has been advanced primarily through support from Solar and Heliophysics programs at NASA, while climate modeling aspects are often supported by the NSF. Funding to continue and maintain this long-term solar data record is currently not well covered well by any one agency: NASA does not monitor long-term irradiances, but NOAA has yet to institute a solar monitoring capability, and the research aspects providing physical understanding are diversely spread and sporadic. It is critical that agencies work together to ensure that the measurements transition to operational status, and that research funding is provided for the long time periods needed for climate understanding. 2. Background Solar radiation is the Earth s primary source of energy, exceeding by four orders of magnitude the next largest source, radioactive decay from the Earth s interior [Sellers, 1965]. At the top of the atmosphere (TOA) the radiative balance between the incoming solar radiation and outgoing scattered solar and emitted infrared radiation defines the planet s radiative effective temperature of 255 K, which is significantly lower than the average surface temperature (288 K) because of the atmosphere s greenhouse effect. Energy from the Sun establishes the basic structure of the Earth s surface and atmosphere and defines its external environment. Solar radiation powers the complex and tightly coupled atmospheric circulation and chemistry, and the interactions among the atmosphere, oceans, ice, and land that maintain the terrestrial environment as humanity s habitat. Natural variability on a wide range of temporal and spatial scales is ubiquitous in the Earth system, and this constant change combines with anthropogenic influences to define the net system state in the past, present, and future. For this reason a reliable, continuous record of solar irradiance (total and spectral) is essential for climate change understanding and attribution, a fact that is recognized by a number of funding agencies and international organizations. For example, NASA identified solar irradiance as one of 23 crucial measurements for the NASA Earth Observing System (EOS) program over ten years ago [EOS Science Plan, 1999]. More recently, the Global Climate Observing System (GCOS, a joint undertaking of the World Meteorological Organization, the Intergovernmental 2

3 Oceanographic Commission of the United Nations Educational Scientific and Cultural Organization, the United Nations Environment Programme, and the International Council for Science) has designated solar irradiance (total and spectral) as one of 27 Global Essential Climate Variables and recommends its continuity in the Implementation Plan for the Global Observing System for Climate [2010]. Solar irradiance a designated Environmental Data Record (EDR) by the now-reorganized National Polar-orbiting Operational Environmental Satellite System (NPOESS) and remains a required Climate Data Record (CDR) under the NOAA-NASA Joint Polar Satellite System (JPSS). A continuous 32-year record of total solar irradiance exists from space-based observations, as shown in Figure 1. Evident in this combined record is an 11-year cycle with peak-to-peak amplitude of approximately 0.1% and variations a factor of two to three times greater associated with the short-term transits of sunspots over the disk of the Sun. Variability in TSI occurs over a broad range of time scales, from day-to-day variations through the 11-year solar cycle and longer. Because the Sun s energy input to the Earth is so large, even the small relative fluctuations that occur during the 11-year solar activity cycle can cause detectable climate responses [e.g., Haigh, 2003; van Loon et al., 2007]. Similar variability levels are likely to occur on longer time scales, and may have been the chief contributor to warming in the first half of the twentieth century [e.g., Tett et al., 2002; Stott et al., 2003]. However, the amplitude of long-term change must be deduced indirectly from proxy records tied to the existing TSI data record, which is too short to fully identify the long-term physical mechanisms of solar variability. The Fourth Assessment Report of the Intergovernmental Panel on Climate Change [IPCC AR4, 2007] estimates the direct radiative forcing due to changes in the solar output since 1750 to be 0.12 W m 2 (from its baseline value of 1361 W m -2 ) with a factor of two uncertainty and a low level of scientific understanding. Shortly prior to 1750, the Maunder Minimum, which corresponded with the Little Ice Age in Europe, may have caused even greater changes in solar forcing. Figure 1. The space-borne TSI database. Space-borne measurements of the TSI show ~0.1% variations with solar activity on 11-year and shorter time scales. Offsets between instruments are the results of calibration differences. 3

