|Active Cavity Radiometer Irradiance Monitor
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Total Solar Irradiance (TSI) Monitoring
Figure 1 TSI monitoring by satellite experiments
Updated April, 2013
The Earth’s weather and climate regime is determined by the total solar irradiance (TSI) and its interactions with the Earth’s atmosphere, oceans and landmasses. Evidence from both 34 years of direct satellite monitoring and historical proxy data leaves no doubt that solar luminosity in general, and TSI in particular, are intrinsically variable phenomena. Subtle variations of TSI resulting from periodic changes in the Earth's orbit (Milankovich cycles: ~20, 40 and 100 Kyrs) cause climate change ranging from major ice ages to the present inter-glacial, clearly demonstrating the dominance of TSI in climate change on long timescales. TSI monitoring, cosmogenic isotope analyses and correlative climate data indicate that variations of the TSI have been a significant climate forcing during the current inter-glacial period (the last ~ 10 Kyrs.). Phenomenological analyses of satellite TSI monitoring results, TSI proxies during the past 400 years and the records of surface temperature show that TSI variation has been the dominant forcing for climate change during the industrial era. The periodic character of the TSI record indicates that solar forcing of climate change will likely be the dominant variable contributor to climate change in the future.
A series of Active Cavity Radiometers (ACRs), a new generation of sensors with the precision required for compiling a long term TSI database for climate, were developed at the Jet Propulsion Laboratory (JPL) of the California Institute of Technology under the direction of ACRIM Principal Investigator, Dr. Richard C. Willson. Their use in a series of Active Cavity Radiometer Irradiance Monitor (ACRIM) space flight experiments has provided a precise and traceable component of the TSI database during more than 90 % of its 34 year history. The ACRIM Science Team moved its operation to Columbia University in 1995 and then back to the Jet Propulsion Laboratory under contract in 2008. The ACRIM Instrument Team, directed by ACRIMSAT Program Manager Sandy Kwan, operates the ACRIMSAT satellite, ACRIM instrument and ground telemetry instrumentation at JPL.
A contiguous TSI database of satellite observations extends from late 1978 to the present, covering more than three solar magnetic cycles. It's comprised of the observations of seven independent experiments: Nimbus7/ERB1, SMM/ACRIM12, ERBS/ERBE3, UARS/ACRIM24, SOHO/VIRGO5, ACRIMSAT/ACRIM36 and SORCE/TIM7. A composite database combining these results using overlapping, in-flight comparisons has begun to provide new insights into the variability of total solar irradiance and its implications for climate change.
Monitoring TSI variability is clearly an important component of climate change research, particularly in the context of understanding the relative forcings of natural and anthropogenic processes. The requirements for a long-term, climate TSI database can be inferred from a National Research Council study which concluded that gradual variations in solar luminosity of as little as 0.1 % was the likely forcing for the ‘little ice age’ that persisted in varying degree from the late 14th to the mid 19th centuries. A centuries-long TSI database will have to be calibrated by either precision or accuracy to a small fraction of this value to be of use in assessing the magnitude of solar forcing. The current TSI database is shown in Figure 1.
Instrumentation used in spaceflight TSI monitoring to date has utilized sensors operating at ambient temperature (~ 20C). They provide the only practicable satellite instrument technology presently available for extended flight experiments. The ‘native scale’, on which the results of each experiment’s TSI observations are reported, is based on the metrology of their individual cavity sensor properties in the International System of units (SI).
The de-facto, redundant, overlap TSI monitoring approach that has provided a contiguous record since 1978 resulted from the deployment of multiple, overlapping TSI satellite experiments. The traceability of this database is at the mutual precision level of overlapping experiments. This is typically orders of magnitude smaller than the ‘absolute uncertainty’ of observations in the international system of units (SI). ACRIM3 results have demonstrated a residual annual traceability of ~ 5 ppm during its 13 year mission. A carefully implemented redundant, overlap strategy should therefore be capable of producing a climate timescale (decades to centuries and longer) TSI record with useful traceability for assessing climate response to TSI variation.
