Monthly Newsletter

August 2003

A Closer Look: The TIM Instrument – 

The Total Irradiance Monitor (TIM) measures the solar radiant power density incident at the top of the Earth’s atmosphere, continuing a 25-year record of such measurements from several spaceborne instruments. The total solar irradiance (TSI) measurements show solar variability due to activity on the Sun (Figure 1) and provide important information about the primary external driver of the Earth’s climate. The TIM reports four TSI measurements per day, with data currently available through NASA Goddard’s DAAC website at and through the SORCE website at

Figure 1: The Sun’s output varies with solar activity, as this plot of TIM data correlated with images of the solar disk shows. Dark sunspots decrease the Sun’s irradiance, while associated magnetic activity increases it. (Solar disk images are courtesy of SOHO/MDI consortium.)

The SORCE was launched on January 25, 2003, and the TIM was powered on a few days thereafter. The TIM began acquiring solar data in early March, after a one-month period of spacecraft and instrument commissioning. The instrument is fully functional, with all detectors and mechanisms working properly. Regular solar observations with the TIM’s primary electrical substitution radiometer (ESR) provide nearly continuous TSI measurements during the sunlit side of each SORCE orbit. Duty cycling the other three ESRs gives an intermittent relative comparison against the primary ESR for tracking degradation; so far, we see no change attributable to degradation, although these analyses continue. Ground

processing of the TIM data uses phase-sensitive detection, a first for a spaceborne TSI instrument, to convert the measured ESR power into solar irradiance while reducing sensitivity to noise and spurious thermal signals.

TIM is a radiometer that measures TSI with 0.01% absolute accuracy and a relative stability of 0.001% per year. Greg Kopp is the TIM instrument scientist at LASP responsible for data analyses and instrument calibration.

Initial intercomparisons between the four TIM ESRs were less consistent than expected. It was soon found through testing of an identical TIM instrument here in the lab that a small non-linearity correction needs to be applied. This was verified for the SORCE instrument on orbit, and now after making the correction based on the lab unit, the agreement between TIM channels is greatly improved with deviations of about 0.03%. Initial analyses of the TIM data yield TSI measurements about 0.3% lower than the other currently-operating instruments. This discrepancy indicates either higher uncertainties in the characterizations/calibrations of previous TSI measurements or a possible TIM calibration error; this discrepancy is the subject of much current TIM analysis.

Special Session at AGU Meeting –

A special session relevant to SORCE has been accepted for the AGU Fall Meeting in San Francisco, December 8-12. Falling under the Solar and Heliospheric

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