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Solar Spectral Irradiance (SSI) Variations NIST Workshop — February 28 to March 1, 2012
Introduction and Overview of Our Challenge
Gary Rottman LASP (retired)
2/27/12 1 SSI Variations Workshop
Goals of SSI Workshop(s)
• Examine SSI instruments, their capabilities, their observations, and the uncertainties associated with the measurements.
• Consider how these data were analyzed. How are solar variations separated from instrument effects.
• Establish an understanding of SSI differences. Refine their uncertainties.
• Make plans for the future — studies, calibrations and future meetings.
2/27/12 2 SSI Variations Workshop
Incoming Solar Radiation
2/27/12 4 SSI Variations Workshop
2.0
1.5
1.0
0.5
0.0
I r r a
d i a n
c e ( W
m - 2
n m - 1
)
2000 1500 1000 500
Wavelength (nm)
Top of Atmosphere
At Surface
10 m Below Ocean Surface
The total Solar irradiance (TSI) or radiant flux density is the radiant flux across a surface element, dA :
{W / m2}
Basic Radiometric Quantities- 1
2/27/12 5 SSI Variations Workshop
Record of Total Solar Irradiance
1% 1) At most one of these data sets is correct 2) The mean of all four is almost certainly not correct
1) Apply Hooke’s Law: — restoring force is proportional to the displacement from equilibrium
2) Emphasize corrections that move the data in the desired direction
2/27/12 6 SSI Variations Workshop
8
Model of TSI
Solar variability on all temporal and spatial scales is intimately connected with variations of the solar magnetic field
2/27/12 SSI Variations Workshop
Achievable
Required
Model estimates of Solar Variations vs. Wavelength
Solanki and Unruh
2/27/12 9 SSI Variations Workshop
The Solar Spectral Irradiance (SSI), Eλ, is the radiant flux density per unit wavelength interval: {W / m3}
Basic Radiometric Quantities- 2
Solar Irradiance Digital Data to the Ground
[Σ=1361 W/m2]
NOTE: the Total Solar Irradiance, TSI, is the integral over all wavelengths of the Solar Spectral Irradiance.
2/27/12 10 SSI Variations Workshop
First, do everything you can to reduce optical instability
— maintain cleanliness, select materials, reduce exposure
Goal is to establish (in-flight) change in responsivity and correct solar data accordingly
1. Conduct in-flight calibrations: • Carry an irradiance standard — FEL lamp, D2 lamp, etc. • Use an astronomical standard — stars, the moon, the Sun (!)
2. Use redundant systems — instruments, optical channels, detectors, etc. Employ varying duty cycles to solar exposure (1, 0.1, 0.01, etc.) and then
build an exposure/time dependent model of the responsivity, R(,t)
3. Return instrument from space and repeat calibration. From pre-flight and post-flight calibrations, interpolate responsivity to time of solar observation.
In-flight Change in Instrument Responsivity
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Ground Data Processing detector data
Processing algorithm or — data transform or — measurement equation Pre-launch knowledge (σ’s)
and assumptions (σ’s)
• Instrument data • Spacecraft data • Orbit/attitude
E(λ,t) =C(t)
T(t,λ)D(t,λ)As∆(λ)− SL + diff ± ?
