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Active Cavity Radiometer Irradiance Monitoring (ACRIM)*


A series of Active Cavity Radiometers (ACRs) were developed for TSI measurement 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 TSI database during more than 90 % of its 34 year history, covering three solar cycles since 1978.

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 redundant 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.

ACRIM and PMOD TSI Composite

Continuous time series of total solar irradiance (TSI) observations have been constructed from the set of redundant, overlapping total solar irradiance (TSI) measurements made by satellite experiments during the past 34 Years. One, the ACRIM composite [Willson & Mordvinov, 2003 (Fig. 1 (upper))], displays a significant upward trend in TSI of 0.04 percent per decade during solar cycles 21-23. Another, the PMOD composite [Frohlich & Lean, 1998 (Fig. 1 (lower))], displays no significant trend over this period using different combinations of TSI data sets, computational philosophy and assumptions. Both time series demonstrate no significant trend over the two decade period separating the first and third solar activity minima.

Figure 1.The ACRIM (upper) and PMOD (lower) TSI composites.

The TSI Observational Record

The potential significance of ACRIM's upward trend during solar cycles 21-23 as a climate forcing makes it important to explore the ACRIM-PMOD trend difference to determine which of the two composites best represents the TSI measurement database. Two types of experiment have provided TSI data: self-calibrating, precision TSI monitors and Earth radiation budget (ERB) experiments.

TSI monitoring experiments are designed to provide state of the art accuracy and precision. The ERB experiments (both Nimbus7/ERB and ERBS/ERBE) are designed to lower standards for TSI observations appropriate to the requirements of earth radiation budget investigations. TSI monitors are solar pointed so they can observe continuously (or during the sunlit part of every orbit) and are capable of self-calibrating the degradation of their sensors. Nimbus7/ERB viewed the sun while it passed through its field of view every orbit three days of every four. The ERBS/ERBE viewed the sun as it passed through its field of view during one orbit every two weeks. Neither ERB experiment could self calibrate the degradation of their sensors. Because of their multi-sensor design, TSI monitoring experiments are able to self-calibrate their sensor degradation and provide significantly greater  precision, accuracy and  traceability for their observations than the ERB experiments. Both the Nimbus7/ERB and ERBS/ERBE met their observational requirements but but their results are not competitive in traceability with those of TSI monitoring experiments.

An optimum composite TSI time series would utilize the results of monitoring experiments to the maximum extent possible because of the smaller uncertainties of their data. Some ERB results must be used, however, since there are two periods during the past 34 years during which only ERB observations are available.

The first ERB period extended from November 1978 to early 1980 during which time the Nimbus7/ERB TSI observations were the only measurements made. This period ended in February 1980 with the launch of ACRIM1 on the Solar Maximum Mission. ACRIM1 was the prototype TSI monitoring experiment that established the standards and approach used by succeeding experiments.

The second ERB period was during a two year gap in the TSI monitoring record between the ACRIM1 and ACRIM2 experiments (1989 - 1991). ACRIM2 was originally intended to be launched by the space shuttle in 1989 on the NASA Upper Atmosphere Research Satellite. The launch was delayed more than two years by the Challenger disaster. Compilation of a continuous record over the entire 34 years of satellite observations would not be possible were it not for the availability of ERB results during the gap. The relationship between ACRIM results across the gap can be derived using the overlapping ERB data sets: the Nimbus7/ERB or the ERBS/ERBE. These two choices are embodied in the construction of ACRIM and PMOD composites, respectively, shown in Figure 1.

* This content is summarized from