NASA has asked the National Institute of Standards and Technology (NIST) to help resolve a dispute among scientists stemming from disparate readings by a pair of instruments designed to measure the intensity of sunlight reaching the Earth.
At issue is which sensor is more accurate: the Total Irradiance Monitor on NASA’s Solar Radiation and Climate Experiment, launched in January 2003; or the Active Cavity Radiometer Irradiance Monitor 3 aboard the AcrimSat satellite launched in December 1999. Scientists on both sides of the debate are standing by their instruments.
“We’re very confident,” said Greg Kopp, a scientist at the University of Colorado’s Laboratory for Atmospheric and Space Physics in Boulder, which built the Total Irradiance Monitor. “We tried to make the instrument much better than the others in terms of absolute accuracy.”
AcrimSat scientists counter that their readings track well with the satellite record and with simultaneous readings from two radiometers aboard the Solar and Heliospheric Observatory, a NASA-European Space Agency spacecraft that watches the sun from an orbit 1.5 kilometers from Earth.
“I believe everyone else is right, and they’re wrong,” said Roger Helizon, a scientist at the Jet Propulsion Laboratory in Pasadena, Calif., which built the AcrimSat instrument.
Solar irradiance is a key measurement as scientists attempt to determine the role of solar variability in global warming. The stakes are high, because Total Irradiance Monitor instruments are slated to be aboard NASA’s $250 million Glory satellite, scheduled to launch in late 2008, as well as aboard the second of a new generation of U.S. polar-orbiting weather satellites slated to start launching around 2010.
“We want to make sure we’re flying the best calibrated instruments we can,” said Jim Butler, head of calibration efforts for NASA’s Earth Observing System, who made the request to NIST. The Earth Observing System is a series of satellites designed to study global climate trends and how they are affected by human activity.
NASA gathered the dueling instrument scientists at a workshop at NIST headquarters in Gaithersburg, Md., last July, but the two-and-half-day meeting only fanned the flames of the controversy . NASA then asked NIST to propose a plan to compare the accuracy of the instruments by testing spares of them with funds provided by NASA.
This coming May, NIST will propose obtaining flight spares or engineering models of the world’s four operational solar irradiance sensors and comparing their abilities to measure the intensity of a reference laser, said NIST physicist Joe Rice, who would lead the Total Solar Irradiance Intercomparison effort. Technicians also would use a laser measuring tool and microscope to inspect the openings, or apertures, of the instruments for geometric flaws that might be skewing the readings. Rice said the total cost of the two-year-long test program would be “at most hundreds of thousands of dollars.”
Scientists have been measuring the sun’s strength from the vantage point of space since 1978. The instruments all use the same general approach, with differences in materials and specific designs, solar scientists said. Sunlight enters a hollow cavity or cavities through small apertures. The light is trapped and converted into thermal energy.
Because no one knows how to directly measure the intensity of sunlight to scientific accuracy, the instruments rely on a trick that has been around for about 100 hundreds years, Rice said. The instruments measure the amount of electricity that is required to bring a reference chamber to the same temperature as the cavity that is receiving the sunlight. Using this so-called electrical substitution, the scientists can calculate the strength of the sun over a given area, called irradiance, and express that as watts per square meter, Rice said.
The readings from the Total Irradiance Monitor and the AcrimSat sensor are consistently separated by 5 watts per square meter, which works out to 0.4 percent of the sun’s intensity at the top of the atmosphere, or about 1,360 watts per square meter, the scientists said. Opinions differ on whether that disparity is so large that it should concern climatologists.
“That will matter when their models and their measurements get to be a bit more refined. Right now they’re not at a level where this is really driving or stopping them from doing anything,” said Kopp. The AcrimSat scientists said the disparity shouldn’t matter at all because the readings from the radiometers track each other very well relative to the sun’s fluctuations.
“Quite frankly I don’t really care [about the disparity.] I’ve never cared. [AcrimSat ] makes very precise and repeatable measurements,” said Columbia University physicist Richard Willson, the AcrimSat principal investigator. Willson said the NIST experiments would be “mostly a NASA self-justification activity, in my opinion.”
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