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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 daac.gsfc.nasa.gov/upperatm/sorce/
and through the SORCE website at
tsi_data.html.
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
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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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