• A redundant, overlapping TSI measurement strategy using existing ‘ambient temperature’ instrumentation can provide the long term traceability required by a TSI database for climate change on climate time scales.
The state of the art measurement uncertainty for flight observations on an ‘absolute scale’ in the international system of units (SI) has not been demonstrated to be significantly less than 1000 parts per million (ppm). The results of TSI monitoring experiments are reported on their 'native scales' as defined in SI by the ‘self-calibration’ features of their sensor technologies. Systematic uncertainties in the metrology used to relate their observations to SI caused the ± 0.25 % spread of results during the first decade of monitoring. The tighter clustering of results after 1990 is attributable to dissemination of more accurate sensor metrology among the various experiments and national standards labs.
A new approach to calibrating TSI sensors has been developed by several laboratories including Absolute Cryogenic Infrared Radiometry at the at the National Institute of Standards and Technology (NIST) LBIR facility and the TSI Radiometer Facility (TRF) at the Laboratory for Atmospheric and Space Physics (LASP) of the University of Colorado. High powered lasers are calibrated in SI units using self-calibrating cryogenic irradiance detectors, similar in design to the self-calibrating ambient temperature sensors employed by satellite TSI monitors, but operated at LHe temperature. Self-calibrating irradiance sensors are thermal detectors that compare the heating effects of solar irradiance and electrical heating on a cavity detector. The uncertainty of their ability to define irradiance in SI units is temperature dependent. When cooled to LHe temperatures self-calibrating irradiance sensors can define irradiance at the 1 TSI level with uncertainties approaching a few hundred ppm. The calibrated lasers are used as transfer standards to irradiate ambient temperature satellite TSI sensors and compare their basic 'self-calibrated' SI scales to that defined by the LHe cryogenic detector's. The effects of scattering and diffraction on sensor calibrations can also be determined by varying the beam size of the laser. The SI uncertainty of TRF calibrations can be on the order of 500 parts per million (ppm) or less, with the SI scale traceable to NIST. However, the ability of satellite TSI sensors to reproduce TRF calibrations on orbit has yet to be determined experimentally. The LASP TSI Radiometry Facility (TRF) has been used to calibrate TSI sensors equivalent to the SORCE/TIM, ACRIMSAT/ACRIM3 and SOHO/VIRGO satellite sensors. The results calibrate the basic scale of these sensors' operation in SI as well as the scattering and diffraction effects of their field-of-view defining instrumentation.
Preliminary LASP/TRF testing of ACRIM3 flight backup instrument found a net ~ 5000 ppm difference between ACRIM3 and the TRF cryo-radiometer defined SI scale caused by scattering (~ 3500 ppm), diffraction (~1200 ppm) and a basic SI scale difference (~300 ppm). Application of the TRF corrections to the ACRIM3 observations has resulted in close scale agreement with those of the SORCE/TIM experiment. Similar results have been obtained for the SOHO/VIRGO instrument. Additional testing is planned to decrease the uncertainties of the ACRIM3 results and apply the same type of TRF characterizations to the results of representative sensors of the SMM/ACRIM1 and UARS/ACRIM2 satellite instruments.
• Preflight calibration of satellite TSI monitors does not guarantee in-flight observations with the laboratory level of SI uncertainty. Flight experiments with LHe cryogenic sensors need to be implemented to calibrate the effects of launch and flight environments on preflight calibrations.
1. Nimbus7/Earth Radiation Budget experiment (1978 - 1993)
2. Solar Maximum Mission/Active Cavity Radiometer Irradiance Monitor 1 (1980 - 1989)
3. Earth Radiation Budget Satellite/Earth Radiation Budget Experiment (1984 - 1999)
4. Upper Atmosphere Research Satellite/Active cavity Radiometer Irradiance Monitor 2 (1991 - 2001)
5. Solar and Heliospheric Observer/Variability of solar Irradiance and Gravity Oscillations (1996 --->)
6. ACRIM Satellite/Active cavity Radiometer Irradiance Monitor 3 (2000 --->)
7. Solar Radiation and Climate Experiment/Total Irradiance Monitor (2003 --->)