Model estimates
Other observations
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1980
1985
1990
1995
2000
2005
2010
Extended Time Series from Multiple Instruments
Lean, J., and M. DeLand, 2012: How Does the Sun’s Spectrum Vary?, J. Climate Doi:10.1175/JCLI-D-11-00571.1
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1980
1985
1990
1995
2000
2005
2010
Extended Time Series from Multiple Instruments
σ = 2%
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Internal Instrument Changes
aging temperature electronic drifts dosage shifts in optics
scattered light overlapping orders contamination solar exposure +++++++
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Environment/Operational Changes
— changes in solar pointing — atmospheric absorption (SZA) — airglow (SZA) — off-axis scattered light
a. limb scattering b. f.o.v. intrusions
— energetic particles in space — spacecraft outgassing — plus others
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Exposure Changes
UV-A UV-B
UV-C
work functions of most materials
• Outgassing throughout the interior of the instrument is likely ongoing
• Solar exposure of optical surfaces and/or contamination on these surfaces may change transmission
• Most of the “train wreck” happens at the first optical surface
• Some scattering may proceed from there to the walls of the spectrometer
• Fluorescence may occur • Photo-polymerization (or other
chemistry) may occur at the first optic • UV is used to smooth Plexiglas
Pick your poison
2/27/12 19 SSI Variations Workshop
What Can this Workshop Accomplish?
• Wait to see what we accomplish in three days. What new insight?
• Networking — Expand the number of experts who understand the SSI instruments and their data processing
• Evaluate assumptions made about instrument performance — postulate alternate assumptions and approaches to reconcile differences
• Evaluate uncertainties presently associated with the SSI data sets
• Advise the instrument teams on methods of reducing uncertainties 2/27/12 20 SSI Variations Workshop
• Model results are based on TSI and UV observations
• The UARS instruments provided the required accuracy for:
λ < 300 nm
• SIM provides the required accuracy for:
200 < λ <2000 nm
Estimates of Solar Cycle Variability (model results of Solanki and Unruh)
Required Capability
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TSI Measurements
• TSI varies by ~ 0.1% over the solar cycle • Solar cycles 21, 22 and 23 are roughly the same amplitude • Standard uncertainties have steadily improved from 5000 ppm to about 500 ppm
• TSI measurements require random uncertainty (Type A, or precision) on the order 50 ppm
• A single instrument can measure TSI variability even with large systematic uncertainty (Type B, or bias)
• Individual data sets are limited to about 5 years. — with overlap additional observations can extend the time base. Without overlap observations require combined standard uncertainties of ~ 100 ppm.
21 22 23
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Spectral Irradiance Measurements
Note: For measurements of spectral irradiance, all requirements are wavelength dependent. • If a single instrument is used, the systematic uncertainty can be large, as long as the random uncertainty is small (< .1 of variation — 10% in EUV, 1% in the UV, and 0.01% in the visible)
• For multiple data sets (again limited to < 5 years), if the measurement sets overlap the data can be combined. If the sets do not overlap, the measurements must have a combined standard uncertainty of less than 0.1 (ideal) to 0.3 (acceptable) of the solar variation.
(From Solanki et al.)
Solar cycle variation of spectral irradiance — 300 < λ < 2000 nm
2/27/12 23 SSI Variations Workshop
10 ppm
.01%
.1%
1%
10%
UARS Capability
SIM Capability
2000 1500 1000 500 Wavelength (nm)
M a x
i m u m
/ M i n
i m u m
- 1
To Control Type A Uncertainty
• Limit detector noise (phase-lock detection)
• Limit electronic noise (phase-lock detection)
• Repeatable and stable mechanisms
• Thermal stability
• Pointing stability and knowledge
• others terms ?
2/27/12 24 SSI Variations Workshop
To Control Type B Uncertainty Establish the responsivity of a “flight” instrument relative to International System of
Units (SI)
1) Transfer calibration from a known “standard” instrument ( > 1%) — the resulting uncertainty of the “flight” unit is the combined uncertainty of
the “standard” + uncertainty of the transfer technique + uncertainty from instrument unique parameters and corrections.
2) Measure “flight” instrument response against an “irradiance standard” ( > 1%) — the resulting uncertainty of the “flight” unit is the combined uncertainty of
the “irradiance standard” + uncertainty from instrument unique corrections.
3) Characterize the “flight” instrument as an “absolute sensor” ( < 1%) — characterize each term in the measurement equation. Roll-up the table of
the uncertainties in each term — may be a unit level calibration or a calculation.
2/27/12 25 SSI Variations Workshop