4 The measurements made by individual radiometers providing the data in Figure 1 [prepared by G. Kopp, 2010] exhibit a spread of nearly 1% that is of instrumental rather than solar origin and far exceeds the 11-year or rotational solar variability. The individual TSI datasets in Figure 1 from 1978 to the present time include observations made by ERB on Nimbus-7; ACRIM-I on SMM, ACRIM-II in UARS, and ACRIM III on ACRIMSAT; ERBS on the ERBE satellite; SOVA on EURICA; VIRGO on SOHO; and TIM on SORCE [Kyle et al., 1993, Willson, 1994, Froelich, 1994, Lee et al., 1995, Froelich, 1996, Willson, 2001, and Kopp et al., 2005]. While instrument offsets are large, each instrument has high precision and is able to detect small changes in the TSI caused by variability in solar activity. Increases of 0.1% in TSI during times of high solar activity over the 11-year solar cycle are unambiguous. Short term changes of 0.3% are directly attributable to variations in solar magnetic activity [Fligge, et al., 2000]. These data were all recorded with ambient temperature sensors, each of which has its own stated instrumental uncertainty, typically on the order of 0.1% (1000 ppm), with the exception of the Total Irradiance Monitor on SORCE, which has a 350 ppm uncertainty [Kopp et al., 2005]. Most of these instruments have internal degradation tracking methods, giving them the best stability of any on-orbit solar sensor so that long-term (secular) changes in solar variability can be monitored given measurement continuity. The differences between current on-orbit TSI measurements are approximately 0.35%, which significantly exceeds the true solar changes. A 2005 workshop conducted at the National Institute of Standards and Technology (NIST) in Gaithersburg [Butler et al., 2008] has led to investigations into the effects of diffraction and of aperture area measurements on the differences between instruments. Additional recommendations resulting from the workshop included power and irradiance calibrations, and comparisons with a standard cryogenic electrical substitution radiometer. In response to these issues, a new TSI Radiometer Facility (TRF) has been established to provide such calibrations [Kopp et al., 2007]. The TRF is the only facility in the world to allow direct irradiance comparisons between a TSI instrument and a reference cryogenic radiometer at full solar power levels and under flight-like vacuum conditions. New understanding of radiometer performance and the quantification of all sources of uncertainties will allow improved constructions of composite irradiance records for CDRs. In addition to total solar irradiance, reliable knowledge of solar spectrum changes is crucial for the irradiance Climate Data Record. Because of selective absorption and scattering processes in the Earth s atmosphere, different spectral regions contribute to the vertical distribution of deposited energy in distinct ways (Figure 2) [Pilewskie et al., 2005]. Various possible mechanisms of climate response to solar variability have been identified and are dependent on wavelength [Lean et al., 2005]. The solar radiation in the Figure 2. Top panel: SORCE Solar Irradiance Monitor [SIM; Harder et. al., 2005; Harder et al., 2009] top-of-atmosphere solar irradiance, downwelling spectral irradiance near surface, and at 10 m below ocean surface. Lower panel: Spectral optical thickness for model atmosphere showing major contributors through near-ultraviolet to near-infrared. 4

5 ultraviolet, at wavelengths shorter than 300 nm, plays a major role in the photochemistry, dynamics, temperature, composition, and structure of the middle atmosphere. The direct heating of the Earth s surface is a consequence of absorption of solar irradiance in the near ultraviolet, visible, and near infrared spectral regions. When averaged over the globe, roughly half of the incoming solar radiation is either absorbed in the atmosphere or scattered back into space, leaving the remaining half to be absorbed at the surface. Because scattering and absorption are wavelength-dependent, accurate knowledge of the spectral dependence of solar variability is essential to resolving many issues related to solar influences on climate and to determining the mechanisms of climate response to solar forcing. Solar spectral irradiance inputs are essential for the next generation of Global Climate Models (GCMs), which include parameterizations of atmospheric processes from the surface to the mesosphere on increasingly finer altitude resolution grids and with interactive ozone photochemistry. In particular, while the GCMs in IPCC AR4 primarily used the total solar irradiance to specify solar forcing of climate change, updated state-of-the art models planned for use in IPCC AR5 will require solar spectral irradiance binned onto their respective wavelength grids. The requirements for functional solar irradiance Climate Data Records therefore include accurate time series of total and spectral irradiance in the ultraviolet, visible and infrared spectrum on time scales of days to centuries. 3. History of the Total and Spectral Solar Irradiance Sensor The Total and Spectral Solar Irradiance Sensor (TSIS) is a dual-instrument package that will acquire solar irradiance in the present decade. The TSIS is comprised of the Total Irradiance Monitor, or TIM [Kopp et al., 2005], which measures the total solar irradiance that is incident at the top of the atmosphere, and the Spectral Irradiance Monitor, or SIM TIM [Harder et al., 2005], which measures solar spectral irradiance over 96% (in integrated irradiance) of the solar spectrum. The TSIS TIM and SIM are heritage instruments to those currently flying on the NASA Solar Radiation and Climate Experiment (SORCE). Both were selected as part of the TSIS because of their unprecedented measurement accuracy and stability, and because both measurements are essential to constraining the energy input to the Earth s climate system and for interpreting the response of climate to external forcing. The measurements of TSI and SSI from the TSIS were originally mandated under the National Polar-orbiting Operational Environmental Satellite System (NPOESS) in the 1990 s. TSIS was de-manifested from NPOESS following a 2006 restructuring, and re-manifested (as Government Furnished Equipment) in 2008 following a 2007 National Academy of Science workshop, Options to Ensure the Climate Record from the NPOESS and GOES-R Spacecraft. In early 2010 NPOESS underwent yet another major restructuring, with the National Oceanic and Atmospheric Administration taking responsibility for one of the two NPOESS polar orbits. NASA and NOAA have since established the Joint Polar Satellite System (JPSS), which will provide stewardship for TSIS and all other original NPOESS climate sensors. The TSIS instrument package underwent Preliminary Design Review in May, 2009, and Critical Design Review in December, It is on schedule for delivery for spacecraft integration in However, the spacecraft, launch vehicle, and launch date have yet to be determined. The JPSS afternoon satellite will utilize an identical spacecraft bus to that of the NASA NPOESS 5

