| A
Closer Look: The SIM Instrument –
The Spectral
Irradiance Monitor (SIM) is making the first continuous record
of the top of the atmosphere spectral solar irradiance in the
visible/near infrared region. The SIM provides spectral irradiance
measurements with a newly developed prism spectrometer, which
includes a miniaturized electrical substitution radiometer (ESR).
It provides spectral measurements in the near-ultraviolet, visible,
and near infrared – from 200 to 2000 nm. Jerry Harder is
the SIM instrument scientist at LASP.
The SORCE
spacecraft has been on orbit for about 300 days, and SIM’s
performance has met expectations for the full duration of that
time; all of the detectors and mechanisms are performing to design
specifications. Nonetheless, at this point in time scientists
are still learning how to best use the instrument and are ‘tweaking’
the instrument’s operation plans to maximize the scientific
output of the flight instrument. Laboratory studies at LASP are
continuing to refine the in-flight instrument calibration. Of
particular scientific interest is the new insight researchers
are gaining from SIM’s infrared measurements. Juan Fontenla
(a member of the SIM analysis team since October 2002) is the
lead author on a paper entitled “The Signature of Solar
Activity in the Infrared Spectral Irradiance” that describes
a comparison of solar irradiance trends predicted by a solar irradiance
model and observed by SIM. This comparison indicates that in the
visible there is agreement between SIM and the model, but the
same model is inaccurate in the infrared because it predicts darkening
of plage/faculae regions near 1.6 microns where SIM observations
indicate an increase in the spectral irradiance at these wavelengths.
This paper has just been submitted to The Astrophysical
Journal Letters. Interested readers can obtain preprints 
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Spectral
IrradianceMonitor (SIM)
of
the paper after it passes through the initial review process.
To
demonstrate the solar variability that SIM can now observe,
Figure 1 shows concurrent data for TIM (blue data on right hand
axis 6-hour averaged data) and SIM (red data on left-hand axis
at 885.7 nm) for a time period covering a single solar rotation
period in June of 2003. The graph is set up so that fractionally
the full-scale reading is the same for both instruments. Notice
that the amplitude and phase of the two data sets compare very
well, and scientists will be able to study the time series for
each wavelength cover by SIM. Further study of the SIM diode
responsivity function is still needed to understand the small
discontinuous jumps seen in these data sets. The ESR data is
still being analyzed and has yet to be fully incorporated in
these diode data.
Figure
1. (to the left) Concurrent time series for TIM and SIM in June
of this year. The irradiance scale for the TIM solar irradiance
is on the right, and the spectral irradiance for SIM at 885.71
nm is on the left. Notice the concurrence in amplitude and phase
of the time series showing that both instruments are following
sunspot irradiance modulation during this time period.
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