Solar Spectral Irradiance (SSI) Variations
NIST Workshop — February 28 to March 1, 2012
Introduction and Overview of Our Challenge
Gary Rottman
LASP (retired)
gary.rottman@lasp.colorado.edu
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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.
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Why it’s Important to measure the solar irradiance
— TSI and SSI
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Incoming Solar Radiation
Top of Atmosphere
-2
-1
Irradiance (W m nm )
2.0
1.5
At Surface
1.0
10 m Below
Ocean Surface
0.5
0.0
500
1000
1500
2000
Wavelength (nm)
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Basic Radiometric Quantities- 1
The total Solar irradiance (TSI) or radiant flux
density is the radiant flux across a surface
element, dA :
{W / m2}
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Record of Total Solar Irradiance
1%
2) Emphasize corrections that move
the one
dataof
in these
the desired
1) At most
data direction
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
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Success Story of the TSI Measurement Program
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Model of TSI
Solar variability on all temporal
and spatial scales is intimately
connected with variations of
the solar magnetic field
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Model estimates of
Solar Variations vs. Wavelength
Achievable
Required
Solanki and Unruh
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Basic Radiometric Quantities- 2
The Solar Spectral Irradiance (SSI), Eλ, is the radiant flux
density per unit wavelength interval:
{W / m3}
[Σ=1361 W/m2]
NOTE: the Total Solar Irradiance, TSI, is the
integral over all wavelengths of the Solar Spectral
Irradiance.
Solar Irradiance
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Digital Data to the Ground
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In-flight Change in Instrument Responsivity
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.
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Ground Data Processing
detector data
E( λ,t) =
C(t)
− SL + diff ± ?
T(t, λ )D(t, λ)As∆( λ)
• Instrument data
• Spacecraft data
• Orbit/attitude
Model estimates
Other observations
Processing algorithm or
— data transform or
— measurement equation
Pre-launch knowledge (σ’s)
σ
and assumptions ( ’s)
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Example of a time series
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Extended Time Series from Multiple Instruments
198
0
198
5
199
0
199
5
200
0
200
5
201
0
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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Extended Time Series from Multiple Instruments
σ = 2%
198
0
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5
199
0
199
5
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0
200
5
201
0
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Extended Time Series from Multiple (one) Instrument
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Internal Instrument Changes
scattered light
overlapping orders
contamination
solar exposure
aging
temperature
electronic drifts
dosage
shifts in optics
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+++++++
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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
Pick your poison
• 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
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UV-C
UV-A
UV-B
work functions
of most materials
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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
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Estimates of Solar Cycle Variability
(model results of Solanki and Unruh)
• 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
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
21
22
23
• 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.
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Spectral Irradiance Measurements
Maximum/Minimum - 1
(From Solanki et al.)
Solar cycle variation of
spectral irradiance —
300 < λ < 2000 nm
10%
1%
UARS Capability
.1%
.01%
SIM Capability
10 ppm
500
1500
1000
2000
Wavelength (nm)
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.
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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 ?
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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.
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