SORCE SIM Version 23 and
SIM and OMI Comparisons
Tom Woods
tom.woods@lasp.colorado.edu
SIST May 2017 Meeting

SIM Version 23 released this week
SIM A
SIM B
SIM A – SIM B

• The SIM A-B difference
trends are improved in
V23 primarily in the
first year of the mission
(2003) and in the DOOp mode (2014present).
• The SIM A-B differences
are shown for Version 22
and 23.

335 nm, V23 data

656 nm, V23 data

V23 A-B

V23 A-B

V22 A-B

V22 A-B

SIM-SOLSTICE Overlap
• SORCE SOLSTICE version 15 and SIM versions 22-23 data have improved
corrections and consequently have improved comparisons.
SOLSTICE and SIM overlap in the 240-305 nm range.
SIM has small offset to normalize to SOLSTICE at
solar cycle minimum in 2009.

245 nm

260 nm

280 nm

290 nm

Their trends agree very well between
2008 and 2017 and are within their
stability uncertainty of ±2.5% (0.35%/year
over 7 years) for 2003 to 2008.
There are on-going studies to better
understand the early SORCE mission years.

SIM and OMI Comparison
Measurements and models currently agree at
some wavelengths and disagree at others.
Figure 2 from Marchenko et al. (JSWSC, 2016) is
good example for such comparisons.
• SORCE, OMI, and model trends agree over 2009-2015
for 267.5 nm, 280 nm, and 285 nm.
• OMI and model trends agree but these disagree with
SORCE for 313 nm, 340 nm, and 390 nm.
• SORCE and OMI trends agree but these disagree with
models for 442 nm, 471 nm, and 500 nm.

SORCE SSI Quality for TSIS-1 Overlap
• SORCE SIM data quality for the TSIS-1 overlap is acceptable at most wavelengths,
but there are limitations at some wavelengths.

One indication of SSI data quality is the ratio of the
solar variability to the measurement noise, called the
variability-to-noise ratio (VNR). The variability used
for this ratio is the median of the monthly peak-topeak solar variability over the SORCE mission. Quality
categories for the SSI comparisons are as follows:
• VNR > 5: Excellent for comparison
• 3 < VNR < 5: Acceptable for comparison
• VNR < 3: Marginal for comparison

Variability-to-Noise Ratio (VNR)

• Each of the ~1800 wavelengths in the SSI have to be considered as independent data sets that
need comparisons / validation with multiple data sets at the same wavelength.

Category

Excellent

Acceptable

Marginal

Number Wavelengths

601

294

268

Percent of Total Wavelengths

52%

25%

23%

Comparison of SORCE SIM and OMI Quality

• OMI solar irradiance measurements provide additional validation in the 260-504 nm range.

Variability-to-Noise Ratio (VNR)

• SORCE Variability to Noise Ratio (VNR) values are better than OMI for λ < 305 nm and for
λ > 420 nm. OMI VNR values are higher for 305-420 nm range.

2017 has had 28 spot-free days (SSN=0)
• Solar Cycle minimum is expected in 2019-2020.