6 Preparatory Project, scheduled for launch in September, In its current configuration, the JPSS bus cannot accommodate TSIS. Several options for flying TSIS have been studied, including an optimal Sun-pointing free-flyer option, but a final decision has yet to be made. Continuity with existing TSI missions, most notably the NASA Glory mission, demands that a TSIS flight decision is acted upon soon in order to meet a planned 2014 launch. As a free-flyer, TSIS is a small mission; estimated mission cost is $150M including launch, the TSIS TIM and SIM instruments, spacecraft, mission operations, and science data processing. The free-flyer concept utilizes a SORCE-like spacecraft, which lowers the overall cost, cost uncertainty, and mission risk. 4. TSIS Requirements The Solar Irradiance Environmental Data Record (EDR) requirements were specified in the original NPOESS Integrated Operational Requirements Document (IORD) and updated in version II [2002] and in the NPOESS Technical Requirements Document [2002]. Solar irradiance is now recognized as a Climate Data Record (CDR) rather than an EDR to distinguish it from higher latency, lower accuracy requirements of operational weather data. The requirement for TSI accuracy is 100 ppm, the same as it is for the NASA Glory mission. This is approaching a level that will help mitigate the risk of a data gap by reducing measurement offsets as shown in Figure 1 and improve our ability to link the Glory and TSIS TSI data records to future measurements, providing that the future measurements are made with similar or better accuracy. The current record relies on measurement overlap and good instrument stability to link independent measurements. The TSIS Level I requirements (see Table 1) appropriately reflect the TSI accuracy and instrument stability measurement requirements to determine long-term solar variability needed for climate studies. Similarly, the requirements for the newer SSI record address current needs for solar spectral irradiance to improve our understanding of the climate system response to solar variability over shorter-duration time scales. The NOAA Climate Data Record Program is updating and refining the requirements for all Global Essential Climate Variables based on improved understanding of the climate system and its response to observable trends, and to improvements in metrology. In addition to these ongoing activities, the Achieving Satellite Instrument Calibration for Climate Change (ASIC³) multi-agency sponsored workshop report [2007] provided recommendations for climate observing system requirements (including those for solar irradiance) for the next decade. Specification Total Irradiance Spectral Irradiance Measurement Range 1310 to 1410 W/m W/m 2 /nm Long Term Stability 0.001%/yr 400 nm: 0.05%/yr > 400 nm: 0.01%/yr Measurement Precision 0.002% 0.01% Measurement Accuracy 0.01% (0.15 W/m 2 ) 0.25% Spectral Range Entire spectrum nm < 280 nm: 2 nm Spectral Resolution Integrated 280 nm 400 nm: 5 nm > 400 nm: 45 nm Reporting Frequency 4 6-hourly averages/day 2 spectra/day Table 1. TSIS Level-1 requirements. 6

7 5. The Link to Heliophysics TSIS and the NASA solar irradiance missions before it (ERBE, ACRIMSAT, SORCE, and Glory) have been funded through the NASA Earth Science Division, and rightly so considering their emphasis on climate. The National Academy of Sciences Earth Science decadal survey [Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond, 2007] recommended the Glory and TSIS missions for continuity of the solar irradiance record. Recognizing the importance of these solar measurements, interdisciplinary connectivity, in particular in fields associated with the solar-terrestrial environment, is improving. The continuous measurement of solar irradiance, historically within NASA the domain of Earth science, has important ties to solar physics and space weather. Likewise, terrestrial climate change is now a component of the NASA Heliophysics division within the Living With a Star (LWS) Program. One important cross disciplinary activity funded by the LWS Program is in improving solar models and our understanding of spectral solar variability. For example, the Solar Radiation Physical Model (SRPM) has been developed over several years [Fontenla et al., 2009] and provides the necessary physical basis for calculating high resolution spectra and their variations directly from solar theory and analysis of solar images. Using ground-based images of the solar disk and a set of models of active- and quiet-sun features such as those of Fontenla and Harder [2005], the sources of the UV, visible, and IR spectral irradiance variation observed by SIM are being explored. Further studies are underway to investigate visible and infrared variations observed by SIM, and to improve modeling of SOLSTICE data in the MUV and FUV spectral regions, where large non-lte effects play an important role in solar emissions. The extension and improvement of these studies over the full rise and decay of a solar is needed for understanding the physics of the solar atmosphere in originating the observed SSI variations. This physical insight can guide much more reliable analyses of historical records and future expectations. Finally, it should be noted that while there is broad international support for the continuity of solar irradiance measurements from space, and that NASA Earth Science has supported and managed most of the missions, it appears that Research and Analysis (R&A) support for solar irradiance studies occasionally falls through the cracks, with no proper place to call home. NASA Earth and Heliophysics divisions both provide some opportunities for solar irradiancebased studies, but generally within broader programs and at a relatively low level of emphasis. We recommend that cross-division collaborative support within NASA is needed to better address the many aspects of the solar-terrestrial environment and to provide the necessary resources to answer some of the outstanding questions in Sun/Earth/global change climate studies. 7

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